Table Of ContentNASA-CR-204846
Effect of hindlimb unweighting on tissue
blood flow in the rat
J ._j
K. S. McDONALD, M. D. DELP, AND R. H. FITTS
Biology Department(cid:0)Biological and Biomedical Research Institute, Marquette University, Milwaukee,
Wisconsin 53233; and Exercise Biochemistry Laboratory, University o[ Georgia, Athens, Georgia 30602
McDONALD, K. S., M. D. DELP, AND R. H. FITTS. E[fect o[ gravity soleus muscle to be most affected (13, 14). Both
hindlimb unweighting on tissue blood flow in the rat. J. Appl. HU and HI caused a significant reduction in soleus peak
Physiol. 72(6): 2210-2218, 1992.--The purpose of this study twitch and tetanic tension (13, 34, 37), and an increased
was to characterize the distribution of blood flow in the rat
fatigability (27, 37). After HI, the atrophied soleus dis-
during hindlimb unweighting (HU) and post-HU standing and played greater glycogen and ATP usage and lactate pro-
exercise and examine whether the previously reported (Witz-
duction during contractile activity {37}. In addition, so-
mann et al., d. Appl. Physiol. 54: 1242-1248, 1983) elevation in
leus muscle fiber glycogen, ATP, and creatine phosphate
anaerobic metabolism observed with contractile activity in the
stores are increased after 2wk HU (17, 18). These results
atrophied soleus muscle was caused by a reduced hindlimb
blood flow. After either 15 days of HU or cage control, blood suggest a shift in the atrophied soleus toward a greater
flow was measured with radioactive microspheres during un- dependence on anaerobic metabolism. This shift oc-
weighting, normal standing, and running on a treadmill {15 curred despite an increased oxidative capacity (per fiber
m/min). In another group ofcontrol and experimental animals, weight) in the soleus after HU and HI (12). Thus the
blood flow was measured during preexercise (PE} treadmill purpose of this study was to use the radiolabeled micro-
standing and treadmill running (15 m/min). Soleus muscle sphere technique to 1) characterize the distribution of
blood flow was not different between groups during unweight- blood flow during HU and post-HU standing and exer-
ing, PE standing, and running at 15m/min. Chronic unweight-
cise and 2) examine whether the increased reliance on
ing resulted in the tendency for greater blood flow to muscles
anaerobic energy production of the atrophied soleus dur-
composed of predominantly fast-twitch glycolytic fibers. With
ing contractile activity results from a decreased muscle
exercise, blood flow to visceral organs was reduced compared
with PE values in the control rats, whereas flow to visceral blood flow.
organs in 15-day HU animals was unaltered by exercise. These The results demonstrate that, compared with the con-
higher flows to the viscera and to muscles composed of predomi- trol standing value, blood flow to the soleus was reduced
nantly fast-twitch glycolytic fibers suggest an apparent reduc- during unweighting in both the control {acute 10-min un-
tion in the ability of the sympathetic nervous system to distrib- weighting) and the 15-day HU group and the 15-day HU
ute cardiac output after chronic HU. In conclusion, because 15 group with normal standing. No differences between
days of HU did not affect blood flow to the soleus during exer- groups were observed in soleus muscle blood flow during
cise, the increased dependence of the atrophied soleus on aner- a 5-min bout of treadmill exercise; however, the redistri-
obic energy production during contractile activity cannot be
bution of visceral flow with the onset of exercise was
explained by a reduced muscle blood flow.
attenuated after 15 days of HU.
energy metabolism; muscle atrophy
METHODS
Animals. Male Sprague-Dawley rats (250-275 g) were
SPACEFLIGHT iS known to produce bone demineraliza- obtained and taught to run on a motor-driven treadmill
tion, atrophy to antigravity muscles, and cardiovascular (10 min/day, 20 m/min for 10 days) and then randomly
changes (4, 36). Land-based animal models, including assigned to the HU or cage control group. The hindlimbs
hindlimb immobilization (HI) and hindlimb unweighting of the HU animals were partially elevated for 15 days
(HU), have been developed to simulate weightlessness with a harness attached to the proximal two-thirds of the
conditions. These models have proven to elicit changes in tail, as previously described in detail {13). The height of
bone composition and muscle structure, function, and hindlimb elevation was adjusted to prevent the hind-
biochemistry that closely mimic those observed with mi- limbs from touching supportive surfaces. The forelimbs
crogravity (13, 14, 16, 23, 26, 28). In addition, HU elicits maintained contact with a grid floor, which allowed the
hemodynamic changes--including a transient increase animal full range of motion. The HU animals were fed
in central venous pressure (31), natriuresis and diuresis Purina rat chow and water ad libitum, whereas control
(6), reduction in blood volume (11, 30), and a decrease in rats were pair fed to maintain weights similar to those of
exercise capacity (10, 30)--similar to those induced by the HU group. Both groups were housed at 23°C with a
spaceflight (4). 12:12-h light-dark cycle per day.
