Low LBNP tolerance in men is associated with attenuated activation of the renin-angiotensin system

Plasma vasoactive hormone concentrations [epinephrine (pEpi), norepinephrine (pNE), ANG II (pANG II), vasopressin (pVP), endothelin-1 (pET-1)] and plasma renin activity (pRA) were measured periodically and compared during lower body negative pressure (LBNP) to test the hypothesis that responsiveness of the renin-angiotensin system, the latter being one of the most powerful vasoconstrictors in the body, is of major importance for LBNP tolerance. Healthy men on a controlled diet (2,822 cal/day, 2 mmol · kg−1· day−1 Na+) were exposed to 30 min of LBNP from −15 to −50 mmHg. LBNP was uneventful for seven men [25 ± 2 yr, high-tolerance (HiTol) group], but eight men (26 ± 3 yr) reached presyncope after 11 ± 1 min [ P < 0.001, low-tolerance (LoTol) group]. Mean arterial pressure (MAP) did not change measurably, but central venous pressure and left atrial diameter decreased similarly in both groups (5–6 mmHg, by ≈30%, P < 0.05). Control (0 mmHg LBNP) hormone concentrations were similar between groups, however, pRAdiffered between them (LoTol 0.6 ± 0.1, HiTol 1.2 ± 0.1 ng ANG I · ml−1 · h−1, P < 0.05). LBNP increased ( P < 0.05) pRA and pANG II, respectively, more in the HiTol group (9.9 ± 2.2 ng ANG I · ml−1· h−1 and 58 ± 12 pg/ml) than in LoTol subjects (4.3 ± 0.9 ng ANG I · ml−1 · h−1 and 28 ± 6 pg/ml). In contrast, the increase in pVP was higher ( P < 0.05) in the LoTol than in the HiTol group. The increases ( P < 0.05) for pNE were nonsignificant between groups, and pET-1 remained unchanged. Thus there may be a causal relationship between attenuated activation of pRA and pANG II and presyncope, with pVP being a possible cofactor. Measurement of resting pRA may be of predictive value for those with lower hypotensive tolerance.