Research in animals has demonstrated that carotid baroreceptor regulation of vascular resistance, heart rate, and humoral mechanisms is influenced by interaction with factors such as cardiopulmonary baroreceptors, somatic receptors, and sodium intake. It is important to study these interactions in humans for several reasons: first, there are significant species differences in reflex control of circulation and second, knowledge of these interactions holds potential for improved understanding of mechanisms and treatment of human cardiovascular disorders such as hypertension, heart failure, and hypotension. In addition, refinements in techniques for study of reflex control afford the opportunity to examine the modulation of carotid baroreflex in humans. These techniques include the new Milan neck chamber for study of responses to inhibition (neck pressure) and stimulation (neck suction) of carotid baroreceptors. The goal of the proposed research is to characterize the modulation and interactions of the carotid baroreflex in humans. There are four major objectives: The first is to study the influence of cardiopulmonary baroreceptor reflexes on carotid baroreflex control of vascular resistance, heart rate, renin and ADH using the neck chamber to alter carotid baroreceptor activity and lower body suction to inhibit cardiopulmonary receptors. The second is to determine if carotid baroreceptors modulate the somatic pressor reflex during sustained handgrip and to see if carotid baroreflex modulation of somatic pressor reflex is impaired in hypertension. The third is to compare the influence of low and high sodium intake on effectiveness of the carotid baroreflex in normotensive and hypertensive humans. The fourth is to test the concept of selective pharmacologic sensory modulation in humans by determining if intravenous infusion of lidocaine can block reflexes originating in cardiac receptors without blocking carotid baroreceptor reflexes.