The general importance of proteins and other factors in the transfer of steroids into the rat epididymis.

The entry of total radioactivity into the perfused tubule of the cauda epididymidis was measured during intravenous infusion of labelled steroids into the anaesthetised rat. The effect of altering the composition of the perfusate on this entry and the nature of radiometabolites was also studied. No radioactivity entered the epididymal lumen during infusions of cholesterol, and in the absence of luminal proteins the entry of isotope was generally low (< 14% plasma levels) for all steroids except dehydroepiandrosterone. C21/22-steroids exhibited a delay of about 30 min before a slow rise in luminal activity reached between 3% and 14% plasma levels. For C19-steroids a low plateau level of radioactivity was reached (at 6%-9% plasma levels) with dihydrotestosterone and androstenedione, whereas luminal activity continued to rise with respect to blood during infusions of dehydroepiandrosterone. When protein was added to the perfusing solution the delay in entry of radioactivity was decreased and plateau levels established for C21/22-steroids (at 14%-38% plasma levels). The extents of entry of dihydrotesterone and androstenedione were raised (34%-47%) and radioactivity continued to rise in the lumen relative to blood, but at a faster rate during infusions of dehydroepiandrosterone and perfusion with protein. Testicular fluid protein had little effect on the entry of activity for any steroid. There was no correlation between the extent of entry of tracer with binding of the steroids to plasma or perfusates, or partition into lipids. Tentative identification of metabolites indicated an entry of both dihydrotestosterone and dehydroepiandrosterone during their respective infusions. Androstenedione and androstanediols were excluded whereas 5 alpha-reduced metabolites appeared during of androstenedione and dehydroepiandrosterone.