Losbanos, Louis

Skeletal muscle insulin resistance contributes to the pathogenesis of type 2 diabetes mellitus. Epidemiological studies have demonstrated that vitamin D deficiency is associated with the presence of type 2 diabetes in humans. To probe the role of the vitamin D receptor (VDR), an intracellular and nuclear protein that mediates the bioactivity of the active vitamin D ligand, 1,25‐dihydroxyvitamin D, in skeletal muscle mediated glucose disposal, we deleted the skeletal muscle VDR in adult mice. We assessed glucose disposal, insulin concentrations and sensitivity, and skeletal muscle gene expression in Vdr knockout and wild‐type Vdr mice. Following the induction of a skeletal muscle‐specific Cre recombinase with tamoxifen in adult male Vdr fl/fl mice, efficient deletion of the Vdr was demonstrated. In skeletal muscle‐specific Vdr knockout mice, basal glucose concentrations were similar to those in wild‐type Vdr mice. In Vdr knockout mice, blood glucose concentrations decreased more rapidly following the administration of intraperitoneal glucose than in control mice expressing wild‐type Vdr (Figure 1A and Figure 1B). The AUC for glucose concentrations in Vdr fl/fl .actin iCre mice treated with tamoxifen was reduced by 20% (p=0.015) compared to the AUC of blood glucose concentrations prior to tamoxifen treatment. Pre‐and post‐treatment AUC’s for blood glucose in Vdr fl/fl .actin iCre mice treated with vehicle and in Vdr fl/fl mice treated with tamoxifen were unchanged (p=0.97,0.23). Plasma insulin concentrations were identical in knockout and wild‐type Vdr mice following a bolus of glucose, demonstrating that deletion of the Vdr in skeletal muscle did not influence the release of insulin in response to glucose (one‐way ANOVA, p=0.45). Insulin sensitivity was similar in Vdr knockout and wild‐type Vdr mice. Gene expression analysis of skeletal muscle mRNA derived from Vdr knockout mice and wild‐type mice demonstrated an increase in the expression of genes associated with oxidative phosphorylation, fatty acid oxidation and adipogenesis pathways that could potentially alter glucose utilization. We conclude that selective deletion of the Vdr in the skeletal muscle of adult mice results in more efficient glucose disposal than in wild‐type Vdr mice; basal and glucose‐stimulated insulin serum concentrations are identical in knock‐out and wild type VDR mice with no changes in insulin sensitivity. The data suggest that modulation of the vitamin‐D endocrine system is unlikely to improve glucose utilization.

Abstract Background The precise concentrations of full-length parathyroid hormone (PTH1-84) and the identity and concentrations of PTH fragments in patients with various stages of chronic renal failure are unknown. Methods We developed a liquid chromatography-high resolution mass spectrometry (LC-HRMS) method to characterize and quantify PTH1-84 and PTH fragments in serum of 221 patients with progressive renal dysfunction. Following capture by matrix-bound amino-terminal or carboxyl-terminal region-specific antibodies and elution from matrix, PTH1-84 and PTH fragments were identified and quantitated using LC-HRMS. PTH was simultaneously measured using an intact PTH (iPTH) immunoassay. Results Full-length PTH1-84 and 8 PTH fragments (PTH28-84, 34-77, 34-84, 37-77, 37-84, 38-77, 38-84, and 45-84) were unequivocally identified and were shown to increase significantly when an eGFR declined to ≤17-23 mL/min/1.73m2. Serum concentrations of PTH1-84 were similar when measured by LC-HRMS following capture by amino-terminal or carboxyl-terminal immunocapture methods. In patients with an eGFR of <30 mL/min/1.73 m2, serum PTH concentrations measured using LC-HRMS were significantly lower than PTH measured using an iPTH immunoassay. PTH7-84 and oxidized forms of PTH1-84 were below the limit of detection (30 and 50 pg/mL, respectively). Conclusions LC-HRMS identifies circulating PTH1-84, carboxyl-terminal PTH fragments, and mid-region PTH fragments, in patients with progressive renal failure. Serum PTH1-84 and its fragments markedly rise when an eGFR decreases to ≤17-23 mL/min/1.73 m2. PTH concentrations measured using LC-HRMS tend to be lower than those measured using an iPTH immunoassay, particularly in severe chronic renal failure. Our data do not support the existence of circulating PTH7-84 and oxidized PTH1-84.