MODULATION OF A CALCIUM-SENSITIVE NONSPECIFIC CATION CHANNEL BY CLOSELY ASSOCIATED PROTEIN-KINASE AND PHOSPHATASE-ACTIVITIES

Regulation of nonspecific cation channels often underlies neuronal bur sting and other prolonged changes in neuronal activity. In bag cell ne urons of Aplysia, it recently has been suggested that an intracellular messenger-induced increase in the activity of a nonspecific cation ch annel may underlie the onset of a 30-min period of spontaneous action potentials referred to as the ''afterdischarge.'' In patch clamp studi es of the channel, we show that the open probability of the channel ca n be increased by an average of 10.7-fold by application of ATP to the cytoplasmic side of patches. Duration histograms indicate that the in crease is primarily a result of a reduction in the duration and percen tage of channel closures described by the slowest time constant, The i ncrease in open probability was not observed using 5'-adenylylimido-di phosphate, a nonhydrolyzable ATP analog, and was blocked in the presen ce of H7 or the more specific calcium/phospholipid-dependent protein k inase C (PKC) inhibitor peptide((19-36)). Because the increase in acti vity observed in response to ATP occurred without application of prote in kinase, our results indicate that a kinase endogenous to excised pa tches mediates the effect, The effect of ATP could be reversed by exog enously applied protein phosphatase 1 or by a microcystin-sensitive ph osphatase also endogenous to excised patches. These results, together with work demonstrating the presence of a protein tyrosine phosphatase in these patches, suggest that the cation channel is part of a regula tory complex including at least three enzymes. This complex may act as a molecular switch to activate the cation channel and, thereby, trigg er the afterdischarge.