L-dopa absorption and the pituitary-hypothalamic axis.

Administration of oral L-dopa is often used as a neuropharmacological probe to evaluate the pituitary hypothalamic axis. The effect of gastrointestinal absorption of L-dopa on the changes in plasma GH, PRL, and body temperature which occur after ingestion of this amino acid is unknown. Plasma L-dopa, GH, PRL, and rectal and skin temperatures were measured in 14 male volunteers after oral administration of 1.0 g measured in 24 men after random administration of L-dopa and a placebo. L-Dopa levels rose to 1.85 +/- 1.33 microgram/ml (mean +/- SD), but maximum plasma levels occurred at variable times from 30-230 min after drug administration. Plasma GH levels increased to 23.7 +/- 14.7 ng/ml, while PRL levels fell to 46.7 +/- 12.3% of the mean basal values. Rectal temperature decreased significantly in 3 of the men after L-dopa ingestion. Plasma GH levels after L-dopa correlated with the absorption of the drug (P less than 0.05) and inversely with the basal level of GH before L-dopa administration. There was no correlation between the basal PRL level or basal body temperature and the magnitude of the fall in PRL or body temperature after L-dopa administration. The variability of responses in GH, PRL, and body temperature after oral L-dopa ingestion is not the result of differences in absorption in the amino acid alone, and indicate either that there is a different sensitivity in the mechanisms that stimulate GH secretion and lower plasma PRL and body temperature, or that L-dopa acts at different sites to bring about each of these changes.