Blood Cell Versus Vascular Endothelial Cell Contributions to Nitrite Homeostasis and Blood Pressure Regulation

Mice genetically deficient in constitutive nitric oxide synthase (eNOS) are hypertensive compared to normal C57Bl6 (wild type) mice, indicating the importance of constitutively produced nitric oxide (NO•) to blood pressure regulation and vascular homeostasis. The objective of this study was to use molecular methodologies to determine the contribution of eNOS in circulating blood cells to the intravascular pool of nitrite, a metabolite and storage form of nitric oxide (NO•) with powerful vasorelaxant activity, and to the regulation of blood pressure under physiological conditions. We used bone marrow transplant to create chimeric mice genetically deficient in eNOS in either circulating blood cells (−/+) or vascular endothelium (+/−), confirmed by flow cytometry, western blot, RT-PCR and immunohistochemistry. Nitrite concentrations in whole blood and plasma of chimeras were quantified using ozone-based reductive chemiluminescence. Mean arterial and diastolic blood pressures of chimeras were assessed in the absence/presence of NOS stimulation with oral L-Arginine or NOS inhibition with oral L-Name. A highly significant inverse correlation between plasma nitrite concentrations and blood pressures was noted across all groups of chimeric mice. Importantly, in agreement with higher blood pressures in −/+ chimeras compared to eNOS positive controls (+/+ chimeras globally competent for eNOS), significantly reduced whole blood and plasma nitrite concentrations were also measured. Blood pressure responses to NOS inhibition or stimulation were intact in all chimera groups except eNOS negative controls (−/− chimeras globally deficient for eNOS) and, importantly, blunted in −/+ chimeras compared to eNOS positive controls. These findings indicate a functional blood cell eNOS that is a major contributor to circulating nitrite concentrations and plays a critical role in vascular homeostasis.