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Role of prolactin in osmotic and ionic regulation of the marine form (trachurus) of the threespine stickleback, gasterosteus aculeatus l. in fresh water Lam, Toong Jin

Abstract

The role of prolactin in osmotic and ionic regulation of the marine threespine stickleback (Gasterosteus aculeatus L., form trachurus) in fresh water has been investigated in winter (or late autumn) and spring. Sticklebacks in late autumn or early winter, when transferred from sea water to fresh water, suffered a high mortality which could be reduced by prolactin treatment. The fish also displayed a greater fall in plasma osmolality and a smaller fall in urine osmolality than late-spring fish transferred to fresh water in the same way; this seasonal difference was apparently triggered by photoperiodic changes and could be eliminated by prolactin treatment of the late-autumn or early-winter fish. Similarly, a seasonal difference exists in the histological picture of the glomeruli of late-autumn and late-spring sticklebacks, and this difference could be eliminated by prolactin treatment of the former fish. The fall in plasma osmolality in late-autumn and winter fish after transfer to fresh water was paralleled by a rapid drop in plasma sodium and chloride, which could be corrected by a single injection of prolactin given 24 hr before the transfer. Plasma potassium, however, seemed unaffected by prolactin treatment. The evidence suggests that prolactin is essential for freshwater survival of sticklebacks and that prolactin undergoes seasonal changes in secretion associated with photoperiodic changes; the secretion is minimal in the autumn and winter when the fish live in sea water or brackish water, and maximal in the spring and summer when the fish migrate to (or are in) fresh water, to breed. Thus prolactin may be involved in the freshwater migration of sticklebacks. Next, the mechanism of action of prolactin was studied. Prolactin seems to exert its effects (in fresh water) on the three recognized organs of osmotic and ionic regulation in teleosts, viz. kidneys, gills and gut. In the kidneys, prolactin increased urine flow, apparently as a result of an increased GFR. Prolactin reduced the apparent increase in intracapsular space in the glomeruli of the late-autumn and winter sticklebacks, and, consequently, increased the percentage frequency of glomeruli with no evident intracapsular space. The data are interpreted to mean that prolactin rendered glomeruli more functional or more glomeruli fully functionaland, hence, increased GFR. Since the increase in urine flow and GFR was paralleled by a decrease in urine osmolality and urine concentrations of sodium and chloride, prolactin must also increase renal tubular reabsorption of sodium and chloride (A) and/or decrease water reabsorption (B); and since the total renal loss of sodium and chloride did not appear to be significantly increased despite an increase in GFR, A must occur with or without B. Prolactin, however, apparently increased the total renal loss of potassium and did not affect the tubular potassium reabsorption, although there was a suggestion that prolactin actually decreased tubular reabsorption of potassium. In the gills (or other regions around the head), prolactin reduced the net osmotic influx of water and the net loss of sodium, chloride and C¹⁴ (from injected C¹⁴ -inulin); the latter was probably because prolactin reduced the outflux. These changes were accompanied by the behaviour of the gill mucous cells, which were increased in density by prolactin treatment, suggesting a cause-or-effect relationship. In the gut, prolactin reduced water absorption and, at the same time, seemed to reduce the freshwater drinking rate. Thus, it appears that prolactin was able to reduce or prevent osmotic flooding of sticklebacks in fresh water by reducing extrarenal osmotic influx of water and increasing renal loss of water via an increase in GFR and urine flow, and also, possibly, by reducing drinking rate and water absorption by the gut; at the same time, prolactin reduced extrarenal loss of sodium and chloride but did not apparently affect renal loss of the ions, which was small compared to the extrarenal loss. By these mechanisms, prolactin maintained plasma osmolality and sodium and chloride levels after transfer of the fish to fresh water, and, consequently, was able to promote freshwater survival of the fish in the autumn and winter.

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