The effects of simulated weightlessness on muscle Surgical procedures. After 15 days the HU and control
properties have been well characterized. Whole muscle rats were anesthetized with methoxyflurane (Metofane)
and single fiber studies have shown the postural, anti- anesthesia. The HU group were anesthetized while re-
2210 0161-7567/92 $2.00Copyright ,:_:1_992the American Physiological Society
HINDLIMB UNWEIGHTING EFFECTS ON TISSUE BLOOD FLOW 2211
maining unweighted to avoid any weight-bearing muscu- TABLE 1. Tissue weights
lar activity. A catheter (Silastic; 0.6 mm ID, 1.0 mm OD)
was surgically implanted in the ascending aorta via the Tissue Weight, Tissue Weight/Body
mg Weight, mg/g
right carotid artery as previously described (20). This
catheter was subsequently used for recording arterial Tissue C HU C HU
pressure and the infusion of radiolabeled microspheres
Soleus 152+4 88+3* 0.48_+0.01 0.30_+0.01"
for blood flow measurements. A second catheter (polyure-
Plantaris 352_+8 298_+8* 1.12_+0.02 1.00_+0.02"
thane; 0.36 mm ID, 0.84 mm OD) was inserted _4 cm
Gastrocnemius 1,775_+44 1,490_+38* 5.64_+0.13 4.99_+0.10"
into the caudal artery as described by Chiueh and Kopin Tibialis anterior 676_+16 659-+11 2.14-+0.04 2.21_+0.02
(4) with several modifications. This catheter was used to EDL 174-+4 169_+3 0.55_+0.01 0.57+0.01
withdraw blood for the blood flow measurements. Both Vastus intermedius 120_+7 86_+4* 0.38_+0.02 0.29_+0.01"
Vastus lateralis 1,202+28 1,107+27" 3.82-+0.09 3.74_+0.07
catheters were filled with a heparinized (200 U/ml) saline
Adrenal medulla 36_+2 40_+2 0.11_+0.01 0.14_+0.01"
solution and externalized on the dorsal cervical region. Adrenal cortex 25+2 29+2 0.07+0.01 0.10-+0.01"
Experimental protocol. After 12 h of recovery from Left kidney 1,285_+28 1,317-+24 4.10_+0.12 4.44_+0.11
catheter implantations, control and HU animals were in- Spleen 650_+21 688_+33 2.07_+0.07 2.31_+0.11
strumented for blood flow measurements. The hindlimbs Stomach 1,680_+42 1,564_+28" 5.33+0.10 5.23_+0.10
Duodenum 471_+25 511_+25 1.50_+0.08 1.71_+0.07
of the control rats were elevated, and after a 10-min equil-
Femur 1,119_+18 1,020-+13" 3.56-+0.06 3.44_+0.06
ibration period, blood flow was measured in both groups.
Values are means + SE for 18 control and 21 hindlimb-unweighted
The rats were then lowered to a standing hindlimb-
rats. C, control group; HU, 15-day hindlimb-unweighted group; EDL,
weight-bearing position, and blood flow was determined
extensor digitorum longus. *HU significantly different from control
after 10 min of normal standing. Finally, the animals value, P < 0.05.
were run at 15 m/min on a motorized treadmill, and
blood flow measured at the 5th min of running. In an- as a post hoc test to estimate differences among means.
other group of control and experimental animals, the To test for differences between control and 15-day HU
rats were placed directly on the treadmill after the 12-h mean values, a Student's unpaired two-tailed t test was
recovery from catheter implantations, and blood flow used. In all cases, the level of significance was set at
measured after 10 min of standing. In the text, this mea- P < 0.05.
surement is referred to as the preexercise (PE) blood
flow. In this group, asecond blood flow measurement was
RESULTS
made during the 5th rain of running at 15 m/min. Be-
cause the blood flow measurements during exercise were Tissue weights. Average values of tissue weights on
not affected by the type of PE standing (normal vs. which blood flow was measured are shown in Table 1.
treadmill), these values were pooled (see Table 3). After Tissue weights are expressed in both absolute units and
the final blood flow measurement, pentobarbital sodium normalized per body weight. In agreement with previous
was infused through the carotid catheter to induce deep reports (10, 13, 14) the postural, antigravity soleus mus-
anesthesia. The rats were then decapitated; and tissue cle exhibited the greatest atrophy of the hindlimb mus-
samples were excised, weighed, and placed in counting cles after HU. The soleus muscle weight was reduced
vials for flow determination. 42% after HU [154 _+4 vs. 88 + 3 (SE) mg], and when
Blood flow measurements. Radiolabeled (85Sr, ll3Sn, normalized the average soleus muscle-to-body weight ra-
and 'SaGd) microspheres (New England Nuclear) with a rio was 38% lower than the control. The plantaris and
15-tLm diam were used for blood flow measurements as gastrocnemius muscles also showed significant atrophy
previously described (21). Radioactivity of the samples after HU. In addition, the deep knee extensor, vastus
was measured with a gamma counter (Packard Auto- intermedius, was markedly smaller (tissue/body weight,
Gamma 5780), and flows were computed (IBM-PC com- 76% of control) after HU. The mean value of the vastus
puter) from counts per minute and tissue wet weights. To lateralis (mg) was significantly smaller in the HU group;
ensure that microsphere-blood mixing was adequate, however, when expressed per body weight, no significant
flows to the right and left kidneys were compared for difference existed. The adrenal medulla and cortex hy-
each animal. Mixing was considered adequate if both pertrophied after 15 days of HU and when expressed per
kidney flows were within 15% of each other. body weight were significantly larger than control (Table
Central hemodynamic measurements. Mean arterial 1). In addition, both the stomach and femur weights (mg)
pressure and heart rate were recorded from the carotid were significantly reduced after HU; however, when ex-
catheter just before each microsphere infusion using a pressed per body weight the difference was not signifi-
pressure transducer (Narco Bio-Systems) and recorder cant.
