Pressure-induced hypertrophy of cat right ventricle. An evaluation with the force-interval relationship.

We evaluated the force-interval relationship for papillary muscle isolated from two groups of cats, one sham-operated (control group), the other having undergone pulmonary artery constriction (hypertrophy group) 18.6 2.9 weeks prior to sacrifice. The right ventricular free wall muscle mass and the peak systolic right ventricular pressure were significantly greater in the hypertrophy than in the control group. The peak force and maximum rate of rise of force (F max ) per cross-sectional area were not significantly different for the two groups. The qualitative features of the force-interval relationship, in particular postextrasystolic potentiation, were the same for both groups. There were quantitative differences between the groups, however. The amount of potentiation expressed, the ratio of F max of the potentiated to that of the previous regular contraction (i.e., the force-interval ratio), was significantly greater in the hypertrophied than in the control group. In both groups, the force-interval ratio was independent of muscle length, yet was altered by changing the inotropic state of the muscle (e.g., by alterations in calcium concentration). Increasing and decreasing the calcium concentration decreased and increased, respectively, the force-interval ratios in both groups. The application of these results to theories about the mechanisms underlying the alterations in mechanical performance induced by hypertrophy is discussed.