Role of the sympathetic nervous system in mediating the renin response to head-up tilt. Their possible synergism in defending blood pressure against postural changes during sodium deprivation.

To investigate the role of the sympathetic nervous system in controlling the response of renin to upright posture and the physiologic significance of this activation in cardiovascular homeostasis, the effects of 30 minutes of 65 ° head-up tilt were assessed before and during acute beta blockade with propranolol in patients with essential hypertension studied under conditions of low, normal and high sodium intakes. Before beta blockade mean blood pressure was always unchanged during tilt, whereas the absolute increases in renin were inversely related to the state of sodium balance. During beta blockade, the renin responses to tilt were always abolished and mean blood pressure was unchanged in sodium-loaded patients, whereas blood pressure decreased progressively during the last 15 minutes of tilt on normal and low sodium intakes. The hypotension was much greater in sodium-deprived patients, all of whom fainted.These results indicate that the increase in renin during upright tilt is mediated by beta adrenergic receptors independently of the state of sodium balance, and they suggest that the postural renin response can be an effector pathway of the sympathetic nervous system for supporting blood pressure during prolonged tilt. This pathway has minimal significance when sodium is present in excess, but it becomes more important when sodium is restricted.