In SLO-permeabilized isolated nerve endings from the rat neurohypophysis, GTP, guanosine 5′[y-thio]triphosphate (GTPyS) and guanosine 5′(ßy-imido]triphosphate (GMPPNP) inhibit the Ca2+-evoked vasopressin release. Pretreatment with pertussis toxin enhances the inhibitory effects of both GTP-analogues. Omission of Mg2+ overcomes the effect of GMPPNP and reverses the inhibitory effect of GTP and GTPyS. In the absence of Mg2+, GTP and GTPyS now potentiate Ca2+-evoked secretion.

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