Relationship between the Accumulation of Pituitary Growth Hormone and Nuclear Occupancy by Triiodothyronine in the Rat

Studies were undertaken in hypothyroid rats in an effort to define the kinetics of growth hormone (GH) accumulation in response to i.v. pulse injections of triiodothyronine (T₃) and to calculate the relationship between nuclear occupancy by T₃ and the instantaneous rate of accumulation of pituitary GH. Results were contrasted to the findings in previous studies of the induction of hepatic mitochondrial α-glycerophosphate dehydrogenase (α-GPD) and malic enzyme (ME) by T₃. The dose of T₃ required to achieve half-maximal accumulation of GH in 24 h was 0.6 μg/100 g body wt, a value 15-fold less than the half-maximal dose for α-GPD and ME induction at a comparable time after injection. Although significant increase in pituitary GH were evident as early as 3 h after injection of maximally effective doses of T₃, the rate of increase became linear only 12 h after injection. After achievement of peak values, the pituitary content of GH decayed with a similar terminal t_½ of 3.9 days and 4.1 days in two groups of animals injected with a single dose of 1.0 and 50 μg T₃/100 g body wt, respectively. In vivo isotopic displacement studies carried out at the equilibrium time point indicated that the pituitary nuclear binding capacity was 5.5 ng T₃/g tissue and that the plasma concentration at which one-half of the nuclear sites are occupied is 1.0 ng/ml. Nuclear occupancy as a function of time was calculated from the estimated plasma T₃ concentration after injection of the dose and the half-occupancy plasma concentration. These data were then analyzed by application of the mathematical model previously developed to ascertain the relationship between nuclear occupancy and the rate of hepatic enzyme induction. Results indicated that the pituitary nuclear occupancy-response relationship was generally linear, in marked contrast to the highly amplified relationship between nuclear occupancy and the response of ME and α-GPD to T₃ in the liver. In supplementary experiments, euthyroid rats received daily injections of 200 μg of T₃ for 7 days to keep nuclear sites nearly saturated for the duration of the experiment. No significant increase in the pituitary GH content above euthyroid base-line levels was noted. This also contrasts with the marked increase above euthyroid levels in α-GPD and ME observed in previous studies. Our findings suggest the existence of major differences between the specific mechanisms which lead to the induction of pituitary GH and the hepatic enzymes by T₃.

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