(Narco physiograph). Only mean arterial pressure was Distribution of blood flow. The absolute blood flows
reported because of the damping effect of the catheters. (ml/min) and blood flows expressed per 100 g tissue
Heart rates were determined from the pulsatile pressure weight (ml. min-'. 100 g-l) during unweighting and qui-
recordings. escent standing are shown in Table 2,whereas blood flow
Data analysis. A one-way analysis of variance with re- during PE standing and running (15 m/min) on the
peated measures was used to compare heart rates, mean treadmill are presented in Table 3. In both the control
arterial pressures, and blood flows within tissues across (unweighted acutely for 10 min) and 15-day HU animals,
unweighting, standing, PE, and 15-m/min running con- flow to the soleus was significantly lower during un-
ditions. Duncan's new multiple-range test was then used weighting compared with normal quiescent standing
2212
HINDLIMB UNWEIGHTING EFFECTS ON TISSUE BLOOD FLOW
TABLE 2. Blood flows in organs and tissues during (control, 8_+1vs. 36 + 5; HU, 10 + 2vs. 23 +_4 ml. min -1.
hindlimb unweighting and standing 100 g-l). The flow to the red portion of the tibialis ante-
rior (TA), an ankle flexor muscle, was significantly
Blood Flow greater during unweighting than while standing in the
chronically unweighted animals (35 + 8 vs. 14 + 3
ml.min 1.10 _ ml.min 1.100 g
ml. min -_. 100 g-l). This increase in TA blood flow is
Tissue C HU C HU likely due to plantar flexion of the foot and subsequent
stretching of the TA, which occurs frequently during
Soleus (11) (11)
Unwt 1.6+0.5 0.8_+0.2 8_+1 10_+2 chronic HU. Also during unweighting, the flow to the
(11) (10) white portion of the vastus lateralis was significantly
Stand 5.3+0.7¢ 1.9+0.4t 36+_5t 23_+4t*
greater in 15-day HU animals than in control animals
Plantaris
(2 + 1vs. 8_+2 ml. min -1. 100 g-l) (Table 2, Fig. 1). This
Unwt 2.3-+0.7 2.7_+0.8 7-+2 12-+3
Stand 2.3_+0.4 5.3_+1.8 7+_1 18_+6 was the only tissue that exhibited a significant difference
Red gastrocnemius between the two groups during unweighting.
Unwt 0.8_+0.2 1.4-+0.3 8_+3 14_+3
During normal quiescent standing, the blood flow to
Stand 1.9_+0.1 3.5-+1.2 20_+2 38-+15
White gastrocnemius the soleus was found to be significantly lower in the HU
Unwt 1.3_+0.3 1.3+0.4 7-+1 8_+3 than in the control group (36 _+5vs. 23 + 4ml. min -_. 100
Stand 0.9_+0.2 0.7_+0.3 4_+1 5_+2
g-_; Fig. 2). Although the PE standing blood flow (ml/
Mixed gastrocnemius
Unwt 11.2_+3.3 10.9_+1.7 7_+2 9_+2 rain) in the soleus was significantly lower than control
Stand 10.9+2.6 15.6_+4.2 8_+2 13_+3 soleus (0.12 + 0.02 vs. 0.05 _+ 0.01 ml/min), flow in
Tibialis anterior red ml. min -1. 100 g-1 was not different between the two
Unwt 3.0_+1.1 3.6_+0.8 27_+9 35_+8
Stand 3.3_+0.8 1.5_+0.3 34_+7 14_+3t* groups during unweighting, PE, or 15 m/min running
Tibialis anterior white (Fig. 2). As with the soleus, blood flow to the red portion
Unwt 7.5_+2.0 9.6_+2.4 13_+4 15_+4 of TA during standing was significantly lower
Stand 6.4_+1.0 4.1_+0.6 12_+2 8+1
(34 _+7 vs. 14 _+3 ml- min -1. 100 g-l) in 15-day HU rats
EDL
Unwt 1.4+0.3 1.6_+0.6 8_+2 10-+4 than in controls; and flows during unweighting, PE, and
Stand 1.3+0.3 1.3_+0.4 8_+1 8-+3 15 m/min running were not different between groups
Vastus intermedius
(Fig. 3). Tissue blood flow in ml. min -1. 100 g-1 during
Unwt 2.3_+0.5 1.8+0.5 20_+5 21_+5
Stand 4.0_+0.8 4.0_+1.1 37_+7 44-+11 PE standing was not different between groups except in
Vastus medialis the kidneys, where flow was significantly less in the HU
Unwt 7.4_+1.8 6.9_+2.0 13_+3 14_+3 group (Table 3; see Fig. 5).
Stand 6.3_+2.4 6.0_+1.3 11_+4 14_+3
Both groups showed an exercise-induced increase in
Vastus lateralis red
Unwt 5.1_+1.4 2.5_+0.6 32-+9 17_+4 hindlimb musculature blood flow. However, only the
Stand 4.3_+1.1 4.0_+1.3 28_+7 27-+8 control group underwent a consistent decrease in blood
Vastus lateralis white
flow to visceral organs (Table 3, Figs. 4and 5). For exam-
Unwt 0.3_+0.1 1.6_+0.5" 2_+1 8-+2*
Stand 2.5_+2.1 1.1_+0.3 2_+1 5_+1 ple, exercise in control rats was associated with a marked
Vastus lateralis mixed drop in duodenum and kidney blood flow compared with
Unwt 6.8-+1.7 7.9_+1.8 8_+2 11_+3 the PE standing value, whereas flow to these two organs
Stand 4.4_+1.1 6.1_+1.7 6_+1 9-+2
in HU animals was unaltered by exercise (Figs. 4 and 5).
