Cyclic AMP elicits biphasic current whose activation is mediated through protein phosphorylation in snail neurons

The involvement of protein phosphorylation in cAMP-induced transmembrane current was tested electrophysiologically and pharmacologically in identified neurons of the Japanese land snail, Euhadra peliomphala. Intracellular injection of cAMP elicited a biphasic transmembrane current (cAMP current) which consisted of inward and outward components. The inward component was blocked with Na(+)-free, Ca2(+)-free saline and the outward component abolished by either application of tetraethylammonium or a long-lasting exposure to caffeine in Ca2(+)-free saline. The cAMP current was completely suppressed by the protein kinase inhibitors, protein kinase inhibitor isolated from rabbit muscle or isoquinoline sulfonamide (H-8). The catalytic subunit of cAMP-dependent protein kinase transiently restored the cAMP current suppressed by H-8 nearly to pre-H-8 level. These findings suggest that protein phosphorylation may be an intermediate step in the activation process of the cAMP current.