Abstract
In isolated neurons of the land snail, a high threshold voltage-activated calcium current (ICa) was recorded using the two microelectrode voltage clamp method. Iontophoretic intracellular injection of cAMP or extracellular application of dbcAMP evoked a rapid and reversible decrease in peak amplitude of ICa in 30 out of 54 cells tested. The effects of cAMP on ICa and IBa were similar. cGMP injected into the same cell from another barrel of a multi-barreled micropipette did not mimic the effect of cAMP. The cyclic nucleotide phosphodiesterase activator, imidazole, caused an augmentation of ICa in 8 out of 13 cells tested. Intracellular injection of the Ca2+-chelator, EGTA, had no influence on the extent of cAMP-evoked depression of ICa. The inhibitors of kinase A, tolbutamide and H-8, evoked an effect resembling the effect of cAMP, i.e. depression of ICa. The results suggest that the down-regulation of ICa by cAMP is achieved through neither kinase A activation nor through cytoplasmic Ca2+ elevation.
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