Treatment with pyrophosphate inhibits uremic vascular calcification

Abstract

Pyrophosphate, which may be deficient in advanced renal failure, is a potent inhibitor of vascular calcification. To explore its use as a potential therapeutic, we injected exogenous pyrophosphate subcutaneously or intraperitoneally in normal rats and found that their plasma pyrophosphate concentrations peaked within 15 min. There was a single exponential decay with a half-life of 33 min. The kinetics were indistinguishable between the two routes of administration or in anephric rats. The effect of daily intraperitoneal pyrophosphate injections on uremic vascular calcification was then tested in rats fed a high-phosphate diet containing adenine for 28 days to induce uremia. Although the incidence of aortic calcification varied and was not altered by pyrophosphate, the calcium content of calcified aortas was significantly reduced by 70%. Studies were repeated in uremic rats given calcitriol to produce more consistent aortic calcification and treated with sodium pyrophosphate delivered intraperitoneally in a larger volume of glucose-containing solution to prolong plasma pyrophosphate levels. This maneuver significantly reduced both the incidence and amount of calcification. Quantitative histomorphometry of bone samples after double-labeling with calcein indicated that there was no effect of pyrophosphate on the rates of bone formation or mineralization. Thus, exogenous pyrophosphate can inhibit uremic vascular calcification without producing adverse effects on bone.

Figure 1. Change in plasma pyrophosphate (PPi) concentration after injection of PPi containing tracer [³²P]PPi

(a) Single subcutaneous injection of 80 μmol/kg sodium PPi in three different rats. (b) Single intraperitoneal injections of 0.026 μmol/kg (solid symbols) or 2.6 μmol/kg (open symbols) sodium PPi in a volume of 5 ml/kg. The increment in plasma PPi concentration was calculated from the specific activity of the injected ³²PPi.

Figure 2. Effect of pyrophosphate (PPi) on aortic calcium content in uremic rats

Rats were fed a diet consisting of 2.5% protein, 1.06% phosphorus, 0.92% calcium, and 0.75% adenine for 4 weeks and received sodium PPi in Hank’s buffered saline or saline alone by daily intraperitoneal injection of 5 ml/kg. Each point represents a single animal.

Figure 3. Plasma pyrophosphate (PPi) concentration after single intraperitoneal injections of 160 μmol/kg sodium PPi in a volume of 40 ml/kg

Results are means±s.e.m. of four animals. *P<0.01, **P<0.001, ^#P<0.05 vs baseline.

Figure 4. Effect of peritoneal infusion of pyrophosphate (PPi) on uremic vascular calcification

Rats were fed a diet consisting of 2.5% protein, 1.06% phosphorus, 0.92% calcium, and 0.75% adenine for 4 weeks and given calcitriol 40 ng/kg by subcutaneous injection 3 times per week. PPi was infused daily through a peritoneal catheter in a volume of 40 ml/kg. Each point represents a single animal. *P<0.005 vs control.

Figure 5. Histology of the diaphragm and parietal peritoneum in uremic rats given sodium pyrophosphate (PPi) by intraperitoneal injection

(a) Vehicle; (b) 0.8 mmol/l PPi; (c) and 4 mmol/l PPi. Hematoxylin and eosin staining.