Central nervous system and pituitary mechanisms in dopaminergic stimulation of growth hormone release in women

In an attempt to evaluate the relative importance of brain and peripheral (i.e., outside the blood-brain barrier) dopamine (DA) receptor stimulation in the regulation of GH release, DA (5 microgram/kg/min infused i.v. for 120 min), L-dopa (500 mg p.o.), L-dopa (100 mg p.o.) plus carbidopa (35 mg p.o.) after pretreatment with carbidopa (50 mg p.o. every 6 h for 1 day), and nomifensine (200 mg p.o.) have been administered to healthy women. Significant elevations in serum GH were induced by all regimens in comparison with a control placebo study, although the effect of DA was only transient in spite of continuing infusion. The between drugs analysis revealed that L-dopa alone was significantly more effective in releasing GH than carbidopa plus L-dopa as well as DA infusion and nomifensine administration. These results suggest that both central nervous system and peripheral mechanisms are involved in dopaminergic stimulation of GH release in man. Since DA does not affect somatotropes directly, increased serum GH levels by DA infusion probably reflect stimulation of median eminence dopaminergic neurons.