Seasonal variation in conduction velocity of action potentials in squid giant axons

9 Pages·2000·3.6 MB·English

by  J J Rosenthal

Reference: Biol. Hull. 1W: 135-143. (October 2000)
Seasonal Variation in Conduction Velocity of Action
Potentials in Squid Giant Axons
JOSHUA J. C. ROSENTHAL AND FRANCISCO BEZANILLA*
Departments ofPhysiology andAnesthesiology, UCLA School ofMedicine,
Los Angeles, California 90095
Abstract. To determine whether the electrical properties tions are not well understood. Do they involve changes at
of the squid giant axon are seasonally acclimated, action the level of the action potential, the synapse, or both?
potentials, recorded at different temperatures, were com- Action potential duration and propagation are strongly in-
pared between giant axons isolated from Loligo pealei fluenced by acute temperature changes (Hodgkin and Katz,
caught in May, from relatively cold waters (-10-12C), 1949), largely due to temperature sensitivity of the under-
and in August, from relatively warm waters (-20C). Pa- lying ion channels (Hodgkin et ai, 1952). If action poten-
rameters relating to the duration ofthe action potential (e.g., tials themselves are a common target for thermal acclima-
maximum rate ofrise, maximum rate offall, and duration at tion, which properties are affected? In the giant nerve fibers
half-peak) did not change seasonally. The relationship be- ofearthworms, cold acclimation speeds the action potential
tween conduction velocity and temperature remained con- duration, conduction velocity, and refractory period vs. tem-
stant between seasons as well, in spite of the fact that May perature relationship, but not to the extent that, when mea-
axons were significantly larger than August axons. When sured at rearing temperatures, the kinetics of cold- and
normalized to the fiber diameter, mean May conduction warm-acclimated worms are equivalent (Lagerspetz and
velocitieswere 83% oftheAugustvalues atall temperatures Talo, 1967; Talo and Lagerspetz, 1967). In goldfish cardiac
tested, and analysis of the rise time of the action potential muscle cells, the action potential's duration is reduced in
foot suggested that a change in the axoplasmic resistivity cold-acclimated fish (Ganim etai, 1998). IncertainAplysia
was responsible for this difference. Direct measurements of neurons, however, early potassium current kinetics are not
axoplasmic resistance further supported this hypothesis. affected by rearing temperature (Treistman and Grant.
Thus seasonal changes in the giant axon's size and resistiv-
1993).
ity are not consistent with compensatory thermal acclima- The squid giant axon, long a model for understanding the
tion, but instead serve to maintain a constant relationship basic physiology ofvoltage-dependent ion channels, is, for
between conduction velocity and temperature.
a variety of reasons, an excellent system for examining
temperature-dependent acclimation of the action potential.
Introduction
First, its output participates in a known function the jet-
Signal transduction in the nervous system is profoundly propelled escape response (Prosser and Young, 1937;
temperature sensitive. Acclimation of higher order nervous Young, 1938; Otis and Gilly, 1990) and presumably it is
function to variation in environmental temperature has been advantageous for this response to be rapid. Second, the
the subject of many investigations (see Prosser and Nelson, dimensions ofthe giant axon permit action potentials, mac-
1981); however, the precise mechanisms of such acclima- roscopic ionic currents, gating currents, and single-channel
currents to be measured from the same preparation
Received 3 March 2000; accepted 7 July 2000. E-mail: fbezanil (Hodgkin and Huxley, 1952; Armstrong and Bezanilla,
UCLA.edu 1973; Conti and Neher, 1980; Bezanilla, 1987). Third. Na
*To whom correspondence should he addressed. and K currents, which underlie the action potential, have
MitRyA;Rbib.\.r.emveaixatxetirimnoanulsm:rerCsaitmse,taomnfecemr;ibsrer;,a.n/?i,e.n,tceearxpntaaeclrintraaelsnicrseet:sainMsctRei.vFi.ty;ma/?x,,iimnutemrnraaltereosfisftailvl-: b</e/.e.n19e9x0t)e.nsFiovuerltyh,chgairaancttearxioznedNianathnidsKsycshtaenmne(slesehGaivlebebrteeent
