CHARACTERIZATION OF ADRENOCEPTORS INVOLVED IN THE ELECTROGENIC CHLORIDE SECRETION BY CULTURED RAT EPIDIDYMAL EPITHELIUM

1 Short-circuit current (SCC) technique was used to study the adrenoceptors involved in the electrogenic chloride secretion by cultured cauda epididymal epithelium of rats. Stimulation of the epithelium with noradrenaline (primarily beta-1-adrenoceptor selective agonist), salbutamol (beta-2-adrenoceptor selective agonist) and adrenaline (non-selective beta-adrenoceptor agonist) led to a rise in SCC. At a low chart-speed (2 mm min-1), the response profile to these agonists consisted of a peak followed by a sustained response considerably higher than the basal SCC. 2 The EC50s (doses of agonist producing 50% maximum response) of noradrenaline, salbutamol and adrenaline were 300, 115 and 10 nm respectively. Pretreating the tissues with 1-mu-M atenolol (beta-1-selective antagonist) and 10-mu-M butoxamine (beta-2-selective antagonist) shifted the dose-response curves of noradrenaline (shifted EC50 = 4000 nm) and salbutamol (shifted EC50 = 1050 nm) to the right. Atenolol (1-mu-M) and butoxamine (10-mu-M) shifted the dose-response curve of adrenaline to the right with new EC50s of 30 nM and 115 nM, respectively. 3 The rapidly rising phase of the SCC response to noradrenaline and adrenaline observed at low chart-speed consisted of a brief and transient retraction followed by a rebound increase in SCC. At a high chart-speed (1 mm s-1), the retraction and rebound phenomenon manifested as a fast initial spike which could be blocked by phentolamine (non-specific alpha-adrenoceptor antagonist) in a dose-dependent fashion. Similar initial spikes were observed when the tissues were stimulated with phenylephrine (alpha-1-selective agonist) but not with isoprenaline (non-selective beta-agonist) or forskolin (activator of adenylate cyclase). The response of the initial spike triggered by noradrenaline was dose-dependent and the EC50 was 2000 nM. 4 The present study showed that the electrogenic chloride secretion by rat epididymis could be stimulated by alpha-1-, beta-1- and beta-2-adrenoceptor agonists. The alpha-1-mediated response had a faster onset and more transient action than the beta-counterpart. It is postulated that epididymal chloride secretion might be regulated by neural (noradrenaline-mediated) and humoral (adrenaline-mediated) controls and that the stimulus-secretion coupling mechanisms might involve both Ca2+ (alpha-1-mediated response) and adenosine 3':5'-cyclic monophosphate (beta-mediated response) as intracellular second messengers.