Volume regulating hormone responses to repeated head-up tilt and lower body negative pressure

We hypothesized the existence of different hormonal response patterns to repeated lower body negative pressure (LBNP) and head-up tilt (HUT) in healthy males. We compared hormonal, cardiovascular and plasma volume changes from rest to stress within- and between-LBNP and HUT applications. Hormones investigated included adrenocorticotropic hormone (ACTH), aldosterone, plasma renin activity (PRA), atrial natriuretic peptide (ANP) and arginine vasopressin (AVP).

Three sequential 30-min bouts of LBNP at -55mmHg (n=14) or 70° HUT (n=9) were preceded by 30-min supine rest, and a 60-min supine rest followed the 3rd stimulus.

Plasma renin activity increases above baseline, in relation to aldosterone, were larger with LBNP than with HUT. The 3rd HUT application resulted in a greater increase in aldosterone compared to LBNP. Mean arterial blood pressure was elevated significantly during 1st and 3rd HUT application. ACTH responses were highly correlated with those of aldosterone in both LBNP and HUT (r(2) =0·96). AVP responses, in contrast to ANP, to the three consecutive stress situations were not significantly different, both with LBNP and HUT.

We speculate that the observed differences in blood pressure and hormonal responses to LBNP and HUT are caused by divergent effects of blood pooling in the splanchnic region, despite similar reductions in splanchnic perfusion. Apparently with repeated central hypovolaemia, especially by the 3rd application of stress, plasma aldosterone levels rise (along with ACTH), conceivably increasing its volume-guarding effect.