Angiotensin II (Ang II), the potent agonist of the renin-angiotensin system (RAS), initiates its actions via specific receptors. Two subtypes of Ang II receptors have been cloned in the human, and these are designated AT1R and AT2R. When activated both these transmembrane receptors initiate the stimulation of different signalling pathways and produce contrasting effects, as the action of AT1R antagonises the effect of AT2R and vice versa (De Gasparo et al. 2000). Although its exact role is uncertain, the existence of a placental RAS is well recognised (Nielsen et al. 2000). In cows, AT1R is predominantly expressed in the uterus and AT2R in the fetal compartment, whereas in the human, AT2R is predominantly expressed in the uterus and AT1R in the placenta. This reflects the suggestion that its distribution is tissue and species specific (Nielsen et al. 1996).
To determine the expression of AT2R subtype in human placenta, three polyclonal antibodies raised against three different epitopes of the receptor protein were used. Two of these antibodies are directed against sites close to N-terminal of the receptor protein and the other to a site close to the C-terminal. With ethical approval, placenta from early (first trimester, n = 6), second trimester (n = 5) and term placenta (n = 8) were collected, fixed in formalin and embedded in paraffin wax. The Dako Envision Plus System and the avidin-biotin complex methods were then used to investigate the immunolocalisation of the receptor.
With all three antibodies, immunoreactivity for AT2R was present in the syncytiotrophoblast and the cytotrophoblast in all three periods of placental growth. Expression of the receptor in perivascular areas was only evident using the antibody closest to the N-terminal and only in the first trimester samples. Overall, the distribution of AT2R is similar to that reported for AT1R in the human placenta (Cooper et al. 1999).
We have shown for the first time using immunohistochemistry with three different antibodies acting on three different epitopes of the receptor protein that human placenta expresses AT2R. The different expression of AT2R with different antibodies may reflect some unknown characteristic of this receptor.
In conclusion, these studies add support to the hypothesis that Ang II has a physiological role in human placenta through stimulation of AT1R and AT2R. However, which receptor Ang II activates and what factors determine which receptor is activated remain to be elucidated.
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