Positive Feedback Mechanism Serves as Information Cascade in Renin-Angiotensin Aldosterone System
Hypertension is a chronic cardiovascular condition in which arterial blood pressure is at an elevated state. This illness is exacerbated by a positive feedback mechanism in which an enzyme from the kidney receives information from the body and reacts accordingly, initiating the production of a hormone that facilitates a cascade of anatomical responses that affect blood pressure.
A key component in blood pressure regulation is the renin-angiotensin aldosterone system. This system responds to sodium and ion concentration in the blood and blood volume. Should either of these decrease, the result could be hypotension, or decreased blood pressure. To prevent this from occurring, renin, an enzyme released by the kidney, signals the adrenal gland in the brain to produce a hormone called aldosterone. This hormone increases the reabsorption of ions into the blood stream, thus increasing sodium concentration in the blood. Renin also cleaves angiotensin, a serum globulin found in the liver. All but the first ten amino acid residues are removed, forming angiotensin I. Angiotensin I serves as a substrate and interacts with Angiotensin Converting Enzymes (ACE). The angiotensin I fits into the active site of the enzyme, which cleaves the substrate even further. Two C-terminal residues are removed from the molecule, forming angiotensin II. This process takes place primarily in the lungs, but is also present in the kidneys (where renin is present) or the liver (where the ACE is produced). Angiotensin II stimulates a protein in vascular muscles cells. The layer of muscle tissue in the arterial walls constricts, leading to an increase in blood pressure. ACE also breaks down bradykinin, a peptide vasodilator. When present, bradykinin allows blood vessels to open up and allow blood to flow at a lower pressure. However, ACE removes bradykinin from the system, once again promoting blood vessel constriction and high blood pressure.
This system operates on a positive feedback mechanism, which is a form of information cascade. In this situation, the decrease in ion concentration or blood volume serves as the information. The renin receives this information and makes a decision. Using its “knowledge”, the enzyme “decides” to signal the adrenal gland to produce the aldosterone, which leads to the cleavage of angiotensin, the degradation of bradykinin, and an increase in blood pressure.
The first few renin molecules all receive similar information, as the ion concentration does not immediately increase. Based off of the information they receive, they make the decision to signal the production of the hormone. However, once the aldosterone is present, other renin molecules recognize that the first several enzymes made the decision to alert the adrenal gland. Thus, they are more likely to make the decision to do the same. This leads to a large production of aldosterone and consequentially, angiotensin II. Blood pressure continues to increase.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1253591/