Origin of the angiotensin II secreted by cells

Circulating angiotensin II is unique in that it is formed in the blood by the interaction of circulating proteins. There are in addition many local renin-angiotensin systems in tissues in which angiotensin II is apparently secreted by various types of cells. This brief review considers the possible pathways for synthesis of locally produced angiotensin II in the brain, the anterior pituitary, the testes, the ovaries, the adrenal cortex, the kidneys, the heart, blood vessel walls, and brown and white fat. Synthesis by cells in culture is also reviewed. The possibility that certain cells contain a complete intracellular renin-angiotensin system is not ruled out, but there are problems with this hypothesis. Proteases other than renin may be involved, and there may be different pathways in different tissues. However, it appears that at least in some tissues, angiotensinogen is produced in one population of cells and transported in a paracrine fashion to other renin-containing cells, where it serves as the substrate for production of angiotensin II.