PREGNANCY-INDUCED ALTERATIONS OF NEUROGENIC CONSTRICTION AND DILATIONOF HUMAN UTERINE ARTERY

The responses to electrical field stimulation (EFS) of perivascular ne rves in human uterine arteries were characterized. The arteries were r emoved from pregnant and nonpregnant patients undergoing hysterectomy. Tetrodotoxin, guanethidine, and phentolamine blocked EFS (2 min, 80 V , 0.1-ms duration)induced constriction. The constrictions and the endo genous norepinephrine levels were lower (P < 0.01) in uterine arteries from pregnant than from nonpregnant patients. When arterial rings wer e precontracted, the response to EFS was biphasic, consisting of an in itial constriction followed by a postconstriction relaxation. The EFS- induced relaxation was endothelium independent and was greater (P < 0. 01) in uterine arteries from pregnant than from nonpregnant patients. The relaxation was enhanced by guanethidine and superoxide dismutase, inhibited by nitric oxide synthase inhibitors, blocked by tetrodotoxin , and unaffected by atropine, propranolol, or indomethacin. The result s demonstrate that human uterine arteries respond to EFS with contract ion and relaxation and that these responses may be mediated, respectiv ely, by norepinephrine and, in part, by nitric oxide released from per iarterial nerves. The decrease in neuronally mediated uterine arterial constriction and the increase in dilation could be physiological mech anisms for ensuring appropriate uteroplacental perfusion.