22-Oxacalcitriol prevents progression of endothelial dysfunction through antioxidative effects in rats with type 2 diabetes and early-stage nephropathy

Vitamin D deficiency is associated with endothelial dysfunction in type 2 diabetes patients, but the effectiveness of vitamin D supplementation remains controversial. We assessed whether 22-oxacalcitriol (OCT) could prevent endothelial dysfunction in type 2 diabetes mellitus (DM) rats. DM rats with early-stage nephropathy were treated for 10 weeks with OCT (0.2 g/kg) three times per week or by an implanted insulin pellet. Endothelial dysfunction was assessed by femoral flow-mediated dilation (FMD). Insulin significantly improved FMD as blood glucose levels normalized. OCT also improved FMD without hypercalcemia or hyperphosphatemia and without affecting blood glucose or blood pressure. In femoral arteries, OCT significantly suppressed the elevated expression of p22(phox), a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit, and improved the endothelial nitric oxide synthase (eNOS) dimer-to-monomer ratio. In cultured endothelial cells, OCT significantly inhibited high-glucose (HG)-induced reactive oxygen species (ROS) production. Simultaneously, OCT significantly suppressed HG-induced p22(phox) expression and improved eNOS uncoupling as was observed in the in vivo study. In DM rats, OCT improved endothelial dysfunction, at least in part, by suppressing ROS generation through p22(phox) expression, which might contribute to improving eNOS uncoupling.