ISOLATION AND EXPRESSION OF A NOVEL ANGIOTENSIN-II RECEPTOR FROM XENOPUS-LAEVIS HEART

A Xenopus laevis heart cDNA library was screened using the human angio tensin type 1 (AT1) receptor cDNA coding sequence as a hybridization p robe A cDNA was isolated that encodes a protein of 363 amino acids tha t shares 63% sequence identity with the human AT1 receptor. Radioligan d binding studies with the cloned receptor expressed in COS cells indi cated that it is an angiotensin II receptor that possesses pharmacolog ical properties distinct from those of the two known mammalian recepto r subtypes, AT1 and AT2. Electrophysiological studies with the recombi nant receptor expressed in X. laevis oocytes revealed that the amphibi an receptor, like the mammalian AT1 receptor, can functionally couple to a second messenger system, leading to the mobilization of intracell ular stores of calcium, However, nonpeptide antagonists selective for the mammalian AT1 and AT2 receptors do not block angiotensin II-stimul ated functional responses in injected oocytes, which confirms that the amphibian receptor is a pharmacologically unique angiotensin II recep tor. Nevertheless, based on conservation of structural features and mo tifs and similarity in coupling mechanisms, we speculate that the clon ed Xenopus receptor is the amphibian counterpart of the mammalian AT1 receptor, having acquired its unique pharmacology as a consequence of evolutionary divergence.