E L S E V I E R Neuroscience Letters 169 (1994) 188 190

NEUROSCIfNC[ LETTERS

Amflutizole, a xanthine oxidase inhibitor, inhibits free radical generation in the ischemic/reperfused rat cerebral cortex

John W. Phillis*, Souvik Sen, Xianghui Cao

Department of Physiology, Wayne State University School of Medicine, 540 E. Canfield, Detroit, MI 48201, USA

Received 10 December 1993; Accepted 17 January 1994

Abstract Free radical generation and release from the cerebral hemispheres of rats subjected to four-vessel occlusion elicited cerebral

ischemia/reperfusion was monitored using a cortical cup technique with the spin-trapping agent ~-(4-pyridyl-1-oxide)-N-tert-butylni- trone (POBN). Electron spin resonance (ESR) was used to detect the presence of free radical adducts of POBN in the cortical superfusates. Thirty min of ischemia plus reperfusion resulted in the release of "OH radical adducts during ischemia and especially in the initial stages of reperfusion. Pretreatment with the xanthine oxidase inhibitor, amflutizole (30 mg/kg) virtually abolished free radical formation and release. These results are consistent with earlier evidence that xanthine oxidase activity contributes to free radical formation in the ischemic/reperfused rat brain.

Key words: Ischemic injury; ESR; Cerebral cortex; Hydroxyl radical; Free radical; Amflutizole

Recent evidence suggests that oxygen free radical-in- duced tissue damage and membrane lipid peroxidation play a key role in ischemia/reperfusion-induced cerebral injury [6]. Oxygen free radicals can damage endothelial cells, disrupting the blood-brain barrier and also directly injure brain cells, causing edema and neuronal loss. Free radical scavengers and trapping agents have been re- ported to reduce anoxic or ischemic neuronal injury [4,11]. Spin trapping and electron spin resonance tech- niques have recently been used to provide evidence of oxygen-derived free radical formation in ischemic/reper- fused rat brains [14,16].

The enzyme xanthine oxidase has been implicated as a potential generator of superoxide radicals in ischemic/ reperfused tissues [3,8]. Superoxide is sequentially con- verted to H 2 0 2 and the highly reactive hydroxyl radical ( 'OH). An involvement of xanthine oxidase in the gener- ation of free radicals in ischemic/reperfused brains would be consistent with the observation that oxypurinol, an inhibitor of xanthine oxidase, attenuates "OH radical production in the ischemic/reperfused rat cerebral cortex [13]. Oxypurinol also effectively reduces ischemia/reper-

*Corresponding author. Fax: (1) (313) 577-5494.

0304-3940194/$7.00 © 1994 Elsevier Science Ireland Ltd. All rights reserved S S D I 0 3 0 4 - 3 9 4 0 ( 9 4 ) 0 0 0 7 2 - I

fusion evoked cerebral injury [7,10,12]. However, the existence of in vitro data suggesting that oxypurinol can act as a scavenger of hydroxyl radicals [5,9] raises the possibility that the free radical quenching action of this compound may not be due to inhibition of xanthine oxidase, and opens to question the role of xanthine oxi- dase in cerebral ischemia/reperfusion-induced free radi- cal formation. To resolve the issue, this study was per- formed to evaluate the effects of a chemically unrelated, potent, inhibitor of xanthine oxidase, amflutizole [15], on free radical release from the ischemic/reperfused rat cer- ebral cortex. The spin trap agent 4-pyridyl-l-oxide-N- tert-butylnitrone (POBN) was used to detect the release of free radicals into cerebral cortical superfusates of rats subjected to cerebral ischemia/reperfusion.

Adult male Sprague-Dawley rats were anesthetized with halothane. After insertion of a tracheal cannula, anesthesia was maintained with a mixture of methoxyflu- rane in air. Body temperature was maintained at 37°C with an abdominal heating pad controlled by a rectal probe. One femoral artery was cannulated to monitor mean arterial blood pressure (MABP) and for the with- drawal of blood samples for arterial pH and blood gas analysis. The right and left common carotid arteries were isolated and separated from their accompanying nerve

J. ~ Phillis et al./Neuroscience Letters 169 (1994) 18~190 189

trunks. A length of dental floss was looped around each artery and exteriorized through the neck incision. Fol- lowing placement of the animal's head in a Narashige head holder, both vertebral arteries were electrocoagu- lated through the alar foramina of the first cervical verte- bra. The dorsal surfaces of both cerebral hemispheres were exposed and cortical cups were placed over each hemisphere, so that the frontal, parietal and occipital cortices were enclosed within the cups. The dorsal sur- face of the head was then covered with 4% agar in saline. Monopolar EEG leads were placed in both cups with the tip of each electrode adjacent to the cortical surface. The cups were filled with 310/zl of artificial CSF (ACSF) [14] containing 100 mM POBN, which had been bubbled with a mixture of 5% CO2 in nitrogen. The cup fluid was maintained at 37°C with the cups shielded from light. POBN (50 mg/kg b.wt., i.p.) was administered 20 min prior to the onset of cerebral ischemia.

Amflutizole (30 mg/kg; dissolved in 0.3 ml 8% sodium bicarbonate) was administered intravascularly 20 min prior to the onset of cerebral ischemia. In both the con- trol (n = 9) and amflutizole (n = 5) groups of animals, ACSF containing 100 mM POBN was sampled at identi- cal intervals independently from the cups covering each cortex. After a 20 rain period for equilibration with the ACSF POBN (during which time the ACSF was replaced three times), a control basal sample was obtained after a 10 rain collection period. Two ischemia superfusate samples were obtained after 15 min collection periods. Three reperfusion samples (10 min) were obtained dur- ing the initial 30 min reperfusion period and two further (10 rain) samples were obtained following successive 20 min intervals. The 310/11 of ACSF sample collected from each cup was immediately frozen in dry ice and kept on dry ice until it was thawed and loaded into an ESR flat quartz cell.

