Angiotensin II is an endogenous potent vasoconstrictor peptide. The amino acid sequence of angiotensin II is Asp-Arg-Val-Tyr-Ile-His-Pro-Phe | His-Leu. Angiotensin II increases blood pressure by stimulating the Gq protein in vascular smooth muscle cells (which in turn activates an IP3-dependent mechanism leading to a rise in intracellular calcium levels and ultimately causing contraction). In addition, angiotensin II acts at the Na/H exchanger in the proximal tubules of the kidney to stimulate Na reabsorption and H excretion which is coupled to bicarbonate reabsorption. This ultimately results in an increase in blood volume, pressure, and pH. Angiotensin II is degraded to angiotensin III by angiotensinases located in red blood cells and the vascular beds of most tissues. It has a half-life in circulation of around 30 seconds, whereas, in tissue, it may be as long as 15–30 minutes.