Evidence for Cl−-dependent K+ secretion by the blood–testis barrier of brook trout

The transepithelial transport of 86Rb+, a tracer for K+, was examined in the isolated sperm duct of brook trout (Salvelinus fontinalis). Fluxes of 86Rb+ at 1.0 mM Rb+ in voltage-clamped ducts bathed with Ringer's solution on both sides revealed net secretion of Rb+ averaging 31–58 nequiv.∙cm−2∙h−1 after stimulation by 1.0 mM dibutyryl-cAMP. Unstimulated tissues had no net Rb+ transport. The stimulated Rb+ transport was reduced 70% by bilateral replacement of Cl− with gluconate, indicating that Rb+ secretion is dependent on Cl−. Ba2+ (2.0 mM) added to the luminal (mucosal) side had no effect on Rb+ secretion rate, suggesting that apical Ba2+-sensitive K+ channels are not involved. When added on the blood side (serosal), Ba2+ stimulated Rb+ net flux by 68%, possibly as a result of blockade of basal K+ channels and increased intracellular [K+]. Injection of the antiandrogen cyproterone acetate (3 × 0.4 mg∙kg−1 over 7 days in vivo) significantly reduced stimulated Rb+ secretion (in vitro), suggesting that androgens may maintain the active transport characteristics of the blood – testis barrier. The active K+ secretion by the sperm duct accounts for the high concentration of K+ in seminal plasma which in turn is important in maintaining quiescence of developing spermatozoa.