Cardiac refractoriness in rats is reduced by angiotensin II

We investigated the effect of angiotensin II (AII) on cardiac refractoriness in muscle trabeculae isolated from adult rat ventricle. Strength-interval curves were initially obtained under control conditions and after exposure of the muscle to Tyrode's solution containing 10(-9) M AII. AII displaced the strength-interval curves to the left. The minimal current intensity needed to elicit a propagated response was reduced by AII for all intervals used. The effect of AII was not influenced by propranolol 10(-6) M or phentolamine 10(-7) M but was blocked by 250 microM DuP 753. No change in resting potential was observed with 10(-9) M AII, but action potential duration at 50% APD50 of its amplitude was reduced by 25% and conduction velocity was appreciably decreased (41%). The effect of the peptide on APD was blocked by DuP 753. Spontaneous discharges of APs were elicited by a single stimulus in fibers exposed to 10(-9) M AII, supporting the view that AII has an arrhythmogenic action.