The in vitro effect of leptin on growth hormone secretion from primary cultured ovine somatotrophs.

Although existing data suggest an influence of leptin on circulating levels of growth hormone (GH), the action site and properties of leptin are still controversial. Using primary cultured ovine pituitary cells, we studied the direct effect of leptin on the secretion of GH. Pituitary cells were dissociated by collagenase and subjected to Percoll gradient centrifugation to enrich the somatotroph population to 60-80% of cells. Treatment of primary cultured ovine somatotrophs with leptin (10(-9)-10(-7) M) for 30 min did not affect basal, GH-releasing hormone (GHRH) (10(-7) M)- or GH-releasing peptide-2 (GHRP-2)(10(-7) M)-stimulated GH secretion. Following treatment of cells for 1-3 d with leptin, GHRH-stimulated GH secretion was reduced and GHRP-2-stimulated GH secretion increased. The combined effect of GHRH and GHRP-2 on GH secretion was not altered by the treatment of cells with leptin for 3 d. GHRH receptor mRNA levels in cultured somatotrophs were decreased but GHRP receptor mRNA levels were increased by 3-d leptin treatment. These results suggest that leptin has a long-term effect on somatotrophs to reduce GHRH receptor synthesis leading to a decrease in GHRH-stimulated GH secretion. Leptin appears, however, to have an opposite effect on GHRP receptor synthesis leading to an increase in GHRP-stimulated GH secretion.