Alterations of subunit composition and ATPase activity of myosin in early hypertrophied right ventricles of dogs with mild experimental pulmonic stenosis

Mild pulmonic stenosis was performed in dogs by banding of the pulmonary artery to evaluate the effect of systolic pressure overload on the enzymatic activity and subunit composition of myosin in early hypertrophied right ventricles. Three weeks following pulmonary constriction, 12 hypertrophied dogs were sacrificed and compared to 12 control animals. The weight of the hypertrophied right ventricles (HRV) relative to body weight was 46% greater than the weight of normal right ventricles (NRV) (P < .001) in dogs of similar body weights. Myosin ATPase activity (V_max values in μmol phosphate/mg min) was 25% higher in the stressed ventricles for both K⁺ and Ca²⁺ activated myosin (P < .001). Since the V_max values for the enzymatic activity of myosin with NH₄ ⁺ or Mn²⁺, as the activator cations, was the same in HRV and NRV, the augmented K⁺ and Ca²⁺ activity in HRV was not due to an increased concentration of more enzymatically active myosin. Associated with the increase in myosin activity there was a 33% decrease in the percent of light chains present in myosin from HRV as compared to myosin from NRV (P < .001). There was approximately 4 mol of myosin light chains/mol of myosin in NRV; in contrast, there was approximately 2 mol of myosin light chains/mole of myosin in HRV, similar to the proportion observed in NLV. The proportion of light chain C₁ to light chain C₂ did not change in myosin from HRV. Of the C₁ light chains analyzed on 2-dimensional gel electrophoresis, there was significantly less C_1d as compared to C_1c in HRV.