Dehydroascorhic acid uptake by coronary artery smooth muscle: Effect of intracellular acidification

Dehydroascorbic acid (DHAA) enters cells via Na⁺-independent glucose transporters (GLUT) and is converted to ascorbate. However, we found that Na⁺ removal inhibited [¹⁴C]DHAA uptake by smooth-muscle cells cultured from pig coronary artery. The uptake was examined for 2-12 min at 10-200 μM DHAA in either the presence of 134 mM Na⁺ or in its absence (N-methyl D-glucamine, choline or sucrose replaced Na⁺). This inhibition of DHAA uptake by Na⁺ removal was paradoxical because it was inhibited by 2-deoxyglucose and cytochalasin B, as expected of transport via the GLUT pathway. We tested the hypothesis that this paradox resulted from an inefficient intracellular reduction of [¹⁴C]DHAA into [¹⁴C]ascorbate upon intracellular acidosis caused by the Na⁺ removal. Consistent with this hypothesis: (i) the Na⁺/H⁺-exchange inhibitors ethylisopropyl amiloride and cariporide also decreased the uptake, (ii) Na⁺ removal and Na⁺/ H⁺-exchange inhibitors lowered cytosolic pH, with the decrease being larger in 12 min than in 2 min, and (iii) less of the cellular ¹⁴C was present as ascorbate (determined by HPLC) in cells in Na⁺-free buffer than in those in Na⁺-containing buffer. This inability to obtain ascorbate from extracellular DHAA may be detrimental to the coronary artery under hypoxia-induced acidosis during ischaemia/reperfusion.