ISOFLURANE-INDUCED CEREBRAL HYPEREMIA IN NEURONAL NITRIC-OXIDE SYNTHASE GENE DEFICIENT MICE

Background: Nitric oxide (NO) has been reported to play an important r ole in isoflurane-induced cerebral hyperemia in vivo. In the brain, th ere are two constitutive isoforms of NO synthase (NOS), endothelial NO S (eNOS), and neuronal NOS (nNOS). Recently, the mutant mouse deficien t in nNOS gene expression (nNOS knockout) has been developed The prese nt study was designed to examine the role of the two constitutive NOS isoforms in cerebral blood flow (CBF) response to isoflurane using thi s nNOS knockout mouse. Methods: Regional CBF (rCBF) in the cerebral co rtex was measured with laser-Doppler flowmetry in wild-type mice (129/ SV or C57BL/6) and nNOS knockout mice during stepwise increases in the inspired concentration of isoflurane from 0.6 vol% to 1.2, 1.8, and 2 .4 vol%. Subsequently, a NOS inhibitor, N-omega-nitro-L-arginine (L-NN A), was administered intravenously (20 mg/kg), and 45 min later, the r CBF response to isoflurane was tested again In separate groups of wild -type mice and the knockout mice, the inactive enantiomer, N-omega-nit ro-D-arginine (D-NNA) was administered intravenously in place of L-NNA . Brain NOS activity was measured with radio-labeled L-arginine to L-c itrulline conversion after treatment with L-NNA and D-NNA. Results: Is oflurane produced dose-dependent increases in rCBF by 25 +/- 3%, 74 +/ - 10%, and 108 +/- 14% (SEM) in 129/SV mice and by 32 +/- 2%, 71 +/- 3 %, and 96 +/- 7% in C57BL/6 mice at 1.2, 1.8, and 2.4 vol%, respective ly. These increases were attenuated at every anesthetic concentration by L-NNA but not by D-NNA. Brain NOS activity was decreased by 92 +/- 2% with L-NNA compared with D-NNA. In nNOS knockout mice, isoflurane i ncreased rCBF by 67 +/- 8%, 88 +/- 12%, and 112 +/- 18% at 1.2, 1.8, a nd 2.4 vol%, respectively. The increase in rCBF at 1.2 vol% was signif icantly greater in the nNOS knockout mice than that in the wild-type m ice. Administration of L-NNA in the knockout mice attenuated the rCBF response to isoflurane at 1.2 and 1.8 vol% but had no effect on the re sponse at 2.4 vol%. Conclusions: In nNOS knockout mice, the cerebral h yperemic response to isoflurane is preserved by compensatory mechanism (s) that is NO-independent at 2.4 vol%, although it may involve eNOS a t 1.2 and 1.8 vol%. It is suggested that in wild-type mice, eNOS and n NOS contribute to isoflurane-induced increase in rCBF. At lower concen trations (1.2 and 1.8 vol%), eNOS may be involved, whereas at 2.4 vol% , nNOS may be involved.