135
136 J. J. C. ROSENTHAL AND F. BE7.ANILLA
defined on a molecular level (Rosenthal and Gilly, 1993; tial. Voltage signals were recorded at two points along the
M
Rosenthal et it/., 1996). Finally, and perhaps most impor- axon using two additional micropipettes (3 KC1. 1-3
tant, squid of the genus Loligo (and other genera that Mil) connected to two high-impedance, capacitance-com-
contain "giant" axons) inhabit a wide variety of thermal pensated electrometers. The chamber was grounded using
environments. two Ag+/AgCl coils embedded in 10 K" ASW + 3%
Squid of the species Loligo penlei live off the eastern agarose and connected to virtual ground. Signals were col-
seaboard of North America, where inshore water tempera- lected using software written in-house and a PC44 (Inno-
tures undergo large seasonal fluctuations. By examining the vative Technologies) signal processor board connected to a
axons from these squid in both May and August, the present PC compatible computer. Sampling rates varied between
study seeks to identify those properties ofthe action poten- 200 kHz and 1 MHz, depending on the temperature, and
tial that change on a seasonal basis. signals were filtered at Vw of the sampling rate. Axon
diameters and distances between micropipettes were mea-
Materials and Methods sured with an eyepiece micrometer. Data were analyzed
only from electrode impalements with resting potentials
Squid collection and water temperatures more hyperpolarized than 55 mV.
Adult specimens ofLoligopealeiwere collected from the
waters surrounding Woods Hole, Massachusetts, in 1997 Capacitance and resistance measurements
and 1998 during May and August. In May, specimens were Membrane capacitance was measured in the voltage-
jigged from the town dock, and in August they were caught clamp configuration as previously described (Bezanilla et
by trawls in Vineyard Sound. During two trawls, water <;/., 1982a. b) with minor modifications. Signals were col-
temperature was measured at the net opening with a data lected as described in the previous section. A 75-/am plat-
logger. Daily temperatures, recorded near the Marine Bio- inum wire for passing current and an internal measuring
logical Laboratory (MBL) seawater intake system at adepth electrode consisting ofan 80-/am glass capillary filled with
MBL M
of 15 feet, were kindly furnished by Janice Hanley. 0.6 KC1 and containing a floating 25-/im platinum wire
water quality technician. Data used in Figure 2 were re- were inserted into the axon in a piggy-back configuration
corded by the National Oceanic and Atmospheric Admin- (Hodgkin ft<//.. 1952). A wide-aperture glasscapillary filled
istration (NOAA) at the Woods Hole Oceanographic Insti- with 10 K* ASW + 3% agar was used as an external
tution pier (tide station number 8447930, available on the reference. Capacity transients were generated by brief
NOAA website). Squid were maintained in flowing seawa- pulsesfromaholding potential of -80 mV to -90mV.The
ter, whose temperature was kept within 1 degree of the average of 10 suchrecords was used foranalysis. Datawere
intake temperature, and were used within 2 days ofcapture. collected at 500 kHz and filtered at 50 kHz. For resistance
Animals were killed by rapid decapitation, and hindmost measurements, axons were carefully cleaned ofall adhering
stellar nerves were removed and carefully cleaned of small accessory fibers and placed on a platform of clear acrylic-
fifiibeedr,swienrseeapweartefro.rmAeldl ienxp1e0riKme+ntsar,tiufinclieaslssoetahweartweirse(AsSpeWc;- pploaosltsic.cTonhteaianxiongn e5n0d0s wmerMe tKh-egnluctuatmaatned.di1p0peHdEiPnEtoS.two5
composition in millimoles: 430 NaCl, 10 KC1, 50 MgCl2. 10 EGTA, pH 7.5. These pools were voltage clamped, and
Cad,, 10 HEPES. pH 7.5, 970 mOsm). signals were collected, as described above. In all cases, data
were analyzed using in-house software. Errorbars represent
Action potential measurements the standard error ofthe mean (SEM). and a Student's t test
was used where probabilities are indicated in the text.
