The regulation of prolactin release from the pituitary of the tilapia, Oreochromis mossambicus, by cortisol and environmental salinity

Borski, Russell John
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Prolactin is an essential hormone in the freshwater osmoregulation of the euryhaline teleost fish, tilapia, Oreochromis mossambicus. Cortisol, on the other hand, is important in seawater adaptation in the tilapia. The present studies address how cortisol and environmental salinity regulate prolactin cell function in the tilapia. During in vitro incubations, prolactin release is inhibited in a dose-related manner by cortisol. This action is mimicked by the synthetic glucocorticoid agonist dexamethasone but not by other classes of steroids tested. Perifusion studies indicate that physiological concentrations of cortisol inhibit prolactin release within 20 min. cortisol reduces cAMP and Ca2+ accumulation in the tilapia pituitary within 15 min, a time-course similar to the one over which cortisol inhibits prolactin release. These studies suggest that the rapid inhibition of prolactin release by cortisol is a specific glucocorticoid action that is mediated, in part, by the cAMP and Ca2+ second-messenger systems. Previous studies have shown that reductions in medium osmotic pressure, which reflect the blood osmotic pressure of a tilapia adapting to fresh water, rapidly stimulate prolactin release, while elevations in medium osmotic pressure inhibit prolactin release. The present studies clearly indicate that exposure to reduced osmotic pressure increases intracellular free Ca2+ in single, tilapia prolactin cells. This hyposmolar-induced elevation in intracellular free Ca2+ occurs within 30 seconds, is sustained as long as the cells are exposed to hyposmotic medium, and can be reduced to prestimulated levels by exposure to hyperosmotic medium. Taken together, these data suggest a mediatory role for Ca2+ in the induction of prolactin secretion by osmotic pressure. Consistent with prolactin's role in freshwater osmoregulation, the quantity of the two tilapia prolactins (tPRL188 and tPRL177) and their release are greater from the pituitaries of freshwater tilapia compared with that from seawater fish. Nonetheless, the relative content of the two tilapia prolactins (tPRL188/tPRL177) in the pituitaries of freshwater tilapia was significantly higher (1.5:1) than that seen in seawater fish (0.75:1). These studies indicate that the processing of the tPRL188 and tPRL177 may be differentially sensitive to environmental salinity and that they may be undergoing functional divergence.
Thesis (Ph. D.)--University of Hawaii at Manoa, 1992.
Includes bibliographical references (leaves 116-129).
xiv, 129 leaves, bound ill. 29 cm
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Zoology; no. 2802
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