The vas deferens forms part of the male reproductive tract and extends from the cauda epididymis to the prostate. Using the patch clamp technique, we have identified a Ca2+-activated, voltage-dependent, maxi K+ channel on the apical membrane of epithelial cells cultured from human fetal vas deferens. The channel had a conductance of ∼250 pS in symmetrical 140 m m K+ solutions, and was highly selective for K+ over Na+. Channel activity was increased by depolarization and by an elevation of bath (cytoplasmic) Ca2+ concentration, and reduced by cytoplasmic Ba2+ (5 m m) but not by cytoplasmic TEA (10 m m). Channel activity was also dependent on the cation bathing the cytoplasmic face of the membrane, being higher in a Na+-rich compared to a K+-rich solution. We estimated that up to 600 maxi K+ channels were present on the apical membrane of a vas cell, and that their density was 1-2 per μ2 of membrane. Activity of the channel was low on intact cells, suggesting that it does not contribute to a resting K+ conductance. However, fluid in the lumen of the human vas deferens has a high K+ concentration and we speculate that the maxi K+ channel could play a role in transepithelial K+ secretion.
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