Angiotensin is a peptide hormone that is part of the renin-angiotensin system responsible for regulating fluid homeostasis and blood pressure. It is involved in various means to increase the body's blood pressure, hence why it is a target for many pharmceutical drugs that treat hypertension and cardiac conditions. Angiotensin II, the primary agent to inducing an increased blood pressure, is formed in the general circulation when it is cleaved from a string of precursor molecules. Angiotensinogen is converted into angiotensin I with the action of renin, an enzyme secreted from the kidneys. From there, angiotensin I is converted to the central agent, angiotensin II, with the aid of angiotensin-converting enzyme (ACE) so that it is available in the circulation to act on numerous areas in the body when an increase in blood pressure is needed. Angiotensin II can act directly on receptors on the smooth muscle cells of the tunica media layer in the blood vessel to induce vasoconstriction and a subsequent increase in blood pressure. However, it can also influence the blood pressure by aiding in an increase of the circulating blood volume. Angiotensin II can cause vasopressin to be released, which is a hormone involved in regulating water reabsorption. Vasopressin is created in the supraoptic nuclei and they travel down the neurosecretory neuron axon to be stored in the neuronal terminals within the posterior pituitary. Angiotensin II in the cerebral circulation triggers the release of vasopressin from the posterior pituitary gland. From there, vasopressin enters into the systemic blood circulation where it eventually binds to receptors on epithelial cells in the collecting ducts of the nephron. The binding of vasopressin causes vesicles of epithelial cells to fuse with the plasma membrane. These vesicles contain aquaporin II, which are proteins that act as water channels once they have bound to the plasma membrane. As a result, the permeability of the collecting duct changes to allow for water reabsorption back into the blood circulation. Angiotensin II also has an effect on the hypothalmus, where it helps trigger a thirst sensation. Correspondingly, there will be an increase in oral water uptake into the body, which would then also increase the circulating blood volume. Another way that angiotensin II helps increase the blood volume is by acting on the adrenal cortex to stimulate aldosterone release, which is responsible for increasing sodium reuptake in the distal convoluted tubules and the collecting duct. It is formed when angiotensin II binds to receptors on the zona glomerulosa cells in the adrenal cortex, which triggers a signaling cascade that eventually activates the steroidogenic acute regulatory (StAR) protein to allow for cholesterol uptake into the mitochondria. Cholesterol then undergoes a series of reactions during steroidogenesis, which is a process that ultimately leads to the synthesis of aldosterone from cholesterol. Aldosterone then goes to act on the distal convoluted tubule and the collecting duct to make them more permeable to sodium to allow for its reuptake. Water subsequently follows sodium back into the system, which would therefore increase the circulating blood volume. In addition, potassium and hydrogen are also being excreted into the urine simultaneously to maintain the electrolyte balance.

Angiotensin Metabolism References