Propagated action potentials were measured by mounting
a freshly dissected axon (4-6 cm) in a long, rectangular Results
glass chamber filled with 10 K.* ASW. Temperature was
regulated using two Peltier units mounted under the cham- In the giant axon, action potential duration and conduc-
ber and was measured with a hand-held thermocouple po- tion velocity are strongly affected by temperature (Hodgkin
sitioned directly adjacent to the axon. After equilibration of and Katz, 1949; Chapman. 1967). Figure 1A shows three
the chamber, temperatures at all points along the axon were action potentials, recorded at different temperatures, from a
within 0.5C of the recorded temperature. Action poten- single electrode inserted into a giant axon. Clearly, the
tials were stimulated intracellularly at one end of the axon durations ofthe rising and falling phase both increase as the
using brief current pulses (2-20 juA for 200-400 jus) ad- temperature is decreased. The half-width of the action po-
ministered through a 0.4-0.6 Mfi micropipette filled with 3 tential recorded at 2()C is 365 /as. This number increases to
M
KC1. and connected to the output of the online D/A 850 /as at 10.2C. and to 2400 /as at 1.8C. Furthermore, the
converter. Stimuli, which varied from axon to axon, were conduction velocity shows a similar temperature depen-
the minimum required to produce a consistent action poten- dence. In Figure IB. action potentials, evoked by a single
SEASONAL CHANGES IN THE GIANT AXON 137
B. 25 n
1 8C 20 -
5C
a is -\
2
40mVL u
10
e
I
20C 5 -
20.0C
Figure 1. Action potentials recorded al different temperatures. (A)
Propagated action potentials from a single position in a single giant axon Figure 2. Seasonal temperature variation in the Woods Hole Passage.
amteatswuorepdosaittitohnrseeinteampesrinagtluereas.xo(nB)meParsopuargeadteadt atcwtoiontepmopteernattiuarless.recAoxrodnesd Hlionuer)layndte1m9p9er8a(tduorteterdecloirnedls.fTreommpeArpartiulrethsrwouegrheSteakpetnemfbreormfNorO1A9A97st(astoiloind
werebathed in 10 K+ artificial seawater. Brieftransients at the beginning number 8447930 located at the Woods Hole Oceanographic Institution at
ofrecords are stimulus artifacts. 41 31.5' N, 70 40.3' W.
stimulus, are recorded at two points, separated by 19.6 mm, between squid captured in May and in late August. Figure 3
along the same axon. From this experiment the conduction compares maximal rates of rise (MRR) and fall (MRF) of
5C
velocity is calculated tobe 9.56 m/s at and 18.94 m/s at the action potential, at various temperatures, for May and
20C. The axons used in this figure were dissected from a August squid. No significant difference exists for either
squid captured in May. measurement between groups. MRRs have a nearly linear
Water temperatures in the Woods Hole Passage and in
Vineyard Sound, where squid for these studies were cap- 900 -
tured, change dramatically between the spring and summer.
In May, squid were collected by jigging from waters di- 800 -
rectly measured to be 10-12C. In late August, squid were
captured by trawling, and on two trawls the average tem- 700 -
perature at the net opening was recorded to be 20.4C and
21.2C. Figure 2 shows hourly watertemperatures recorded | 600-
NOAA
by the tidal monitoring station (adjacent to the -
g 500
Woods Hole Oceanographic Institution pier in the Woods
Hole Passage) between April and October, 1997 and 1998. 5 400 ~
During May, water temperatures vary between about 10 T3
and 12C. In August they average slightly greater than 300 -
20C. These values are corroborated by daily temperature
MBL 200
records taken near the seawater intake system at a
depth of 15 feet. In 1998, May and August average temper-
atures (SD) were 12.4C 1.2 and 21.8C 0.49.
