Effects of treadmill running in a rat model of chronic kidney disease.

Chronic kidney disease (CKD) progression results in musculoskeletal dysfunction that is associated with a higher likelihood of hospitalization and is predictive of hospitalizations and mortality. Despite this, there is a lack of effective interventions to treat the musculoskeletal dysfunction. We studied treadmill running as an intervention to improve musculoskeletal health in a translational rat model that has slowly progressive CKD. CKD rats were subjected to treadmill exercise or no treadmill exercise for 10 weeks (n = 8 each group). Animals ran for 60 min, 5 times per week starting at a speed of 8 m/min and ending at 18 m/min (1 m/min increase/week). Treadmill training had no effect on muscle strength (assessed as maximally stimulated torque), half-relaxation time (time from peak torque to 50%) or muscle cross-sectional area. Overall, there were no biochemical improvements related to CKD progression. Skeletal muscle catabolism was higher than non-exercised animals without a concomitant change in muscle synthesis markers or regeneration transcription factors. These results suggest that aerobic exercise, achieved via treadmill running was not protective in CKD animals and actually produced potentially harmful effects (increased catabolism). Given the high prevalence and dramatic musculoskeletal mobility impairment in patients with CKD, there is a clear need to understand how to effectively prescribe exercise in order to benefit the musculoskeletal system.

The mechanisms of uremic serum-induced expression of bone matrix proteins in bovine vascular smooth muscle cells.

We have previously found that uremic human serum upregulates RUNX2 in vascular smooth muscle cells (VSMCs), and that RUNX2 is upregulated in areas of vascular calcification in vivo. To confirm the role of RUNX2, we transiently transfected a dominant-negative RUNX2 (DeltaRUNX2) construct in bovine vascular smooth muscle cells (BVSMCs). Blocking RUNX2 transcriptional activity significantly decreased uremic serum induced alkaline phosphatase (ALP) activity (268+/-34 vs 188+/-9.5 U/g protein, P<0.05) and osteocalcin expression (172+/-17 vs 125+/-9 ODU, P<0.05). To determine the mechanism by which uremic serum upregulates RUNX2, we examined cell signaling pathways. BVSMCs were incubated in the presence or absence of inhibitors and RUNX2 expression and ALP activity were determined. The results demonstrate that the cyclic AMP (cAMP)/protein kinase A (PKA), but not protein kinase C, signaling pathway is involved in uremic serum-induced RUNX2 expression and ALP activity in BVSMCs. To examine potential uremic 'toxins', we measured bone morphogenetic protein (BMP)-2 concentration and found that uremic serum contained increased BMP-2 (uremic serum=169+/-33 pg/ml, normal serum=117+/-15 pg/ml, P<0.05). The incubation of BVSMCs with noggin, an inhibitor of BMP, decreased RUNX2 expression. In addition, BMP-2 secretion progressively increased during calcification and uremic serum enhanced its secretion compared to normal serum. In conclusion, this study demonstrates that RUNX2 transcriptional activity is critical in uremic serum-induced bone matrix protein expression in BVSMCs and that the cAMP/PKA pathway is involved. BMP-2 is also increased in uremic serum and can upregulate RUNX2 and calcification in vitro in VSMCs.

Calciphylaxis is associated with hyperphosphatemia and increased osteopontin expression by vascular smooth muscle cells.

Calciphylaxis or calcific uremic arteriolopathy (CUA) is a fatal disease in dialysis patients due to calcification of cutaneous blood vessels. The pathogenesis has been attributed to elevated parathyroid hormone (PTH). However, recent studies evaluating vascular calcification in nondialysis patients have found that the smooth muscle cells play an active role, including production of the bone matrix protein osteopontin. To examine the involvement of various clinical parameters and smooth muscle cells of CUA, we performed a case-control analysis comparing 10 CUA patients with our current dialysis patients. Available histologic sections were immunostained for osteopontin, markers of smooth muscle cells, endothelial cells, and macrophages. Compared with our current dialysis population, patients with CUA were more likely to be obese, white, and female (P < 0.02). Comparison of laboratory values found CUA patients with lower serum albumin, greater serum phosphorus, and greater calcium X phosphorus product (P < 0.01). In contrast, there was no difference in the concentration of PTH or calcium between the 2 groups. Immunostaining of calcified blood vessels showed that all calcified vessels stained positive for osteopontin, whereas all the noncalcifed vessels showed no osteopontin localization. Staining for smooth muscle alpha-actin decreased in the medial layer with calcification, with cells appearing to be sloughed off, leading to near occlusion of the vessel lumen. Our case-control study demonstrates that hyperphosphatemia and an elevated calcium X phosphorus product is associated with CUA. Histologic examination suggests that the calcification is associated with increased expression of osteopontin by smooth muscle cells.