Mechanical catecholamine responsiveness and myosin isoenzyme pattern of pressure-overloaded rat ventricular myocardium

Pressure-overloaded cardiac hypertrophy was induced by abdominal aortic constriction in 10-week-old male Wistar rats. 24–26 weeks after aortic constriction, the hearts were excised and a myocardial mechanical study was performed using isolated left ventricular papillary muscles. There was no significant difference in isometric developed tension (T) between sham-operated control and aortic constriction (AC) rats (control vs AC rats=2.9±0.6 vs 2.7±0.7 g/mm2). dT/dtmax of AC rats, on the other hand, was significantly lower than that of controls (controls vs AC rats=32.8±7.5 vs 26.3±6.1 g/mm2sp<0.05). Myocardial mechanical responses to isoproterenol (10−7 mol/l) were depressed in the group with aortic constriction compared with the control group (ΔT:18.5±6.7 vs 12.1±4.9%,p<0.05, ΔdT/dt: 25.2±6.2 vs 17.5±5.8%,p<0.02). Responses of the parameters to dibutyryl cyclic AMP (10−5 mol/l) were also smaller in the AC group than in the control group (ΔT: 18.0±5.6 vs 13.3±4.0%,p<0.05, ΔdT/dt: 20.4±6.9 vs 14.7±4.1%,p<0.05). Left ventricular myosin isoenzyme pattern, revealed by pyrophosphate gel electrophoresis, shifted towards VM-3 under pressure overload. The present study demonstrates that post-membrane processes may be mainly responsible for the decreased myocardial mechanical catecholamine responsiveness in pressure-overloaded cardiac hypertrophy.