Ethanol directly modulates gating of a dihydropyridine-sensitive Ca2+ channel in neurohypophysial terminals

Abstract

Ingestion of ethanol results in a decreased level of plasma vasopressin, which appears to be caused by inhibition of arginine vasopressin (AVP) release from the neurohypophysis. Activation of membrane voltage-gated Ca2+ channels plays an important role in triggering this neurohormone release. In this article, single-channel recordings are used to demonstrate that ethanol, at concentrations constituting legal intoxication, inhibits dihydropyridine-sensitive "L-type" Ca2+ channels in isolated nerve terminals of the rat neurohypophysis. Ethanol reduced the channel open probability in a concentration-dependent manner. To allow finer resolution of channel openings and to better characterize the mechanisms of ethanol action, Bay K 8644 was used to prolong the openings of L-type Ca2+ channels. In the presence of this dihydropyridine (DHP), the reduction of the channel open probability by concentrations of ethanol of 25 mM or higher could be determined to be due primarily, although not completely, to a shortening of the open duration of this L-channel. Channel conductance was unaffected by ethanol, even at high concentrations. These results are consistent with previous macroscopic data indicating that calcium channels in these peptidergic terminals are targets for ethanol action, and indicate that ethanol acts directly on the gating characteristics of the L-type channel. Furthermore, examination of open and closed state transitions, as well as Hill plot analysis, suggests that ethanol's effects on gating are consistent with the interaction of a single drug molecule with a single target site, possibly the L-channel itself.