Dopaminergic Regulation of Growth Hormone (GH) Secretion in Normal Man: Correlationof L-Dopa and Dopamine Levels with the GH Response*

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Abstract
Although it is clear that dopamine (DA) can affect PRL [prolactin] and GH [growth hormone] release at the level of the hypothalamus and median eminence/pituitary gland, the relative importance of the site of control remains unclear. Central or peripheral DA levels were differentially altered by administering L-dopa (250 mg) orally to 6 healthy male volunteers after pretreatment with the peripheral decarboxylase inhibitor carbidopa (central action), i.v. DA (4 .mu.g/kg .cntdot. min, peripheral action), and L-dopa (500 mg orally, both central and peripheral actions). The changes in GH and PRL observed were correlated with the concentrations of L-dopa and DA measured by the radioenzymatic technique. Peak L-dopa and DA levels were 1.2 .+-. 0.3 .mu.g/ml and 8.4 .+-. 1.9 ng/ml after L-dopa and 2.5 .+-. 0.6 .mu.g/ml and 1.7 .+-. 0.5 ng/ml, respectively, when half the L-dopa dose was combined with carbidopa. While DA and L-dopa concentrations were significantly correlated after L-dopa (r = 0.9), this relationship was abolished by carbidopa. Peak GH levels were comparable after both drug regimens, but the GH response correlated with L-dopa (r = 0.85) or DA (r = 0.70) concentrations only when L-dopa was given without carbidopa. During the infusion of DA, circulating DA levels reached 49.4 .+-. 5.4 ng/ml and GH increased to 10.4 .+-. 3.5 ng/ml. PRL concentrations fell comparably after each of the 3 regimens, but baseline values were significantly increased by carbidopa pretreatment, suggesting that basal PRL is modulated by circulating DA. Since treatment with carbidopa plus L-dopa has a predominant central effect, DA has a peripheral effect, and L-dopa has both peripheral and central effects, the data strongly support both peripheral (pituitary and/or median eminence) and central modulation of GH as well as PRL secretion in normal man.