Mannitol-induced hyperosmolality and cerebrospinal fluid vasopressin in anesthetized cats

Arginine-vasopressin (AVP) has been found to influence brain water. Since AVP is released by hyperosmolality into plasma we determined the role of AVP in controlling cerebrospinal fluid (CSF) pressure. Adult cats were anesthetized with pentobarbital and samples of plasma and cisternal CSF were collected 1 or 2 h before i.v. infusion of 2 g/kg of mannitol and for 2 h afterwards. We found a significant increase in plasma osmolality from 320.0 +/- 1.6 to 331.6 +/- -3.4 mOsm/l (mean +/- S.E.M.), while CSF osmolality was unchanged. Prior to mannitol infusion, AVP was elevated to 105 +/- 19 pg/ml in plasma and to 136 +/- 19 pg/ml (mean +/- S.E.M.) in CSF. After infusion of mannitol AVP levels were unchanged in either plasma or CSF. The reduction of CSF pressure by mannitol is independent of AVP in the anesthetized cat.