HALF-LIFE OF OXYTOCIN IN BLOOD OF PREGNANT AND NON-PREGNANT WOMEN

Abstract

ABSTRACT Using tritium-labelled oxytocin with a high specific activity, the halflife in the blood and the urinary excretion of intravenously injected oxytocin were followed in the female. The following groups of patients were studied: normally menstruating women during different phases of the menstrual cycle, women using a combination of gestagenic and oestrogenic hormones for oral contraception, and pregnant women in the first and second trimester. The pregnant women were admitted to the hospital for legal abortion in the 10th–20th week of gestation. In the proliferative phase, t½ was 272 seconds (n = 14), in the secretory phase 221 seconds (n = 5), and in women using oral contraceptives 199 seconds (n = 10). In pregnant women during the first trimester, t½ was 178 seconds (n = 6). The corresponding value in women examined during the 14th–17th weeks and during the 18th–20th weeks of gestation was 295 seconds (n = 6) and 282 seconds (n = 6), respectively. T½ was also determined within 24 h of abortion in patients in the second trimester, where the abortion was induced by intra-amniotic instillation of 50% glucose. In all cases a decrease in t½ was found. The decrease was most marked in women during the 18th–20th weeks of gestation. Altogether 25–50% of the radioactivity injected was recovered in the urine from pregnant women within 3 h of the injection. Thin-layer chromatography of the urine did not reveal the presence of any intact oxytocin. The results demonstrate that the disappearance of oxytocin from the blood seems to be influenced by the sex hormones. Thus, an oestrogendominated stage shows a lower disappearance rate, whereas gestagens produce the reverse effect. The pronounced decrease in t½ in pregnant women immediately after abortion might be due to a change to a more progesterone-dominated stage induced by the death of the foetus, or by an alteration in the affinity of oxytocin to the myometrium.