Contraction and resting stiffness of isolated cardiac muscle: effects of inotropic agents.

The purpose of this study was to test the hypothesis that either hypoxia and its combined effects with extracellular calcium (Ca), digoxin, and ouabain, or these positive inotropic agents acting alone or in combination, influence contraction and resting stiffness of isolated papillary muscle. Stiffness was measured utilizing the sinusoidal forcing function technique. Neither an increase in extracellular calcium concentration (from 2.5 to 4.0 mM) nor digoxin or ouabain in either Ca concentration altered contraction or resting stiffness in the well-oxygenated environment. Resting stiffness for any given resting tension was increased at the end of hypoxia only in the presence of digoxin, and this occurred in both 2.5 mM Ca (P less than 0.02) and in 4.0 mM Ca (P = 0.05). Contraction stiffness for any given tension was increased in 2.5 mM Ca by hypoxia alone (P less than 0.05) and by hypoxia in the presence of digoxin (P less than 0.005) and ouabain (P less than 0.02), but was not increased in any experiments conducted in 4.0 mM Ca. The conclusions from these data are that certain experimental conditions of the study evoked different directional changes in stiffness and contractility. Further, changes in contraction stiffness are not always paralleled by changes in resting stiffness.