Isolated while heart calorimetry: energetics of length-dependent activation.

Myocardial energy expenditure measured by whole heart calorimetry has in the past been used to demonstrate that the thermal accompaniment of contractile activity is indicative of both force-related and force-independent activities. A mechanical technique for demonstrating the length dependence of activation in the right ventricular papillary muscle has been extended to the intact but isolated whole left ventricle of the heart. A functional relationship between the degree of length-dependent activation and force-independent energy expenditure has been observed. Potentiation of isovolumic cardiac mechanical activity by inotropic stimulation with extracellular calcium is thermally more economic than similar potentiation by length-dependent activation, particularly at large ventricular volumes (long muscle fiber lengths). A hypothesis is formulated that explains both the current myothermic observations and the aequorin calcium transients observed by Allen and Kurihara. Important analytic limitations of whole heart calorimetry are acknowledged.