Effect of Mild Pressure Overload on Mechanoenergetics of the Rabbit Heart

Summary

We determined the effect of a mild pressure overload (perinephritic hypertension) on cardiac mechanoenergetics in New Zealand white rabbits (n = 7). Results were compared to sham operated controls (n = 8) and rabbits rendered hypothyroid (n = 5). Using an isovolumically contracting, red blood cell perfused, isolated heart preparation, mechanoenergetics were assessed as the inverse slopes of the ${{\text{V}}_{{0_2}}}$-pressure-volume area relation (contractile efficiency) and the ${{\text{V}}_{{0_2}}}$-stress time integral relation (contractile economy), where ${{\text{V}}_{{0_2}}}$ is myocardial oxygen consumption. The energy cost of excitation-contraction coupling was assessed as unloaded ${{\text{V}}_{{0_2}}}$ minus basal ${{\text{V}}_{{0_2}}}$ per beat. Morphologic measurements demonstrated no significant increase in left ventricular or heart-weight-to-body-weight ratio in pressure overload hearts. In controls, percent V₃ isomyosin was 80% ± 7% (±SD), while in hypothyroid hearts V₃ averaged 100% ± 1% and in pressure overload 98% ± 1% (both P < 0.01 vs controls). Both efficiency and economy demonstrated trends toward higher values in pressure overload compared to either hypothyroid or control hearts. Considered as a function of percent V₃ isomyosin and compared to control and hormonally modified (hyper- and hypothyroid) hearts, values for efficiency and economy were clearly elevated. In contrast, ${{\text{V}}_{{0_2}}}$ used for excitation-contraction coupling was not significantly different among the three groups. Thus, a very mild pressure overload results in increased efficiency and economy. It is proposed that the most likely explanation for this finding is an alteration at the level of the contractile proteins, independent of changes in isomyosin composition.