Environmental control of prolactin synthesis in the teleost fish Oreochromis (formerly Sarotherodon) mossambicus.

In the cichlid fish Oreochromis mossambicus prolactin cell activity is inversely related to the osmolarity and the Ca2+ concentration of the ambient water. Prolactin cell activity was estimated, at the end of a 3-week experimental period, by determination of the rate of prolactin synthesis during incubation of the rostral parts of the pituitary gland, in the presence of [3H]lysine. Since the secretory activity of isolated prolactin cells is known to be inversely related to the osmolarity of the incubation medium, the possibility was investigated that the effects of changes in ionic composition of the ambient water on prolactin secretion in vivo are mediated by changes in osmolarity of the blood plasma. No support was found for this hypothesis. In fish exposed to high water osmolarities prolactin cell activity was reduced, while plasma osmolarity increased. In contrast, at high Ca2+ concentrations of the water, when prolactin secretion was inhibited to a similar extent, plasma osmolarity was significantly reduced. Although direct effects of plasma osmolarity on prolactin cells cannot be excluded completely, it is unlikely that plasma osmolarity is the predominant factor in the control of prolactin cell activity in situ. The physiological significance of the capacity of isolated prolactin cells to respond to changes in osmolarity of the ambient medium--a capacity shared with some other endocrine cell types--is therefore unclear.