Effect of prolactin versus growth hormone on islet function and the importance of using homologous mammosomatotropic hormones

The purpose of this study was to determine the effects of homologous rat PRL (rPRL) and rat GH (rGH) on islet B-cell function in neonatal and adult rat islets in vitro. In neonatal rat islets, exposure to rPRL for more than 24 h was necessary for a stimulatory effect on insulin secretion. By day 4, insulin secretion was 3.8-fold greater in the islets cultured with rPRL. rGH had a modest effect on insulin secretion, and this effect was additive with that of rPRL. Both rPRL and rGH increased islet insulin content and 3H]thymidine incorporation. After removal of rPRL, more than 24 h were necessary to detect a reversal in the level of insulin secretion. In addition, even after 5 days without rPRL the previously treated islets still had elevated levels of insulin secretion. In a dose-response study of rPRL on insulin secretion, a detectable effect was observed at 62.5 ng/ml, with a half-maximal effect of approximately 100 ng/ml. Glucose oxidation by neonatal islets was enhanced by rPRL treatment, but not by rGH treatment. In adult rat islets, exposure to rPRL, but not to rGH, enhanced insulin secretion. In contrast, when using heterologous human GH, results similar to those obtained with rPRL, but not rGH, were observed. The results from these experiments indicate that rPRL and rGH have both individual and shared regulatory effects on rat islets. However, it is rPRL and not rGH that has the primary influence on insulin secretion. When interpreting studies examining the effect of GH and PRL on islet function, it is important to consider whether homologous hormones are used.