Dehydroascorbic 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 [C-14]DHAA uptake by smooth-muscle cells cultured from pig coronary artery. The uptake was examined for 2-12 min at 10-200 muM 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 [C-14]DHAA into [C-14]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 C-14 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.