Physiological and Pharmacological Influences on Thyroxine to 3,5,3′-Triiodothyronine Conversion and Nuclear 3,5,3′-Triiodothyronine Binding in Rat Anterior Pituitary

R. G. Cheron; M. M. Kaplan; P. R. Larsen

doi:10.1172/JCI109598

Physiological and Pharmacological Influences on Thyroxine to 3,5,3′-Triiodothyronine Conversion and Nuclear 3,5,3′-Triiodothyronine Binding in Rat Anterior Pituitary

R. G. Cheron, M. M. Kaplan, and P. R. Larsen

Published November 1, 1979 - More info

View PDF

Abstract

Our recent in vivo studies have suggested that intrapituitary l-thyroxine (T₄) to 3,5,3′-triiodo-l-thyronine (T₃) conversion with subsequent nuclear binding of T₃ is an important pathway by which circulating T₄ can inhibit thyrotropin release. The present studies were performed to evaluate various physiological and pharmacological influences on these two processes in rat anterior pituitary tissue. Intact pituitary fragments were incubated in buffer—1% bovine serum albumin containing 0.14 ng/ml [¹³¹I]T₃ and 3.8 ng/ml [¹²⁵I]T₄. Nuclei were isolated after 3 h of incubation and the bound iodothyronines identified by paper chromatography. There was 0.3-1% [¹²⁵I]T₃ contaminating the medium [¹²⁵I]T₄, and this did not change during incubation. Nuclear [¹²⁵I]T₄ was not decreased by 650-fold excesses of medium T₃ or T₄, suggesting that it was nonspecifically bound. The ratio of nuclear to medium [¹³¹I]- and [¹²⁵I]T₃ were expressed as nuclear counts per minute per milligram wet weight of tissue:counts per minute per microliter medium. Intrapituitary T₄ to T₃ conversion was evidenced by the fact that the nuclear:medium (N:M) ratio for [¹³¹I]T₃ was 0.45±0.21, whereas that for [¹²⁵I]T₃ was 2.23±1.28 (mean±SD, n = 51). A ratio (R), the N:M [¹²⁵I]T₃ divided by the N:M [¹³¹I]T₃, was used as an index of intrapituitary T₄ to T₃ conversion. Increasing medium T₃ concentrations up to 50 ng/ml caused a progressive decrease in the N:M ratio for both T₃ isotopes, but no change in the value for R, indicating that both competed for the same limited-capacity nuclear receptors. Increasing concentrations of medium T₄ caused no change in the N:M [¹³¹I]T₃ but did cause a significant decrease in R in three of four experiments. These results suggest saturation of T₄-5′-monodeiodination occurred at lower T₄ concentrations than saturation of nuclear T₃ binding sites. In hypothyroid rats, the N:M ratios for both [¹³¹I]T₃ and [¹²⁵I]T₃ were increased (P < 0.005), but R was three-fold higher than in controls (P < 0.005). Animals given 10 μg T₄/100 g body wt per d for 5 d had significantly decreased N:M ratios for both [¹³¹I]T₃ and [¹²⁵I]T₃, as well as a decreased value for R. In fasted rats, neither N:M ratio was depressed, although hepatic T₄ to T₃ conversion in the same animals was 50% of control (P < 0.005). Iopanoic acid (13 μM), but not 6-n-propylthiouracil (29 μM), decreased the N:M [¹²⁵I]T₃ with a significant decrease in the value for R (P < 0.025 or less). Neither sodium iodide (6 μM) nor thyrotropin-releasing hormone (7-700 nM) affected the T₃ N:M ratios. These results indicate that intrapituitary T₄ to T₃ conversion is stimulated in hypothyroidism and depressed in T₄-treated animals, whereas opposite changes occur in hepatic T₄-5′-monodeiodination. Unlike liver, anterior pituitary T₄-5′-monodeiodination is not affected by fasting or incubation with 6-n-propyl-2-thiouracil, but T₄ to T₃ conversion is inhibited in both by iopanoic acid. These results indicate that there are important differences between anterior pituitary and other tissues in the regulation of T₄-5′-monodeiodination.