Growth Hormone and Prolactin Action in a Teleost Anabas testudineus (Bloch): Effect of the Time of Administration

Circadian rhythms play a very important role on metabolic process and have considerable effects on growth, especially in ectotherms. Like variation in hormone levels, the sensitivity of target cells may show diurnal or seasonal fluctuations. The aim of this study was to compare the effects of morning versus evening injections of growth hormone and prolactin on malic enzyme, glucose-6-phosphate dehydrogenase, isocitrate dehydrogenase, aspartate aminotransferase, alanine aminotransferase and Na⁺,K⁺-ATPase in a teleost Anabas testudineus. Activities of malic enzyme, glucose-6-phosphate dehydrogenase and isocitrate dehydrogenase of the two control groups themselves differ significantly at morning and evening. Early morning administration of growth hormone increases malic enzyme, glucose-6-phosphate dehydrogenase and isocitrate dehydrogenase activities while evening administration of growth hormone does not effect these enzymes. Transaminase activities were stimulated by morning and evening administration of GH and PRL. Na⁺,K⁺-ATPase activity was stimulated by morning administration and inhibited by evening treatment of both hormones. The results reveal that a given hormone may provide a different message to the target tissues at different periods of the day.