Several phosphate transport processes are present in vascular smooth muscle cells

We have studied inorganic phosphate (Pi) handling in rat aortic vascular smooth muscle cells (VSMC) using P-32-radiotracer assays. Our results have revealed a complex set of mechanisms consisting of 1) well-known PiT1/PiT2-mediated sodium-dependent P-i transport; 2) Slc20-unrelated sodium-dependent P-i transport that is sensitive to the stilbene derivatives 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS); 3) a sodium-independent P-i uptake system that is competitively inhibited by sulfate, bicarbonate, and arsenate and is weakly inhibited by DIDS, SITS, and phosphonoformate; and 4) an exit pathway from the cell that is partially chloride dependent and unrelated to the known anion-exchangers expressed in VSMC. The inhibitions of sodiumin-dependent P-i transport by sulfate and of sodium-dependent transport by SITS were studied in greater detail. The maximal inhibition by sulfate was similar to that of P-i itself, with a very high inhibition constant (212 mM). SITS only partially inhibited sodium-dependent P-i transport, but the K-i was very low (14 mu M). Nevertheless, SITS and DIDS did not inhibit P-i transport in Xenopus laevis oocytes expressing PiT1 or PiT2. Both the sodium-dependent and sodium-independent transport systems were highly dependent on VSMC confluence and on the differentiation state, but they were not modified by incubating VSMC for 7 days with 2 mM P-i under nonprecipitating conditions. This work not only shows that the P-i handling by cells is highly complex but also that the transport systems are shared with other ions such as bicarbonate or sulfate. NEW & NOTEWORTHY In addition to the inorganic phosphate (P-i) transporters PiT1 and PiT2, rat vascular smooth muscle cells show a sodium-dependent P-i transport system that is inhibited by DIDS and SITS. A sodium-independent P-i uptake system of high affinity is also expressed, which is inhibited by sulfate, bicarbonate, and arsenate. The exit of excess P-i is through an exchange with extracellular chloride. Whereas the metabolic effects of the inhibitors, if any, cannot be discarded, kinetic analysis during initial velocity suggests competitive inhibition.