Circadian effects of prolactin on lipid metabolism and hydromineral regulation in intact and hypophysectomized goldfish,Carassius auratusL.

Abstract

Intact and hypophysectomized goldfish (Carassius auratus L.) were given intra‐coelomic injections of ovine prolactin at cither 09.00 h or 16.00 h (2 h and 9 h after the onset of the photoperiod, respectively). The effects of prolactin on plasma Na⁺, K⁺, Cl^‐ and osmotic pressure, total body water content, liver lipid content, total body lipid content and liver glycerol‐3‐phosphate acyltransferase activity were determined in an attempt to detect circadian variability in the response to exogenous prolactin. There were no significant changes in plasma K⁺, total body water content, liver lipid content or total body lipid content in response to hypophysectomy or prolactin administration. Plasma Cl⁺ and plasma osmotic pressure values fell significantly after hypophysectomy. Prolactin injections given at both 09.00 h and 16.00 h were equally effective in partially restoring the plasma Cl^‐ and plasma osmotic pressure values to pre‐hypophysectomy levels. Plasma Na⁺ values were significantly increased in both intact (p+ levels in fish killed at 16.00 h but not at 09.00 h. The data suggest a diurnal variation in the plasma Na⁺ response to hypophysectomy and prolactin administration. There were no significant differences in hepatic glycerol‐3‐phosphate acyltransferase activity in any of the groups injected at 09.00 h whereas both groups of hypophysectomized goldfish injected at 16.00 h (saline‐ and prolactin‐injected) had significantly lower enzyme activity than the comparable sham‐operated fish (p<0.01 for comparisons of the saline‐injected groups and p<0.05 for comparisons of the prolactin‐injected groups). Moreover, prolactin significantly (p<0.05) elevated the acyltransferase activity in hypophysectomized animals which were injected at 16.00 h. Possible mechanisms by which these diurnal responses to prolactin might be mediated are discussed.