Adrenal medulla
Unwt 3.6_+0.7 4.3_+0.7 100_+15 105+21 Heart rate and blood pressure. Mean heart rate and arte-
Stand 3.0_+0.5 3.7-+0.5 90_+12 97-+18 rial blood pressure values for control and HU animals
Adrenal cortex
during the four conditions are presented in Table 4. In all
Unwt 14.0_+4.0 21.0_+6.0 482_+126 683-+168
Stand 13.0_+3.0 22.0_+6.0 471_+98 732_+189 four conditions mean heart rates were higher in the HU
Left kidney group, with significant differences observed during PE
Unwt 750.0-+80.0 650.0_+60.0 576_+58 488_+45
and exercise (Fig. 6). Mean arterial blood pressure did
Stand 800.0_+61.0 690.0_+60.0 611_+46 527_+46
not differ between control and HU groups except during
Right kidney
Unwt 750.0_+80.0 680.0_+60.0 575_+64 510_+49 PE standing.
Stand 800.0_+58.0 710.0_+70.0 623-+44 540-+50
Spleen
Unwt 80.0_+27.0 68.0-+21.0 124_+42 71_+12 DISCUSSION
Stand 91.0-+13.0 91.0_+21.0 143_+22 123_+23
Stomach A major goal of this study was to characterize the dis-
Unwt 110.0_+20.0 110.0_+20.0 67_+13 71-+12
tribution of blood flow between cage control and 15-day
Stand 130.0-+23.0 120.0_+20.0 80_+15 78-+10
Duodenum HU rats during unweighting, standing, and a bout of ex-
Unwt 100.0-+20.0 100.0-+20.0 232_+43 217_+46 ercise. The important findings were I) blood flow was
Stand 100.0_+13.0 120.0_+10.0 215_+25 255_+26 significantly reduced to the postural antigravity soleus
Femur
Unwt 21.0_+2.0 18.0_+3.0 18_+2 17-+3 muscle during both acute (10-min) and chronic (15-day)
Stand 23.0_+2.0 22.0_+2.0 20_+1 22_+2 HU compared with flow during standing; 2) after 15 days
of HU, blood flow to the soleus was significantly reduced
Values are means _+SE for number of observations shown in paren-
theses. Unwt, 10-min acute and 15-day hindlimb unweighting for con- during normal standing but not during a bout of exercise
trol and HU groups, respectively; Stand, 10-min standing after the at the same absolute work load; and 3) 15 days of HU
unweighting in both groups. *HU significantly different from control altered the redistribution of blood flow generally elicited
values, P < 0.05; t standing significantly different from unweighted
values, P < 0.05. by exercise.
HINDI,IMB UNWEIGHTING EFFECTS ON TISSUE BLOOD FLOW 2213
TABLE 3. Blood/lows to organs and tissues during preexercise treadmill standing and
treadmill running (I5 re(cid:0)rain)
BloodFlow Blood Flow
ml- min _.102 ml. min a.100g ml.min _-10_ ml-min _.100g
Tissue C HU C HU ']['issue C HU C H[
Soleus (5) (6) Vastus lateralis red
PE 12.0+2.0 5.0_+1.0" 69+_13 55+_11 PE 12.0_+3.0 13.0_+3.0 83-+21 107-+33
(9) (7) 15 m/min 28.0_+5.0 24.0_+5.0 192-+33 189-+41
15 m/min 14.0_+2.0 9.0_+2.0 85+8 105+21 Vastus lateralis
Plantaris white
PE 8.7-+1.8 9.7+_4.8 22-+5 31+16 PE 1.8_+0.4 2.2_+0.9 9_+2 9_+4
15 m/min 26.0+6.0 17.0+3.0 69+15 58-+9 15 m/rain 1.9_+0.6 3.:3_+0.9 10_+3 16_+4
Red gastrocnemius Vastus lateralis
PE 9.0+2.0 7.0_+2.0 94_+24 76+-22 mixed
15m/min 15.0_+3.0 10.0-+1.0 156+25 108_+12 PE 23.0_+5.0 17.0_+7.0 25_+5 21_+9
White 15 m/min 57.0_+17.0 42.0_+8.0 63_+18 56_+10
gastrocnemius Adrenal medulla
PE 0.8-+0.2 1.0+0.3 5+1 7-+2 PE 4.7-+1.0 6.7+_1.0 131+36 197-+51
15m/rain 1.9+0.4 3.3_+1.0 10_+2 26+8 15 m/rain 3.6_+1.0 5.6-+2.8 102±29 190_+107
Mixed Adrenal cortex
gastrocnemius PE 18.0±7.0 40.0_+16.0 898±336 1479±425
PE 34.0+-7.0 30.0+18.0 21+4 23_+10 15 m/rain 19.0_+7.0 17.0+_3.0 963_+415 758_+219
15m/rain 88.0+21.0 87.0+_16.0 57+_14 70+-11 Left kidney
Tibialis anterior red PE 1,170.0-+90.0 740.0_+120.0" 944+93 556+_84*
PE 5.7_+2.6 3.1+2.1 46+15 31+22 15 m/min 520.0_+80.0 640.0+_110.0 416+_71 493+_87
15m/min 16.0_+3.0 9.0+_2.0 159_+31 95_+17 Right kidney
Tibia|is anterior PE 1,220.0_+110.0 750.0_+90.0* 974_+92 556_+66*
white 15 m/rain 530.0_+90.0 600.0-+110.0 415+_75 469±_92
PE 13.0+_4.0 14.0_+10.0 20+-6 23+-16 Spleen
15m/rain 27.0-+5.0 30.0_+6.0 45_+9 55+_l0 PE 230.0_+40.0 130.0_+40.0 ',352_+72 207_+62
EDL 15m/rain 76.0_+7.0 28.0_+3.0 104_+41 45_+11
PE 3.7_+1.2 1.4+0.6 20-+7 8+_4 Stomach
15m/min 3.9_+0.7 5.1+1.5 22+4 30_+8 PE 340.0_+60.0 190.0_+70.0 185+28 122±42
Vastus intermedius 15m/rain 72.0_+13.0 75.0_+16.0 41_+7 49_+9
PE 19.0_+5.0 3.9_+1.0" 125_+33 47-+13 Duodenum
15m/min 22.0+_4.0 12.0+_3.0 160+_29 134+_29 PE 220.0_+50.0 220.0+_50.0 390+_76 341+_49
Vastus medialis 15m/min 41.0±8.0 150.0_+30.0" 79_+10 285_+43*
PE 17.0+4.0 7.2+3.0 34+10 17+7 Femur
15m/min 45.0_+15.0 32.0_+5.0 90+_30 73_+12 PE 26.0+_7.0 21.0_+6.0 '26_+6 21±6
15m/min 18.0_+3.0 20.0_+5.0 17_+2 21_+4
Values are means +_SE fornumber of observations shown inparentheses. PE. preexercise. *HU significantly different from control values, P<
0.05.
