RELATIONSHIP BETWEEN DOSE, MODE OF ADMINISTRATION AND EFFECTS OF TRIIODOTHYRONINE ON 2 HEPATIC RESPONSIVE ENZYMES

We have examined serum 3,3',5-triiodo-L-thyronine (T3) levels and the activity of two hepatic responsive enzymes [malic enzyme (ME) and α-glycerophosphate dehydrogenase (α-GPD)] in the livers of hypothyroid rats, under conditions where different doses of T3 (1 and 2.5 μg/100 g b.w.) were administered daily for one week either by intraperitoneal injection or by continuous infusions. In infused animals, serum T3 concentrations were constant for the whole period of treatment while in injected groups, widely oscillating diurnal levels were observed. The injection of 2.5 μg/100 g b.w. resulted, at the end of the treatment, in serum T3 levels which were higher than in animals receiving the same dose by infusion. No significant differences were observed when the administered dose was 1 μg/100 g b.w. The basal levels of α-GPD and ME, which were markedly reduced in the livers of hypothyroid rats, were returned to normal both in infused rats (both with the dose of 1 μg and 2.5 μg/100 g b.w. of T3) and in rats injected with a dose of 1 μg/100 g b.w.). On the other hand, the dose of 2.5 μg/100 g b.w. when administered by injection, resulted in α-GPD and ME activities which were significantly higher even than those found in normal ones. The results indicate that both the diurnal T3 prefile and the activity of the two hepatic T3 responsive enzymes are dependent not only on the dose but also on the administration mode.