Role of the exercise pressor reflex in rats with dilated cardiomyopathy

Background - In heart failure, there is a sympathetically mediated hyperkinetic cardiovascular response to exercise that limits tolerance to physical activity. Alterations in skeletal muscle morphology and metabolism have led to the hypothesis that the exercise pressor reflex (EPR) becomes hyperactive after the development of cardiomyopathy and contributes to the exaggerated circulatory response elicited. Methods and Results - To test this hypothesis, Sprague-Dawley rats were divided into the following groups: control, sham, and dilated cardiomyopathy (DCM, induced by ischemic injury). Using transthoracic echocardiography, left ventricular fractional shortening was 47±2%, 44±1 %, and 24±2% in control, sham, and DCM rats, respectively. Activation of the EPR by electrically induced static muscle contraction resulted in significantly larger increases in mean arterial pressure and heart rate in DCM animals (32±2 mm Hg, 13±1 bpm) compared with control (20±1 mm Hg, 8±1 bpm) and sham (20±2 mmHg, 8±1 bpm) rats. Comparable results were obtained with selective stimulation of the mechanically sensitive component of the EPR by passive muscle stretch. The augmentations in EPR and mechanoreflex activity in DCM occurred progressively over a 10-week period, becoming greater as the severity of left ventricular dysfunction increased. Conclusions - In DCM, the potentiated cardiovascular response to static muscle contraction is mediated, in part, by an exaggerated EPR. The muscle mechanoreflex contributes significantly to the EPR dysfunction that develops.