Relationship Between Dose, Mode of Administration and Effects of Triiodothyronine on Two Hepatic Responsive Enzymes

Abstract

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 rat, 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 profile and the activity of the two hepatic T₃ responsive enzymes are dependent not only on the dose but also on the administration mode.

Key words

Thyroid Hormone - Triiodothyronine - Malic Enzyme - α-Glycerophosphate Dehydrogenase - Liver