Control of the delayed outward potassium currents in bursting pace‐maker neurones of the snail, Helix pomatia.

Abstract

The net outward current in bursting pace-maker neurons of the snail H. pomatia during sustained and repeated voltage clamp pulses was studied. The properties of currents remaining in Co-Ringer or after TEA [tetraethylammonium] injection were compared with those in untreated cells. With sustained voltage clamp depolarizations the net outward current first increased to a maximum at 150 ms and then declined to .ltoreq. 60% of its peak intensity. This depression which was greater during repetition of short pulses (e.g., 100 ms pulses at 0.5 s intervals), represented a true decrease in the outward flow of K+ (designated IK) and was not due to a decreased driving force resulting from extracellular K+ accumulation. The steady-state current-voltage (I-V) relationship for IK was N-shaped (Heyer and Lux, 1976) A component of IK persisted when Ca and Mg in the medium were replaced by Co (ICo-res). With voltage clamp depolarizations ICo-res increased rapidly to a maximum and then partially inactivated with voltage dependent time constants of hundredths or tenths of seconds. Repolarization removed the inactivation. Repeated stimulation with short pulses did not increase the depression of ICo-res. ICo-res (e.g., measured during voltage steps from holding potentials of -50 to near 0 mV) was smaller in test pulses preceded by depolarization and larger in pulses preceded by hyperpolarization. The steady state I-V relationship was not N-shaped. ICo-res was blocked by intracellular injection of TEA. Repeated voltage clamp depolarization to .apprx. 0 mV with 100 ms pulses for neurons with large Ca currents in normal Ringer produced a long-term depression which was maximal with 300-400 ms repolarizations (to -50 mV) between pulses. This corresponded with stimulus parameters for the maximum Ca current (Heyer and Lux, 1976). Intracellular injection of Ca2+ (also Ba2+ and Co2+) reduced the total net outward current and especially the delayed outward current under voltage clamp. The IK component removed by Co was identified as Ca dependent and designated IK(Ca). With single voltage clamp pulses IK(Ca) followed the approximate time course and voltage dependence of the slow inward Ca current (Iin slow; Heyer and Lux, 1976). Several lines of evidence suggested that Ca2+ moving through the membrane activated IK(Ca). Part of IK could not be blocked by intracellular TEA injection. In different neurons the magnitude of the IK component resistant to TEA (ITEA-res) was approximately proportional to the relative magnitudes of Iin slow. ITEA-res did not inactivate with sustained depolarization and showed pronounced long-term depression with repetitive stimulation at intermediate intervals and an increased outward current at the onset of the 2nd and subsequent pulses following short repolarizations. The steady-state I-V relationship was N-shaped. ITEA-res was abolished by extra cellular Co. A net inward current with low depolarizations could be measured after TEA injection. TEA seemed to slow the onset of Iin slow and delayed its inactivation. The K current was discussed as consisting of 2 components. One was Ca independent and probably had the voltage and time dependent properties of ICo-res. It is compared with K conductances in such preparations as the squid giant axon. The 2nd component was dependent on Ca both for its activation (a transient effect of Ca2+ moving through the membrane) and inactivation (a long lasting effect of increased concentrations of Ca at the inner surface of the membrane). It probably had many properties similar to ITEA-res and may be related to other TEA resistant K currents. Its properties were contrasted with those of previously described membrane conductances.