Mechanism of calcium-dependent inactivation of a potassium current in Aplysia neuron R15: interaction between calcium and cyclic AMP.

In the preceding paper (Kramer and Levitan, 1988), we presented evidence that an inwardly rectifying K+ current (IR) is inactivated by Ca2+ influx accompanying spontaneous bursting activity in the Aplysia neuron R15. In this paper we examine the mechanism that enables Ca2+ to inactivate IR. Since IR is enhanced by cyclic AMP in neuron R15 (Drummond et al., 1980; Benson and Levitan, 1983), we examined the Ca2+-dependent inactivation of IR after application of either serotonin (5-HT), the adenylate cyclase activator forskolin, or a membrane-permeable cAMP analog, all agents that increase cAMP and hence the magnitude of IR. Even though more active IR channels are available under these conditions, less Ca2+-dependent inactivation is observed. This is contrasted with the Ca2+-dependent inactivation of the voltage-gated Ca2+ current (ICa). Elevating cAMP enhances ICa in R15 and also increases its Ca2+-dependent inactivation. Hence the mechanisms whereby Ca2+ inactivates IR and ICa appear to differ from each other. Elevating internal Ca2+ by repeatedly depolarizing the neuron suppresses the response of IR to brief applications of 5-HT, and speeds the relaxation of the response, suggesting that Ca2+ can interfere with the cAMP-dependent activation of IR. One biochemical site where Ca2+ can reduce cellular cAMP is by activating the Ca2+/calmodulin-sensitive form of phosphodiesterase. We have detected such enzyme activity in homogenates of Aplysia abdominal ganglia and extracts of single R15 somata. Inhibitors of the phosphodiesterase activity suppress the Ca2+-dependent inactivation of IR. Finally, we have used a radioimmunoassay to measure cAMP in individual R15 somata, and have found that R15 neurons hyperpolarized for prolonged periods contain more cAMP than do R15 neurons allowed to burst, consistent with the hypothesis that Ca2+ influx reduces cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)