Plasma osmolality (Posmol) decreases in pregnancy, possibly because of hemodynamically mediated arginine vasopressin (AVP) secretion, i.e., inadequate vascular filling and/or decreased blood pressure. This hypothesis was tested in Sprague-Dawley rats treated on gestational days 1-18 or 13-18 with 1) deoxycorticosterone acetate (DOCA) + standard chow (0.5% Na), 2) vehicle + standard chow, or 3) high-Na (1.25%) diet. Renal sodium "escape" and suppression of the renin-aldosterone system suggested "effective volume expansion," yet Posmol was similar in all pregnant groups, 7-10 mosmol/kg below levels in virgin controls, and plasma AVP was unaltered. Apparent osmotic thresholds for AVP secretion, similar in control and DOCA-treated gravid animals, were 8-10 mosmol/kg below those of untreated virgin rats. Norepinephrine + DOCA, administered to gravid animals consuming normal or high-Na chow, increased blood pressure approximately 10% above control levels, but this also failed to alter Posmol. These data suggest that mechanisms other than hemodynamically mediated AVP secretion are responsible for the osmoregulatory alterations accompanying rodent pregnancy.
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