Stress As an Index of Metabolic Cost in Papillary Muscle of the Cat

Active stress, stress-time integral (STI) and total heat production of cat right ventricular papillary muscles were recorded during brief trains of isometric twitch contractions at muscle lengths less than or equal to optimal length. Individual muscles were subjected to a 10 degree C change in temperature, a change of stimulus frequency and the addition of isoprenaline sulphate (10(-7) mol. 1(-1). The STI-heat and stress-heat data were subjected respectively to linear and quadratic regression analyses. For both relations, the intercept (stress-independent heat) was unaffected by the frequency change, doubled by the temperature decrease and trebled by the addition of isoprenaline. None of the treatments had a significant effect on the first or second order coefficients of the stress-heat relation. The slope of the STI-heat relation was halved by lowering the temperature, increased 50% by the addition of isoprenaline and unaffected by stimulus frequency. Thus the energetic cost of a given stress increment was constant across conditions while that for a given STI increment was not. Stress is the better mechanical index of myocardial energy cost when the inotropic state is changing.