EXOGENOUS GROWTH-HORMONE INHIBITS BOVINE BUT NOT MURINE PITUITARY GROWTH-HORMONE SECRETION INVITRO - EVIDENCE FOR A DIRECT FEEDBACK OF GROWTH-HORMONE ON THE PITUITARY

To test the hypothesis that one aspect of growth hormone (GH) regulation involves a direct effect of GH on its own secretion at the level of the pituitary, we evaluated the effects of human GH on basal and GH-releasing factor (GRF)-induced GH secretion in rat and bovine pituitary cell cultures. Both 4 h (acute) and 24 h (chronic) exposure of bovine pituitary cells to exogenous human GH significantly (p < 0.001) reduced GH secretion. Acute treatment resulted in a decrease in GH secretion from 579 +/- 24 to 500 +/- 17 ng/ml 4 h (mean +/- SEM, n = 24) while chronic treatment resulted in a GH decrease from 595 +/- 21 to 483 +/- 17 ng/ml/4 h. In contrast, human GH treatment of rat somatotropes had no effect on basal GH secretion. GRF-stimulated GH secretion was not affected by either acute or chronic GH exposure in either species. These results demonstrate that there are species-specific differences in the ability of GH to regulate its own secretion and provide evidence for a direct inhibitory effect of GH at the level of the bovine pituitary.