Involvement of the adrenal glands in the hypotensive response to bromocriptine in spontaneously hypertensive rats.

1 The dopamine agonist, bromocriptine, produced a hypotensive response following oral administration to conscious normotensive and spontaneously hypertensive (SH)-rats. 2 In SH-rats the dose-related falls in blood pressure to bromocriptine, 3 to 30 mg/kg orally or intraperitoneally, were biphasic, an initial fall at 1 h being followed by some recovery at 2 h and a subsequent fall in blood pressure at 4 and 6 h. 3 The dopamine antagonists, metoclopramide, sulpiride, haloperidol and pimozide, had little or no effect on the hypotensive response to bromocriptine, 10 mg/kg orally, in SH-rats. 4 Pretreatment with alpha-methyl-p-tyrosine augmented the hypotensive response to bromocriptine, 10 mg/kg orally, in SH-rats. 5 In adrenal demedullated SH-rats, the hypotensive response to bromocriptine, 3 to 30 mg/kg orally, was abolished. 6 In SH-rats the hypotensive response to bromocriptine, 10 mg/kg orally, was prevented by the beta-adrenoceptor blocking drugs, propranolol and oxprenolol, but was unaffected by (+)-propranolol and by the cardio-selective beta-adrenoceptor blocking drug, atenolol. 7. In SH-rats pretreated with bromocriptine, 10 mg/kg orally, and then anaesthetized, the pressor responses to low doses of intravenous adrenaline were reversed to depressor, indicating that bromocriptine possesses alpha-adrenoceptor blocking activity. 8 The results suggest that hypotensive response to bromocriptine in conscious SH-rats is mediated by adrenaline released from the adrenal medullae which, in the presence of alpha-adrenoceptor blockade, stimulates vascular beta-adrenoceptors producing vasodilatation.