Peripherally detectable hormones--their relation to the increased uterine activity during standing in pregnant women

Two-thirds of women in late pregnancy in standing position show marked cyclic accelerations in heart rate with concomitant increase in the uterine activity. As the regulating mechanism of these contractions has not been investigated the aim of the present study is to see if variations in the concentrations of peripheral venous circulating hormones could account for the accelerations of the heart rate and the uterine contractions. In four healthy pregnant women, 25 to 27 years old and in the 33rd-38th weeks of gestation, and in three healthy nonpregnant women, 29 to 30 years old, venous blood was intermittently collected from a cubital vein. The women were investigated in the left lateral as well as in the standing postures. The plasma concentrations of norepinephrine (NE), prostaglandin E2(PGE2), prostaglandin F2 alpha (PGF2 alpha), 6-k-prostaglandin F1 alpha (6-k-PGF1 alpha) thromboxane B2 (TxB2), aldosterone (A), and the plasma renin activity (PRA) were measured by specific and sensitive assays. Significant differences in level and dynamics of the various substances were found between pregnant and nonpregnant subjects. However, no correlation could be found between the fluctuations in the concentration of hormones and heart rate accelerations and the occurrence of uterine contractions, respectively. Local changes of these substances in the uterus may not be reflected in the peripheral venous blood. Therefore our measurements can neither prove nor disprove the hypothesis that these hormonal substances are involved in the regulatory mechanism of uterine contractions occurring in standing.