METABOLISM OF TOAD VENTRICLE DURING ALTERATIONS TO THE INOTROPIC STATE

The heat produced by toad ventricle during manipulations of the inotro pic state was measured using thermopiles, and some comparisons made to rat ventricle. The tension-independent heat, peak stress, and the ten sion-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/mm (2), and 0.49 +/- 0.06 units; about 25% of the tension-independent hea t may relate to the Na+-K+ pump. At similar [Ca2+](o), rat ventricle p roduced a smaller tension-independent heat (1.6 +/- 0.2 mJ/g), and act ive heat per unit stress (0.22 +/- 0.01 units) than toad. Tension-inde pendent heat, stress, and tension-dependent heat were increased by orc iprenaline, and decreased by BDM. Ouabain increased the stress and ten sion-dependent heat but not the tension-independent heat. Five millimo lar [Ca2+](o) in HEPES buffer decreased the stress but increased the t ension-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 re sults suggest that in toad ventricle (a) the sarcoplasmic reticulum pl ays only a minor role in activation and relaxation, (b) the Na+-K+ pum p contributes substantially to activation metabolism, (c) active metab olism is stimulated by increases in [Ca2+](o) and (d) there is a large r tension-independent heat, a larger active metabolism per unit stress , and a lower basal metabolism than in rat papillary muscle. The energ y cost of removing intracellular Ca2+ through the sarcolemma appears t o be greater than uptake into sarcoplasmic reticulum.