ACUTE EFFECTS OF HEAD-DOWN TILT AND HYPOXIA ON MODULATORS OF FLUID HOMEOSTASIS

In an effort to understand the interaction between acute postural flui d shifts and hypoxia on hormonal regulation of fluid homeostasis, the authors measured the responses to head-down tilt with and without acut e exposure to normobaric hypoxia. Plasma atrial natriuretic peptide (A NP), cyclic guanosine monophosphate (cGMP), cyclic adenosine monophosp hate (cAMP), plasma aldosterone (ALD), and plasma renin activity (PRA) were measured in six healthy male volunteers who were exposed to a he ad-down tilt protocol during normoxia and hypoxia. The tilt protocol c onsisted of a 17 degrees head-up phase (30 minutes), a 28 degrees head -down phase (1 hour), and a 17 degrees head-up recovery period (2 hour s, with the last hour normoxic in both experiments). Altitude equivale nt to 14,828 ft was simulated by having the subjects breathe an inspir ed gas mixture with 13.9% oxygen. The results indicate that the postur al fluid redistribution associated with a 60-minute head-down tilt ind uces the release of ANP and cGMP during both hypoxia and normoxia. Hyp oxia increased cGMP, cAMP, ALD, and PRA throughout the protocol and si gnificantly potentiated the increase in cGMP during head-down tilt. Hy poxia had no overall effect on the release of ANP, but appeared to att enuate the increase with head-down tilt. This study describes the acut e effects of hypoxia on the endocrine response during fluid redistribu tion and suggests that the magnitude, but not the direction, of these changes with posture is affected by hypoxia.