Two widely used models simulating weightlessness, twitch oxidative (SO) muscle (3). This was likely due to a
HU and HI, have been documented to effect soleus mus- reduction in muscular activity and a corresponding de-
cle structure, function, and metabolism. Witzmann et al. crease in the release of vasodilatory metabolites in the
(37) found 42 days of HI to increase the rate of fatigue soleus, which is near maximally active during postural
induced by electrode stimulation of the in situ soleus. maintenance (19, 33). In other muscles composed pre-
Additionally, the atrophied soleus exhibited a greater dominantly of fast-twitch oxidative-glyeolytic (FOG)
glycogen and ATP usage and higher lactate production and fast-twitch glycolytic (FG) fibers, blood flow did not
than did the control muscle (37). It is also known that change with acute unweighting in control animals, pre-
HU elicits an elevated glycogen content in the soleus sumably because of insignificant changes in muscular ac-
(17). Recent single fiber data have found 2 wk of HU to tivity, because these muscle types are relatively inactive
increase soleus muscle fiber glycogen, ATP, and creatine during standing (19, 33). However, blood flow to predomi-
phosphate (18). These findings suggest an atrophied-in- nantly FG muscle (i.e., white regions of the gastrocne-
duced shift in metabolism to a higher reliance on carbo- mius and vastus lateralis) after chronic unweighting was
hydrate and anaerobic (glycolytic) energy production. 157% higher than that of acutely unweighted muscle
This observed shift occurs despite an increased oxidative (Fig. 7). This may mechanistically be due to a decreased
capacity (per fiber weight) after both HU and HI (12, 16). responsiveness to sympathetic activation and, subse-
These observations led us to hypothesize that the in- quently, a decreased vascular tone and increased blood
creased reliance of the atrophied soleus on anaerobic en- flow. Overton and Tipton (29) observed an attenuated
ergy production might result from a reduced muscle mesenteric artery pressor response to exogenous sym-
blood flow, leading to a low O2 and free fatty acid (FFA) pathomimetic agents after 9 days of HU; chronic un-
delivery to exercising hindlimb muscles. weighting may produce a prolonged stimulus to sympa-
Both acute (10-min) and chronic (15-day) HU de- thetic drive (11, 29), as suggested by the observed hyper-
creased blood flow to the soleus, a predominantly slow- trophy of the adrenal glands (Table 1), resulting in a
2214 HINDLIMUBNWEIGHT1ENFGFECTOSNTISSUEBLOOFDLOW
o Control o Control
2O
• 15d HU 160
"-'18
v--I I 140 • 15d HU I
o16 o
o o 120
v,-"14
v--I 'lidI
_12 100
E E
E 10 E 80
8
3= ]_ 60
o o
_- 6
"to "Io 40
o o
o 4. o
,,n 20
2
0 I 1 I I 0 I I I I I
Unwt Stand PE 15m/mln Unwt Stand PE 15m/mln
FIG. 1. White vastus lateralis blood flow (mean _+SE) during 4con- FIG. 3. Blood flows (mean + SE) to red region of tibialis anterior
ditions. Unwt, acute 10-min hindlimb unweighting (HU) for control during 4conditions, x-Axis abbreviations, same as Fig. 1. *15-day HU
group and after 15days of HU for the HU group; Stand, normal stand- value significantly different from control, P < 0.05.
ing; PE, preexercise standing on treadmill; 15 m/min, treadmill run-
ning at 15 m/min. *15-day HU value significantly different from con-
trol value, P < 0.05. 500 o Control
450 • lSdHU T
o Control
v--
I
140 • 15d HU o_400
I
I _. 350
120
oo I
_=300
•- 100
I
¢- E 250
"E 80
E o_200
_- 150
6O '1o
o o
o 100
•v 40 m O+
o 5O
o
20 0 I I I I 1
Unwt Stand PE 15m/mln
0 I I I I a FIG. 4. Duodenum blood flow (mean +_SE) during 4conditions, x-
Axis abbreviations, same as Fig. 1.t15 m/min blood flow significantly
Unwt Stand PE 15m/rain
different from PE value, P < 0.05. *15-day HU value significantly dif-
F[O. 2. Soleus muscle blood flow (mean _+SE) during 4conditions. ferent from control, P < 0.05.
