A simplified index of the plasma sodium threshold for arginine vasopressin secretion—Morning fasting, euhydrated sodium levels

Abstract

Objective: Human pregnancy results in a reduction in plasma osmolality and thus a reduction in the osmotic threshold for arginine vasopressin secretion. Although the functional characteristics of the osmoregulatory system controlling arginine vasopressin secretion have been carefully defined, determination of the osmotic threshold requires a complex, labor-intensive protocol of an intravenous hypertonic saline infusion. To aid in studies of osmotic threshold resetting in pregnancy, we sought to develop a simplified method for determination of this value. Study Design: Ten healthy nonpregnant women between the ages of 18 and 40 years were studied over 2 days. All patients were hospitalized, and morning euhydration was ensured by oral water hydration (5-10 mL/kg) the evening before the study. On the first study day, patients were fed a standard no-salt-added diet; plasma osmolality and sodium values were checked just before and 1 and 2 hours after meals. On the second study day, after fasting blood samples were obtained, patients received an intravenous infusion (0.06 mL · kg –1 · min –1 for 120 minutes) of hypertonic (5%) saline to gradually increase the plasma sodium level. Blood samples were obtained every 15 minutes for measurement of plasma electrolytes and arginine vasopressin. Plasma arginine vasopressin concentrations were regressed against plasma osmolality and sodium concentration to calculate the osmotic threshold for arginine vasopressin secretion. Results: Hypertonic saline injection significantly increased plasma sodium (from 139 ± 1 to 149 ± 1 mEq/L) and osmolality (from 284 ± 2 to 304 ± 2 mOsm/kg H 2O). Plasma arginine vasopressin significantly increased (from 5 ± 1 to 30 ± 10 pg/mL). The mean sodium and osmolality thresholds for arginine vasopressin secretion were calculated as 137 ± 2 mEq/L and 285 ± 15 mOsm/kg H 2O. The mean morning fasting sodium level was nearly identical to the calculated sodium threshold, whereas the morning fasting osmolality value was significantly different. Conclusions: The morning fasting, euhydrated sodium level can be used as a simplified index for the plasma osmotic threshold for arginine vasopressin secretion. This index may provide a useful predictive measure for pregnant women in whom the plasma volume does not expand. (Am J Obstet Gynecol 2000;183:933-6.)