Sympathetic Nerve Activity and the Skeletal Myopathy of Heart Failure

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DESCRIPTION (provided by applicant): The concept that exercise limitation in patients with chronic heart failure (HF) is directly due to elevated filling pressures or inadequate cardiac output during exercise has been revised. It is now recognized that the skeletal myopathy of HF is an important contributor exercise limitation in HF. The skeletal myopathy of HF is a predictable sequela to chronic cardiac dysfunction. Oxygen transport is a multi-step pathway, and according to the concept of "coordinated adaptation," the development of an abnormality in one step of a multi-step pathway initiates a readjustment of all other steps in the pathway to achieve a new equilibrium, so that all capacities are matched However, the mechanism(s) of "matching capacities," in this case the down- regulation of skeletal muscle capacity and the development of a skeletal myopathy in the setting of severe cardiac disease, remain unknown. We hypothesize that muscle sympathetic activation mediates the skeletal myopathy of HF. This may in part explain why cardiac resynchronization therapy (CRT), a therapy that improves abnormalities of cardiac function, which are not directly related to exercise dysfunction, nonetheless produces an irrefutable increase in exercise capacity. We have preliminary evidence that CRT significantly decreases muscle sympathetic nerve activity (MSNA). The overall goal of this project is to investigate the role of MSNA as the instigator of the skeletal myopathy of chronic HF. Aim 1. To determine the mechanisms of chronic sympathoinhibiton with CRT in HF, and the effect of this sympathoinhibition on the skeletal myopathy. 1.1 Does CRT reduce resting MSNA chronically in advanced HF? 1.2 Is the skeletal myopathy reversed in HF patients in whom MSNA is decreased following CRT? 1.3 Does an increase in baroreceptor gain underlie this decrease in MSNA following CRT? 1.4 Does reversal of the augmented muscle mechanoreceptor sensitivity in HF contribute to the decrease in MSNA following CRT? Aim 2. To determine if another sympatholytic therapy, clonidine, reverses the skeletal myopathy of HF. 2.1 Is the sympatholysis with clonidine associated with a reversal of the skeletal myopathy in HF? 2.2 What is the effect of the improved skeletal myopathy on muscle mechanoreceptor sensitivity in HF?