Cerebral Endothelial Nitric Oxide Synthase Expression is Reduced After Very Low Flow Bypass

Background In previous studies we have shown that delayed capillary reperfusion after low flow bypass predicts neurologic injury. In this acute study, we hypothesized that low flow reduces endothelial nitric oxide synthase (eNOS) expression, which may lead to more profound inflammatory response and delayed capillary perfusion. Methods Twelve piglets (13.2 ± 0.7 kg) had a cranial window placed over the parietal cerebral cortex for direct examination of the microcirculation using intravital fluorescence microscopy. Animals were cooled to 15°C or 34°C on cardiopulmonary bypass (pH stat, hematocrit 30%, pump flow 100 mL/kg/minute) followed by 2 hours of low flow (50 mL/kg/minute) or very low flow (10 mL/kg/minute). Rhodamine staining was used to observe adherent and rolling leukocytes in postcapillary venules. The eNOS protein expression was determined by Western immunoblotting. Results High temperature and low flow rate correlated with significantly reduced eNOS expression (p< 0.01). Univariate comparisons based on Student t tests indicated that eNOS protein levels were lower at 34°C than at 15°C (0.7 ± 0.6 vs 1.7 ± 0.5, p< 0.01) and at 10 mL/kg per minute compared with 50 mL/kg per minute (0.8 ± 0.7 vs 1.6 ± 0.5, p = 0.03). Moreover, two-way analysis of variance revealed that temperature (F = 21.6, p< 0.001) and flow rate (F = 13.8, p = 0.005) were independent multivariate predictors of eNOS expression. During low flow bypass, eNOS was inversely correlated with numbers of adherent (p = 0.002) and rolling (p = 0.006) leukocytes, following an exponential decay curve closely. Conclusions eNOS expression is reduced after very low flow bypass, particularly at a higher bypass temperature. This is associated with delayed capillary reperfusion. Reduced eNOS is also associated with increased white cell activation which may lead to greater neurologic injury.