The dose-related effects of nitric oxide synthase inhibition on cerebral blood flow during isoflurane and pentobarbital anesthesia

Background: Recent work in animals suggests that nitric oxide may play a role in the cerebral blood flow (CBF) changes produced by anesthetics, particularly the vasodilation seen with volatile anesthetics. It is not clear, however, whether nitric oxide causes the flow increase or simply plays some constitutive role. To distinguish between these possibilities, we studied the dose-related effects of nitric oxide synthase inhibition in rabbits with varying baseline CBFs, produced by anesthesia with isoflurane, low-dose pentobarbital, or high-dose pentobarbital. Methods: New Zealand White rabbits were anesthetized with 1 MAC isoflurane, low-dose pentobarbital (50-mg/kg load, 7.5-mg · kg^-1 · h^-1 infusion), or high-dose pentobarbital (50-mg/kg load, 20-mg · kg^-1 · h^-1, deep burst- suppression on the electroencephalogram), and prepared for the measurement of CBF using radioactive microspheres. The confluence of sinuses was also exposed to permit sampling of cerebral venous blood and the determination of cerebral metabolic rate for O₂ (CMR(O₂)). Normocapnia and normothermia were maintained throughout. After baseline measurements, animals were sequentially given a cumulative total of 3, 13, and 43 mg/kg intravenous Nω̄-nitro-L- arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase. CBF and CMR(O₂) were recorded ≃10 min after each dose. Results: L-NAME produced a dose-related reduction in CBF in all three anesthetic groups. Statistical examination indicated that the dose response curves were parallel. For example, in isoflurane-anesthetized rabbits, CBF decreased from 77 ± 19 to 47 ± 11 ml · 100 g^-1 · min^-1 after the 43 mg/kg L-NAME, whereas in high-dose pentobarbital animals, CBF decreased from 42 ± 15 to 26 ± 8 ml · 100 g^-1 · min^-1 (all values mean ± SD). Decreases in CMR(O₂) did not quite achieve significance (P = 0.08), and the CBF/CMR(O₂) ratio decreased in all animals, suggesting that the CBF reductions were due primarily to direct vasoconstriction. There were no electroencephalographic changes. In separate groups of isoflurane-anesthetized rabbits given 3 mg/kg L-NAME, treatment with 300 mg/kg L-arginine partially reversed the decreases in CBF. By contrast, the effects of 43 mg/kg L-NAME were not reversible with 430 mg/kg L-arginine. Conclusions: Although L-NAME reduced CBF in all three anesthetic conditions, it did not alter the relative differences among them: CBF in the presence of isoflurane remained much higher than that seen with the barbiturates. This suggests that nitric oxide may not the primary mediator of anesthetic CBF effects, but rather acts to influence background vascular tone in these anesthetized animals.