Effects of stimulation frequency on myocardial extensibility.

Whether a change in myocardial eontractihty is accompanied by a change in diastolic extensibility was studied by altering stimulation frequency, including paired stimulation, as the inotropic intervention. Isometric and isotonic contractions of isolated cat papillary muscles were examined. Apparent changes in diastolic extensibility were observed with paired stimulation in isometric preparations. Rapid changes were seen only in muscles that had spontaneous aftercontractions between driven beats. These changes were believed not to reflect true extensibility changes but rather a change in timing between the relaxation of a beat driven by a paired stimulus and the onset of an aftercontraction. In continuous isotonic (not afterloaded) preparations, end-diastolic length depended on beat frequency. The muscle crept to a longer length when diastole was prolonged by decreasing the heart rate. In isotonic experiments without aftercontractions myocardial extensibility remained unchanged during paired electrical stimulation and changes in beat frequency. ADDITIONAL