Abstract 1574: Low Myocardial Protein Kinase G Activity Raises Cardiomyocyte Resting Tension in Diastolic Heart Failure
Cardiomyocytes (CM) of diastolic heart failure (DHF) patients (pts) are hypertrophied and have high diastolic resting tension (RT), which elevates left ventricular (LV) diastolic stiffness. In rodent LV pressure overload models, development of pathological CM hypertrophy has been related to low myocardial protein kinase G (PKG) activity. Myocardial PKG activity and in-vitro effect of PKG on RT of isolated CM were therefore determined using LV endomyocardial biopsies procured from DHF pts. All DHF pts (n=36) had been admitted to hospital for worsening heart failure (NYHA 3– 4), were free of coronary artery disease, had a LV ejection fraction > 50%, a LV end-diastolic volume index < 97 ml/m2, a LV end-diastolic pressure > 16 mmHg and no histological evidence in the biopsies of myocardial infiltration or inflammation. Myocardial PKG activity was assessed by immunohistochemistry using antibodies against the specific PKG substrate vasodilator stimulated phosphoprotein (VASP) and against phosphorylated VASP ((P)-VASP). Myocardial PKG activity corresponded to the ratio of (P)-VASP/VASP. CM were isolated from the biopsies, treated with Triton X-100 to remove all membranes, attached to a force transducer and stretched to 2.2 μm to measure RT. In DHF pts, (P)-VASP/VASP ratio (0.67±0.15) was lower than in a control group (n=22) (0.98±0.19; p<0.0001). CM isolated from LV myocardium of DHF pts had higher RT (7.2±0.5 kN/m2 ) than CM of a control group (4.8±0.2 kN/m2; p<0.0001). In-vitro administration of PKG lowered RT of CM of DHF pts to levels similar to the control group (PKG: 4.0±0.7 kN/m2; p<0.01). Subsequent treatment with PKA did not further decrease RT (3.8±0.9 kN/m2). DHF patients had low myocardial PKG activity. This low myocardial PKG activity could have elevated RT of CM as in-vitro administration of PKG corrected the high RT. Absence of an additional in-vitro effect of PKA after PKG suggested that high RT results from hypophosphorylation of myofilamentary proteins, whose phosphorylation sites are targets for both PKA and PKG. In pts with DHF, phosphodiesterase 5A inhibition could raise myocardial PKG activity and thereby correct their high CM RT, which is an important determinant of their high diastolic LV stiffness.