Dual action of dopamine on growth hormone release in vitro

The effect of dopamine (DA) on growth hormone (GH) release was studied in perifused freshly dispersed rat anterior pituitary cells. Pulses of DA (0.01-100 nmol/l), each applied for 30 min, resulted in a prompt rise in GH release. This effect was reversible, concentration-dependent and partially antagonized by metoclopramide, a DA antagonist. The effect of DA was further tested on GH-stimulated secretion by human GH-releasing factor (hGRF). Perifusion with hGRF (6.25 pmol/l) for 2 min elicited an immediate rapid increase in GH release which lasted 20 min. Pretreatment of cells with DA (100 nmol/l) for 10 min and a subsequent hGRF challenge during continuation of DA perifusion significantly reduced the effect of hGRF pulses on GH release. The present data suggest that DA has direct opposite actions at the somatotroph level, stimulating the basal GH release and inhibiting the hGRF-induced GH secretion, and may thus be an important modulator of GH release.