Inhibitory action of ethanol on L-type Ca2+ channels and Ca(2+)-dependent guanosine 3',5'-monophosphate accumulation in rat pinealocytes.

It has previously been shown that the K+ potentiation of vasoactive intestinal peptide-stimulated cAMP and cGMP responses was inhibited by ethanol in rat pinealocytes, suggesting an inhibitory action of ethanol on the voltage-dependent Ca2+ channels (VDCC). In this study, using the whole cell version of the patch clamp technique, we found that ethanol reduced the amplitude, but did not change the voltage dependence or the time course of activation or inactivation of the L-type VDCC. The inhibitory effect of ethanol on this current was concentration dependent, and ethanol (100 mM) resulted in a 40% inhibition of this current. However, in fura-2-loaded cells, total increases in intracellular Ca2+ ([Ca2+]i) caused by ethanol and BayK 8644 did not differ from the [Ca2+]i signal caused by BayK 8644 alone, suggesting that the inhibitory action of ethanol on VDCC may not be related to a reduction in [Ca2+]i. Although there was no change in the total [Ca2+]i signal, ethanol (25-200 mM) dose-dependently inhibited the potentiation effects of depolarizing concentrations of K+ and BayK 8644 on the isoproterenol-stimulated cGMP, but not the cAMP, response. Therefore, the cGMP response appears to be more sensitive to the inhibitory action of ethanol, and a site distal to elevation of [Ca2+]i of importance to the potentiation mechanism may be inhibited by ethanol. This was confirmed by the finding that ethanol was effective in inhibiting the A23187 potentiation of isoproterenol-stimulated cGMP response. These results suggest that 1) the L-type VDCC was inhibited by ethanol; 2) the Ca(2+)-mediated potentiation of the isoproterenol-stimulated cGMP response was sensitive to the inhibitory action of ethanol; and 3) although ethanol inhibits the VDCC, it alone cannot explain the inhibitory effect of ethanol on BayK 8644- and K(+)-mediated potentiation of the isoproterenol-stimulated cGMP response.