respectively. The few data points available for this site in
1997 are similar to the 1998 values (10.2C on May 5, 10 15 20 30
12.8C on May 28, 22.0C on August 8, and 21.TC on Temperature (-C)
August 26). Thus, water temperatures during the study Figure 3. Action potential maximum rates of rise and fall do not
period in both 1997 and 1998 changed by about 10C. change seasonally. Action potential records from squid in May (filled
Acute temperature changes ofthis magnitude would clearly symbolslandAugust(opensymbols)weredifferentiated numerically,and
affect the giant axon's electrical properties (see Fig. 1). Do maximum values(ratesofrise;circles)and minimum values (ralesoffall,
these properties change to compensate for seasonal temper- absolute values; triangles)wereplottedagainsttemperature. Actionpoten-
atuTroe vaanrsiwaetriotnh?is question, action potentials were compared rtsietacalonsrdwdaesrrd(e6erraerxocoronrosdf)edtfhoaerttAmwueogaunps;ots.inti=ons1alornegcoarndsaxo(5n.aExrornosr)bfaorrsrMeapyresaenntdt1he1
138 J. J. C. ROSENTHAL AND F. BEZANILLA
temperature relationship, while that of the MRFs is expo- duration (e.g., rise time, fall time, and duration at half-peak
nential. In addition, the MRFs have a higher temperature amplitude) also showed no seasonal difference.
coefficient. Q10 values for the MRF are 3.7 and 2.5 for the In contrast, resting potentials and conduction velocities
temperature ranges 0-12.5C and 12.5-25C. respec- did exhibit significant seasonal variation. Figure 4A com-
tively. For the same intervals, (2lo's for the MRR are 2.4 pares the resting potential vs. temperature relationship for
and 1.5. Other measurements relating to the action potential May and August axons. Between and 15C, both groups
A B
-52- 30
-54-
&
25
'8
-56-
20
g -58 tj ^g^
-
tin
-60 6
. .5
t/1
I
-62
10
-64
5 10 15 20 25 30 5 10 15 20 25 30
Temperature(C) Temperature (C)
1600
1400
o
o 1200
a
o 1000
i
800
O .
a
N 600
13
O 400
5 10 15 20 25 30
Temperature (C)
Figure4. Resting potentialsandconduction velocitieschangeseasonally. (A) Resting potential i'i. temper-
ature relationship for May axons (filled symbols) and August axons (open symbols). Error bars represent the
standarderrorofthe mean (SEM);n = 10 foreachseason. (B)Conduction velocities vs. temperature forMay
and August axons (same symbol convention) as in A. (C)Conduction velocities, normali/ed to the square root
ofthediameter,r.v.temperatureforMayandAugustaxons(samesymbolconvention).ErrorbarsrepresentSEM;
= 6 foreach season for both B and C.
SEASONAL CHANGES IN THE GIANT AXON 139
become more hyperpolarized as the temperature is raised. In from squid (Tayloretnl.. 1962) and was used toextrapolate
this temperature range, the May axons are on average ~3 all experimental values to 15C. ForMay and Augustaxons,
mV more depolarized. At temperatures greater than 15C, mean capacitance was determined to be 1.03 0.039 /xF/
May axons become progressively more depolarized, while cm2 (SEM, n = 6) and 0.96 0.027 /LiF/cnr (SEM. n =
August axonscontinue to hyperpolarize until approximately 6), respectively. A statistical difference between these
20C, after which they level out. Thus at 25C the resting means is not well supported (P - 0.18). Thus differences
potential disparity reaches 5 mV. in capacitance are not sufficient to account for the Cln(Rl +
Figures 4B and 4C show the relationship between con- /?,.) data from the previous section.
