Circulating hypotensive factor in pregnant spontaneously hypertensive rats.

1. Blood pressure decreased significantly at a late stage of pregnancy in spontaneously hypertensive rats (SHRs) but this reduction tended to disappear shortly after delivery. Since acute and chronic administration of progesterone, prolactin and estrone failed to reduce blood pressure, these hormones were not responsible for a fall of blood pressure at a late stage of pregnancy in SHRs. Exogenous administration of prostaglandin E2 (PGE2) significantly reduced blood pressure, but endogenous prostaglandin E1 (PGE1), PGE2 and prostaglandin F2 alpha (PGF2 alpha) in plasma did not increase at a late stage of pregnancy. 2. Plasma from pregnant SHRs produced a significant reduction of blood pressure in male and female SHRs, but the plasma from male and non-pregnant female SHRs failed to reduce blood pressure. Plasmas from pregnant SHRs were less effective in reducing blood pressure when tested in Wistar Kyoto rats (WKYs). 3. It is concluded that a hypotensive factor is circulating at a late stage of pregnancy in SHRs.