Prolactin inhibition test with L-dopa: decrease and restoration of plasma prolactin levels in the rat by a peripheral process.

Abstract

L-DOPA, within 30 min after administration, induced a highly significant decrease of plasma prolactin levels (phase 1) in a number of groups of rats, differing in age and/or endocrine status, apparently by direct inhibition of prolactin release from the pituitary. Three hours after administration of L-DOPA these low plasma prolactin concentrations in treated animals had increased (phase 2) and did not differ significantly from levels in control animals, indicating that the effect of L-DOPA on plasma prolactin levels is only of short duration. During this process some interesting phenomena were observed, especially in the animals treated with oestrone. The elimination rate of prolactin from plasma was very high (t 1/2 = 2.8 min), as indicated by decreasing concentrations of the hormone during phase 1. Pituitary prolactin content did not change during phase 1, suggesting that prolactin synthesis was also stopped. Notwithstanding the high elimination rate, plasma prolactin regained initial concentrations in phase 2, suggesting release of a substantial part of the pituitary prolactin content. The latter,however, remained constant during the whole experiment (i.e. before L-DOPA administration and during phase 1 as well as phase 2). The results suggested another working mechanism of L-DOPA in decreasing plasma prolactin levels, namely by stimulating the uptake of this hormone in the periphery. After the effect of L-DOPA had ceased, most of the prolactin from the periphery returned into the bloodstream, causing a rapid restoration of plasma prolactin levels without substantial release from the pituitary. The nature of the processes responsible for the peripheral uptake of prolactin is discussed.