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 (T-3) levels and the activity of two hepatic responsive enzymes [malic enzyme (ME) and cx- glycerophosphate dehydrogenase (alpha-GPD)] in the livers of hypothyro id rats, under conditions where different doses of T-3 (1 and 2.5 mu g /100 g b.w.) were administered daily for one week either by intraperit oneal injection or by continuous infusions. In infused animals, serum T-3 concentrations were constant for the whole period of treatment whi le in injected groups, widely oscillating diurnal levels were observed . The injection of 2.5 mu g/100 g b.w. resulted, at the end of the tre atment, in serum T-3 levels which were higher than in animals receivin g the same dose by infusion. No significant differences were observed when the administered dose was 1 mu g/100 g b.w. The basal levels of a lpha-GPD and ME, which were markedly reduced in the livers of hypothyr oid rats, were returned to normal both in infused rats (both with the dose of 1 mu g and 2.5 mu g/100 g b.w. of T-3) and in rats injected wi th a dose of 1 mu g/100 g b.w.). On the other hand, the dose of 2.5 mu g/100 g b.w. when administered by injection, resulted in alpha-GPD an d ME activities which were significantly higher even than those found in normal ones. The that both the diurnal T-3 profile and the activity of the two hepatic T-3 responsive enzymes are dependent not only on t he dose but also on the administration mode.