SELECTIVE UPTAKE OF RADIOACTIVITY BY THE ADRENAL CORTEX OF DEXAMETHASONE-TREATED GUINEA-PIGS AFTER THE ADMINISTRATION OF 131I-LABELLED α1–24 ADRENOCORTICOTROPHIN

Abstract

Normal and dexamethasone-treated guinea-pigs were injected intravenously with 50 ng ¹³¹I-labelled α^1–24 adrenocorticotrophin (ACTH). The concentration of radioactivity in the adrenals of dexamethasone-treated animals was significantly greater than in the untreated group but no difference was observed among the other tissues studied. There was no change in the concentration of ¹²⁵I-labelled insulin, ¹²⁵I-labelled poly-l-tyrosine, ¹³¹I-labelled bovine serum albumin (BSA) or [¹³¹I]iodide in any tissue including the adrenal. Intra-adrenal radioactivity after the injection of ¹³¹I-labelled α^1–24 ACTH into normal animals showed a peak approximately localized at the glomerular-fascicular border with a second peak in the zona reticularis. Dexamethasone treatment significantly enhanced the size of both peaks. There was no difference in the intra-adrenal distribution of ¹²⁵I-labelled BSA, ¹²⁵I-labelled insulin and ¹²⁵I-labelled poly-l-tyrosine after dexamethasone administration. The effect of other proteins on the increased uptake of ¹³¹I-labelled α^1–24 ACTH by the adrenal of dexamethasone-treated animals was also studied. Human luteinizing hormone (100 ng), human growth hormone (100 ng), poly-l-tyrosine (50 ng) and glucagon (10, 100 and 1000 ng) were without effect. Excess α^1–24 ACTH (100 μg) and insulin (100 and 1000 ng) reduced adrenal radioactivity to the concentrations found in control animals.