ALBUMINURIA AND OVERALL CAPILLARY-PERMEABILITY OF ALBUMIN IN ACUTE ALTITUDE HYPOXIA

The mechanism of proteinuria at high altitude is unclear. Renal functi on and urinary excretion rate of albumin (U-alb) at rest and during su bmaximal exercise and transcapillary escape rate of I-125-labeled albu min (TER(alb)) were investigated in 12 normal volunteers at sea level and after rapid and passive ascent to 4,350 m. The calcium antagonist isradipine (5 mg/day; n = 6) or placebo (n = 6) was administered to ab olish hypoxia-induced rises in blood pressure. Lithium clearance and u rinary excretion of beta(2)-microglobulin were used to evaluate renal tubular function. High altitude increased U-alb from 2.8 to >5.0 mu g/ min in both groups (P < 0.05). In the placebo group, high altitude sig nificantly increased filtration fraction (P < 0.05), but this response was abolished by isradipine. Lithium clearance and urinary excretion of beta(2)-microglobulin remained unchanged by hypoxia in both groups. Exercise did not reveal any further renal dysfunction. In both groups , high altitude increased TER(alb) from 4.8 to >6.7 %/h (P < 0.05). In conclusion, acute altitude hypoxia increases U-alb despite unchanged tubular function and independent of effects of isradipine on filtratio n fraction. The elevated TER(alb) suggests an overall increase in capi llary permeability, including the glomerular endothelium, as the criti cal factor in high-altitude induced albuminuria.