BIPHASIC SHORT-CIRCUIT CURRENT RESPONSE TO NORADRENALINE MEDIATED BY CA2+ AND CAMP IN CULTURED RAT EPIDIDYMAL EPITHELIUM

A study was carried out to investigate the short-circuit current (I-sc) response to noradrenaline (NA) and the signal transduction mechanisms involved in cultured rat cauda epididymal epithelium. In normal Krebs-Henseleit solution, NA (1O mu mol.l(-1)) added basolaterally elicited a biphasic I-sc response consisting of a transient spike followed by a second sustained response. The biphasic response was almost abolished by removing ambient Cl-. Preloading the tissues witha cell-permanent Ca2+ chelator. 1,2-bis(2-aminophenoxy) ethane-N,N,N',N',-tetraacetic acid acetoxymethyl ester (BAPTA/AM), or pretreating them with thapsigargin (Tg), a microsomal adenosine triphosphatase inhibitor abolished the initial spike in the I-sc response to NA, but had Little effect on the second component. Pretreating the tissues with a non-selective beta-antagonist, nadolol, reduced the second I-sc response in a dose-dependent fashion but the initial spike was not affected. Microfluorimetric studies showed that NA (100 mu mol.l(-1)) elicited single Ca2+ spikes in isolated epididymal cells, which could be abolished by prior treatment with Tg. Biochemical assays showed that NA (100 mu mol.l(-1)) increased intracellular cyclic adenosine monophosphate concentration ([cAMP](i)) and the response was abolished by prior treatment with nadolol (50 mu mol.l(-1)). The results showed that NA elicited a biphasic I-sc response mediated by a rise in intracellular Ca2+ concentration ([Ca2(+)](i)) followed by a rise in [cAMP](i). The Ca2+-mediated I-sc response had a faster onset and more transient action than the cAMP counterpart. It is suggested that NA released from noradrenergic nerve endings regulates transepithelial Cl- secretion in the epididymis thereby providing the specialized millieu vital for sperm storage and maturation.