Growth-associated hyperphosphatemia in young recipients accelerates aortic allograft calcification in a rat model.

OBJECTIVES: Cardiovascular allografts in the young have limited durability because of early graft calcification. The objective of this study was to examine the hypothesis that growth-associated hyperphosphatemia in youth accelerates aortic allograft calcification by osteogenic transformation of graft medial smooth muscle cells (SMCs).

METHODS: The descending aortas of donor rats were subcutaneously transplanted into recipients. Syngeneic (Lewis-to-Lewis) transplantations between 3-week-old "young" (Y) rats and between 10-week-old "adult" (A) rats were combined with standard (ST, 0.9% phosphate) and low-phosphate (LP, 0.2%) diets, resulting in Y-ST, Y-LP, and A-ST groups. Allotransplantations (Brown-Norway-to-Lewis) involving these ages and diets were also made. The grafts and sera were retrieved from recipients after 14 days. Cultured rat aortic SMCs were used to analyze the effects of tumor necrosis factor-alpha (TNF-α) and phosphate on SMC calcification.

RESULTS: In vivo, serum phosphate levels were higher in Y-ST (11.5 mg/dL) than those in Y-LP (8.9 mg/dL) and A-ST (8.5 mg/dL). Graft medial calcification appeared severe only in Y-ST. Allotransplants did not affect these outcomes. Graft medial cells showed phenotypic changes (contractile to synthetic) and osteogenic transformation (α-smooth muscle actin to Runx2 and osteocalcin), together with up-regulated proinflammatory TNF-α and sodium-phosphate cotransporter, Pit-1, despite ages and diets. In vitro, TNF-α induced phenotypic changes and osteogenic transformation of SMCs with Pit-1 up-regulation, but SMC calcification occurred only with high phosphate (4.5 mmol/L).

CONCLUSIONS: Growth-associated hyperphosphatemia with inflammatory responses may be essential for accelerating allograft calcification in youth and could be a therapeutic target.