β-Myosin heavy chain myocytes are more resistant to changes in power output induced by ischemic conditions

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Abstract
During ischemia intracellular concentrations of Piand H+increase. Also, changes in myosin heavy chain (MHC) isoform toward β-MHC have been reported after ischemia and infarction associated with coronary artery disease. The purpose of this study was to investigate the effects of myoplasmic changes of Piand H+on the loaded shortening velocity and power output of cardiac myocytes expressing either α- or β-MHC. Skinned cardiac myocyte preparations were obtained from adult male Sprague-Dawley rats (control or treated with 5- n-propyl-2-thiouracil to induce β-MHC) and mounted between a force transducer and servomotor system. Myocyte preparations were subjected to a series of isotonic force clamps to determine shortening velocity and power output during Ca2+activations in each of the following solutions: 1) pCa 4.5 and pH 7.0; 2) pCa 4.5, pH 7.0, and 5 mM Pi; 3) pCa 4.5 and pH 6.6; and 4) pCa 4.5, pH 6.6, and 5 mM Pi. Added Piand lowered pH each caused isometric force to decline to the same extent in α-MHC and β-MHC myocytes; however, β-MHC myocytes were more resistant to changes in absolute power output. For example, peak absolute power output fell 53% in α-MHC myocytes, whereas power fell only 38% in β-MHC myocytes in response to elevated Piand lowered pH (i.e., solution 4). The reduced effect on power output was the result of a greater increase in loaded shortening velocity induced by Piin β-MHC myocytes and an increase in loaded shortening velocity at pH 6.6 that occurred only in β-MHC myocytes. We conclude that the functional response to elevated Piand lowered pH during ischemia is MHC isoform-dependent with β-MHC myocytes being more resistant to declines in power output.