Facilitation by forskolin of electrically evoked vasopressin release in vitro requires bursting pattern of stimulation.

Electrically stimulated release of vasopressin from isolated rat neurohypophysis was measured in the presence and absence of forskolin. Forskolin inhibited the release of vasopressin evoked by continuous stimulation at constant frequency, and enhanced the release evoked by trains of pulses separated by silent intervals. It is suggested that this biphasic effect of forskolin is consistent with an involvement of cyclic adenosine monophosphate (cAMP) in the regulation of cytosolic Ca2+ in the neurosecretory terminals. These results may partly explain some of the discrepant effects of increasing tissue cAMP observed between those studies using electrical pulses, and those using high K+, for stimulation of vasopressin release.