Effect of Ouabain on the Energy Output of Rabbit Cardiac Muscle

Abstract

The effect of ouabain, 0.3 µg/ml, on the energy output of rabbit papillary muscles has been examined by a myothermic technique. The experiments were conducted at two temperature ranges, 19.2° to 22.8°C and 29.0° to 32.2°C, and both isometric and afterloaded isotonic contractions were studied. Temperature differences alone caused pronounced physiological changes, the higher temperature being associated with lower tension-independent heat and markedly higher active efficiency, external work /(external work + active heat production). The heat versus tension curve was rectilinear at higher temperatures but showed upward curvature at lower temperatures. At 19.2° to 22.8°C, ouabain increased the tension-independent heat by 23%, maximum tension development by 23%, and mean work output by 39%. Ouabain did not significantly alter the slope of the heat versus tension curve and increased mean efficiency only slightly. At 29° to 32.2°C, ouabain did not cause any significant change in the slope of the heat versus tension curve or in mean muscle efficiency. Ouabain produced significant increases in maximum tension development, mean work output, and the tension-independent heat. The effects of ouabain at the higher temperature were examined at two different calcium levels, 2.5 and 1.25 mM. In the isometric studies the effects of ouabain were independent of the calcium level, and the calcium level itself had no significant effect on the heat-tension relationship. In the isotonic studies, ouabain increased work output but more so at the 2.5 mM calcium level. Ouabain did not affect mechanical efficiency at either calcium level but muscle efficiency was higher at the 2.5 mM calcium level. It is concluded that any effects of the cardiac glycosides on energy expenditure are consequences of their inotropic actions and do not represent changes in the energy cost of contraction.