duction velocity and temperature. Absolute (non-normal- Axoplasmic resistancewas measured directly indissected
ized) conduction velocities were equivalent between May axons. Axons were blotted dry and placed on an acrylic
and August squid (4B) at all temperatures tested. Interest- plastic platform; their ends were cut and dipped into two
ingly. May axons had significantly larger diameters (506 reservoirs containing internal solution (Fig. 6A). The reser-
26.8 /urn; mean SEM) than those in August (383 14.68 voirs were then voltage clamped, and after transients had
jam; mean SEM). Conduction velocity in the giant axon subsided, the current flow through the axon was measured
is proportional to the square root of the axon diameter, a and normalized to the axon's cross sectional area and
relationship originally established by Hodgkin and Huxley length. All measurements were conducted at room temper-
(1952) and verified by many othergroups (Chapman, 1967; ature (about 20C). The results from a typical axon segment
Taylor, 1963). Taking this into account, normalized con- of404 /xm diameterand 3.65 cm length are shown in Figure
duction velocities are plotted vs. temperature in Figure 4C. 6B. In this case, the voltage between the reservoirs and the
mV
Conduction velocities from May axons are relatively slow, current flow through the axon were 107.3 and 1.24 /J.A..
being on average 83% 2.5% (SD) of the August values. Thus the resistance (r, + r(.) forthis axon was calculated to
In both seasons the (2io values were equivalent (1.5 be- be 86.5 kfl, and the specific resistance (R, + Rt) was 30.3
tween 10 and 20C). fl*cm. As expected, the current voltage relationship at a
The fact that normalized conduction velocity values variety oftest potentials is linear(Fig 6C). A seriesofaxons
changed from May to August, but the MRR and MRF did from May and August were analyzed in a similar manner
not, suggested that the passive electrical properties of the and their specific resistivities were found to differ. Mean
axon had also changed. The action potential's initial rate of R, + Re was measured to be 35.2 1.3 fl*cm ( SEM,
rise, or "foot," can be used to extract information related to n = 6) and 28.4 2.5 ( SEM, n = 6; P = 0.05) in May
the axon's cable properties by the following relationship: and August, respectively. Thus on average. May values are
22% greater than August values.
Cm(r,.+ c,,) = \/r02
Discussion
where Cm is membrane capacitance, r, is internal resistance,
rf is external resistance, r is the time constant of the rise The present investigations were initiated to identify sea-
time of the action potential foot, and is the conduction sonal changes in the giant axon's electrical properties, and
velocity (Taylor, 1963). Figure 5A shows an example ofan to address whether these changes could compensate for
exponential fit to the foot ofan action potential recorded at seasonal temperature variability. Implicit in these studies is
10C from a May axon. In this case, r was 210 /as, was that the squid do in fact experience seasonal temperature
12.45 m/s, and therefore C,n(r, + re) was 30.7 fl*F*cm. variability. Temperature data from all sources, taken at
Similaranalysis wasextendedtoaction potentialsfrom May various depths, all show atemperature profile similarto that
and August, and all data were normalized to account for in Figure 2 (i.e., an approximate 10C difference between
axons ofdifferent diameters (Fig. 5B). At all temperatures, May and August). The recorded spring temperatures are
normalized C,,,(r, + re) was greater in May than in August probably maximum values and thus are a conservative es-
axons. On average. May values were 31.7% 6% (SD) timate of the squid's environment. Summer temperatures
greater. reported in this paper are probably representative for Vine-
The preceding analysis indicated that the product of re- yard Sound and the Woods Hole Passage, the locations
sistance and capacitance was variable between seasons; where the squid used for these experiments were captured.
however, it did not identify which property changed. To Turbulence, created by large tidal flows, prevents the for-
accomplish this, capacitance and resistance were measured mation of thermoclines in these shallow areas, and temper-
independently. Capacitance was measured at a variety of atures are uniform throughout the water column. Other
temperatures using aconventional axial wire voltage clamp. papers have come to a similar conclusion about mixing and
A <2io of 1.06 was determined for the relationship between report summer temperatures as high as 23C (Summers,
capacitance and temperature in two axons (data not shown). 1968; McMahon and Summers, 1971). Outside ofthe Vine-
This number agrees well with previously published data yard Sound, temperatures are likely to be significantly
140 J. J. C. ROSENTHAL AND F. BEZANILLA
A.
