Modulation of AT1 receptor-mediated contraction of rat uterine artery by AT2 receptors.

The aim of this study was to characterize the angiotensin II receptors in isolated uterine arteries from non pregnant and pregnant rats, since it has been reported from binding studies that ovine uterine arteries contain AT2 receptors. Uterine arterial segments were obtained from virgin, non-pregnant and late pregnant (18-21 days) Sprague-Dawley rats and mounted in small vessel myographs. Concentration-response curves were constructed to angiotensin II (1 nM-10 microM) in the absence and presence of various angiotensin II receptor subtype selective compounds. These included losartan (AT1 antagonist; 1, 10 and 100 nM), PD 123319 (AT2 antagonist; 1 microM) and CGP 42112 (AT2 agonist; 1 microM). Responses to angiotensin II were measured as increases in force (mN) and expressed as a per cent of the response to a K+ depolarizing solution. Losartan (1, 10 and 100 nM) caused significant concentration-dependent rightward shifts of the angiotensin II concentration-response curve in uterine arteries from non-pregnant and pregnant rats. The pA2 values calculated from these data were 9.8 and 9.2, respectively, although the slope of the Schild plot in the non-pregnant group was less than unity. PD 123319 (1 microM) caused significant 6- and 3 fold leftward shifts of the angiotensin II concentration-response curve in uterine arteries from non-pregnant and pregnant rats, respectively. In vessels from pregnant rats, PD 123319 also significantly increased the maximum response to angiotensin II. CGP 42112 (1 microM) attenuated the response to angiotensin II of uterine arteries from non-pregnant rats. This was reflected by a 14 fold rightward shift of the angiotensin II concentration-response curve and a decrease in the maximum response. In uterine arteries from pregnant rats, CGP 42112 (1 microM) caused a 3 fold rightward shift of the angiotensin II concentration-response curve, but had no effect on the maximum response. PD 123319 (1 microM) and CGP 42112 (1 microM) had no effect on the concentration-response curves to phenylephrine (PE) of uterine arteries from non-pregnant or pregnant rats. In addition, CGP 42112 (1 nM-1 mM) had no vasodilator effect on tissues precontracted with phenylephrine. These results suggest that the contractile responses of the rat uterine artery are mediated by the AT1 receptor. Furthermore, in this vascular preparation, the AT2 receptor appears to inhibit the response mediated by the AT1 receptor, although, this is not uniform between the non-pregnant and pregnant states.