Phosphate as a cardiovascular risk factor: effects on vascular and endothelial function.

Background: Hyperphosphataemia is a risk factor for accelerated cardiovascular disease in chronic kidney disease. The mechanism is poorly understood; it is unclear whether phosphate has direct effects or effects mediated via calcification or FGF23. We investigated direct effects of phosphate on endothelial function using myography to study rat and human blood vessels. In addition we assessed the effects of phosphate loading on endothelial function in a clinical study.
Methods: Resistance vessels from patients with (n=12) and without (n=13) chronic kidney disease were incubated in normal or high phosphate. Vasoconstrictor and vasorelaxation responses were measured. Concentration-response curves were constructed and comparisons made. Identical experiments were performed in rat mesenteric vessels with and without phosphodiesterase type 5 inhibitor. A cross-over study was done in 19 healthy volunteers receiving phosphate supplements or binders and endothelial function measured by flow mediated dilatation (FMD). Primary outcome was percent change in FMD from baseline.
Findings: Nine to 13 vessels were used in each group. Endothelium-dependent vasodilatation was impaired in high compared with normal phosphate in rat (mean maximum vasodilatation 64% [SE 9] vs 95 [1], p<0·001) and human vessels with (25·3 [11·1] vs 75·7 [13·6], p<0·001) and without chronic kidney disease (42·9 [12] vs 79·4 [8·2], p=0·003). In rat vessels, these effects were reversed by a phosphodiesterase type 5 inhibitor. In vivo in volunteers, endothelial function was reduced by phosphate loading (median maximum vasodilatation 3·38% [IQR 2·57-5·26] vs 8·4 [6·2-11·6], p<0·001); this effect was independent of serum phosphate concentration but associated with urinary phosphate excretion and serum FGF23 concentrations.
Interpretation: Prolonged exposure to phosphate is associated with endothelial dysfunction, a direct effect of phosphate, which might contribute to cardiovascular risk in chronic kidney disease. In a high phosphate environment, endothelial and vascular dysfunction is evident in blood vessels and in man exposed to prolonged oral phosphate loading. These effects might be mediated by disruption of the NO pathway.
Funding: British Heart Foundation, Darlinda's Charity for Renal Research.
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