Isoprenaline, propranolol, and the energy output of rabbit cardiac muscle

A myothermic investigation has shown that isoprenaline increases the magnitude of the tension-independent heat but does not alter the slope of the heat versus tension relationship. Isoprenaline had no effect on cardiac efficiency at a 2·5 mM calcium level but restored the depressed efficiency values obtained in 0·6 mM calcium to normal levels. Propranolol had no significant effects on energy production in either isometric or isotonic contractions but effectively prevented the mechanical and energetic changes caused by isoprenaline.