Reversible antagonism between acetylcholine and phenoxybenzamine in isolated atria of guinea-pigs.

The antagonism between acetylcholine and phenoxybenzamine on isolated driven left atria of guinea-pigs has been examined. Phenoxybenzamine (10(-6) and 2.5 X 10(-6)M), added to the bath fluid 5 minutes before the administration of acetylcholine (10(-7) - 10(-6)M), caused an almost parallel shift (to the right) in the log dose-response curve of acetylcholine on isolated driven left atria of guinea-pigs. There was no fall in the maximum attainable response induced by acetylcholine. These results were taken to imply a reversible, competitive antagonism between acetylcholine and phenoxybenzamine in isolated atria of guinea-pigs. It is suggested that phenoxybenzamine probably combines reversibly (like atropine) with the muscarinic cholinoceptors on the atrial muscle to antagonise, in a competitive manner, the inhibitory action of acetylcholine on isolated atrial myocardium.