Localization and quantitation of angiotensin AT₁ and AT₂ receptors in the pregnant and non-pregnant sheep uterus

The two angiotensin II receptor subtypes, AT₁ and AT₂, have been reported to be differentially expressed in the myometrial membrane preparations of nulliparous and pregnant sheep, however, their distribution in the sheep reproductive tract has not been reported. The aim of this study is to map the distribution of AT₁ and AT₂ receptors in the anoestrus reproductive tract of the sheep by quantitative in vitro autoradiography and to investigate if the density and distribution of the receptors change during pregnancy. The AT₂ receptor is abundant in a discrete layer in the myometrium of the anoestrus sheep uterus, whilst the AT₁ receptor is expressed at lower levels, predominantly in the endometrium. Near-term pregnant uteri, show a marked change in the expression of angiotensin II receptors: the myometrium no longer expresses detectable AT₂ receptors but rather, expresses low levels of AT₁ receptors. Angiotensin converting enzyme is found in high concentrations in the blood vessels of the pregnant and non-pregnant sheep reproductive system and on the epithelial cells of the fallopian tubes of the non-pregnant sheep. These studies reveal marked reciprocal changes of angiotensin II receptors, with myometrial AT₁ receptors increasing during pregnancy, whilst AT₂ receptors fall markedly. These changes suggest that angiotensin II may be involved in regulating changes of uterine structure and function during pregnancy by interaction with multiple receptor subtypes.