An α-cardiac myosin heavy chain gene mutation impairs contraction and relaxation function of cardiac myocytes

Left Ventricular (LV) myocytes were isolated from 15-wk-old male mice bearing the Arg(403) --> Gln alpha-cardiac myosin heavy chain missense mutation (alpha-MHC(403/+)), a model of familial hypertrophic cardiomyopathy. LV myocytes were classified morphologically: type I, rod shaped with parallel myofibrils; type II, irregularly shaped, shorter and wider than wild-type (WT) control cells, with parallel myofibrils; and type III, irregularly shaped with disoriented myofibrils. Compared with WT myocytes, alpha-MHC(403/+) myocytes had fewer type I cells (WT = 74 +/- 3%, alpha-MHC(403/+) = 41 +/- 4%, P < 0.01) and more type III cells (WT = 12 +/- 3%, alpha-MHC(403/+) = 49 +/- 7%, P < 0.01). In situ histology also demonstrated marked myofibrillar disarray in the alpha-MHC(403/+) hearts. With the use of video edge detection, myocytes were paced at 1 Hz (37 degrees) to determine the effects of the mutation on myocyte function. End-diastolic length was reduced in mutant myocytes, but fractional shortening (% contraction) and sarcomere length were not. Velocity of contraction (-dL/dt(max)) was depressed in mutant cells, but more in type II and III cells (-31%) than in type I cells (-18%). Velocity of relaxation (+dL/dt) was also depressed more in type II and III cells (-38%) than in type I cells (-16%). Using fura 2 dye with intracellular Ca(2+) transients, we demonstrated that in alpha-MHC(403/+) myocytes, the amplitude of the Ca(2+) signal during contraction was unchanged but that the time required for decay of the signal to decrease 70% from its maximum was delayed significantly (WT = 159 +/- 8 ms; alpha-MHC(403/+) = 217 +/- 14 ms, P < 0.01). Sarco(endo)plasmic reticulum Ca(2+)-ATPase mRNA levels in alpha-MHC(403/+) and WT mice were similar. These data indicate that the altered cardiac dysfunction of alpha-MHC(403/+) myocytes is directly due to defective myocyte function rather than to secondary changes in global cardiac function and/or loading conditions.