CATION CHANNEL MECHANISMS IN ET-3-INDUCED VASOPRESSIN SECRETION BY RAT HYPOTHALAMONEUROHYPOPHYSEAL EXPLANTS

Endothelins modulate not only vasoregulation but also neurotransmissio n and hormone secretion, specifically vasopressin (AVP) secretion. The present studies were designed to ascertain the site of action and the participation of membrane cation channels mediating endothelin-3-indu ced AVP release. Experiments were performed using standard and compart mentalized hypothalamo-neurohypophysial explants. The stimulatory acti on of endothelin-3 on AVP release occurred at the neural lobe, consist ent with the failure of sodium channel blockade to decrease AVP secret ion. Calcium channel antagonism or chelation of extracellular calcium inhibited neurohormone release, but blockade of calcium mobilization f rom intracellular stores with 8-(diethyl-amino)octyl 3,4,5-trimethoxyb enzoate hydrochloride (TMB-8) did not. Inhibition of the calcium-activ ated potassium channel with charybdotoxin increased AVP levels dose de pendently. Potassium ionophore abolished this response, as did TMB-8, but inhibition of calcium entry failed to do so. A subthreshold dose o f charybdotoxin potentiated AVP secretion to submaximal stimulation wi th endothelin-3 that was prevented only by concomitant blockade of cal cium influx and intracellular mobilization. The data support interacti on between calcium and potassium channels at the secretory terminal. C ollectively, these data are consistent with endothelin-3 receptor acti vation at the secretory terminal initiating calcium entry, thereby lea ding to depolarization independent of sodium conductances. This mechan ism is opposed by hyperpolarizing forces linked to calcium accumulatio n, namely, the charybdotoxin-sensitive calcium-activated potassium cha nnel. Interaction of the depolarizing and repolarizing systems enables graded AVP secretion from the neural lobe. These findings do not prec lude the participation of other systems as well.