CYCLICAL EFFECTS OF TRIIODOTHYRONINE ON BLOOD-FREE FATTY ACID AND GLUCOSE CONCENTRATIONS IN WARM-ADAPTED AND COLD-ADAPTED RATS

Abstract

Effects of triiodothyronine (T₃)(100μg/100g, sc) on blood-free fatty acid (FFA) and glucose concentrations were investigated in warmand cold-adapted rats. Blood specimens were obtained from the tail locally anesthetized with lidocaine. In warm-adapted rats FFA rose 3 to 6hr after administration of T₃, but returned to the pre-injection level 12hr later. Thereafter, blood FFA level showed a repeated cyclical rise and fall every 12hr for up to 60hr, while blood glucose level did not increase until 12hr later and this level was maintained for up to 36hr. After that time it also showed a repeated rise and fall every 12hr for up to 60hr. Both FFA and glucose levels returned to the initial values after 72hr. Changes induced by T₃ in blood levels of these metabolites were reciprocal. In cold-adapted rats the patterns of responses to T₃ were essentially the same as those observed in warm-adapted ones, except that T₃ provoked a simultaneous rise in blood glucose as well as FFA 6hr after injection, although an extent of FFA increment was less than in warm-adapted rats. Reserpine pre-treatment caused a considerable reduction in the FFA mobilizing action of T₃. The FFA and glucose mobilizing action of T₃ was also observed at dosages of 25 and 6.25μg/100g, although to lesser extent. These results indicated for the first time the cyclical action of T₃ on blood FFA and glucose concentrations, and a changed sensitivity to T₃ in coldadapted animals.