The effects of acute and chronic inotropic interventions on tension independent heat of rabbit papillary muscle.

We have used the myothermal method to noninvasively monitor the amount of calcium cycled during a single isometric twitch of rabbit papillary muscle. Experiments were designed to test the working hypothesis that changes in peak twitch tension caused by pharmacological agents or changing haemodynamic conditions are accompanied by parallel changes in the tension independent heat (TIH) signal associated with Ca2+ cycling. We isolated the TIH signal by eliminating the tension dependent component of initial heat with a hyperosmotic Krebs solution containing 2,3-butanedione monoxime. Contrary to the working hypothesis, positive or negative inotropic effects on twitch tension caused by pressure overload hypertrophy, thyrotoxic hypertrophy, isoproterenol, and UDCG115 were not accompanied by parallel changes in TIH. Alternative explanations for the relation between peak twitch tension and TIH are explored.