Comparison of plasma oxytocin and catecholamine concentrations with uterine activity in pregnant rhesus monkeys.

Pregnant rhesus monkeys exhibit diurnal changes in uterine activity (UA), with episodes of increased UA during the early hours of darkness. The estrogenic environment during late pregnancy serves a permissive role in the maintenance of nocturnal UA episodes and may involve myometrial interactions with oxytocin (OT) and/or alpha-adrenergic stimuli. In the present study we have used chronically catheterized pregnant rhesus monkeys to measure diurnal changes in maternal plasma OT, epinephrine, norepinephrine, and dopamine. We also determined the effects of infusing an OT antagonist (ORF 22164) and the alpha-adrenergic antagonist phentolamine on nocturnal UA episodes. Animals were exposed to a 16-h light, 8-h dark photo-period, with the hours of darkness between 2300-0700 h. Maternal plasma samples were collected at 3-h intervals for 36 h and analyzed by RIA for OT and by high performance liquid chromatography for catecholamines. Plasma OT was correlated with UA in animals that displayed nocturnal UA episodes (r = 0.76; P less than 0.01). Maximal OT concentrations occurred at 2400 h in these animals; plasma OT was higher during the hours of darkness compared to levels during the light phase (10.4 +/- 1.9 and 3.0 +/- 0.3 pmol/L, respectively; n = 4). Some animals did not display nocturnal episodes of increased UA and showed no increase in OT concentrations during the hours of darkness. Maternal plasma catecholamine concentrations were not correlated with nocturnal UA and were maximal during the light phase. Nocturnal UA was abolished within 30 min of infusion of the OT antagonist, but phentolamine infusions had no effect on nocturnal UA. We conclude that 1) changes in maternal plasma catecholamine concentrations are not involved in the generation of nocturnal UA; 2) the presence of episodes of increased UA at night results from increased maternal plasma OT concentrations; and 3) the absence of nocturnal UA in some animals can be explained by a reduced level of OT secretion.