Characterization of the AT1 angiotensin II receptor expressed in guinea pig liver

In guinea pig hepatocytes angiotensin II induced phosphorylase a activation. This effect was mimicked by other angiotensins with the potency order: angiotensin II (EC50 approximately 1 nM) > angiotensin III (EC50 approximately nM) > angiotensin I (EC50 approximately 300 nM). The effect of 10 nM angiotensin II was blocked by the angiotensin II receptor AT1-selective antagonists irbesartan and losartan (Ki values of approximately 1 nM and approximately 10 nM for irbesartan and losartan respectively) but not by the AT2-selective antagonist PD123177. Similar data were obtained when the production of [3H]IP3 from [3H]myo-inositol-labeled cells was studied Angiotensin II induced a dose-dependent increase in [3H]IP3 production; the maximal effect (approximately 3-fold) was observed at a concentration of 10 microM. This effect of angiotensin II was completely blocked by the AT1-selective antagonists irbesartan and losartan, but only in a very limited fashion by PD123177. [125I][Sar1-Ile8]angiotensin II bound with high affinity (approximately 3.8 nM) to a moderately abundant number of sites (approximately 660 fmol/mg protein) in guinea pig liver membranes. Binding competition experiments indicate the following orders of potency for agonists: angiotensin II (approximately 1.5 nM) > angiotensin III (approximately 7 nM) > angiotensin I (approximately 176 nM), and for antagonists: irbesartan (approximately 0.5 nM) > losartan (approximately 36 nM) > > PD123177 (> > 10000 nM). The functional and binding data strongly indicate that the effects of angiotensin II were mediated through AT1 receptors. Expression of the mRNA for these receptors was confirmed by RT-PCR and hybridization of the reaction product with a radiolabeled rat AT1 receptor cDNA probe.