Effects of bromocriptine on plasma catecholamines in normal men

Administration of a single oral 2.5 mg dose of bromocriptine (Brc), a dopamine (DA) receptor agonist, to normal male volunteers results in comparable marked decreases in resting supine plasma concentrations of DA (decrement 2 h after drug administration was 67 +/- 5%; mean +/- SE), norepinephrine (NE; 63 +/- 3%) and epinephrine (E; 65 +/- 5%); decreases in all three plasma catecholamine (CA) concentrations were significant at p less than 0.001. This effect of Brc on plasma CAs was initiated more rapidly and was of briefer duration than the suppressive effect on serum prolactin (Prl). When Brc was administered daily for 1 week, its effect on plasma CAs had terminated within 4 h of its adninistration. A further oral dose after administration of Brc for 1 week produced identical suppressive effects on plasma CAs to those observed following the 1st dose. Brc did not prevent the incremental response of plasma DA, NE and E to standing. Also, orthostatic hypotension occurred on occasion, in spite of normal plasma CA responses to standing. Brc did not affect basal serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone or dihydrotestosterone. These data describing a suppressive effect of Brc on plasma DA, NE and E suggest that Brc inhibits release of these CAs from peripheral sympathetic nerve endings and adrenal medulla, and acts at least in part through presynaptic CA receptors in the brain. The suppressive effect of Brc on plasma CAs may prove useful as a test of normal presynaptic CA mechanisms.