Properties of a Lactate-Induced Relaxation in Human Placental Arteries and Veins

Objective: Our purpose was to determine the vasoactive effects and mechanism of action of lactate in human placental vessels by means of isometric muscle bath studies. Study Design: Isolated 1 to 2 mm human placental arteries and veins from normal term pregnancies, precontracted with prostaglandin F2α and incubated under a Po2 of ∼35 torr were exposed to lactate, 1 to 10 mmol/L, (pH 7.4), pyruvate, hydrogen peroxide, nitroglycerin, or forskolin. The effects of endothelium removal or inhibitors of cyclooxygenase (indomethacin 10 μmol/L) and L-arginine metabolism (nitro-L-arginine 30 μmol/L) on the response to lactate and the effects of an antagonist of guanylate cyclase activation (methylene blue 10 μmol/L), cyanide (1 mmol/L), and hypoxia (Po2 8–10 torr) on responses to all agents were determined by analysis of variance and t test statistics. Results: Lactate-elicited dose-dependent relaxation was not inhibited by endothelium removal, indomethacin, or nitro-L-arginine but was attenuated by methylene blue, cyanide, and hypoxia. Relaxation to hydrogen peroxide was inhibited by methylene blue and cyanide but not hypoxia. Relaxation to nitroglycerin was inhibited only by methylene blue, and relaxation to forskolin was not inhibited by these probes. Pyruvate did not produce a significant relaxation. Conclusions: These findings suggest that lactate causes relaxation in the human placental vessels by an oxygen and cyclic guanosine-3′:5′-monophosphate-dependent mechanism, which may involve the generation of hydrogen peroxide but not the metabolism of arginine. Lactate-induced dilatation may be of importance during labor and in situations of acute and chronic fetal hypoxia