Effects of pregnancy on spontaneous contraction and barium responsiveness of the rat portal vein.

Spontaneous activity and BaCl2 responsiveness of portal veins from non-pregnant and from 1-, 2-, and 3-week pregnant rats were compared. Isometric tension was recorded from vessels suspended in oxygenated, 37 degrees C Krebs solution. After 60-90 min equilibration, peak isometric tension, contraction frequency, and duration were measured. Then responses to cumulative additions of BaCl2 were determined. Initial isometric tension development of veins from pregnant rats was not significantly different from that of the non-pregnant rats. However, the frequency of contractions was reduced at all three stages of pregnancy and the duration of major contractions was increased. Barium caused dose-related increases in tension and reduced contraction frequencies of all vessels. Maximum increases in tension of veins from all pregnant rats were significantly less, however, than those of veins from non-pregnant rats, while barium-induced decreases in contraction frequencies were significantly greater only at 3-weeks gestation. Thus, pregnancy decreases the frequency of the spontaneous contractions of the rat portal vein throughout pregnancy, and inhibits the barium-induced increases in tension development. These changes in venous smooth muscle activity are similar to pregnancy-induced changes that have been reported to occur in arterial, uterine, and gastrointestinal smooth muscle, and may be dependent on a general smooth muscle inhibitory effect of elevated progesterone during pregnancy.