The importance of testicular interstitial fluid in the transport of injected hCG to the Leydig cells.

High levels of hCG occur in testicular interstitial fluid (IF) following injection of the hormone, and the proportion that is taken up by the testis has been assessed by comparing the concentrations in IF of [125I]hCG and [125I]BSA (i.e. an inert protein) in rats treated with the same total dose of hCG. Adult male rats were injected with a total dose of either 0.5 or 45.5 IU hCG and which included either [125I]hCG or [125I]BSA, and were killed between 2 and 40 h later when the concentrations of the labelled proteins in plasma IF and testicular tissue were assessed. Treatment with either dose of hCG increased IF levels in the testis at 4-24 h after injection although this increase was much greater (350% v 61%) with the higher dose of hCG. In rats injected with [125I]hCG, the ratio of radioactivity in testicular tissue to that in IF (T/IF) varied between 0.3 and 2.1 depending on the total injected dose of hCG and the time after injection, whilst in rats injected with the same amounts of unlabelled hCG together with [125I]BSA, the T/IF ratio never exceeded 0.09 and remained constant with time. In the latter group, the IF/plasma ratio of [125I]BSA was always significantly higher than in rats injected with [125I]hCG, irrespective of the total dose of hCG injected. The difference between the IF/plasma ratios for [125I]BSA and [125I]hCG gives a measure of how much of the hCG has been removed by testicular uptake. The results show that following injection of either 0.5 or 45.5 IU hCG, the temporal pattern of testicular hCG-uptake from IF is similar, although proportionately more (50-59%) of the hCG in IF has been taken up within 24 h of injection of the low dose of hCG than with the high dose (32-52%). It is concluded that testicular IF is an important medium for the transport of injected hCG from the blood to the Leydig cells.