mV
20
= 210us ' 0.5 ms
B
0.005
0.0045
0.004
a
(Q-F-cm)
0.0035
0.003
0.0025
5 10 15 20 25
Temperature (C)
Figure 5. Product ofresistance and capacitance changes seasonally. (A) Example ofmeasurement. Dotted
lineisanactionpotentialrecordedat 10CfromaMayaxon;solidlineisanexponentialtittotheactionpotential
fSoeoet.te(xBt)fVoraldueersivoatfi(o/n?,of+(/R?,t.+)CmKevls.Cmte.mEprerroartubraresfroerprMeasyent(ttihlleedsttarnidaangrldese)rraonrdofAtuhgeusmtea(no;penn=tri6anfgolresM)aayxoannsd.
5 for August.
lower. Schopf (1967) reports the presence of thermoclines quired to reach deep oceanic waters (depending on the point
and maximum annual bottom-watertemperatures of 13C in ofdeparture in Vineyard Sound). Second, the August squid
the waters off Nantucket. were routinely captured during the day. In other cephalo-
The migration patterns of Loligo pealei are not well pods, daily migrations involve a nocturnal shift to shallow
understood. On a seasonal basis this species is reported to waters (Boyle. 1983; Hanlon and Messenger, 1996). There-
winter near the break of the continental shelf where it can fore it is likely that the squid spend a significant portion of
8C
avoid temperatures below (Summers, 1969). In the their time at the water temperatures specified in this report.
spring these squid move inshore when waters warm past Various reports document the presence of L. pealei in yet
about 10C (Summers. 1969; Mesnil, 1977). The first group warmer waters (e.g.. 22-29C in the Gulf of Mexico; see
to arrive are the 2-year-olds, normally in early May, fol- Boyle. 1983).
lowed by the 1-year-olds in June (Summers, 1971). This This study presents no evidence for a seasonal compen-
sequence ofevents probably explains the larger diameterof sation in the propagated action potential's duration, as there
MRR MRF
the May axons. Migration on a shorter time scale has not is no change in the curve of or vs. temperature.
been reported for this species, and therefore it is unknown These data also suggest that the properties ofthe underlying
whether these squid migrate to colder oceanic waters on a ionic current do not change, and studies using voltage-
daily basis. We consider such a migration unlikely for two clamped giant axons support this conjecture (data not
reasons. First a substantial horizontal shift would be re- shown). Therefore it is predicted that in vivo, the durationof
SEASONAL CHANGES IN THE GIANT AXON 141
V
I. V, I.
Axon
B
Ou.A-
IOiA)
4uA
mV
40
-200 -150 -100 - 50 100 150 200
2ms
V(mV)
OmV- V
Figure 6. Direct resistance measurements. (A) Schematic ofexperimental setup. A defined length ofaxon
was placed on aclearplastic platform, and each end was cut and placed in a bath containing internal solution
(in mM: 500 K-glutamate, 10 HEPES, 2.5 EGTA, pH 7.5). The interbath voltage was clamped using a
home-built squid axon apparatus, and the resulting current was measured. (B) An example of the current
resulting from a 100-mV voltage step (May axon). (C) Current-voltage relationship from the same axon. All
experiments performed at temperatures between 18 and 20C.
the action potential in May squid is over twice as long as it found to be statistically significant. It is probable that resis-
pisoteinntiAaulsgursetcosrqdueiddat(s1ee0CFiga.nd1 2f0oCr).exaUmnlpilkees tohfe aaccttiioonn RtievetochoaunrgmeseaasruerdeumeenttoschisanegxepsecitnedRtt,oabsethveercyonstmrailblu.tiGorneaotf
potential duration, the conduction velocitydoes appeartobe care was taken to blot the axon's external surface prior to
regulated between seasons. However, the direction of the recording, thus the layer of adhering seawater would be
change is not consistent with a compensatory thermal ac- quite small comparedtothe cross-sectional areaofthe axon.