x-Axis abbreviations, same as Fig. 1.tStanding blood flow significantly
different from unweighted value, P < 0.05. *15-day HU value signifi- during quiescent standing. This is in agreement with
cantly different from control value, P < 0.05. LeBlanc et al. (23), who observed a decreased percent
uptake of microspheres by the soleus after 14-90 days of
downregulation of a-receptor number and/or sensitivity HU. However, soleus muscle blood flow measured while
{38). In the rat, muscle composed primarily of FG fibers the rats of the second study were standing on the tread-
has the potential for greater control of vascular tone by mill (Table 3) was not different between controls and
the sympathetic nervous system than that composed of 15-day HU animals (control 69 _+ 3; HU 55 + 11). Two
SO or FOG fibers (9, 21). Thus a reduction in sympathe- factors may account for the discrepancy in the standing
tically mediated vascular tone would be expected to pri- soleus muscle flows between the two studies. First, both
marily increase blood flow to muscle composed of FG control and 15-day HU animals in the first study were
fibers.
behaviorally very quiescent after being lowered from an
After 15days ofHU, blood flow to the soleus was signif- unweighted to standing position (on laboratory bench).
icantly reduced (23 +_4 vs. 36 _+5 ml. min-_. 100 g 1) The relaxed posture maintained by these animals may
HINDLIMB UNWEIGHTING EFFECTS ON TISSUE BLOOD FLOW 2215
o Control o Control
1000 • 15d HU 600 • 15d HU
m,,-
I
I ,/I
o
o 800 550
m,-.
I
c
"_ 600 500
E
o
3= 400 450
o
i 1=
N=- o
o zO
o 200 400
m
0 I I I I I 350 I l i I i
Unwt Stand PE 15m/mln Unwt Stand PE 15m/mln
FIG.5. Bloodflow (mean _+SE)to right kidney during 4conditions. FIG. 6. Heart rate (mean+SE)(luring 4conditions, x-Axis abbrevia-
x-Axis abbreviations, same asFig. 1.+15m/min hlood flowsignificantly tions, same as Fig. 1.*15-day HU significantly different from control
different from PE value, P < 0.05.*15-day HU value significantly dif- values, P < 0.05.
ferent from control, P <0.05.
265I
TABLE 4. Heart rate and blood pressure
MeanArterialBleod
: 255
HeartRate,beats/min Pressure,mmHg
26°I
Condition C HU C HU
LLI _ 250Y
Unweighted 433±14 (11) 460+18 (12) 1_8_+2(7) 138+_4(9)
Standing 394_+10(ll) 429+14 (12) 127+_3(8) 127+_4(10)
Preexercise 428±17 (5) 509+17' (6) 117+_4(5) 135±5" (6) A
Exercise 478+16 (9) 583_+11"(8) 1:),8+6(9) 143±4 (9)
(15m/rain) 40
Values are means + SE fornumber of observations shown inparen- i
theses. *HU significantly different from control values, P < 0.05. 50 FOG FG Duo SO FOG FG Duo SO FOG FG Duo
UNWEIGHTED STANDING RUN 15 rn/min
account for the low flow in the postural antigravity so- FIG. 7. Percent differences in blood flows (BF; ml•min-]•100g ')
between chronically unweighted (u) and control (c) rats for extensor
leus muscle compared with the other standing/preexer-
muscles composed primarily of slow-twitch oxidative (SO, soleus and
else flows in this and other studies (7, 19). Additionally, vastus intermedius muscles), fast-twitch oxidative-glycolytic (FOG,
reposturing to relieve the atrophied soleus muscle from red portions ofgastrocnemius and vastus lateralis muscles), and fast-
as much weight bearing as possible may have caused the twitch glycolytic (FG, white portions of gastrocnemius and vastus la-
teralis nmscles) fibersand duodenum (Duo)during hindlimb unweight-
lower soleus muscle blood flow in the 15-day HU animals
ing,preexercise standing, and exercise.
of the first study. Second, the higher standing/preexer-
else soleus muscle blood flows in the second study may be those of Laughlin and Armstrong (19), who demon-
due to an anticipatory response, because flow was mea- strated that exercise-induced increases in hindlimb
sured after the animals were placed on the treadmill and blood flow are preferentially directed to muscles primar-
anticipated an ensuing bout of exercise. Previous studies ily composed of FOG fibers. However, chronic unweight-
have shown that rats conditioned (1) or trained (2) to run ing did influence the distribution of cardiac output dur-
on a treadmill have increased standing flows in several ing treadmill running. During low-intensity exercise, ox-
muscles. Thus standing blood flows may have been in- idative (SO and FOG) muscle blood flow increases and
creased by placing the animals of the second study, which glycolytic (FG) muscle and visceral flows decrease (19).
had been previously conditioned to treadmill running, on After chronic unweighting, duodenum (Fig. 4) and kid-
the treadmill. The higher heart rate of these rats, which ney (Fig. 5) blood flows remained elevated with exercise.
is indicative of anticipation to exercise (1, 2), supports As mentioned above, this failure to redistribute the car-
this hypothesis. diac output to the active muscle may be due to a downreg-
There were no significant differences between chroni- ulation of _-receptors and a subsequent attenuation in
cally unweighted and control animal muscle blood flows responsiveness to sympathetic activation (29). Although
per 100 g tissue during treadmill running at 15 m/min. less apparent, unweighting may also result in a decrease
Additionally, our findings show patterns of muscle fiber in sympathetically mediated vascular tone in muscle com-
recruitment during low-intensity exercise similar to posed of FG fibers during exercise (Fig. 7). This could
2216 HINDLIMUBNWEIGHTINEGFFECTOSN
TISSUE BLOOD FLOW
explain the 60 and 160% greater blood flow observed in becomes even more impressive, because visceral blood
the white portions of vastus lateralis and gastrocnemius flow is inversely related to exercise intensity (19).
