Daily Variations in Protein Synthesis and Tissue Responsiveness to Growth Hormone, Prolactin and Insulin-Like Growth Factor-I in the Gulf Killifish, Fundulus Grandis Baird and Girard.

Circadian variations of \sp{14}C-glycine incorporation into protein and the responsiveness of muscle tissue to growth hormone, prolactin and insulin-like growth factor-I were examined in gulf killifish, Fundulus grandis. Incorporation of label during a 5 h period following \sp{14}C-glycine injection was tested at different times of day (LD 12:12). Incorporation varied during the day in all the tissues examined (scale, muscle, liver and intestine). The times of peak incorporations differed among the tissues. Scales showed peak incorporation during the dark hours. Peak incorporation by muscle occurred at the onset of light. These patterns were similar in male and female fish. The daily pattern of incorporation by liver and intestine differed between the two sexes. Growth hormone and prolactin injections enhanced incorporation of the label in muscle but the responses varied as a function of the time of day of injections. Both hormones were most effective at light onset and least effective at light offset. In tissue culture study, recombinant human insulin-like growth factor-I (rhIGF-I) stimulated \sp{14}C-glycine incorporation by muscle in a dose-dependent manner. The minimal effective dose was 0.1 ng/ml of incubation medium. Maximal stimulation was obtained with 1 ng/ml. In a time-course study using 1 ng/ml of rhIGF-I, the greatest stimulation was observed 6 h after the onset of a 3 h exposure to rhIGF-I. Responsiveness of muscle to a 3 h exposure to rhIGF-I examined at 3 different times of a day was greatest at light onset and lowest at light offset. Binding studies using partially purified membrane preparations of muscle demonstrate daily variations in the binding of \sp{125}I-rhIGF-I. Greatest binding was observed during the dark period. Daily variations in dissociation constant (Kd) and maximal binding (Bmax) were found at three times of day tested. These studies provide additional evidence that tissue-responsiveness to stimuli vary during a day. Thus, circadian rhythms of tissue- responsiveness coupled with rhythms of growth promoting hormones may produce a temporal interaction between stimuli and responses and play an important role in the regulation of protein synthesis and growth in fish