ATP STIMULATES CA-2+ RELEASE FROM A RAPIDLY EXCHANGING POOL IN CULTURED RAT EPIDIDYMAL CELLS

A study was carried out to investigate an ATP-sensitive Ca2+ pool in r at epididymal cells and its role in transepithelial Cl- secretion. In normal buffered solution containing 2.5 mM free Ca2+, ATP triggered si ngle calcium spikes in a dose-dependent fashion. In nominally Ca2+-fre e solution, the peaks of successive Ca2+ spikes diminished after repea ted ATP stimulations. Addition of Sr2+ (2.5 mM) to Ca2+-free solution after ATP stimulation did not cause changes in fluorescence signals. H owever, in the presence of Sr2+, ATP gave rise to apparent repetitive Ca2+ spikes of similar magnitudes after repeated stimulations. Increas ing the time of exposure in Ca2+-free solution containing 50 muM ethyl ene glycol-bis(beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid rapi dly decreased the intracellular Ca2+ concentration ([Ca2+]i) response to subsequent ATP stimulation. On the other hand, increasing the time of exposure in Sr2+-containing solution in Ca2+-depleted cells rapidly increased the apparent [Ca2+]i response to subsequent ATP stimulation . These observations suggested the existence of a Ca2+ pool that was r apidly exchanging with the extracellular compartment. Apical applicati on of ATP elicited a transient rise in short-circuit current across th e epididymal epithelium in a dose-dependent fashion, and the response was reduced by prior stimulation with thapsigargin. Ca2+ released from a rapidly exchanging ATP-sensitive store might stimulate Cl- secretio n in the epididymis, thereby maintaining the electrolyte contents and fluidity of the epididymal microenvironment.