Renin, aldosterone, and converting enzyme during exercise and acute hypoxia in humans

The possibility that hypoxia might inhibit the secretion of angiotension-converting enzyme (ACE) would explain the low concentrations of aldosterone reported in humans at high altitude. To observe the effect of such a reduction in ACE concentration on the plasma aldosterone concentration (PAC) four subjects performed mild exercise throughout a 2-h study so as to elevate their plasma renin activity (PRA). After the first 60 min breathing air they were switched to breathing 12.8% O2 (4,000 an altitude equivalent). Venous samples were taken at intervals for hormone analysis. Results showed the expected rise of PRA and PAC both tending toward a plateau after about 45 min. There was no significant change in ACE activity (F = 0.065). Hypoxia produced a further 50% rise in PRA but a fall in PAC and a 30% reduction in ACE activity. Angiotensin I concentrations closely followed PRA throughout (r = 0.984). These results indicate that during exercise acute hypoxia changes the usual close relationship between PAC and PRA by reducing ACE activity.