Measurement of renin activity in human plasma

• Many methods for measurement of renin activity have been described since Tigerstedt and Bergman1 first made an extract of kidney and showed that when injected it raised blood pressure. Those methods which rely on the rise of blood pressure following such injec-tions suffer from three main disadvantages. First, the magnitude of response to a given dose may vary and tachyphylaxis can be elicited. Second, relatively large quantities of extracts are required to obtain a meas-urable response. Third, renin is an enzyme and not a pressor substance as thought at first, and this type of assay does not measure its enzymatic activity as discussed originally by Plentl and Page.2 Methods in which renin is allowed to react with substrate in vitro, and in which the re-sultant angiotensin is measured, avoid these difficulties.2 Angiotensin does not produce tachyphylaxis except in large doses. The sen-sitivity of renin measurement can be increased by prolonging the time of incubation. Fur-thermore, these methods allow control of numerous factors which affect the rate of formation, or destruction, of angiotensin such as pH, temperature, substrate concentration, presence or absence of inhibitors or activa-tors, and angiotensinases. Control of these factors can be achieved by separating renin as much as possible from other plasma con-stituents, and then adding to it a unifor