Effects of thyroid status on plasma adrenaline and noradrenaline concentrations in sheep during acute and chronic cold exposure

Abstract

In the first of two experiments, plasma concentrations of adrenaline (A), noradrenaline (NA), and glucose were determined in intact sheep and in surgically thyroidectomized sheep treated intramuscularly with either 0.125 or 0.25 mg triiodothyronine (T₃) per day. On day 28 of exposure to temperatures of 22–25 or 2–5 °C, overall mean plasma concentrations of A were 0.07 and 0.15 ng/ml, respectively, and of NA were 0.30 and 0.45 ng/ml, respectively. Plasma A concentrations were higher in intact compared with thyroidectomized sheep on T₃ therapy. Plasma glucose concentrations were increased by exposure to 2–5 °C and by T₃ treatment. In a second experiment thyroidectomized sheep were kept at 22–26 °C and were either T₃ treated (0.07 mg T₃/day, im) or untreated. After 3 weeks, mean concentrations in the untreated sheep before acute cold and during the last hour of cold exposure (−23 °C) were, respectively: for A, 0.09 and 0.47 ng/ml; for NA, 0.46 and 3.15 ng/ml; and for glucose, 62.1 and 122.1 mg/100 ml. In T₃-treated sheep the mean concentrations before and during cold were, respectively: for A, 0.07 and 0.22 ng/ml; for NA, 0.30 and 1.71 ng/ml; and for glucose, 59.6 and 82.3 mg/100 ml. The untreated sheep showed greater increases in plasma concentrations of A, NA, and glucose and in hematocrit values than T₃-treated sheep, but had slightly smaller increases in metabolic rate, greater decreases in rectal temperature, and similar heart rates during cold exposure.