Resource sharing in rat gestation: role of maternal cardiovascular hemodynamics

There is a substantial decrease in blood pressure (BP) in late pregnancy in the laboratory rat. It is so pronounced that manipulations that produce sustained elevations in BP in nonpregnant animals have little or no effect during pregnancy. It is commonly believed that this decrease in BP is a consequence of a large decrease in total peripheral resistance resulting from the passive combination of the placental vasculature with a preexisting maternal vasodilation. An alternative view is presented here. We suggest that, in small mammals like the laboratory rat, pregnancy severely challenges the ability of the maternal cardiovascular system to meet its metabolic demands, so that during the last stages of maturation of the low-resistance placental circulation delivery of vital metabolic or nutritional substances to the maternal vasculature becomes marginal. When the so-called maternal hemodynamic preservation threshold is reached, a pronounced and wide-spread vasodilation occurs to maintain adequate perfusion of maternal organs. The late-gestational decrease in BP thus reflects a dynamic interaction between the maternal and placental circulations rather than reflecting their passive combination. The hypothesis provides a framework for the integrated discussion of a number of important phenomena: the fact that hypertensive rats exhibit a larger decrease in BP in late gestation than normotensive rats; the existence of a positive association between litter size and the magnitude of the late-gestational decrease in BP; and, finally, the well-established ability of the food-restricted pregnant rat to compartmentalize its nutritional resources.