ATP stimulates Ca 2+ release from a rapidly exchanging pool in cultured rat epididymal cells

Abstract

A study was carried out to investigate an ATP sensitive Ca 2+ pool in rat epididymal cells and its role in transepithelial Cl - secretion. In normal buffered solution containing 2.5 mM free Ca 2+, ATP triggered single calcium spikes in a dose-dependent fashion. In nominally Ca 2+-free solution, the peaks of successive Ca 2+ spikes diminished after repeated ATP stimulations. Addition of Sr 2+ (2.5 mM) to Ca 2+-free solution after ATP stimulation did not cause changes in fluorescence signals. However, in the presence of Sr 2+, ATP gave rise to apparent repetitive Ca 2+ spikes of similar magnitudes after repeated stimulations. Increasing the time of exposure in Ca 2+-free solution containing 50 μM ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid rapidly decreased the intracellular Ca 2+ concentration ([Ca 2+](i)) response to subsequent ATP stimulation. On the other hand, increasing the time of exposure in Sr 2+-containing solution in Ca 2+-depleted cells rapidly increased the apparent [Ca 2+](i) response to subsequent ATP stimulation. These observations suggested the existence of a Ca 2+ pool that was rapidly exchanging with the extracellular compartment. Apical application of ATP elicited a transient rise in short-circuit current across the epididymal epithelium in a dose-dependent fashion, and the response was reduced by prior stimulation with thapsigargin. Ca 2+ released from a rapidly exchanging ATP-sensitive store might stimulate Cl - secretion in the epididymis, thereby maintaining the electrolyte contents and fluidity of the epididymal microenvironment.