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.

SUMMARY1. Isolated neurones from the snail Helix pomatia were investigated under voltage clamp at 21-23 'C. The cells were internally dialysed and the current through small electrically isolated patches of the membrane was measured. The area of the patches was 30-500 ,1m2 ( 1/1000-1/100 of the cell surface). The internal resistance of the membrane patches was 1090101o Q.2. In order to obtain the maximum conductance of the calcium channels an external solution containing 130 mM-Ba2+ or Ca2+ and an internal solution containing Tris glutamate and 5 mM-EGTA were used. Fluctuations due to the activity of calcium channels have been detected and analysed.3. The power density spectra of barium current fluctuations were calculated for conductance values from 3 % to 30 % of the maximum conductance in the frequency band 1-1000 Hz. They were fitted to a spectral density function of the Lorentz form. The half-power frequency of the spectra was 227 + 60 Hz (S.E.). It did not reveal any distinct voltage dependence. 4. The current flowing through a single calcium channel was calculated from the variance-to-mean relationship. Its value for the transfer of Ba2+ ions is iB1 = 0.20 + 0-02 pA (S.E.). Single channel current was not affected by the membrane potential (since the equilibrium potential was high) nor by inactivation. The maximum calcium inward current which flows through a single calcium channel is about 0-1 pA and corresponds to a conductance Yca = 0 5 pS (calculated for an equilibrium potential of 200 mV). This estimate gives an upper limit to yea 5. The parameters of calcium channels modified by external EGTA have been evaluated. Measurements were performed in an external solution containing 200 mMNa+. The current carried by a single modified calcium channel is 'Na = 1.0 + 0-2 pA (S.E.) (YNa8-0 pS).

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