- Email: [email protected]
Mechanism of Pi accumulation in matrix vesicles and its potential role in the calcification process
306
Mechanism of Pi accumulation in matrix vesicles and its potential role in the calcification process C. MONTESSUIT, J. CAVERZASIO and J.P. BONJOUR Division ofPathophysiology, Department of Medicine, the University Cantonal Hospital, 1211 Geneva 4. Switzerland
ABSTRACT The mechanism by which Ca*+ and Pi accumulate into matrix vesicles (MV) remains ly unknown. Recent observation in our laboratory indicated that osteoblastic cells were endowed with a Na-dependent Pi transport system. We therefore examined the possibility that MV derived from the plasma membrane of osteogenic cells would be equipped with a similar Pi transport system. The uptake of QPi into MV isolated from chicken growth plate cartilage was selectively stimulated by extravesicular sodium. The Na-dependent component of Pi uptake was saturable with respect to the extravesicular concentrations of sodium and Pi. The affinity constants for sodium (1062 11 mM) and Pi (0.68 f 0.16 mM) were in the physiological range, indicating that the Na-Pi cotransporter is functionally active under physiological conditions. The Na-Pi cotransport was inhibited by As04 (Ki = 7.21 f0.64 mM) and phosphonoformic acid (Ki-0.188 &0.035), two competitive inhibitors of epithelial Na-dependent Pi transport systems. L-tetramisole (5 mM), a specific inhibitor of the alkaline phosphatase, was inactive on the Pi transport in MV. The Na-Pi cotransport system was expressed in MV isolated from mildly rachitic growth plates. In these MV, calcification as assessed by 45Cauptake was dependent on extravesicular sodium. In MV isolated from normal growth plates, which already contain mineral deposits, the activity of the Na-Pi cotransport was markedly reduced and extravesicular sodium was not required for the calcification process. In conclusion these observations strongly suggest the presence of a Na-Pi cotransport system in MV, which could play a key role in the early steps of CaP accumulation by MV.
Mechanism of Pi accumulation in matrix vesicles and its potential role in the calcification process C. MONTESSUIT, J. CAVERZASIO and J.P. BONJOUR Division ofPathophysiology, Department of Medicine, the University Cantonal Hospital, 1211 Geneva 4. Switzerland
ABSTRACT The mechanism by which Ca*+ and Pi accumulate into matrix vesicles (MV) remains ly unknown. Recent observation in our laboratory indicated that osteoblastic cells were endowed with a Na-dependent Pi transport system. We therefore examined the possibility that MV derived from the plasma membrane of osteogenic cells would be equipped with a similar Pi transport system. The uptake of QPi into MV isolated from chicken growth plate cartilage was selectively stimulated by extravesicular sodium. The Na-dependent component of Pi uptake was saturable with respect to the extravesicular concentrations of sodium and Pi. The affinity constants for sodium (1062 11 mM) and Pi (0.68 f 0.16 mM) were in the physiological range, indicating that the Na-Pi cotransporter is functionally active under physiological conditions. The Na-Pi cotransport was inhibited by As04 (Ki = 7.21 f0.64 mM) and phosphonoformic acid (Ki-0.188 &0.035), two competitive inhibitors of epithelial Na-dependent Pi transport systems. L-tetramisole (5 mM), a specific inhibitor of the alkaline phosphatase, was inactive on the Pi transport in MV. The Na-Pi cotransport system was expressed in MV isolated from mildly rachitic growth plates. In these MV, calcification as assessed by 45Cauptake was dependent on extravesicular sodium. In MV isolated from normal growth plates, which already contain mineral deposits, the activity of the Na-Pi cotransport was markedly reduced and extravesicular sodium was not required for the calcification process. In conclusion these observations strongly suggest the presence of a Na-Pi cotransport system in MV, which could play a key role in the early steps of CaP accumulation by MV.