Superficial Hand Vein Responses to N^G-Monomethyl-l-Arginine in Post-Partum and Non-Pregnant Women

1. During human pregnancy marked vasodilatation occurs in arterial and venous vascular beds. The mechanisms responsible for this change remain unclear. The contribution of increased nitric oxide activity to vasodilatation associated with pregnancy was determined by examining superficial hand vein responses to N^G-monomethyl-l-arginine, an inhibitor of nitric oxide synthase, in post-partum women 24–48 h after delivery when vasodilatation remains at levels present during the third trimester.

2. Seventeen healthy women, 24–48 h post partum, and 13 healthy non-pregnant women were studied. Eight of the post-partum women underwent repeat studies 12–16 weeks after delivery.

3. N^G-monomethyl-l-arginine (100 nmol/min) resulted in venoconstriction in non-constricted veins (baseline, 0%; 5 min, 26 ± 9%; 10 min, 14 ± 8%; 15 min, 8 ± 7%; means ± SEM) and noradrenaline-constricted veins (5 min, 30 ± 7%; 10 min, 24 ± 10%; 15 min, 14 ± 11%). No constrictor response to N^G-monomethyl-l-arginine was present in the same women 12 weeks post partum (5 min, 1 ± 4%; 10 min, 0 ± 3%; 15 min, 1 ± 4%) or in the non-pregnant control subjects in non-constricted (5 min, 2 ± 3%; 10 min, 4 ± 3%; 15 min, 2 ± 2%) or noradrenaline-constricted veins (Smin, − 2 ± 7%; 10 min, 1 ± 9%; 15 min, − 5 ± 7%).

4. These findings indicate that nitric oxide activity is increased in the immediate post-partum period in venous vasculature, and support the hypothesis that increased nitric oxide activity may be responsible for the vasodilatation observed during normal pregnancy.