Effects of nitroglycerin and nitroprusside on the uterine vasculature of gravid ewes.

The effects of nitroglycerin (TNG) and sodium nitroprusside (SNP) on mean aortic pressure (MAP), uterine blood flow (UBF), uterine vascular conductance (UVC), and pulse rate (PR) were compared when the two agents were infused to prevent and treat hypertension induced by norepinephrine (NE) in gravid ewes. When infused alone, TNG, 19 microgram/kg/min, decreased MAP 19 per cent and increased PR 33 per cent from control values (P less than 0.05), but did not significantly change UBF or UVC. In comparison, SNP, 3 microgram/kg/min, decreased MAP 20 per cent and increased PR 43 per cent (P less than 0.05), and did not significantly change UBF or UVC. When given alone, four successive 2-min infusions of NE produced dose-related increase in MAP and decreases in UBF, UVC, and PR; values were significantly different from control with the two higher doses of NE. Although MAP, UBF, and UVC were still significantly changed from control levels when NE was given in the presence of the above infusions of TNG or SNP, MAP was lower and UBF and UVC were higher compared with when NE was given alone (P less than 0.05). When given to control hypertension induced by a continuous infusion of NE, TNG or SNP produced uterine vasodilatation and significantly increased UBF. Nitroglycerin and SNP were equally effective in counteracting the maternal hypertension and antagonizing the uterine vascular effect of NE. It is concluded that TNG and SNP counteract uterine vasoconstriction resulting from alpha-adrenergic stimulation and do not produce a shunt of blood flow away from the uterine vasculature when used to control hypertension in gravid ewes.