A high plasma renin activity (PRA) is expected in patients

A high plasma renin activity (PRA) is present in patients who are taking angiotensin-conveing enzyme inhibitors.Renin is a proteolytic enzyme that is synthesized and stored in the juxtaglomerular (JG) apparatus, which consists of the juxtaglomerular cells (specialized myoepithelial cells) and the cells of the macula densa, located in the distal tubule. The JG cells surround the afferent aerioles of glomeruli in the kidneys. Once released, renin acts on angiotensinogen, which is synthesized in the liver, and conves it into the decapeptide angiotensin I. Angiotensin I is then enzymatically transformed into angiotensin II (All), an octapeptide, by conveing enzymes located in the kidney and the pulmonary vascular endothelium in the lungs. AII is a potent peripheral vasoconstrictor that directly acts on aeriolar smooth cells in the resistance vessels. In addition, it stimulates the efferent aerioles in the kidneys and has a negative feedback on renin release. All, along with the nonapeptide AIII, also stimulates the release of aldosterone from the zona glomerulosa in the adrenal coex. Aldosterone enhances the distal and early collecting tubule reabsorption of sodium in exchange for potassium and/or hydrogen ions.An angiotensin-conveing enzyme inhibitor, such as captopril, increases PRA because it is the negative feedback inhibition of All on renin release. This treatment is used to test for stimulation of renin release, paicularly in renovascular hypeension, in which there is an exaggerated increase in PRA.