Cold Adaptation and it's Endocrine Control in Teleosts.

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The seasonal variations of plasma osmolality and Na concentration in the plasma of the saffron cod were investigated. And with our previous data on the kidney structure and the pituitary immunohistochemistry, the mechanisms of cold adaptation for the saffron cod were discussed.Plasma osmolality was increased during the winter than during the summer. This high plasma osmolality of the winter fish may be due to the increased antifreeze glycoprotein content. Na concentration showed a higher value in the plasma of the winter fish than in those of the summer fish. The significance of such high Na concentration in the winter fish is need further study.In the fish collected during the summer, the positive immunoreactions with both growth hormone(GH) and prolactin(PRL) were remarkable than during the winter. However, there are no seasonal differences on the cross-sectional areas of GH- and PRL-secreting cells and their nuclei between the winter fish and the summer fish.It has been shown that an intact pituitary is necessary for the disappearance of antifreeze protein(AFP) from the plasma of the winter flounder during the summer(Fletcher et al.,'78). It is also evident recently that GH controls the concentration of AFP in the winter flounder reasoning its inhibitory effect of mRNA transcription in the liver, and PRL has no effect(Idler et al.,'89). In the teleost, however, it is known that PRL increases both urine flow and glomerular filtration rate involving an effect as the glomerular tuft is enlarged(Lam and leatherland,'69).These results reconfirmed the previous consideration(Ogawa et al.,'93) in which PRL may act possibly on clearing of antifreeze glycoprotein by increasing glomerular filtration during the summer.