Effects Of Blood Withdrawal And Angiotensin II On Prolactin Release In The Tilapia, Oreochromis Mossambicus

Abstract

Reflecting their important role in freshwater (FW) osmoregulation, plasma levels of prolactins (PRL188 and PRL177) in the euryhaline tilapia, Oreochromis mossambicus, are always higher in fish acclimated to FW than in those in seawater (SW). Repeated blood withdrawal (5% of estimated blood volume at 0, 1,4, 8,24,48, 76, and 120 h) from the tilapia acclimated to FW resulted in a marked increase in plasma levels of PRLs. The increase seemed to be correlated with a decrease in plasma osmolality, but the increased PRL levels were more marked than the levels expected from the change in plasma osmolality alone. Repeated blood withdrawal from the fish in SW, on the other hand, did not cause any change in plasma PRL levels, although a significant increase in plasma GH accompanied an increase in plasma osmolality. Blood withdrawal resulted in a significant reduction in hematocrit values regardless of the environmental osmolality, suggesting hemodilution. The reduction in plasma osmolality after blood withdrawal in FW and the increased osmolality in SW suggest that the blood volume is restored at least in part by drinking environmental water. In a separate experiment, a single blood withdrawal (20% of total blood) stimulated drinking in the fish regardless of whether they were held in FW, 30% SW, or SW. Plasma levels of both PRLs were also elevated following a single blood withdrawal in the fish acclimated to FW and 30% SW, but not in the fish in SW. Activation of the renin-angiotensin system after blood withdrawal and dipsogenic action of angiotensin II (ANG II) is well established in fish. Intraperitoneal injection of ANG II (0.1 and 1.0 µg/g) into the fish in FW significantly increased plasma levels of the tilapia PRLs after 1 h. ANG II at concentrations of 10-1000 nM was also effective in stimulating PRL secretion in vitro. There was no effect of ANG II on GH release. These results suggest that the marked increase in PRL concentration after blood withdrawal from the fish in FW is due to a facilitative interaction between ANG II and a reduced plasma osmolality.