Dopaminergic mechanisms and luteinizing hormone (LH) secretion. II. Differential effects of dopamine and bromocriptine on LH release in normal women.

After our observation of the failure of bromocriptine to inhibit LH secretion in hyperprolactinemic women, we have investigated the effects of bromocriptine and dopamine (DA) on LH secretion in normal ovulatory women within 5 days of the midcycle LH peak. Both bromocriptine (2.5 mg, orally) and DA (4 micrograms/kg . min, iv) for 4 h lowered serum PRL levels by more than 80%. Bromocriptine was ineffective in suppressing LH secretion and, in fact, resulted in a slight but significant stimulation, while, in contrast, DA was effective in lowering circulating LH levels by approximately 30% and reducing spontaneous LH fluctuations. The dissociation of the effects of these two dopaminergic agents on LH secretion may be explained by the existence of multiple DA receptors. The slight stimulatory effect of bromocriptine might be due to a preferential presynaptic action of bromocriptine inhibiting endogenous DA secretion in the hypothalamus and thus reducing this catecholamine's tonic inhibitory influence on LH secretion.