muscles, respectively, after chronic unweighting. Twenty days of deconditioning bed rest in humans in-
Although cardiac output was not measured in this duced tachycardia at rest and during submaximal exer-
study, a few simple calculations using regional flows can cise {32). Prolonged HU similarly induced PE standing
be made to estimate the effects of hypodynamia on this and exercise tachycardia in this study, but this was in
variable during exercise. Using estimates of total muscle contrast to another unweighted rat study of shorter dura-
[39% total body mass (8)], bone [6% total body mass (8)], tion (30). Exercise training also has been shown to induce
and visceral [8% total body mass (8)] mass multiplied by bradycardia during exercise in rats (2, 15). These obser-
the ratio of summed tissue blood flows (ml/min, Table 3) vations indicate that the chronotropic state of the heart
and summed tissue masses (Table 1) as representative of adapts to both chronic increases and decreases of activity
average muscle, bone or visceral blood flow per gram of in rats.
tissue, then total flows to these tissues, can be deduced. Rat HU provides a model for muscular hypodynamia.
Muscle, bone, and visceral blood flow would be 96, 3, and The effects of prolonged reductions in muscular weight-
58 ml/min, respectively, in control rats and 85, 4, and 67 bearing and locomotory activity on hindlimb vascular
ml, respectively, in chronically unweighted rats during morphology and tone might be the inverse of those after
exercise. Total blood flow to these three body compart- chronic increases in contractile activity. For example,
ments would be 157 ml/min in control and 156 ml/min in exercise training increased rat skeletal muscle vascular
unweighted animals. These calculations are only esti- capacity for flow (22, 24), a response that was likely in
mates and assume that relative body compositions are part caused by an increased capillarity (25). Despite an
similar, but they suggest that cardiac output at the same increased vascular capacity after training, total hindlimb
absolute exercise intensity is unaffected by chronic un- blood flow did not change during treadmill running (2).
weighting if blood flows to other body compartments, However, there was a marked redistribution of flow
e.g., skin and fat, are not different. Saltin and co-workers within the hindlimb, with the trained animals having
(32) have previously reported that cardiac output in hu- higher blood flows to high-oxidative (SO and FOG) mus-
mans during submaximal exercise does not change after cles and lower flows to low-oxidative (FG) muscle (2). All
20 days of bed rest. muscle types had elevated succinate dehydrogenase activ-
The above calculations indicate that total muscle ities after training, so it appears that the vascular adapta-
blood flow (ml/min) during exercise was less after pro- tions to training were fiber type specific; i.e., vessels in
longed unweighting. There were no significant differ- high- and low-oxidative muscles adapted with a de-
ences in individual muscle flows during exercise between creased and increased vascular tone, respectively.
control and chronically unweighted animals. However, of Chronic reductions in activity via HU induced a de-
the 13 muscles and muscle parts sampled, 9had absolute creased capillary-to-fiber ratio in soleus muscle (10). If
decreases in flow and 4, all having high populations of FG the number of fibers in soleus muscle did not change {35),
fibers, had absolute increases in blood flow. The sum then this would indicate that a reduction in the absolute
effect of small insignificant changes in blood flow to indi- number of capillaries, and presumably a reduction in the
vidual muscles can result in significant changes in total absolute vascular transport capacity, occurred. However,
muscle flow (8). The presumed reduction in total muscle capillary density increased with unweighting (10), indi-
blood flow could quantitatively be accounted for by the cating the decrease in fiber mass was greater than the
lack of a reduction in visceral flows. The apparent inabil- decrease in capillary number. Therefore vascular capac-
ity to redistribute blood flow away from inactive to active ity for flow relative to muscle mass may be elevated after
tissue could have significant adverse consequences on hypodynamia. Although the effects of prolonged inactiv-
total body aerobic metabolism and exercise performance. ity on vascular transport capacity have not been directly
For example, Overton et al. (30) reported that maximal tested, the results of this study support the contention
oxygen consumption (Vo 2m_.) is reduced after 6-9 days that HU does not adversely affect blood flow per unit
of head-down or horizontal unweighting. If visceral flows mass of muscle during submaximal exercise. However, as
continue to remain elevated during high-intensity exer- with exercise training, it appears that adaptations in the
cise and a lower percentage of maximal cardiac output is control of vascular tone occurred with chronic unweight-
directed to active muscle, then the loss of ability to redis- ing, and the mechanisms involved in this adaptation pro-
tribute cardiac output after chronic unweighting may cess merit further investigation.
have a causal role in reducing Vo 2max" The observation that soleus blood flow during exercise
If VO2maxwere reduced in our rats, then the chronically was not different between groups leads us to reject the
unweighted rats in this study were exercising at a higher hypothesis that a reduced blood flow caused the elevated
relative 02 consumption during the 15 m/min exercise glycogen depletion and lactate production observed with
bout. Because soleus muscle blood flows between groups contractile activity in atrophy soleus muscle (37). The
were the same during exercise at these potentially differ- question remains as to what triggers the altered sub-
ent relative work loads, the atrophied soleus muscle strate metabolism. One possibility is a HU-induced de-
blood flow might have been lower than the control value crease in the responsiveness to sympathetic activation
had both groups been exercising at the same relative (29). As previously stated, this isconsistent with our find-
work load. Additionally, the apparent lack of control by ing that blood flow to muscle composed of predominantly
the sympathetic nervous system to direct cardiac output FG fibers is elevated during unweighting and flow to the
HINDLIMB UNWEIGHTING EFFECTS ON TISSUE BLOOD FLOW 2217
duodenum and kidneys remained elevated during exer- I0. DESPLANCHES, D., M. H. MAYET, B. SEMPORE, AND R. FLAN-
cise in HU rats. A reduced responsiveness to sympathetic DROIS. Structural and functional responses to prolonged hindlimb
drive could inhibit the work-induced mobilization of suspension in rat muscle. J. Appl. Physiol. 63: 558-563, 1987.
