Stimulation of liver mitochondrial α-glycerophosphate dehydrogenase activity by L-thyroxine in thyroidectomized rats : comparison with the suppression of pituitary TSH secretion

The rate of liver mitochondrial α-glyce-rophosphate dehydrogenase (GPD) induction was compared to the suppression of pituitary thyrotropin (TSH) secretion in thyroidectomized rats submitted to prolonged administration of small amounts of L-thyroxine (T4). With both 350 and 530 ng T4/100 g bw/day, liver α-GPD activity remained at post-thyroidectomy level (mean ± SE: 0.030 ± 0.002 and 0.034 ± 0.001 Δ A/mg prot/ min, respectively) throughout all experiment. A sharp increase in enzyme activity was observed after 3 weeks of treatment in rats receiving 715 ng T4 / 100 g bw / day (mean ± SE: 0.086 ± 0.003 Δ A/mg prot/min). In contrast, serum TSH levels were lower than pretreatment values (199 ± 8ng/dl) in rats receiving 350 ng T4/100 g bw/day (mean ± SE: 104 ± 15 ng/dl; t = 7.48, p < 0.001), decreased progressively with increasing T4 doses (m ± SE:530 ng T4/100 g/day = 54 ± 5 ng/dl; 715 ng T4/100 g/day = 36 ± 7 ng/dl), after only 48 h of treatment and were not significantly modified thereafter. The data are in agreement with the hypothesis of a nonlinear relationship between the degree of thyroid hormone receptor occupancy and the rate of liver mitochondrial α-GPD induction.