Pregnant rats were injected with triiodothyronine (T3) on the 19th and 20th days of pregnancy. This elevated fetal T3 plasma values and caused a large rise in fetal liver phosphoenolpyruvate carboxykinase (PEPcK) activity. At the same time PEPcK activity in fetal brown fat was inhibited. Essentially the same effect of T3 was also noted when injected into postnatal rats. T3 injections also elevated fatty acid synthetase activity in brown fat of weanling rats but, in contrast to corticosteroids, had no effect earlier in life. No effect was noted in the liver, except a slight decrease in the fetus. T3 injections to suckling rats elevated plasma levels of insulin and glucagon within 24 h.
Rats were weaned on the 18th or 30th postnatal day to a high-fat, high-carbohydrate or atherogenic diet. Twenty-four hours later, hepatic levels of cyclic guanosine 3',5'-monophosphate (cyclic GMP) and adenosine, 3',5'-cyclic monophosphate (cyclic AMP) were found to be higher in male animals aged 31 days fed the high-fat than those fed the high-carbohydrate diet. Prematurely weaned rats (day 18) reacted in the same way. However, feeding either diet resulted in higher hepatic cyclic nucleotide levels than found in rats kept with the dam. The atherogenic diet was least effective in raising these levels. After a 24-hour fast, cyclic nucleotide levels in liver and brown fat were elevated and hepatic levels could not be lowered by 6 hours of feeding the high-fat or atherogenic diet. In male 40-day-old rats, however, feeding for 2 hours was sufficient to lower these levels, more so with a high-carbohydrate than a high-fat diet. Since blood levels of glucagon are high, and those of insulin are low in 18-day-old rats that are not weaned and since their hepatic cyclic nucleotide levels are low, it is suggested that other factors, in addition to blood hormone levels, play a role in regulating cyclic nucleotide levels.
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