FFA, decreasing blood FFA and thus their delivery, up- 11. DUNN, C. D. R., P. C. JOHNSON, AND R. D. LANGE. Regulation of
hematopoiesis in rats exposed to antiorthostatic hypokinetic/hy-
take, and oxidation by skeletal muscle. This hypothesis is
podynamia. If. Mechanisms of the "anemia." Aviat. Space Environ.
supported by the observed hypertrophy of the adrenal
Med. 57: 36-44, 1986.
medulla and cortex after 15 days of HU. A second possi- 12. FI'I'rS, R. H., C. J. BRIMMER, A. HEYWOOD-COOKSEY, AND R. J.
bility for the greater reliance of hindlimb muscles on gly- TIMMERMAN. Single muscle fiber enzyme shifts with hindlimb sus-
colytic metabolism after HU is a reduced blood 02-carry- pension and immobilization. Am. J. Physiol. 256 ((?ell Physiol. 25):
C1082 C1091, 1989.
ing capacity. Reductions in plasma volume and erythro-
13. FITTS, R. H., J. M. METZGER, D. A. RILEY, AND B. R. UNSWORTH.
cyte mass have been reported after unweighting (6, 11,
Models of disuse: a comparison of hindlimb suspension and immo-
30). The relative importance of these two possibilities in
bilization. J. Appl. Physiol. 60: 1946-1953, 1986.
altering the exercise metabolism of the atrophied soleus 14. GARDETTO, P. R., J. M. SCHLUTER, AND R. H. FITTS. Contractile
awaits an analysis of blood gases and FFA content during function of single muscle fibers after hindlimb suspension. J. Appl. "
exercise of chronically unweighted animals. Physiol. 66: 2739-2749, 1989.
In conclusion, on unweighting, blood flow to the soleus 15. GLEESON, T. T.. W. J. MULLIN, AND K. M. BALDWIN. Cardiovascu-
lar responses to treadmill exercise in rats: effects of training. 3..
is significantly reduced. After 15 days unweighting, flow
Appl. Physiol. 54: 789-793, 1983.
to the soleus is depressed during normal standing but not
16. HAUSCHRA, E. O., R. R. ROY, AND V. R. EDGERTON. Size and meta-
during a bout of treadmill running. Hence, the increased bolic properties of single muscle fibers in rat soleus after hindlimb
dependence of atrophied soleus on glycogen and anaero- suspension. J. Appl. Physiol. 62:2338 2347, 1987.
bic energy production during contractile activity cannot 17. HENRIKSEN, E. J., M. E. TISCHLER, AND D. G. JOHNSON. Increased
be accounted for by an attenuated blood flow. The fact response to insulin of glucose metabolism in the 6-day unloaded rat
soleus muscle. J. Biol. (!hem. 261:10707 10712, 1986.
that chronic HU blunts the redistribution of visceral
18. HEYWOOD-COOKSEY, A., AND R. H. FITTS. The eifect of hindlimb
blood flow during exercise suggests a reduced cellular
suspension on the subst rate profile of tast and slow skeletal muscle
response to catecholamines. Consequently, with the on- fiber types (Ahstract). Meal Set. Sports Exercise 23: S129, 1991.
set of exercise, the epinephrine-mediated increase in 19. LAUGHLIN, M. H., AND R. B. ARMSTRONG. Muscular blood flow
plasma FFA might be inhibited, producing an increased distribution patterns as a function of running speeds in rats. Am. J.
dependence on muscle glycogen in the atrophied soleus. Physiol. 243 (Heart ('ire. Physiol. 12): H296 H306, 1982.
20. LAUGHLIN, M. H., AND R. B. ARMSTRONG. Adrenoceptor effects on
rat muscle blood flow during treadmill exercise. J. Appl. Physiol.
We thank Barbara DeNoyer for help in the preparation of the man- 62: 1465-1472, 1987.
uscript and Dr. Robert B. Armstrong for advice in the design of the
21. LAUGHLIN, M. H., R. B. ARMSTRONG, J. WHITE, AND K. ROUK. A
blood flow experiments.
method for using microspheres to measure muscle blood flow in
This research was supported by National Aeronautics and Space
Administration Grant NAG 2-212. exercising rats. d. Appl. Physiol. 52:1629 1635, 1982.
Address for reprint requests: R. H. Fitts, Biology Dept., Marquette 22. LAUGHLIN, M. H., AND J. RIPPERGER. Vascular transport capacity
University, Milwaukee, WI 53233. of hindlimb muscles of exercise-trained rats. J. Appl. Physiol. 62:
438-443, 1987.
Received 19 December 1990; accepted in final form 16 December 1991. 23. LEBLANC, A., C. MARSH, H. EVANS, P. JOHNSON, V. SCHNEIDER,
AND S. JHINGRAN. Bone and muscle atrophy with suspension of the
rat. J. Appl. Physiol. 58:1669 1675, 1985.
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