Metabolic Clearance Rates, Production Rates and Concentrations of Progesterone in Pregnant Rhesus Monkeys

Abstract

It has been reported of rhesus monkeys that uterine vein levels of progesterone decrease by 75% during the 2nd half of pregnancy while maternal peripheral vein levels remain fairly constant. Whether this difference could be explained by declining placental and/or ovarian progesterone production rates (PRs) occurring along with declining metabolic clearance rates (MCRs) of progesterone was investigated in 8 rhesus monkeys (Macaca mulatta). Values for PR MCR and peripheral levels of progesterone on Day 151 of pregnancy were not markedly different from those on Day 72 of gestation. Others have reported a 22-fold increase in uterine blood flow in pregnant macaques over nonpregnant control values. Thus as PR and MCR did not change during late pregnancy it is concluded that declining uterine vein levels of progesterone near term reflect increased uterine blood flow which delivers to the peripheral blood an unchanged amount of progesterone in an increased amount of blood which accounts for declining concentrations.