Local regulation of vasopressin and oxytocin secretion by extracellular ATP in the isolated posterior lobe of the rat hypophysis

It is now widely accepted that ATP functions as a signalling substance in t he nervous system. The presence of P-2 receptors mediating the action of ex tracellular ATP in brain regions involved in hormonal regulation raises the possibility that a similar role for ATP might also exist in the neuroendoc rine system. In this study, the release from the rat isolated neurohypophys is preparation of endogenous ATP, oxytocin and vasopressin (AVP) were measu red simultaneously using luciferin-luciferase and RIA techniques. After 70 min preperfusion, electrical field stimulation caused a rapid increase in t he amount of ATP in die effluent and the release of AVP and oxytocin also i ncreased stimulation-dependently. Inhibition of voltage-dependent Na+ chann els by tetrodotoxin (1 mu M) reduced the stimulation-evoked release of AVP and oxytocin; however, the evoked release of ATP remained unaffected. The effect of endogenous ATP on the hormone secretion was tested by suramin (300 mu M), the P-2 receptor antagonist. Suramin significantly increased t he release of AVP, and the release of oxytocin was also enhanced. ATP, when applied to the superfusing medium, decreased the release of AVP, but not t hat of oxytocin, and its effect was prevented by suramin. ATP (60 nmol), added to the tissues, was readily decomposed to ADP, AMP and adenosine measured by HPLC combined with ultraviolet light detection, and the kinetic parameters of the enzymes responsible for inactivation of ATP ( ectoATPase and ecto5'-nucleotidase) were also determined (K-m = 264 +/- 2.7 and 334 +/- 165 mu M and v(max) = 6.7 +/- 1.1 and 2.54 +/- 0.24 nmol/min p er preparation (n = 3) for ectoATPase and ecto5'-nucleotidase respectively) . Taken together, our data demonstrate the stimulation-dependent release, P-2 receptor-mediated action and extracellular metabolism of endogenous ATP in the posterior lobe of the hypophysis and indicate its role, as a paracrine regulator, in the local control of hormone secretion.