DIFFERENTIAL-EFFECTS OF LOW-INTENSITY AND HIGH-INTENSITY LOWER-BODY NEGATIVE-PRESSURE ON NORADRENALINE AND ADRENALINE KINETICS IN HUMANS

1. Lower body negative pressure provides a means to examine neurocircu latory reflexive responses to decreases in venous return to the heart, We assessed whether the pattern of catecholaminergic responses to low er body negative pressure depends on the intensity of the stimulus (-1 5 versus -40 mmHg), 2. In 14 healthy subjects, responses of forearm bl ood flow and noradrenaline spillover and of total body noradrenaline a nd adrenaline spillover were assessed during infusion of [H-3]noradren aline and [H-3]adrenaline during -15 and -40 mmHg of lower body negati ve pressure, 3. During lower body negative pressure at -15 mmHg, heart rate and pulse pressure did not change, but forearm vascular resistan ce increased by 25-50%. Forearm noradrenaline spillover increased by a bout 50%, from 0.63 +/- 0.16 to 0.94 +/- 0.23 pmol min(-1) 100 ml(-1) (P < 0.05), Total body noradrenaline spillover did not change, and tot al body adrenaline spillover increased significantly by about 30%. Cle arances of noradrenaline and adrenaline were unchanged, 4. During lowe r body negative pressure at -40 mmHg, heart rate increased and pulse p ressure decreased, Forearm vascular resistance increased by about 100% , and forearm noradrenaline spillover increased by 80%, from 0.73 +/- 0.19 to 1.32 +/- 0.36 pmol min(-1) 100 ml(-1) (P < 0.05), Total body n oradrenaline spillover increased by 30%, and total body adrenaline spi llover increased by about 50%, Clearances of both noradrenaline and ad renaline decreased, 5. The results are consistent with the view that s elective deactivation of cardiopulmonary baroreceptors during low-inte nsity lower body negative pressure increases sympathoneural traffic to forearm skeletal muscle and increases adrenomedullary secretion witho ut a concomitant generalized increase in sympathoneural outflows, Conc urrent deactivation of cardiopulmonary and arterial baroreceptors duri ng high-intensity lower body negative pressure evokes a more generaliz ed increase in sympathoneural activity, accompanied by further increas ed adrenomedullary secretion and decreased plasma clearances of noradr enaline and adrenaline, The findings support differential increases in skeletal sympathoneural and adrenomedullary outflows during orthostas is, with more generalized sympathoneural responses to systemic hypoten sion.