SUMMARY1. Helix aspersa neurones under voltage clamp generate prolonged outward currents (potassium currents) in response to depolarizing command pulses.2. The potassium currents recorded from cell A were reversibly reduced 25-50 % by 10 mm cobalt ions in the bathing medium; I mm lanthanum, 106 g/ml. D-600 and 106 g/ml. iproveratril had similar effects but were only partially reversible. 3. The relationship between the potassium currents and the membrane potential had an 'n' shape in normal saline. In calcium-free saline (containing 25 mm magnesium) the potassium currents were reduced and the 'n' shape was abolished. The effect of calcium-free saline was readily reversible.4. The voltage-dependence of the calcium-sensitive potassium currents was similar to that of the 'late' calcium channel in squid axons (Baker, Hodgkin & Ridgway, 1971).5. When cell A was depolarized by 90 mV for 40 msec and then repolarized the 'tail' currents were made up of two exponentially declining components. The slower of the two components was reduced in calciumfree saline.6. When cell A was depolarized by 150 mV for 10 msec and then repolarized the 'tail' currents were made up of a single rapidly declining component. The reversal potential of this component changed by 58 mV for a tenfold change in the external potassium concentration as predicted by the Nernst equation.

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Until now the only reported effect of depolarization on the intracellular pH (pHi) of excitable cells in an acidification of the cell cytoplasm. It seems unlikely that this could be a direct effect of membrane potential because pHi is known to be regulated by an electroneutral mechanism and in most cells H+ ions are not in equilibrium with the membrane potential (Em). In any case the membrane conductance to H+ ions would be expected to be small because they are at such low concentrations on either side of the cell membrane. But it is possible that the H+ ion permeability of the membrane increases on depolarization just like that of other ions in the bathing medium depolarization just like that of other ions in the bathing medium (Na+, K+ and Ca2+ for example). To test this idea we have made pHi measurements on molluscan neurones under voltage-clamp. Our findings, presented here, provide evidence for a large increase in H+ ion permeability in depolarized cells. We suggest that this increase in proton conductance may be the basis for the "nonspecific' currents previously described in perfused molluscan neurones and we assess the physiological significance of this newly discovered pathway.

SUMMARY1. Cells from the circumoesophageal nerve ring of the pond snail Lymnaea 8tagnali8 were internally perfused with solutions containing Cs aspartate, EGTA and pH buffers. Time-dependent, voltage-dependent 'residual' outward currents were observed at positive potentials. They were found to be carried largely by H+.2. The outward H+ currents were reduced by high internal pH, low external pH, external Cd2+ and 4-aminopyridine. External tetraethylammonium ions reduced the H+ currents but had a more effective blocking action on the K+ currents in these cells. All five agents reduced the maximum H+ conductance. In addition Cd2+, low external pH and high internal pH were found to shift the voltage dependence of the H+ current to more positive potentials.3. There was no significant difference between H+ currents recorded with the internal pCa2+ about 7 and those recorded with the internal pCa2+ near 5.4. It is likely that the H+ channel described here provides the basis for the increase in H+ permeability described by Thomas & Meech (1982) in depolarized Helix neurones.

SUMMARY1. When calcium chloride was injected into Helix aspersa neurones there was a fall in membrane resistance and the membrane potential became hyperpolarized.2. The reversal potential of the response was dependent on the concentration of potassium in the external solution.3. Injection of a calcium-EGTA buffer containing 9 x 10-7 M free calcium reduced the membrane resistance by 25 %. When calcium chloride was injected it was necessary to increase the total intracellular calcium concentration by about 1O-3 M to produce similar change of resistance. 4. In sodium-free (Tris) solution there was a slow fall of membrane resistance as if the intracellular calcium concentration had increased. There was a similar resistance change in the presence of 2,4-dinitrophenol and iodoacetate.5. A series of repetitive depolarizing pulses produced a long lasting reduction in membrane resistance which was enhanced by 2,4-dinitrophenol and iodoacetate.6. It is concluded that (a) injection of calcium causes an increase in potassium permeability, (b) the injected calcium is rapidly pumped from the cytoplasm by a sodium-dependent mechanism and by mitochondria, and (c) 1-2 msec depolarizing pulses stimulate an influx of calcium. This influx is rapid enough to trigger potassium activation during an action potential.