Summary
Ascorbic acid is essential for the formation of bone by osteoblasts, but the mechanism by which osteoblasts transport ascorbate has not been investigated previously. We examined the uptake ofl-[14C]ascorbate by a rat osteoblast-like cell line (ROS 17/2.8) and by primary cultures of rat calvaria cells. In both systems, cells accumulatedl-[14C]ascorbate during incubations of 1–30 min at 37°C. Unlike propionic acid, which diffuses across membranes in protonated form, ascorbic acid did not markedly alter cytosolic pH. Initial ascorbate uptake rate saturated with increasing substrate concentration, reflecting a high-affinity interaction that could be described by Michaelis-Menten kinetics (apparentK m =30±2 μm andV max=1460±140 nmol ascorbate/g protein/min in ROS 17/2.8 cells incubated with 138mm extracellular Na+). Consistent with a stereoselective carrier-mediated mechanism, unlabeledl-ascorbate was a more potent inhibitor (IC50=30±5 μm) ofl-[14C]ascorbate transport than wasd-isoascorbate (IC50=380±55 μm). Uptake was dependent on both temperature and Na+, since it was inhibited by cooling to 4°C and by substitution of K+, Li+ or N-methyl-d-glucamine for extracellular Na+. Decreasing the external Na+ concentration lowered both the affinity of the transporter for ascorbate and the apparent maximum velocity of transport. We conclude that osteoblasts possess a stereoselective, high-affinity, Na+-dependent transport system for ascorbate. This system may play a role in the regulation of bone formation.
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Wilson, J.X., Dixon, S.J. High-affinity sodium-dependent uptake of ascorbic acid by rat osteoblasts. J. Membrain Biol. 111, 83–91 (1989). https://doi.org/10.1007/BF01869211
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DOI: https://doi.org/10.1007/BF01869211