Angiotensin II mediates its action by interacting with membrane receptor, Angiotensin II type 1 receptors. The signal is transmitted via G-proteins that activates a phosphatidylinositol-calcium second messenger system. ATII receptors are integral membrane proteins of ~359-363aa and predicted to contain at least 7 transmembrane domains. The N and C-terminus are predicted to extracellular and cytoplasmic, respectively. Three isoforms of ATII receptors have been cloned. In rat type 1a (359aa) is expressed in the liver kidney, aorta, lung, uterus, ovary, spleen, heart, adrenal and vascular smooth muscles. Rat Type 1b (also known as AT3; 359aa) is found in liver, kidney, and aorta. Type 1c (177aa) has been detected in brain, kidney, liver, spleen and lung.