Components of the renin-angiotensin system in the cerebrospinal fluid of rats and dogs with special consideration of the origin and the fate of angiotensin II.

Abstract

From the in vitro and in vivo measurements of the components of the renin-angiotensin system (RAS) in the cerebrospinal fluid (CSF) of rats and dogs, it was concluded that angiotension II (ANG II) is not generated within the CSF in significant amounts, since renin was found to be unmeasurable in CSF under most circumstances. The specific concentrations of angiotensinogen and of converting enzyme (CE) were high. Angiotensin I (ANG I) concentrations were low in CSF, while ANG II levels were comparable to those measured in plasma under control conditions. Neither ANG I nor ANG II penetrated from the blood into the brain ventricles of rats, provided that no unrealistically high doses of ANG II were administered intravenously. This holds true even if high blood pressure increases were induced by intravenous ANG II infusion in deoxycorticosterone acetate (DOCA) and salt-treated rats. However, increased ANG II concentrations were measured in CSF perfusate, when the blood-brain barrier (BBB) was opened by the intracarotid injection of a hyperosmolar urea solution. The brain ventricular perfusion of increasing concentrations of ANG II revealed constant recovery of less than 40%. CSF did not contain angiotensinase activity, but ANG II degradation was high in some periventricular regions. ANG II, the ANG II antagonist saralasin, and the CE inhibitor captopril, respectively, escaped from CSF into circulation when high doses of these substances were applied intraventricularly. We conclude that ANG II in the CSF does not originate from and is not related to plasma ANG II. It is probably not generated within the CSF. ANG II may be synthetized in the brain tissue and be released into the brain ventricles where its rapid degradation occurs in contact with circumventricular structures.