Water immersion increases urinary excretion of aquaporin-2 in healthy human

Many previous studies have shown that aquaporin-2 (AQP2), the vasopressin-r egulated water channel, is excreted in the urine and that the excretion inc reases in response to vasopressin. Moreover, recently a close correlation b etween AQP2 excretion in urine and kidney AQP2 expression has been demonstr ated, showing that urinary excretion of AQP2 is a reliable indicator for AQ P2 function. As head-out water immersion causes an expansion in the central vascular volume equal to that induced by 2 liters of saline, without modif ying plasma composition, we used immersion in water to evaluate if the resp onse to acute expansion of the central vascular volume could involve vaspor essin (AVP) and AQP2. In healthy subjects, concentrations of plasma atrial natriuretic factor (ANF) and AVP, and urinary AQP2 were measured during a 2 -hour immersion period. In all subjects, immersion caused a prompt and mark ed increase in immunoreactive ANF (23.0 +/- 2.12 pg/ml at second hour vs. 2 .17 +/- 0.42 pg/ml at baseline) and in urinary excretion of AQP2 (23.9 +/- 2.69 pmol/mg creatinine at second hour vs. 4.42 +/- 0.14 pmol/mg creatinine at baseline), while a significant decrease was found in plasma AVP. Recove ry was associated with a prompt return to pre-study levels. These findings demonstrate that heat-out water immersion stimulates urinary excretion of A QP2 in absence of an increase in plasma AVP. Copyright (C) 2000 S. Karger A G, Basel.