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Comparison of Peripheral Thyroid Hormone Metabolism in Normal Rats and in Rats Receiving Prolonged Glucagon Infusion^*

Birmingham Veterans Administration Medical Center and The University of Alabama in Birmingham School of Medicine, Birmingham, Alabama 35294

Address all correspondence and requests for reprints to: Constance S. Pittman, M.D., Division of Endocrinology and Metabolism, Department of Medicine, University of Alabama in Birmingham School of Medicine, University Station, Birmingham, Alabama 35294.

Abstract

An elevated plasma glucagon concentration and reduced T₃ production from T₄ have both been observed in several clinical disorders, including hepatic cirrhosis, uremia, diabetes mellitus, and starvation. The question of whether glucagon has a direct effect on T₃ production was studied in normal rats infused iv with [¹²⁵I]T₄ or [¹²⁵I]T₃ and 3 µg T₄/day, using implanted minipumps. The blood [¹²⁵I]T₄ and [¹²⁵I]T₃ levels maintained a plateau between the fifth and ninth days of infusion. Each animal also received a second minipump, implanted ip, that infused either a diluant solution or 30 µg glucagon/100 g BW· day. After 7 days of continuous infusion, the glucagontreated animals showed a 20% increase in plasma glucose and a 4-fold increase in plasma glucagon from baseline. However, the levels of insulin, T₄) and T₃ remained unchanged. The MCRs and the disposal rates of T₄ and T₃) calculated by the constant infusion method, showed T₄ and T₃ MCRs to be 0.99 ± 0.18 and 11.25 ± 2.52 ml/h-100 g, respectively, and T₄ and T₃ disposal rates to be 68 ± 10 and 9± 2 ng/h · 100 g; there was no difference between the control animals and the glucagon-infused animals. T₃ product on was also determined in vitro from T₄ added to a liver homogenate. Compared to control animals, the liver homogenate prepared from glucagon-infused animals showed a modestly higher T₃ production rate throughout the 60-min incubation period (P = 0.025-0.05). However, the concentration of nonprotein-bound sulfhydryls was similar in the liver, kidney, brain, muscle, and heart of the two animal groups. In conclusion, glucagon does not have an important regulating role on the peripheral metabolism of thyroid hormone and T₃ production in rats.

Footnotes

^* This work was supported by the Research Division of the V.A.

Received August 13, 1981.