Catecholamine, renin, aldosterone, and arginine vasopressin responses to lower body negative pressure and tilt in normal humans: effects of bromocriptine.

We have examined the effects of the dopamine agonist bromocriptine (BEC) on the hormonal and hemodynamic response to graded lower body negative pressure (LBNP) and tilting in five normal volunteers. BEC blunted the plasma norepinephrine (NE), plasma renin activity (PRA), and aldosterone responses to both LBNP and tilting. The inhibitory effects of BEC on the plasma NE response to these maneuvers are likely mediated through presynaptic inhibition of peripheral neuronal release of NE as well as central nervous system effects of the drug. Since the PRA responses to LBNP and tilting are likely mediated through beta-adrenoreceptor stimulation, BEC probably indirectly blunts the PRA and aldosterone responses to those maneuvers through its inhibitory effects on NE secretion. BEC treatment resulted in a hypotensive response to tilting that was accompanied by a rise in plasma potassium and arginine vasopressin (AVP). No such rises in plasma potassium and AVP are observed, in the absence of BEC treatment, following graded LBNP and tilting. The rise in plasma potassium with tilting (BEC treatment) probably resulted from blunting of the NE rise. Thus, the rise in plasma NE may play an important role in preventing a rise in plasma potassium in association with LBNP and orthostatic stress. AVP levels in normal men are not responsive to unloading of cardiopulmonary and sinoaortic baroreceptors. It is only after overt hypotension is produced--as after BEC treatment--that plasma levels of AVP rise.