Hormonal and electrolyte responses of conscious sheep to 96 h of hypoxia

Hypoxia alters the relationship of aldosterone secretion to plasma renin activity. The potential role plasma electrolytes play in this modification is not clear. This study analyzed the interrelationships among renin, aldosterone, vasopressin (ADH), and plasma electrolytes during 96 h of normobaric hypoxia. Eight ewes were exposed, in discrete experiments, to hypocapnic hypoxia [arterial O₂ tension (Pa(O₂) 37-42 mmHg, arterial CO₂ tension (PA(CO₂) 26-28 mmHg] and eucapnic hypoxia (Pa(O₂) 40-43 mmHg, Pa(CO₂) 28-31 mmHg) by N₂ dilution in an environmental chamber. Urine output (24 h) was measured, and arterial plasma samples were collected during the normoxic control period and at 24-h intervals of hypoxia. Plasma Na⁺, K⁺, renin, and ADH levels did not change from the normoxic values during either hypocapnic or eucapnic hypoxia. However, urinary aldosterone excretion [critical significance (ᾱ) < 0.046], and K⁺ excretion (ᾱ < 0.046) decreased markedly during each type of hypoxia. All sheep developed a pronounced negative K⁺ balance by 96 h of hypoxia. These data suggest that plasma K⁺ concentration is preserved by movement of K⁺ out of the intracellular compartment; this change in K⁺ distribution may inhibit aldosterone secretion during hypoxia.