Effect of Alterations in the Stimulus Rate upon Energy Output, Tension Development and Tension‐Time Integral of Cardiac Muscle in Rabbits

The effect of stimulus rate on the isometric energy output of rabbit papillary muscle at room temperature was examined by a myothermic technique. The stimulus rates used were: 1 per 1.5, 2.0, 3.0, 7.0 and 10.0 seconds. Mechanical output at any given rate was varied by altering muscle length. Both peak tension and the time integral of tension were used as mechanical variables against which isometric heat production was plotted. The relationship between heat and peak tension was curvilinear, whereas that between heat and the time integral of tension was rectilinear. Statistically either mechanical index was equally good at predicting isometric heat production. Changes in the stimulus rate did not affect any of the parameters (intercept, slope, curvature) of the polynomials relating heat to tension but increasing the stimulus rate did significantly raise the slope of the relationships between heat and the time integral of tension. The magnitude of the tension-independent heat rose from .596 mcal/g muscle at a stimulus rate of 1 per 10 seconds to 0.744 mcal/g at a rate of 1 per 1.5 seconds.