Structural and mechanical changes of uterine arteries during pregnancy in the pig

A dramatic 15-fold increase in uterine blood flow in pigs occurs during pregnancy in association with marked increases in uterine arterial (UA) diameter. This study was conducted to determine UA collagen and elastin content, diameter, alpha 1- and alpha 2-adrenergic receptor (AR) numbers, norepinephrine (NE) and in vitro reactivity to phenylephrine on d 0, 20, 50, 80 and 110 of pregnancy in the pig. Uterine arterial collagen content declined progressively throughout pregnancy (P less than .01), whereas the content of elastin remained constant from d 0 to d 80, then increased (P less than .05) from d 80 to d 110. The UA collagen to elastin ratio was correlated with UA diameter (r = -.69; P less than .01), which increased from 4.5 mm on d 0 to 9.0 mm on d 110. Uterine arterial alpha 1-AR numbers remained low and constant throughout pregnancy, consistent with its retained ability to contract in response to phenylephrine. Uterine arterial NE content declined (P less than .05) from d 20 to d 80 before increasing slightly to d 110. Uterine arterial alpha 2-AR numbers remained high from d 0 to d 80 before decreasing (P less than .05) to low values on d 110. These data are consistent with a reduced adrenergic neuronal control and increased elasticity of the UA during pregnancy in the pig.