Effect of volume expansion on pressor response to angiotensin II in pregnant ewes

Vascular refractoriness to infused angiotensin II (AII) characterizes normal human and ovine pregnancy. To ascertain whether this refractoriness in the gravid ewe is mediated by either endogenous plasma concentrations of renin and AII or vasomotor reflexes, effects of acute volume expansion (VE) on the pressor response to AII were studied in chronically instrumented nonpregnant and near-term pregnant sheep. Dose-response curves describing the pressor response (ΔBP) were determined before and after infusions in 1.0 1 of the isotonic saline (NS) or 0.5 1 of 10% dextran (D). In nonpregnant sheep, hematocrit (Hct) and plasma renin activity (PRA) fell in all animals after NS (n = 7) and D (n = 6) (P<0.005). After VE with NS and D, ΔBP increased at each dose of AII (P<0.05). The pressor response to AII in pregnant sheep was not altered by NS although decreases in Hct and PRA were comparable to those in nonpregnant sheep. Baroreceptor responses were not altered. Vascular refractoriness to infused AII in pregnant sheep is not due primarily to changes in plasma concentrations of renin-AII but likely to another factor, vassel wall refractoriness. In this respect, the ewe is similar to the human.