Mammalian AT2 Receptors Expressed in <i>Xenopus laevis</i> Oocytes Couple to Endogenous Chloride Channels and Stimulate Germinal Vesicle Break Down

Background and aims: A comparative analysis was performed of the properties of cloned AT1 and AT2 receptors and their ability to induce ion currents and oocyte maturation. Methods: Frog oocytes were injected with cRNA that codes for rat AT1A and AT2 and bovine AT1 receptors and membrane currents were recorded by using the two-microelectrode voltage-clamp technique. Oocyte maturation was scored by observation of the germinal vesicle breakdown (GVBD). Results: Xenopus oocytes expressing AT1 or AT2 generated oscillatory chloride currents by coupling to the diacylglycerol/inositol-3-phosphate (DAG/IP3) cascade. Ang II activation of collagenase-defolliculated oocytes expressing rat AT1A yielded larger chloride currents (9.2±3.4 A) than those generated by oocytes expressing bovine AT1 (3.78±2.6 A). Oocytes expressing rat AT1A were recovered from desensitization after 0.5 h and completely after 10.5 h; whereas oocytes expressing bovine AT1 receptors were unable to do so. Expression of rat AT2 generated smaller chloride currents (32-210 nA). Expression of AT2 receptors triggered GVBD whereas AT1 did not. The proportion of oocytes developing GVBD was greater for folliculated oocytes than for oocytes that were defolliculated. Furthermore, oocytes injected with AT2 secreted into the media, a heat-resistant factor(s) that stimulated GVBD of oocytes. This media elicited inward and outward membrane currents when applied to non-injected oocytes. Conclusion: Activation of AT1 and AT2 receptors couple to similar metabolic cascades in Xenopus laevis oocytes. However, the pathway activated by AT2 diverges to induce oocyte maturation.