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On a demontre que la distension pericardiale produit des changements chronotropiques dans le cœur de grenouille isole en perfusion. Sur 26 cœurs, l'acceleration cardiaque (18 a 148%) se produit dans 12 avec un volume de fluide pericardique atteignant 4 ml. Dans les 14 cœurs restant nous avons observe une bradycardie, des irregularities de conduction, un blocage cardiaque et une arythmie. On en conclut que ces effects chronotropiques et les variations du parametre d'impulsion sont dues aux changements dans l'activite du «pacemaker».

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.

The influence of stretch due to increased intraluminal pressure on the formation and conduction of electrical impulse was studied electrocardiographically in 12 isolated sinoauricular chambers of frog hearts perfused with frogs' Ringer. The impulse frequency was found to be directly proportional to the degree of intraluminal perfusion pressure up to a certain critical head of pressure (2–6 cm Ringer), beyond which the impulse frequency declined. At higher pressures the electrogram showed a diminished amplitude of the action potential and broadening and notching of sinus waves and auricular complexes. The duration and conduction velocity of electrical impulse was increased.

L'etirement mecanique est une force motrice fondamentale pour l'autoregulation intrinseque du rhythme du cœur en modifiant l'activite du «pacemaker» de la meme maniere que la distension du myocarde influe sur le reponse inotropique. L'etirement est un mecanisme biologique de base pour la rythmicite cardiaque et la contractilite et il met en interdependance positive le retour veineux et le debit sanguin. La sensibilite a l'etirement est une propriete des reponses chronotropiques. Elle fait du cœur une pompe autoregulatrice unique en son genre.

A study of the relationship between the actions of acetylcholine and nicotine on frog hearts indicates that both the excitatory and inhibitory effects of acetylcholine are antagonised by nicotine. Hearts stopped by acetylcholine and also spontaneously stopped hearts were restarted by nicotine. It has been suggested that exogenously administered acetylcholine and nicotine act through same receptors in a competitive manner.