climation: despite a seasonal disparity in axon diameter, the Cole and Hodgkin, who employed a similar experimental
relationship between conduction velocity and temperature setup, came to the same conclusion (Cole and Hodgkin,
remains constant, due mostly to resistive changes in the 1939). In addition, our reported values of/?, + Re, partic-
axon. Computer simulations of conduction velocities using ularly those for August (28.7 ll*cm), are consistent with
the Hodgkin and Huxley equations (Hodgkin and Huxley, previously reported values of /?,-. Cole and Hodgkin re-
1952) support this assertion. By substituting the May and ported 29 ficm (Cole and Hodgkin, 1939) and in a separate
August values for Cm(/?; + /?,.). determined by tits to the report. Cole reported the resistivity ofextruded axoplasm to
action potential foots (32.4OFand42.7 flF, respectively, at be 28 fl*cm (Cole, 1975). Using two internal microelec-
10C), there isa 14.2% increase in conduction velocity. The trodes. Carpenter el al. (1975) reported it to be 31 Ocm. It
difference determined from direct measurements ofconduc- is unclear during which season these studies were con-
tion velocities in May and August axons at 10C was ducted.
14.6%. Seawater. which is isosmotic with axoplasm (Gilbert et
Fits to the action potential foot predicted that the product nl., 1990), has a specific resistivity of only 20 ficm
of (/?,. + Re) and C,,, increased by approximately 30% (Cole and Hodgkin, 1939). Why is axoplasm a relatively
between August and May. This is in reasonable agreement poor conductor? First, unlike seawater, axoplasm con-
with direct measurements of (/?,- + /?t,) for the same sea- tains mostly large organic anions that have lower mobil-
sons, which increased by 22%. Capacitive changes were not ities than chloride. In addition, our measurements of
142 J. J. C. ROSENTHAL AND F. BEZANILLA
resistivity consider the axon as a conducting cylinder, Bezanilla, F. 1987. Single sodium channels from the squid giant axon.
and do not take into account the organelles, which occupy Biophys. J. 52: 1087-1090.
caonnutaniknnsowangpoerocdentdeaagle ooffthiemvmoolbuimlee. Fpirnoatleliyn,.axWoeplaosbm- Bezaavnanitdlilokani,noeFft.i,gcasRt.ionfEg.mcTeuamrybrleronatrsn,.eaJnd.ideGleJen.c.tMri.Pch\Fpsoeilroanlr.ainz7ad9te:izo.n2.11-194.802L.ao.ng-Dtiesrtmriibnuatcitoin-
served that axoplasm appears gelatinous in May, whereas Bezanilla, F.,J. Vergara, and R. E. Taylor. 1982b. Voltage clamping
in August it is considerably more liquid, possibly due to ofexcitable membranes. Merit. Exp. Pliys. 20: 445-511.
a decrease in the order of the underlying cytoskeletal Boyle, P. R. 1983. CephalopodLife Cycles. Academic Press. London.
proteins. Differences in any of these parameters could Carpenter, D. O., M. M. Hovey, and A. F. Bak. 1975. Resistivity of
underlie the seasonal differences in axoplasmic re- axoplasm. II. Internal resistivity ofgiantaxonsofsquid andMvxico/a.
