Characterization of alpha(1)-adrenoceptor subtypes mediating noradrenaline-induced contraction of rat epididymal vas deferens in calcium-free medium

The alpha(1)-adrenoceptor subtype mediating noradrenaline (NA)-induced cont ractions of rat epididymal vas deferens in Ca2+-free/EGTA (1 mM) medium was studied using competitive antagonists. The effects of chloroethylclonidine (CEC) was investigated in Ca2+-free and normal Krebs' medium and RT-PCR wa s used to identify a,adrenoceptor specific mRNA in epididymal vas deferens. In Ca2+-free medium, NA evoked sustained contractions but was less potent (pD(2), 5.9) than in normal Krebs' medium (pD(2), 7.3). The contractions in Ca2+-free medium were inhibited by prazosin (pA(2), 9.3), 5-methylurapidil (pA(2), 8.4), spiperone (pA(2), 7.6) and BMY 7378 (pK(B), 6.8) consistent with activation of alpha(1A)-subtype. Repeated pretreatment with CEC (100 m u M) reduced the potency of NA and maximum contractions in normal and Ca2+- free media. CEC-sensitivity in normal Krebs' medium was enhanced by prior t reatment with phenoxybenzamine. mRNA for alpha(1a)- and alpha(1d)- but not alpha(1b)- adrenoceptors were detected in epididymal vas deferens. These re sults suggest that NA contracts the tissue in Ca2+-free medium by the stimu lation of alpha(1A)-adrenoceptors. Two factors affecting CEC-sensitivity of NA-induced contractions in this tissue are discussed.