Activation process of calcium-dependent potassium channel in euhadra neurons: Involvement of calcium/calmodulin and subsequent protein phosphorylation

Abstract

1. 1. The activation process of Ca 2+-dependent potassium channel was studied electrophysiologically and pharmacologically using identified neurons of the land snail, Euhadra peliomphala. 2. 2. Ca 2+-mediated delayed outward K current ( I KD) was dose-dependently reduced by the calmodulin inhibitors, N-(6-aminohexyl)-l-naphthalenesulfonamide ( N-5, week) and N-(6-aminohexyl)-5-chloronaphthalenesulfonamide (W-7, potent). These antagonists also caused a slight membrane depolarization and increase in impulse discharge frequency with decrease in the amplitude of both action potential and after hyperpolarization. 3. 3. The cAMP-dependent protein kinase inhibitor N-[2-(methylamino) ethyl]-5-isoquinoline-sulfonamide (H-8) did not produce any significant effect on I KD and membrane potential. 4. 4. Calmodulin, when injected into the neuron which had been treated with either W-5 or W-7, transiently restored the suppressed I KD nearly to the pretreatment level, and caused hyperpolarization of the cell. In contrast, calcium chloride, intracellularly injected in the same way, had little effect on both the I KD and the membrane potential shifted by these antagonists. 5. 5. Intracellular injection of kinase II, a Ca 2+/calmodulin-dependent protein kinase, caused an increase in the I KD and membrane hyperpolarization. Similar but weak effects were produced when a catalytic subunit (CS) of cAMP-dependent protein kinase was intracellularly injected. However, the neurons pretreated with W-7 no longer had any detectable increase in the I KD and hyperpolarization of the membrane. 6. 6. These results suggest the possibility that Ca 2+/calmodulin-dependent protein phosphorylation may finally mediate the activation of a certain number of potassium channels.