J. Gen. Physiol. 66: 139-148.
sistivity. Chapman, R. A. 1967. Dependence on temperature of the conduction
In the absence of a seasonal acclimation, giant axon velocity of the action potential of the squid giant axon. Nature 213:
action potentials would travel much faster in August than 1143-1 144.
in May. The resistive changes discussed in this paper Cole, K. S. 1975. Resistivity of axoplasm. I. Resistivity of extruded
would not help compensate for temperature changes in Coles,quKid.aSx.o,plaansdm.AJ.. GL.en.HoPdhgyskiionl.. 6169:391.33-M1e38m.brane and protoplasm
conduction velocity and therefore they do not contribute resistance in the squid giant axon. J. Gen. Physiol. 22: 671-687.
to a compensatory acclimation. However, the giant axon Conti,F.,andE.Neher. 1980. SinglechannelrecordingsofK+ currents
is clearly a part of a larger motor system, the pieces of in squid axons. Nature 285: 140-143.
which do not have equal temperature sensitivities. Per- Ganim, R. B., E. L. Peckol, J. Larkin, M. L. Ruchhoeft, and J. S.
haps thermal acclimation involves maintaining specific Cameron. 1998. ATP-sensitive K* channelsincardiac muscle from
ratios between the rates of processes. For example, the cCoolmdp-.accBliiomcahteemd.goPlhdyfsiisohl:.cAhar1a1c9t:er3i9z5at-i4o0n1a.ndalteredresponsetoATP.
temperature dependence of the giant synapse is steep Gilbert, D. L., W. J. Adelman, and J. M. Arnold. 1990. Squid as
compared to the various properties related to the action ExperimentalAnimals. Plenum Press. New York.
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for integration in the nervous system, would be greater in 240-249.
August than in May. The resistive changes discussed in Hodgkin. A. L., A. F. Huxley, and B. Katz. 1952. Measurement of
this paper would help to maintain a more similar ratio Jc.urPrheynsti-ovlo.lt1a1g6e:r4el2a4t-io4n4s8.in the membrane ofthe giantaxon ofLoligo.
between seasons. In support of this conjecture it is note- Lagerspetz, K. Y. H., and A. Talo. 1967. Temperature acclimation of
worthy that the non-normalized relationship between the functional parameters of the giant nerve fibers in Lumhricus ter-
conduction velocity and temperature did not vary be- restris:I.Conductionvelocityandthedurationoftherisingandfalling
tween May and August, in spite of the fact that August Llinpahsa,seR.ofR.act1i9o9n9.poteTnthiealS.qJu.idExpG.iaBnitolS.y4n7a:ps4e7:1-a48M0o.delfor Chemical
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Llinas, R., M. Sugimori, and K. Walton. 1987. Further studies on
Acknowledgments depolarization releasecoupling in squidgiant synapse.Adv. Exp. Med.
Biol. 221: 1-17.
We thank Drs. David Gadsby, Paul De Weer, Robert McMahon. J. J., and W. C. Summers. 1971. Temperature effects on
Rakowski, and Barbara Ehrlich for generously sharing the developmental rate of squid (Loligo pealei) embryos. Biol. Bull.
laboratory space at the Marine Biological Laboratory; 141: 561-567.
Roger Hanlon for help procuring and maintaining squid: Mesniilll,eceBb.ro1s9u7s7,.froGmrotwhtehnaonrdthlwiefsetcyActlleanotfics,quIindt.. LCoo/mimgo.pNeao/retihwaensdtHAllcl..\
and Janice Hanley, Dr. George Hampson, and John Va- Fish. Sel. Pap. 2: 55-69.
lois for providing temperature data. This work was sup- Otis, T. S., and W. F. Gilly. 1990. Jet-propelled escape in the squid
ported by a National Institute of Health Grant (GM Loligo opalescens: concerted control by giant and non-giant motor
30376) and a National Institute of Health NRSA post- axon pathways. Proc. Natl. Acad. Sci. USA 87: 2911-2915.
doctoral training grant (NS 07101-18-19) for Dr. Joshua Prostseemrp,erCa.tuLr.,eaandadptIa).tiOo.nNoeflpsooink.il1o9t8h1e.rms.ThAnenirio.leRoefv.nePrhyvsoiuosl.sy4s3t:em2s81i-n
Rosenthal.
300.
Prosser, C. L., and J. Z. Young. 1937. Responses of muscles of the
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