Central effects of catecholamine antagonists on angiotensin-induced vasopressin secretion in conscious rats

To evaluate the roles for catecholamines in angiotensin II (ANG II)-induced vasopressin (AVP) release, we examined in conscious rats the effects of intraventricular (ivt) administrations of catecholamine antagonists on plasma AVP responses to ivt applications of its agonists and ANG II. Plasma AVP was determined by RIA using trunk blood collected after decapitation. Dopamine (0.15 mumol), phenylephrine (an alpha-adrenergic agonist, 0.15 mumol) or ANG II (48.2 pmol) augmented plasma AVP 90 sec after the injection, whereas after isoproterenol (a beta-adrenergic agonist, 0.15 mumol) plasma AVP was unaffected. The plasma AVP responses to both dopamine and ANG II were significantly (P less than 0.01) inhibited by haloperidol (a dopamine blocker, 0.15 mumol) given 10 min before administration of these agents. Pre-administration of phenoxybenzamine (an alpha antagonist, 0.15 mumol) which was confirmed to abolish the effect of phenylephrine, or propranolol (a beta antagonist, 0.15 mumol) did not block the effect of ANG II. Administration of haloperidol, phenoxybenzamine or propranolol alone was without effect on plasma AVP level. On the basis of these results, we concluded that ANG II-induced AVP secretion may be mediated and/or modulated by dopamine.