Renal extraction, filtration, absorption, and catabolism of growth hormone.

Renal handling of homologous labeled rat growth hormone (125I-rGH) was studied in the intact rat and in a filtering and nonfiltering isolated perfused rat kidney preparation. Plasma disappearance rate, renal accumulation, renal clearance (C), glomerular sieving coefficient (GSC), absorption rates, and fate of absorbed hormones were determined. 125I-rGH is extensively filtered (GSC approximately 0.6) and subsequently absorbed by the tubular epithelium (C/GRF less than 1%). The absorption process of 125I-rGH has a high capacity and is inhibited by iodoacetate. Absorbed 125I-rGH is catabolized and a detectable product of catabolism (125I-monoiodotyrosine) is returned to the circulation. A nonfiltering kidney preparation with adequate renal perfusate flow was developed to study the contribution of the peritubular side to the renal handling of small proteins. Experiments in the nonfiltering kidney show that renal accumulation, extraction, and catabolism of 125I-rGH from the peritubular side is minimal when compared to that occurring from the luminal side. The ratio of the renal extraction rate of 125I-rGH in the isolated kidney and the mean plasma disappearance rate of 125I-rGH in the intact rat was 0.67, demonstrating that the kidneys account for the major fraction of the total plasma turnover of GH in the rat.