The Fate of I131-Labeled Human Growth Hormone in the Rabbit

Abstract

The rate of disappearance from plasma and tissue localization of radioactivity were determined in rabbits following intravenous injections of radioiodinated human growth hormone (HGH*), and radioiodinated human serum albumin (HSA*). Tissue localization of radioactivity after Nal¹³¹ injection was also examined. HGH* was quickly cleared from the blood; during the extremely rapid initial period (between 2 and 10 minutes), the concentration of trichloroacetic acid precipitable plasma radioactivity fell by 61 %. For the interval 10 to 60 minutes, mean half-time of disappearance of 17.6 ±1.8 minutes (standard error) was determined in a group of 11 rabbits. Half-time of disappearance did not differ in young as compared with old animals. Listed in decreasing order, the tissue radioactivity concentrations at 90 minutes were: kidney, liver, lung, spleen, skin, adrenal, testis, intestine, marrow, pancreas, skeletal muscle fat and cerebral cortex. Substantial proportions of the radioactivity found in the liver and kidney as late as 90 minutes after injection were still precipitable with trichloroacetic acid and with a specific antihuman growth hormone antibody. Comparison studies with HSA* indicate markedly different patterns of plasma disappearance and tissue localization; in all tissues tested the HGH* space was larger than the HSA* space. This suggests that extravascular localization of the HGH* had occurred. Tissue distribution of radioactivity after Nal¹³¹ also differed markedly from that found after HGH* injection, indicating that the pattern of tissue radioactivity after HGH* injected was not due solely to localization of inorganic I¹³¹ from the degraded labeled hormone.