Voltage clamp of the Aplysia giant neurone: early sodium and calcium currents

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154

SUMMARY1. Characteristics of the transmembrane ionic currents under controlled changes in ionic composition of extra-and intracellular medium were studied in isolated neurones from the ganglia of molluscs, Helix pomatia, Limnea 8taJnali8 and Planorbis corneus. The neurones were investigated by a new technique which allows for dialysis of their interior and for clamping of the potential at the surface membrane without using micro-electrodes.2. Replacement of K ions by Tris inside the neurones eliminated the outward K current so that the actual time course of the inward current could be measured. The latter was separated into two additive components, one of which was carried by Na ions and the other one by Ca ions.3. Both inward currents were unaltered by tetrodotoxin (TTX); however, Ca current could be separately blocked by externally applied Cd ions (Kd = 7*2 x 10-5 M) and by the use of fluoride as an intracellular anion.4. No reversal of Na inward current could be achieved in neurones dialysed with Na-free solution, indicating the absence of outward current carrying ions through the corresponding channels. With 5 mM-Na inside the cell, the equilibrium potential was close to the value predicted by the Nernst equilibrium.5. A non-specific outward current could be detected in K-free cells at membrane potentials exceeding 20-40 mV. Its time course was proportional to 1-exp (-t/Tnr). Cd ions depressed this current. The presence of the non-specific outward current made an exact measurement of the equilibrium potential for the Ca inward current impossible.6. The kinetics of Na inward currents could be described by m3h and those of the Ca current by m2h law. The corresponding values for Vm = 0 are: rm(Na) = 1.1 + 05 msec, rm(Ca) = 2-4 + 1*0 msec, rh(Na) = 7-9 + 2-0 msec. The inactivation of Ca current included two first-order kinetic processes with Th1 = 50 + 10 msec and Th = 320 + 30 msec.21-2 P. G. KOSTYUK AND 0. A. KRISHTAL 7. The data presented are considered to be a proof of the existence of separate systems of Na and Ca ion-conducting channels in the nerve cell membrane.

Section: Discussionmentioning

confidence: 65%

Separation of sodium and calcium currents in the somatic membrane of mollusc neurones. With an Appendix by Yu A. Shakhovalov

Kostyuk¹,

Krishtal²,

Shakhovalov³

1977

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154

“…A Ca2+ current with slow kinetics is also found in giant molluscan neurones (Eckert & Lux, 1976). We cannot eliminate the possibility that the steady-state inactivation data is complicated by activation of a K+ current at more positive membrane potentials, but the experimental data shows no evidence of the type of deviation that distorts the inactivation curve in giant molluscan neurones at more negative potentials 228 PHOTORECEPTOR OFF RESPONSE (Geduldig & Gruener, 1970) and which has been related to the presence of a fast inactivating K+ current (Standen, 1975).…”

Section: Discussionmentioning

confidence: 73%

Contribution of calcium and potassium permeability changes to the off response of scallop hyperpolarizing photoreceptors.

Cornwall¹,

Gorman²

1979

SUMMARY1. The membrane response of the distal photoreceptors in the retina of the scallop Pectin irradians to the termination of a bright white light (off response) is shown to be composed of the decay of the hyperpolarizing receptor potential and an action potential with slow kinetics.2. The action potential can be produced in darkness in the absence of external Na+ ions by membrane depolarization.3. The action potential is maintained by replacement of external Ca2+ with Sr2+ or Ba2+, but not by Mg2+. In normal external Ca2+ (9 mM), the action potential is abolished by the addition of the Ca2+ inhibitors, La3+, Co2+, and Mn2+ or the organic Ca2+ antagonist D-600.4. Elevated external Ca2+ concentrations increase the rate of rise and peak amplitude of the action potential as well as the rate of repolarization and after hyperpolarization, but decrease the duration.5. The rate of rise and peak amplitude of the action potential are increased by the K+ antagonists tetraethylammonium (TEA) 4-amino-pyridine (4-AP), Ba2+ and procaine. The antagonists have different effects on subsequent phases of the response, however. External TEA and Ba2+ increase the duration, but decrease the rate of repolarization and abolish the after hyperpolarization, whereas external 4-AP and procaine increase the rate of repolarization, decrease the duration and increase the after hyperpolarization.6. The ratio of the Ca2+ to K+ permeability (PC/0PK) estimated from the constant field equation at the peak of the action potential in different external Ca2+ concentrations is close to 1. 7. The maximum rate of rise and the peak amplitude of the action potential are increased by membrane hyperpolarization and decreased by membrane depolarization. They are decreased by background light intensity relative to their value in the dark.8. In normal ASW the action potential can be identified during the off response as a small overshoot of membrane potential relative to its value in the dark.9. The rate of repolarization of the off response in normal ASW is reduced by agents or conditions which inhibit or reduce Ca2+ permeability changes, e.g. external Co2+ and La2+ or zero external Ca2+.

“…5B). A similar selective inactivation of Na and Ca currents has been reported in the membrane of starfish eggs (Hagiwara, Ozawa & Sand, 1975 Geduldig & Gruener (1970), in subsequent studies in Helix neurones (Neher, 1971;Standen, 1974Standen, , 1975 (Vassort, 1975) and uterine smooth muscle (Anderson & Ramon, 1976).…”

mentioning

confidence: 77%

Characteristics of sodium and calcium conductance changes produced by membrane depolarization in an Aplysia neurone.

Adams¹,

Gage²

1979

SUMMARY1. The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neurone R15 in the abdominal ganglion of Aplysia juliana were examined at temperatures of 10-14 'C.2. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant rh. Activation (after correction for inactivation) was reasonably well described by the expression G' (t) = GNI(oo) (1 -exp [-tIrm])3 over a wide range of potentials. Half-maximal Na conductance occurred at a membrane potential of -8 mV and from -15 to -5 mV, a 5 mV change in membrane potential produced an e-fold change in steady-state Na conductance. 4. Steady-state inactivation of Na conductance (hNa(oo)) was voltage dependent with half-inactivation occurring at a membrance potential of -32 mV. Recovery from Na inactivation followed an exponential time course with a voltage-dependent time constant.5. During a maintained depolarization Ca currents activated then decayed (inactivated) more slowly than Na currents. The decay was exponential with time constant rH. The decay of Ca current was not an artifact produced by an outward current. The amplitude of calcium tail currents, produced by voltage steps back to eK at different times during the decay of Ic., decayed also with a time constant close to TH.6. Ca conductance (after correction for inactivation) could be described approximately by the expression G' (t) = G' (oo) ( 1-exp [-t/TM])P but it was necessary to vary p from 1 to 2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents.7. TM and TH were voltage dependent. In the range of potentials from 0 to + 60 mV, TM varied from 9 to 5 msec and TH from 300 to 50 msec (13-5 'C).