Halothane-induced dilatation of intraparenchymal arterioles in rat brain slices: A comparison to sodium nitroprusside

Background: Halothane is a potent dilator of cerebral arteries. The predominant site of cerebrovascular resistance is thought to be intracerebral arterioles, and the effects of halothane on these vessels mere not previously examined. This study compared the effects of halothane with those of the vasodilator and nitric oxide donor, sodium nitroprusside, on intraparenchymal microvessel responsiveness in a brain slice preparation. Methods: Anesthetized Sprague-Dawley rats underwent thoracotomy and intracardiac perfusion and then were decapitated. Hippocampal brain slices were prepared and placed in a perfusion/recording chamber and superfused with artificial cerebrospinal fluid. An arteriole was located within the brain parenchyma and its diameter was monitored with videomicroscopy before, during, and after various concentrations of halothane or sodium nitroprusside were equilibrated in the perfusate. All vessels were preconstricted with prostaglandin F-2 alpha before halothane or sodium nitroprusside treatment. An observer blinded to treatment analyzed vessel diameter changes with a computerized videomicrometer. Results: Baseline microvessel diameter was 18 +/- 2 mu m in the halothane group (n = 14) and 15 +/- 1 mu m in the sodium nitroprusside group (n = 15). Prostaglandin F-2 alpha (0.5 mu M) preconstricted vessels by approximately 15% from resting diameter in both groups. Halothane significantly and dose dependently dilated intracerebral microvessels by 54% +/- 6%, 74% +/- 8%, 108% +/- 13%, and 132% +/- 7% (normalized to the preconstricted diameter) at 0.5%, 1.0%, 1.5%, and 2.5% halothane, respectively. This dilatation corresponds to a decrease in a calculated index of cerebrovascular resistance index of up to 117% +/- 2% at 2.5% halothane. Sodium nitroprusside, in concentrations ranging from 10(-8) to 10(-3) hr, also dose dependently dilated these intraparenchymal vessels by 129% +/- 7% at the highest concentration. These alterations in microvessel diameter corresponded to a decrease in the cerebrovascular resistance index of up to 116% +/- 4% for the largest dose. Conclusions: Halothane produces dose-dependent vasodilatation of intraparenchymal cerebral microvessels, thus predicting marked decreases in cerebrovascular resistance in this in vitro brain slice preparation. The effects of halothane on these cerebral microvessels are similar to those of the potent vasodilator sodium nitroprusside. These findings suggest that direct effects of halothane on cerebral microvessel diameter contribute substantially to alterations in cerebrovascular resistance and now produced by this agent.