Neuroprotective FK506 does not alter in vivo nitric oxide production during ischemia and early reperfusion in rats

Background and Purpose-Previous studies have demonstrated that the immunosu ppressant FK506 provides neuroprotection in experimental brain injury and s uggest that this action may be mediated by suppression of neuronal nitric o xide synthase activation that occurs after ischemic depolarization. We soug ht to determine whether FK506 reduces histological injury after middle cere bral artery occlusion (MCAO) in the rat and whether the neuroprotective eff ect is mediated via suppression of in vivo nitric oxide (NO) production dur ing ischemia or early reperfusion, Methods-Under controlled conditions of normoxia, normocarbia, and normother mia, halothane-anesthetized male Wistar rats were subjected to 2 hours of M CAO by the intraluminal occlusion technique in a blinded, randomized experi mental trial. Ipsilateral parietal cortical laser-Doppler flowmetry was mon itored throughout ischemia.,Animals were randomly assigned to 4 pretreatmen t groups: intravenous FK506 0.3 mg/kg or 1.0 mg/kg, vehicle (cremaphor), or an equivalent volume of saline administered 30 minutes before MCAO. Infarc tion volume was assessed by a triphenyltetrazolium chloride staining at 22 hours of reperfusion. In separate experiments, microdialysis probes were pl aced bilaterally into the striatum. Rats were perfused with artificial cere brospinal fluid containing 3 mu mol/L [C-14]-L-arginine for 3 hours and the n subjected to 2 hours of right MCAO. Intravenous 0.3 mg/kg FK506 or cremap hor was given 30 minutes before right MCAO, Right-left differences between [C-14]-L-citrulline in the effluent were assumed to reflect differences in NO production. Results-All values are mean+/-SE. FK506 at 0.3 mg/kg reduced infarction vol ume in cortex: 40+/-12 mm(3) compared with saline (109+/-15 mm(3)) and crem aphor Vehicle (148+/-23) (P<0.05). Striatal infarction was also reduced by low-dose FK506: 16+/-4 mm(3) versus 36+/-4 mm(3) and 34+/-4 mm(3) in saline and vehicle groups, respectively (P<0.05). High-dose treatment reduced inf arction volume in cortex (61+/-14 mm(3), P<0.05 from saline and vehicle gro ups) and in striatum (22+/-5 mm(3), P<0.05 from saline and vehicle groups). [C-14]-L-citrulline recovery via microdialysis was markedly enhanced in is chemic compared with nonischemic striatum. However, ischemia-evoked [C-14]- L-citrulline recovery was not different in FK506-treated rats compared with vehicle-treated animals. Conclusions-These data demonstrate that FK506 provides robust neuroprotecti on against transient focal cerebral ischemia in the rat. The mechanism of p rotection in vivo is not through attenuation of ischemia-evoked NO producti on during MCAO and early reperfusion.