Vasopressin-induced protein kinase C-dependent superoxide generation contributes to atp-sensitive potassium channel but not calcium-sensitive potassium channel function impairment after brain injury.

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Abstract
Background and Purpose —Pial artery dilation in response to activators of the ATP-sensitive K + (K ATP ) and calcium-sensitive K + (K Ca ) channels is impaired after fluid percussion brain injury (FPI). Vasopressin, when coadministered with the K ATP and K Ca channel agonists cromakalim and NS1619 in a concentration approximating that observed in cerebrospinal fluid (CSF) after FPI, blunted K ATP and K Ca channel–mediated vasodilation. Vasopressin also contributes to impaired K ATP and K Ca channel vasodilation after FPI. In addition, protein kinase C (PKC) activation generates superoxide anion (O 2 − ), which in turn contributes to K ATP channel impairment after FPI. We tested whether vasopressin generates O 2 − in a protein kinase C (PKC)-dependent manner, which could link vasopressin release to impaired K ATP and K Ca channel–induced pial artery dilation after FPI. Methods —Injury of moderate severity (1.9 to 2.1 atm) was produced with the lateral FPI technique in anesthetized newborn pigs equipped with a closed cranial window. Superoxide dismutase–inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O 2 − generation. Results —Under sham injury conditions, topical vasopressin (40 pg/mL, the concentration present in CSF after FPI) increased superoxide dismutase–inhibitable NBT reduction from 1±1 to 23±4 pmol/mm 2 . Chelerythrine (10 −7 mol/L, a PKC inhibitor) blunted such NBT reduction (1±1 to 9±2 pmol/mm 2 ), whereas the vasopressin antagonist l-(β-mercapto-β,β-cyclopentamethylene propionic acid)2-( o -methyl)-Tyr-arginine vasopressin (MEAVP) blocked NBT reduction. Chelerythrine and MEAVP also blunted the NBT reduction observed after FPI (1±1 to 15±1, 1±1 to 4±1, and 1±1 to 5±1 pmol/mm 2 for sham-, chelerythrine-, and MEAVP-treated animals, respectively). Under sham injury conditions, vasopressin (40 pg/mL) coadministered with cromakalim or NS1619 blunted dilation in response to these K + channel agonists, whereas chelerythrine partially restored such impaired vasodilation for cromakalim but not NS1619. Cromakalim- and NS1619-induced pial artery dilation also was blunted after FPI. MEAVP partially protected dilation to both K + channel agonists after FPI, whereas chelerythrine did so for only cromakalim responses (for cromakalim at 10 −8 and 10 −6 mol/L, 13±1% and 23±1%, 2±1% and 5±1%, 9±1% and 15±2%, and 9±1% and 16±2% for sham-, FPI-, FPI-MEAVP–, and FPI-chelerythrine–pretreated animals, respectively). Conclusions —These data show that vasopressin, in concentrations present in CSF after FPI, increased O 2 − production in a PKC-dependent manner and contributes to such production after FPI. These data show that vasopressin contributes to K ATP but not K Ca channel function impairment in a PKC-dependent manner after FPI and suggest that vasopressin contributes to K Ca channel function impairment after FPI via a mechanism independent of PKC activation.