Metabolism of Toad Ventricle During Alterations to the Inotropic State

The heat produced by toad ventricle during manipulations of the inotropic state was measured using thermopiles, and some comparisons made to rat ventricle. The tension-independent heat, peak stress, and the tension-dependent heat increased when [Ca2+]o increased from 0.25 to 2 mM in Ringer. In 2 mM [Ca2+]o, tension-independent heat, peak stress, and tension-dependent heat were 3.1 ± 0.4 mJ/g, 38.4 ± 5.5 mN/mm2, and 0.49 ± 0.06 units; about 25% of the tension-independent heat may relate to the Na+-K+ pump. At similar [Ca2+]o, rat ventricle produced a smaller tension-independent heat (1.6 ± 0.2 mJ/g), and active heat per unit stress (0.22 ± 0.01 units) than toad. Tension-independent heat, stress, and tension-dependent heat were increased by orciprenaline, and decreased by BDM. Ouabain increased the stress and tensio-dependent heat but not tensio-independent heat. Five millimolar [Ca2+] in HEPES buffer decreased the stress but increased the tension-dependent heat compared to 2 mM [Ca2+]o in Ringer. Ryanodine and CPA caused major reductions in force and tension-independent heat in rat, but had little effect on toad ventricle. In conclusion, our results suggest that in toad ventricle (a) the sarcoplasmic reticulum plays only a minor role in activation and relaxation, (b) the Na+-K+ pump contributes substantially to activation metabolism, (c) active metabolism is stimulated by increases in [Ca2+]o and (d) there is a larger tension-independent heat, a larger active metabolism per unit stress, and a lower basal metabolism than in rat papillary muscle. The energy cost of removing intracellular Ca2+ through the sarcolemma appears to be greater than uptake into sarcoplasmic reticulum.