Pulmonary artery rings were prepared from rats (group I to VI, n = 5-7 per group) in order to investigate if a β-adrenoceptor antagonist interferes with the effect of α-adrenoceptor blockade. In I, four cumulative noradrenaline (NA) dose-response curves (10-9 to 10-3 M) were constructed. In II, NA curves were constructed first, without blockers, then in the presence of phentolamine (10-6 M), then phentolamine plus propranolol (10-8 M). III, IV and V were treated similar to II, except that atenolol (10-8 M), ICI 118,551 (10-8 M) and propranolol (10-6 M), respectively, were added instead of propranolol (10-8 M). In VI, NA (10-6 M) was added followed by phentolamine and then propranolol (10-9 to 10-6 M). Phentolamine shifted the NA curve to the right with an increase in the EC50 value. Propranolol but not atenolol or ICI 118,551, reduced the elevated EC50 values caused by phentolamine. In group VI, the increase in force by NA was reduced by phentolamine; the effect of phentolamine was completely abolished by propranolol. Thus, propranolol interferes with the effect of phentolamine in the rat pulmonary artery.
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