Orthostatic hypotension, involving an imbalance between the regulation of vascular volume, cardiac function and vascular responsiveness, has been reported to be more prevalent in endurance (aerobic) exercise trained individuals. This phenomenon appears related to an exercise training induced alteration in autonomic nervous system unction, which modifies the specific role of discrete baroreceptor populations (aortic, carotid and cardiopulmonary) in regulating heart rate and blood pressure during orthostatism. By using experimental protocols that 1) selectively inhibit and/or stimulate baroreceptors or 2) perturb carotid baroreceptors selectively at various levels of central blood volume, we plan to determine the specific roles played by each of the baroreceptors (aortic, carotid and cardiopulmonary) and how they interact to modify blood pressure control during atropine sulphate and metoprolol tartrate, we will isolate the cardias and vascular components of the carotid baroreflex. these procedures will be carried out using a cross-sectional design on two groups of subject designated as trained (maximal aerobic capacity greater than 65 m102/kg/min) and untrained (45 m102/kg/min). Furthermore, training effects will be determined in a longitudinal 2 max investigation by asking a trained and untrained group of subject to participate in a 16 week endurance exercise training program designed to increase aerobic capacity 20%. Responsiveness of each baroreceptor population will be determined before and after the training program and following a 12 week detraining program designed to decrease aerobic capacity 15%. We expect that the results from these investigations will allow us to identify changes in neural control of the cardiovascular system that result from endurance exercise training. Furthermore, the results will enable us to categorize the changes as being due primarily to changes in central blood volume, or to changes in the effector cardiac and vascular responses to baroreceptor stimulation. The information obtained concerning how baroreflexes operate and summate in humans during orthostatism will provide a base upon which to investigate baroreflex function in people with heart failure, hypertension, senescence and bed rest or space flight deconditioning.
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