EFFECTS OF SYMPATHETIC INHIBITION ON BLOOD-PRESSURE AND RENAL RESPONSES TO CENTRAL HYPERVOLEMIA IN NORMAL HUMANS

The purpose of this study was to evaluate the effects of short-term sy mpathetic inhibition with clonidine on blood pressure and renal respon ses to central hypervolaemia induced by thermoneutral head-out water i mmersion. Eleven healthy subjects were randomly studied on two occasio ns. during a 1 h pre-immersion period. 2 h of water immersion and a 1 h post-immersion period. after either placebo or clonidine treatment. Clonidine caused a significant suppression of plasma adrenaline. plasm a noradrenaline. urinary noradrenaline excretion and mean arterial blo od pressure. Blood pressure remained constant during water immersion a fter both placebo and clonidine. compared with the respective pre-imme rsion control values. The suppression pattern of plasma catecholamines and urinary noradrenaline in response to water immersion during place bo was similar after clonidine treatment. Renal volume excretion was n ot affected by clonidine. In contrast. clonidine caused a significant attenuation of the immersion-induced stimulation of natriuresis (maxim um -33+/-12%. P < 0.01. compared with placebo). These data indicate th at the renal capacity to excrete sodium is impaired during moderate bl ood pressure reduction by short-term sympathetic inhibition with cloni dine, whereas the regulation of arterial blood pressure in response to central hypervolaemia is maintained.