Inactive Renin in Human Plasma

Human plasma contains renin, which is enzymatically active at neutral pH (active renin), and a non-dialysable factor, which has renin-like activity after treatment at low pH (inactive renin). In vitro activated plasma-renin and purified human renal renin showed identical enzyme-kinetic properties. Quantitative estimations of inactive renin in renal venous plasma from 5 patients with renal-artery stenosis demonstrated its release by the kidney. Acute stimulation of renin release by isoprenaline, tilting, or diazoxide in 13 normotensive individuals and in 9 patients with essential hypertension increased active plasma-renin and reduced inactive plasma-renin. Inactive plasma-renin was increased and active plasma-renin decreased during suppression of renin release by propranolol in 12 patients with essential hypertension. In 55 patients with various disorders, inactive and active plasma-renin were directly correlated. However, the concentration of inactive renin, for a given value of active renin, varied widely from patient to patient. These results indicate that so-called inactive renin is indeed physiologically related to active renin. They also suggest that inactive renin can be activated not only in vitro, but also in vivo. Different renin assays measure different relative amounts of active and inactive renin. This may call for reinterpretation of results obtained by various methods, especially in situations where changes in plasma concentrations of the two forms of renin are in opposite directions.