The relationship between experimentally determined litter size and maternal blood pressure in spontaneously hypertensive rats.

Pregnancy lowers blood pressure in hypertensive rats. To evaluate the role of the conceptus in maternal blood pressure regulation, we measured the changes in systolic blood pressure b (by tail-cuff plethysmography) throughout gestation and mean arterial pressure, cardiac output, and organ blood flows (with radioactive microspheres) on postmating day 21 for calculation of total peripheral and organ vascular resistances in spontaneously hypertensive rats with litter size surgically adjusted to 0 to 10 conceptuses on postmating day 7. Blood pressure remained elevated in those rats with zero fetuses but decreased during the last week of pregnancy in those rats with three or more fetuses. The magnitude of the decrease was directly related to litter size. At term, cardiac output was positively correlated (r = 0.61; p less than 0.001), whereas mean arterial pressure and total peripheral resistance were negatively correlated (r = -0.74; p less than 0.001 and r = -0.79; p less than 0.001, respectively) with litter size. Resistances of all the vascular beds in the body, except the kidneys, spleen, and hepatic artery were also negatively correlated with fetal number. Thus pregnancy is characterized by a generalized maternal vasodilation, and the fetal/placental unit may play a significant role in modulating maternal vascular tone.