The effect of endogenous Ca on potential-dependent K current, I(KD), was examined in identifiable neurones of Helix aspersa. The suction pipette method of internal perfusion was used along with a combined voltage-clamp method in which the membrane potential was measured by a separate glass micro-electrode and the current was passed by the suction pipette. Activation of potential-dependent A current, I(A), was prevented by using holding potentials of -40 mV where I(A) is inactivated and by the addition of the A-current blocker 4-aminopyridine. Activation of K currents by transmembrane Ca current, I(KCa), was suppressed by Co substitution for Ca ion extracellularly. Under these conditions I(KD) rose to a peak value and then subsided to a steady level. The current-voltage (I-V) relationship for peak I(KD) had an upward bump at about +50 mV and gave it an S-shape. The I-V curve for steady I(KD) rose continuously. Peak and steady I(KD) were reduced by perfusing with EGTA or F ions intracellularly. The EGTA effect occurred at intracellular Ca activity levels below 10-7 M. Increases in the concentration of EGTA(i) at constant Ca(i) had not additional effect; however, recovery experiments do not allow us to rule out some direct action of EGTA on I(KD). Prolonged extracellular perfusion with Co-substituted solutions also reduced I(KD) and the effects occurred more quickly when the solutions were made hypertonic or caffeine was added to them. The peak transient was abolished, and the small remaining steady I(KD) (about 5-10% of normal peak I(KD)) was blocked by tetraethylammonium. I(KD) could be restored by the temporary reintroduction of Ca in the extracellular solution. The S-shape of the peak I-V relationship for I(KD) may be due to Ca released from an endogenous site by membrane depolarization. The reduction of steady and peak I(KD) to very low values by Ca chelators or prolonged perfusion with Ca-free solutions indicates that CA(i) is important for activation of these K channels. Three cellular structures were identified in electron micrographs of freeze-fractured neurones that could be involved in potential-dependent endogenous Ca release. These were a restricted extracellularly space, an intracellular membrane system of endoplasmic reticulum that may be fused to the internal face of the plasma membrane (the subsurface cisterns of Henkart and Nelson, 1979), and intracellular vesicles that also may be fused to the plasma membrane.
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