An ATP-Activated Cation Conductance in Human Epididymal Cells1

An outwardly rectifying conductance was observed in primary cultured human epididymal cells under the patch-clamp whole-cell configuration in KCl pipette and bath solutions. Removal of Cl- from intracellular and extracellular solutions did not affect this conductance, but substitution of K+ with N-methyl-D-glucamine in both solutions completely blocked the current. The current magnitude was also found to be affected by intracellular but not extracellular K+ concentrations. The conductance could be inhibited by the cation channel blockers, barium and tetraethylammonium chloride. Single-channel recordings from the same cell population also revealed the presence of a conductance of about 150 pS with voltage dependence and blocker sensitivity similar to those observed for the whole-cell current. This cation conductance was not affected by changes in solution osmolarity but could be activated by extracellular ATP. ATP activation of the conductance could be mimicked by ionomycin and inhibited by BAPTA-AM, an agent that suppresses intracellular free Ca2+. This ATP-regulated cation conductance may be responsible for secreting K+ across the epididymal epithelium, resulting in an observed K+ concentration much higher in the lumen of the epididymis than in the blood.