Vascular renin in the guinea pig. Suppression by the renin inhibitor remikiren.

Abstract

Angiotensin I and II are generated by the vascular wall. Whether this generation depends on renin or on other enzymes is debated. We tested the hypothesis that remikiren, a highly specific inhibitor of human and guinea pig renin, may inhibit the vascular renin-angiotensin system. Isolated hindquarters from guinea pigs were perfused with an artificial medium, and angiotensin I and II release was measured by high-performance liquid chromatography and radioimmunoassay. Guinea pig hindquarters released angiotensin I (23.8±5.6 fmol/30 min; n = 13) and angiotensin II (95.2±19 fmol/30 min; n=13) spontaneously. Inhibition of the angiotensin I-convert-T he notion that angiotensin I and angiotensin II (Ang I and Ang II, respectively) are formed in and released from extrarenal resistance vessels is now supported by numerous data from studies in isolated perfused organs. '-4 However, nonrenin enzymes could contribute to the local formation of Ang I 5 ' 6 in isolated organs perfused without blood, ie, in the absence of plasma-derived protease inhibitors. We have recently shown that the natural renin substrate angiotensinogen is cleaved in an isolated, perfused resistance vessel bed.3 These data provide indirect evidence for the presence of renin in resistance vessels, since other enzymes would not form Ang I from angiotensinogen at pH greater than 7. 6 To demonstrate the presence of vascular renin directly, inhibition of vascular angiotensin formation by specific renin inhibitors is necessary. Saito et al 2 have shown that vascular angiotensin formation in rat mesentery can be suppressed by a derivative of pepstatin that inhibits rat renin. However, concentrations of pepstatin that inhibit rat renin also inhibit nonspecific proteases such as cathepsin D. 6More specific inhibitors for rat renin are not available.Remikiren (RO 42-5892) is a highly specific renin inhibitor in primates that is 3500 times more potent against renin than against cathepsin D.7 Furthermore, several in vivo studies suggest that remikiren inhibits renin at tissue sites, eg, vascular renin. -8 Therefore, remikiren appears to be well suited to investigate vascular renin. Remikiren is not active against rat renin but is a highly specific inhibitor of guinea pig renin. To test the hypothesis that remikiren inhibits vascular renin, we adapted our rat hindquarter perfusion mod- ing enzyme by captopril (10 nmol/mL) suppressed angiotensin II by 85% and increased angiotensin I by 352% (n=5, P<.05). Infusion of remikiren (1.6 nmol/mL) in addition to captopril decreased angiotensin I release by 68% (P<.05 versus captopril alone, n=5 each). We conclude that renin generates angiotensin I in an isolated guinea pig resistance vessel bed. Our study demonstrates that renin rather than nonrenin enzymes is responsible for the major part of vascular angiotensin formation. 1 -3 for guinea pigs. We investigated whether perfused guinea pig hindquarters release Ang I and Ang II and tested the effects of Ang I-converting enzyme (ACE) inhibition and remikiren. The ren...