Aerobic fitness: I. Response of volume regulating hormones to head-down tilt

We investigated the relationship of aerobic fitness to the response of volume-regulating hormones to acute simulated microgravity. Six untrained (UT) and six endurance-trained (ET) healthy young males were studied in the head-down tilt (HDT) position of -6° for 4 h. Peak oxygen uptake (VO_2pcak) and plasma volume (PV) were significantly greater in the ET (VO_2pcak = 61.7 ± 1.6 ml•min⁻¹•kg⁻¹ and PV = 53.1 ± 2.8 ml•kg⁻¹) than in the UT (VO_2pcak = 38.4 ± 1.7 ml•min⁻¹•kg⁻¹ and PV = 38 8 ± 1.0 ml•kg⁻¹). Plasma concentrations of atrial natriuretic peptide (ANP), arginine vasopressin (AVP), norepinephrine (NE), renin activity (PRA), and aldosterone (PA) were measured prior to HDT and at minutes 2, 5, 15, 30, 60, 120, 180, and 240 during HDT. PRA and PA significantly decreased during the time of HDT in both groups. The changes in ANP and NE concentrations were not significantly different between the groups nor across time. However, in the ET subjects, the changes in PRA and NE were significantly correlated with the changes in ANP (r = 0.49, P < 0.01; and r = 0.86, P < 0.001, respectively); in the UT subjects, the changes in AVP, PRA, and PA were significantly associated with changes in NE (r = 0.34, P < 0.03; and r = 0.59; and r = 0.53, P< 0.01, respectively). PV significantly decreased during HDT, and was primarily related to the decrease in PA in both groups. For a unit change in PA, the decrease in PV was more in the ET than in the UT (P < 0.017). At the end of 4-h HDT, the decrease in PV was significantly greater in the ET than in the UT subjects (P < 0.027). Our data suggest that volume regulation during acute exposure to simulated microgravity may be more hormonally mediated (ANP-linked) in the ET and more neurally mediated (NE-linked) in the UT.