Changes in myocardial distensibility in rat papillary muscle: fibrosis, KCl contracture, hypoxic contracture, oxygen and glucose deficiency contracture, and experimental tetanus.

We describe diastolic properties of the myocardium in terms of stress-strain relations. In a mathematical analysis, the equation sigma = alpha(e beta.epsilon-1) of the stress-strain curve can be changed by an increase in the exponent beta or the multiplicative constant alpha. It can be experimentally shown that in hypertrophied myocardium of rats with essential hypertension and renal hypertension (SHR and Goldblatt rats, respectively), the steepening of the stress-strain curve is associated with an increase in the exponent beta (stiffness constant) and an enhancement of collagen content. On the other hand, in acute hypoxic myocardium, the slope of the stress-strain curve is increased without a significant change of the exponent beta. Our results from heat measurements and quick-release experiments indicate that the hypoxic contracture (H) and the oxygen and glucose deficiency contracture (HG) are in contrast to the results of depolarization contracture (KCl) and experimental tetanus (T). In H and HG, the cross-bridge cycling rate was found to be slowed by a factor of 2,000 compared to KCl and T. This means that ATP demand for force development and maintenance is 2,000 times less in H and HG than in KCl and T. We will further discuss the meaning and implications of these experimental findings.