Effect of Trypsin or Acid Treatment on Dog Plasma Renin Activity Measurements*

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Abstract
This study examines the effect on dog PRA measurements of three methods for activation of inactive plasma renin. PRA was measured as the rate of angiotensin I formation at pH 5.7 and 37 C in the presence of EDTA and phenylmethylsulfonyl fluoride. Cryoactivation, i.e. incubation of plasma for 4 days at -4 C, had no effect. Acid or trypsin treatment both increased the rate of angiotensin I generation but seemed to do so by affecting renin substrate rather than renin. Thus, unlike their effects on human renin substrate, acidification to pH 3.3 failed to destroy dog renin substrate and incubation with trypsin at concentrations above 1000 μg/ml rendered renin substrate dialyzable. Furthermore, the increase in activity did not appear to result from an increase in the concentration of renin, since the optimum H of angiotensin formation in treated plasma from anephric dogs was pH 5 or less, whereas the pH optimum of exogenous dog renin added to the same plasma was above pH 6. In addition, a change occurred in the reaction characteristics of both endogenous renin and exogenous renin in the treated plasma, such that the rate of angiotensin formation at pH 7.4 increased, while at pH 5.7 it was essentially unchanged. These studies show that when acid or trypsin treatment increase the rate of angiotensin generation in dog plasma, the change in rate is probably not due to activation of inactive renin. At pH 7.4, the increased angiotensin formation may be due, in part, to more complete inhibition of the angiotensinases and also to altered renin substrate reacting with renin more rapidly at an alkaline pH. At pH 5.7, the increase appears to be due to a change in the susceptibility of dog renin substrate to cleavage by an acid protease. Thus, unlike treatment of human plasma, acid or trypsin treatment of dog plasma seems to render renin substrate vulnerable to attack by endogenous cathepsin D (pseudorenin). Notwithstanding, these studies failed to reveal inactive renin in dog plasma, using acid activation, trypsin activation, or cryoactivation in conjunction with a conventional enzymatic renin assay.