REVERSIBLE CRYOACTIVATION OF RECOMBINANT HUMAN PRORENIN

Cleavage of prorenin's prosegment causes irreversible formation of renin. In contrast, renin activity is reversibly exposed when prorenin is acidified to pH 3.3. Nonetheless, acidification of plasma results in irreversible activation of prorenin, because endogenous proteases cleave the prosegment of acid-activated prorenin. Chilling of plasma results in irreversible cryoactivation of prorenin. In this study we investigated whether cryoactivation of purified prorenin is reversible. The intrinsic renin activity of recombinant human prorenin was measured by an enzyme kinetic assay using partially purified human angiotensinogen as substrate. Results are expressed as a percent (mean +/- S.E.) of the maximal activity exposed after limited proteolysis by trypsin. The intrinsic renin activity of two pools (0.3 and 0.06 Goldblatt units/ml) was 1.5% +/- 0.3 and 1.2% +/- 0.6 at 37-degrees-C. Activity increased to 19% +/- 0.3 and 26% +/- 0.5 after incubation at 0-degrees-C and to 5.4% +/- 0.5 and 2.1% +/- 1.2 at room temperature. Cryoactivation did not occur in buffers containing more than 1 M NaCl. It took 8 min at 37-degrees-C or 180 min at room temperature for cryoactivated prorenin to lose half of its intrinsic renin activity. It took 48 and 26 h, respectively, at 0-degrees-C for the two pools of prorenin at 37-degrees-C to regain half of their maximum intrinsic activity at 0-degrees-C. A direct immunoradiometric assay that detects active renin but not prorenin was able to detect cryoactivated prorenin. These results show that human prorenin can be reversibly cryoactivated in buffers of low ionic strength and has greater intrinsic activity at room temperature than at 37-degrees-C

DEXAMETHASONE-INHIBITABLE STIMULATION OF PLASMA PRORENIN BY KETAMINE IN CATS

Plasma prorenin in humans is derived from both renal and extrarenal sources, but in the cat most plasma prorenin is normally of renal origin. Sodium depletion and β-adrenergic blockade can increase plasma prorenin in cats, but the effects of sedatives and glucocorticoids are unknown. We examined the effect of ketamine, a centrally acting nonbarbiturate anesthetic, and of the glucocorticoid dexamethasone on plasma prorenin and renin. Intramuscular injection of ketamine (20 mg/kg, three times a day) for 4 days increased plasma prorenin slowly and consistently, from 7.4 ± 1.4 (± SEM) to 11.4 ± 2.2, 17.1 ± 2.5, 20.2 ± 3.0, and 29.2 ± 3.4 ng/ml · h (P < 0.001) on days 1, 2, 3, and 4, respectively, without any effect on plasma active renin. Plasma renin substrate and cortisol were also unchanged. Bilateral nephrectomy reduced both baseline and stimulated plasma prorenin to undetectable levels. Treatment with α1-blocker, β1-blocker, angiotensin-converting enzyme inhibitor, or Ca2+ antagonist did not affect the rise in prorenin induced by ketamine, but dexamethasone completely blocked the response. In contrast, dexamethasone alone had little effect on plasma prorenin. These results demonstrate that repetitive ketamine administration selectively increases plasma prorenin, suggesting that renal prorenin secretion may be regulated independently of active renin. Blockade of stimulated, but not baseline, plasma prorenin by dexamethasone is consistent with a negative effect of glucocorticoids on the regulatory elements of the renin gene