The ESR analysis was conducted with a Varian E-109 spectrometer with the following settings: microwave power 20.0 roW; time constant 0.5 sec; scan rate 12.5G/ min; scan gain 2.0 x 105; microwave frequency 9.0 GHz. The ESR signals were used to calculate ct N, CdH and peak-to-peak (height) amplitude. The mean and stand- ard error of the peak-to-peak amplitudes of the ESR signals obtained at each time interval were calculated. Fenton's reaction, initiated by adding 100/tM H202 and 1 mM ferrous ammonium sulfate to ACSF containing 100 mM POBN, was used as the standard for the analysis of 'OH radical generated ESR spectra. Results obtained after amflutizole administration were compared with those obtained in the control ischemia/reperfusion ani- mals using a Kruskal-Wallis non-parametric test for lev- els of significance.

Physiological variables were monitored in both groups of animals by recordings of mean arterial blood pressure (MABP) and EEG and through the determination of arterial pH and gas tensions in a basal sample obtained

prior to the ischemic period and in a second sample obtained at the end of the reperfusion period (collection 8). No significant differences in pH, p a C O 2 , o r paO2 were noted between these samples in the control or treatment groups, indicating that the animals readily tolerated the period of cerebral ischemia. Occlusion of the carotid arteries consistently resulted in an isoelectric EEG dur- ing the entire ischemic period, with evidence of partial recovery occurring prior to the end of the experiment, signifying that release of the carotid ties had resulted in reperfusion.

Small ESR signals were apparent in some of the ACSF samples (1/9 rats in the control group; 2/5 rats in the amflutizole group) obtained during the initial basal 10 rain collection periods. This signal took the form of a six line spectrum with splitting constant characteristics (aN = 15.4, c ~ = 2.3) comparable to those of the "OH radical adduct of POBN in ACSF generated by Fenton's reaction. The ESR signals increased in amplitude during the two successive ischemic periods, peaking during the first reperfusion period (Fig. 1) and had declined to a low level by the final collection after 90 min of reperfusion (Fig. 2). The ESR signals were generally comparable in ACSF samples from both hemispheres.

2

Fig. 1. ESR spectra obtained from cerebral cortical superfusate samples of a rat ischemia/reperfusion model, at different time points, using POBN as a spin trapping agent. Samples were collected as follows: 1. prior to ischemia; 2. at the onset of reperfusion; 3. after 50 rain of feperfusion.

190 J. W. Phillis et aL /Neuroscience Letters 169 (1994) 188-190

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0 30 60 90 120 150

TIME OF COLLECTION (MIN)

Fig. 2. Variations in ESR signal intensity with time in rat cerebral cortical superfusates prior to, during ischemia (30 rain) followed by 60 rain of reperfusion. The ESR signal intensity (amplitude) was measured peak-to-peak and expressed in cm. The two groups of animals include controls and those treated with amflutizole (30 mg/kg; i.p.) 20 min prior to ischemia. Significance of difference between control and drug-treated animals; *P < 0.05; ***P < 0.001.

Administration of amflutizole 20 min prior to is- chemia resulted in a significant reduction in ischemia! reperfusion evoked "OH radical formation and release (Fig. 2). "OH radical efftux increased minima!ly during the period of ischemia and immediately decreased upon reperfusion. These findings are consistent with the view that free radicals released during ischemia are generated by xanthine oxidase and that the enzyme is blocked by amflutizole. The cessation of blood flow during ischemia would temporarily impede the access of amflutizole to brain xanthine oxidase, allowing for some residual en- zyme activity, which would then be further suppressed during reperfusion with amflutizole-containing blood.

The initial failure of amflutizole to completely inhibit "OH radical formation may indicate either the existence of alternative pathway(s) for the generation of superox- ide radical or the possibility that amflutizole has a lim- ited ability to penetrate the blood-brain barrier prior to ischemia.'Xanthine oxidase in rat brain is preferentially localized in capillary endothelial cells [1] which would be accessible to systemically administered amflutizole. Inhi- bition of endothelial xanthine oxidase, but not of the enzyme in neuronal or glial cells, as a result of a blood- brain barrier, could account for its failure to depress pre-ischemic "OH release, even though ischemia/reperfu- sion evoked "OH release was almost eliminated during the initial 60 min of reperfusion. It may be significant, in this cohtext, that the formation of free radicals by in vitro rat brain microvessels during ischemia/reperfusion has recently been demonstrated [2]. The resultant free radical-evoked destruction of the blood-brain barrier could b e a major contributor to edema formation in ischemic/reperfused brains.

In conclusion, the results of this study demonstrate that the xanthine oxidase inhibitor, amflutizole, effec-

tively prevents ischemia/reperfusion evoked "OH radical formation in the rat brain. Together with the finding of a previous investigation, in which the xanthine oxidase inhibitor, oxypurinol, had a similar action, the present result confirms a role for xanthine oxidase in "OH radical generation in the ischemia/reperfused rat brain.

Supported by PHS Grant NS 26912-05 and a grant from the American Heart Association. We are grateful to the E. Lilly and Co., Indianapolis for the gift of amflutizole.

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