Biphasic short-circuit current response to noradrenalin mediated by Ca 2+ and cAMP in cultured rat epididymal epithelium

Abstract

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 (10 μmol·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 with a cell permeant Ca 2+ 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 β-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 μmol·l -1) elicited single Ca 2+ spikes in isolated epididymal cells, which could be abolished by prior treatment with Tg. Biochemical assays showed that NA (10 μmol·l -1) increased intracellular cyclic adenosine monophosphate concentration ([cAMP](i)) and the response was abolished by prior treatment with nadolol (50 μmol·l -1). The results showed that NA elicited a biphasic I(SC) response mediated by a rise in intracellular Ca 2+ concentration ([Ca 2+](i)) followed by a rise in [cAMP](i). The Ca 2+-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 milieu vital for sperm storage and maturation.