Factors affecting in vitro activity of prolactin cells in the euryhaline teleost Sarotherodon mossambicus ( Tilapia mossambica)

Abstract

The rostral pars distalis (RPD) of the euryhaline teleost Sarotherodon mossambicus ( Tilapia mossambica) was incubated in vitro to study the activity of the prolactin cells. Total prolactin release during 18 hr was measured by disc gel electrophoresis; release of newly synthesised prolactin was followed using a 3H-leucine tracer. Synthetic activity was estimated by calculating the specific activity of the prolactin bands. Prolactin release from the control tissues was always greater in hyposmotic than in hyperosmotic medium, indicating a direct effect of osmotic pressure upon the cells. Dopamine (1 μg/ml) inhibited release, but not synthesis, of prolactin in hyposmotic medium. Octopamine (5 μg/ml and 10 ng/ml) in hyposmotic medium and γ-amino- n-butyric acid (GABA) (100 ng/ml) in hyperosmotic medium had no effect on prolactin secretion. Cortisol (1 μg/ml) inhibited prolactin release in hyposmotic medium and, to a lesser extent, in hyperosmotic medium. Prolactin specific activity was increased only in the latter medium. Estradiol-17β (100 ng/ml) did not alter prolactin release in hyperosmotic medium, but did appear to enhance hormone synthesis. Thyrotropin-releasing hormone (TRH) (100 ng/ml) inhibited prolactin release, but not synthesis, in hyposmotic medium, and did not affect prolactin secretion in hyperosmotic medium. TRH at 100 pg/ml was ineffective in hyperosmotic medium. Somatostatin (300 ng/ml) inhibited prolactin synthesis and release in hyposmotic medium, but inhibited only release in hyperosmotic medium. These results indicate that there may be a complex regulation of the prolactin cells in this teleost. The in vitro responses of teleost prolactin cells to various potential mediators resemble those seen in other vertebrates; however, the differences in detail may have special adaptive significance for the teleost.