AJP - Regulatory, Integrative and Comparative Physiology, Vol 258, Issue 6 1299-R1307, Copyright © 1990 by American Physiological Society
Resource sharing in rat gestation: role of maternal cardiovascular hemodynamics
D. A. Blizard and T. G. Folk
Department of Physiology and Pharmacology, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27103.
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.
