Effects of 5-hydroxytryptamine, dopamine, and aromatic L-amino acids on growth hormone (GH)-releasing factor-stimulated GH release in rat anterior pituitaries.

The importance of monoaminergic mechanisms for the regulation of GH-releasing factor (GRF)-stimulated GH secretion was studied in perifused rat anterior pituitaries. Dopamine (greater than 1 microL) reduced GRF-stimulated GH release, but 5-hydroxytryptamine (5-HT; 1 mL) had no effect. The substrates for L-aromatic amino acid decarboxylase, L-5-hydroxy-tryptophan(L-5-HTP; 10 microM) L-dihydroxyphenylalanine; (1 mM), D,L-o-tyrosine (2 mM), and D,L-m-tyrosine (2 mM), all reduced GRF-stimulated GH release. Inhibition of the L-aromatic amino acid decarboxylase by benserazide (0.1 mM), carbidopa (0.1 mM), or alpha-monofluoromethyldopa (0.1 mM) did not reduce the effect of the decarboxylase substrates on GH secretion. The enzyme inhibitors had no influence on hormone secretion per se. The findings indicate that dopamine may inhibit GRF-induced GH release at the pituitary level and that the precursor amino acids inhibit GH secretion independently of the formation of the corresponding amines.