Changes in Cardiac Contractility Evoked Under Isotonic and Isometric Conditions

Abstract

SUMMARY.Isolated ventricles or ventricular strips from winter toads (Bufo marmtis) were used to study changes in contractile behaviour of cardiac muscle under isometric and isotonic conditions. Variations in the stimulation rate and in tlie Na+ and Ca++ concentrations of the perfusate were used to induce changes in contractility.The changes recorded isotonically differed from those recorded isometrically. The optimum stimulation frequency was different in the two preparations but the rate at which the various responses were produced depended to some extent upon the stimulation rate used. The results are discussed in relation to the hypothesis that stimulation and stretch influence calcium exchangeability.INTRODUCTION.