Sodium-retaining effects of cortisol, prolactin, and estradiol-17β in medaka Oryzias latipes exposed to acid water

To investigate endocrine control of osmoregulation in the fish under acidic environment, sodium-retaining effects of cortisol, prolactin and estradiol-17 beta were examined in the acid-exposed medaka Oryzias latipes. Exposure from neutral fresh water (pH 7.0) to acidic fresh water (pH 3.8) for 24 h caused a significant decrease in plasma sodium levels in both sexes of the medaka. Single injection of cortisol prior to the exposure retained plasma sodium levels and stimulated gill Na+,K+-ATPase activity in acid water. Ovine prolactin also prevented a decrease in plasma sodium levels after acid exposure, whereas gill Na+,K+-ATPase activity was unaffected. In male fish, estradiol-17 beta prevented a decrease in plasma sodium level without affecting gill Na+,K+-ATPase activity. On the other hand, estradiol-17 beta had no effect on plasma sodium in female fish. There was no additive action of estradiol-17 beta on the prolactin-stimulated retention of plasma sodium levels in male fish. These findings showed that cortisol and prolactin have sodium-retaining activity in acid-exposed medaka of both sexes, and estrogen seems to involve in the sex difference in acid tolerance.