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Biophysics of Pericardium, Great Vessels and Chambers of Frog Heart and their Haemodynamic Inter -relationships

Concentration-response curves were obtained for the action of acetylcholine (ACh) on frog hearts perfused under controlled dynamic conditions. The cardiac output, frequency of contractions, and arterial pressure developed by the heart were continuously recorded electronically. The minimum concentration of ACh that affected pace-maker frequency and ventricular tone was about the same, and was unaffected by the presence of anticholinesterases. The action of ACh on pace-maker frequency was potentiated by anticholinesterases, the degree of potentiation increasing with concentration of ACh. There was a limit to the degree of inhibition of ventricular tone attainable with ACh, this limit varying from 40 to 100 per cent in different hearts. Inhibition of ventricular tone by ACh was often unaltered, or even reduced, by anticholinesterases. The sensitivity of the conducting system to ACh varied greatly from heart to heart, and was independent of the sensitivity of either pace-maker or ventricular muscle. The concentration of ACh required to arrest the heart was always less in the presence of anticholinesterases. The results are explained in terms of differences in the amount and type of cholinesterase present in the component tissues of the heart, and by competition between the direct atropine-like action of some anticholinesterases on muscle and their action in inhibiting cholinesterase.