Decreased contractile efficiency and increased nonmechanical energy cost in hyperthyroid rabbit heart. Relation between O2 consumption and systolic pressure-volume area or force-time integral.

Abstract

Both systolic pressure-volume area (PVA) and force-time integral (FM1) have been used as measures of oxygen consumption per beat (Vo2) in the isolated left ventricle. The reciprocal of the slope of the Vo2-PVA relation has been considered to reflect the chemomechanical energy transduction efficiency of the contractile machinery (contractile efficiency), whereas its Vo2 intercept consists of energy cost of excitation-contraction coupling and basal metabolism. To examine whether the increase in myosin isoform V1/V3 ratio in hyperthyroid rabbits decreases contractile efficiency and to determine overall mechanisms of higher oxygen consumption in hyperthyroid hearts, the Vo2-PVA and Vo2-FTI relations as well as the end-systolic pressurevolume relation were assessed in cross-circulated, isovolumically beating hearts isolated from normal, hyperthyroid, and hypothyroid rabbits. Normalized initial slopes of the rising limb of the curvilinearly fitted end-systolic pressure-volume relation (E'm., ventricular contractility index) were similar for normal and hyperthyroid groups. However, the slopes and Vo2 intercepts of the Vo2-PVA and Vo2-Fll relations were greater in hyperthyroid hearts than in normal hearts. Accordingly, in the hyperthyroid hearts, the contractile efficiency (27±6%) was lower and left ventricular Vo2 for excitation-contraction coupling (0.028±0.004 ml 02Jbeat/100 g) was higher than in normal hearts (40±4% and 0.021±0.005 ml 02/beat/100 g, respectively).This decreased contractile efficiency in the hyperthyroid hearts was attributable to myosin isoform alteration rather than to increased (-adrenoceptors because isoproterenol did not affect the slope of the Vo2-PVA relation in all groups. In contrast, the slope of the Vo2-FTI relation was significantly increased by isoproterenol in all groups. Neither the Vo2-PVA nor the Vo2-FH relations in hypothyroid hearts were different from those in normal hearts except for significantly lower Vo2 for basal metabolism. We conclude that in hyperthyroid rabbits, the left ventricle has decreased contractile efficiency and increased energy cost of excitation-contraction coupling and that the decreased contractile efficiency in hyperthyroid hearts is probably due to the increased V1NV3 ratio of the myosin isoform component. In addition, this study demonstrates that the

Endothelin receptor blockade has an oxygen-saving effect in Dahl salt-sensitive rats with heart failure

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Importance of mitral subvalvular apparatus in terms of cardiac energetics and systolic mechanics in the ejecting canine heart.

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Mechanical work and energetic analysis of eccentric cardiac remodeling in a volume overload heart failure in rats

“…The slope of the V O 2 -PVA relation remained unchanged in hypothyroid (11) and diabetic (1) rat hearts, whereas a steeper slope of the V O 2 -PVA relation was reported in hyperthyroid rabbit hearts. It was reported that the slope of the V O 2 -PVA relation was related to the ratio of the myosin isoform component from predominantly V3 to predominantly V1 (i.e., the slope of the V O 2 -PVA relation increases when the V1-to-V3 ratio increases, and the slope of the V O 2 -PVA relation is unchanged when the V1-to-V3 ratio decreases) (6). It was also reported that a progressive decrease in V1 and an increase in V3 were apparent in the LV 8-to 16 wk after AV shunt (21).…”