Quantitation of inactive renin in human and dog plasma: techniques for activation

For human samples quantitation of inactive renin can be carried out by incubation with trypsin under defined conditions, followed by RIA of the activated renin. For dog samples we were unable to obtain evidence for the presence of inactive renin in the plasma by using trypsin, acid or cold to activate. Increases in angiotensin generation did occur with trypsin and acid but they both changed renin substrate such that the rate of angiotensin generation by exogenous renin was increased at pH 7.4, but not at pH 5.7; also following trypsin or acid treatment angiotensin I was cleaved from renin substrate by a plasma acid protease that normally does not cleave renin substrate in plasma. Therefore, for dog samples, it is important to demonstrate that an increase in the rate of angiotensin generation is indeed due to activation of inactive renin and not to changes in pH optimum of renin with angiotensinogen or to the effect of another enzyme.