Luminal secretion of myo-inositol by the rat epididymis perfused in vitro

A technique for perfusing the lumen of rat epididymal tubules maintained in vitro showed that [3H]inulin was largely excluded from the lumen of unravelled tubules from the cauda and tubules from the corpus if the connective tissue capsule was removed. The preparation transported [3H]inositol from the bath fluid for 3 h against a concentration gradient in both regions with activity rising (16-29% of bath fluid values) in the cauda and reaching a plateau (18%) in the corpus epididymidis. HPLC showed that radioactivity was solely associated with inositol and its movement to the lumen was reduced by raising inositol in the bath fluid from 50 microM (plasma levels) to 10 mM, but not affected by reducing the glucose concentration in the bath fluid or introducing physiological concentrations of inositol (30 mM) into the lumen. Secretion into the caudal lumen of unlabelled inositol measured by g.l.c. was maintained for 3 h at concentrations (300 microM) greater than those in the bath fluid and was not reduced when glucose or inositol were removed from the bath. In contrast, glucose was only detectable in the lumen when it was present in the bathing medium, reaching 1% of this concentration. Radioactivity appeared in the epididymal lumen reaching a plateau (19% of bath fluid values) in the corpus and cauda when [3H]glucose was added to the bath fluid, but no radiolabelled inositol was found in the lumen. We conclude that epididymal tissue is a major source of secreted inositol.