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Relation between α1‐adrenoceptor subtypes and noradrenaline‐induced contraction in rat portal vein smooth muscle

1 In vascular smooth muscle, α1‐adrenoceptors have been classified recently into two or three subtypes. We examined which α1‐adrenoceptor subtypes are involved in the noradrenaline‐induced contraction of rat portal vein smooth muscle. 2 Binding studies with [3H]‐prazosin in membranes from equine portal vein smooth muscle revealed the presence of two distinct affinity binding sites. The high‐affinity site for [3H]‐prazosin was also identified in intact strips of rat portal vein. Prazosin, HV723 (α‐ethyl‐3,4,5‐trimethoxy‐α‐(3‐((2‐(2‐methoxyphenoxy)ethyl)‐amino)‐propyl) benzene‐acetonitrile fumarate), WB4101 (2‐(2,6‐dimethoxyphenoxyethyl)aminomethyl‐1,4‐benzodioxane), 5‐methylurapidil, phentolamine and yohimbine antagonized [3H]‐prazosin binding at both types of sites. Pretreatment with 50 μm chloroethylclonidine (CEC) eliminated the high‐affinity sites for prazosin but had no effect on the low‐affinity sites. 3 Noradrenaline produced a concentration‐dependent contraction in the rat portal vein. Pretreatment with 50 μm CEC induced a slight rightward displacement of the concentration‐response curve but the maximal contraction was not significantly affected suggesting that the CEC‐sensitive α1‐adrenoceptors played a minor role in the noradrenaline‐induced contraction. Prazosin, WB4101 and HV723 produced a concentration‐dependent inhibition of noradrenaline‐induced contractions. The inhibition curves were little affected by CEC‐pretreatment and yielded a relative order of potency of WB4101 > prazosin > HV723. 4 In the presence of 0.1 μm isradipine to block voltage‐dependent Ca2+ channels, the noradrenaline‐induced contraction is due to release of Ca2+ ions from agonist‐sensitive intracellular Ca2+ stores. Under these conditions, the noradrenaline‐induced contraction was not significantly affected by pretreatment with 50 μm CEC but was inhibited by the antagonists mentioned above with affinities different from those in the absence of isradipine. The rank order of potency became HV723 ≥ WB4101 > prazosin. 5 The present results indicate the existence of two distinct α1‐adrenoceptor subtypes in rat portal vein smooth muscle, which show high‐ and low‐affinities respectively for each of prazosin, WB4101 and HV723 and correspond to α1H‐ and α1L‐adrenoceptor subtypes. According to recent α1‐adrenoceptor subclassifications, the α1H‐adrenoceptor subtype which is sensitive to inactivation by CEC may correspond to the α1B‐adrenoceptor subtype. The contraction induced by noradrenaline seems to be predominantly mediated through the α1L‐adrenoceptor subtypes which may include the α1N‐adrenoceptor subtype, as recently proposed.