Defining conditions that lead to the retention of water: The importance of the arterial sodium concentration

Abstract

Background. A water diuresis occurs when a large volume of water is ingested rapidly. Nevertheless, water conservation is required to provide a source of water for evaporative heat dissipation throughout the day. Therefore, the objective was to define conditions that permit the retention of ingested water. Methods. Volunteers collected urine q2h plus an overnight specimen; water loading was conducted after overnight food and water restriction; paired arterialized and venous blood samples were analyzed. Results. When 20 mL water/kg was consumed in < 15 minutes, the peak urine flow rate was 11 ± 0.6 mL/min. The volume of water retained after water intake stopped, and when the urine was hyperosmolar, correlated directly with the daily excretion of sodium plus potassium (r2 = 0.63). The plasma sodium concentration (PNa) was 4.0 ± 0.5 mmol/L lower in arterialized than paired venous blood 30 to 40 minutes after water ingestion began (P < 0.01). In preliminary studies, the smallest water load consumed in 15 minutes that would reproducibly cause a water diuresis was defined in each subject. This same acute water load was retained, however, if it contained 150-mmol/L fructose, but not glucose, or if it was consumed slowly (sipping). The arterialized PNa was not significantly lower than in paired venous samples when water was sipped. Conclusion. A large fall in arterialized and not venous PNa best reflected the signal to induce a water diuresis. Although a very large water load can induce a water diuresis, smaller water loads can be retained for future heat dissipation.