Stimulation of liver mitochondrial alpha-glycerophosphate dehydrogenase activity by L-thyroxine in thyroidectomized rats: comparison with the suppression of pituitary TSH secretion.
The rate of liver mitochondrial alpha-glycerophosphate 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 alpha-GPD activity remained at post-thyroidectomy level (mean +/- SE: 0.030 +/- 0.002 and 0.034 +/- 0.001 delta 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 delta A/mg prot/min). In contrast, serum TSH levels were lower than pretreatment values (199 +/- 8ng/dI) in rats receiving 350 ng T4/100 g bw/day (mean +/- SE: 104 +/- 15 ng/dI; t = 7.48, p less than 0.001), decreased progressively with increasing T4 doses (m +/- SE:530 ng T4/100 g/day = 36 +/- 7 ng/dI); 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 alpha-GPD induction.
Costante
G
G
Instituto Pluridisciplinare di Clinica Medica e Terapia Medica Generale e Speciale, Università di Messina, Italy.
Crupi
D
D
Catalfamo
R
R
Trimarchi
F
F
D000704:Analysis of Variance; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D005993:Glycerolphosphate Dehydrogenase / Q000378:metabolism*; D008297:Male; D008930:Mitochondria, Liver / Q000187:drug effects* / Q000201:enzymology; D010902:Pituitary Gland / Q000187:drug effects* / Q000378:metabolism; D011863:Radioimmunoassay; D051381:Rats; D011919:Rats, Inbred Strains; D013965:Thyroidectomy; D013972:Thyrotropin / Q000097:blood / Q000378:metabolism*; D013974:Thyroxine / Q000097:blood / Q000494:pharmacology*
D013972:Thyrotropin; D005993:Glycerolphosphate Dehydrogenase; D013974:Thyroxine