Changes in cardiac heat production with agents that alter contractility.

Abstract

SummaryThe isometric response of isolated rabbit papillary muscles was altered either by changing the external calcium concentration or by exposing the muscles to adrenaline 1·0 μg./ml. Changes in cardiac heat production were measured using a myothermic technique. Increasing the calcium concentration from 0·6 mM to 5·0 mM did not alter the slope of the relationship between total active heat production and developed tension. The rate at which heat was liberated increased as the calcium concentration was raised. Adrenaline always increased resting heat production (16 to 37%) but active heat production, for a given level of tension development, only increased at the higher calcium concentrations. This effect was due to an increase in the magnitude of the tension ‐ independent heat. At all calcium concentrations adrenaline increased the rate at which heat was liberated. The fast phase of heat production associated with the contractile response was sometimes decreased by adrenaline, but total heat production (fast plus slow phases) measured in a steadily beating preparation never decreased. The decrease in fast phase heat production was associated with the rapid mechanical relaxation induced by adrenaline.