Dihydropyridine calcium channel blockers inhibit ascorbic acid accumulation in human intestinal Caco-2 cells

We investigated the effect of commonly used medications on the accumulation of ascorbic acid in human intestinal Caco-2 cells. Although ascorbic acid is negatively charged at physiological pH, anionic compounds including drugs and metabolites had little effect on its accumulation. On the other hand, hydrophobic 1,4-dihydropyridine compounds (nifedipine and nicardipine), but not other structurally unrelated calcium channel blockers, were found to be potent inhibitors. They inhibited both Na+-dependent and Na+-independent (K+ substituting Na+) accumulation of ascorbic acid. The inhibition was non-competitive with a Ki of 108 microM and 9 microM for nifedipine and nicardipine, respectively. The efflux of ascorbic acid from cells was not affected. Previously, we reported a similar inhibition of ascorbic acid accumulation by estrogens. When nifedipine and estrogens were included in the buffer together, the combined inhibitory effect was less than additive implying that they may act through the same mechanism. The potential clinical significance of dihydropyridine usage on ascorbic acid status in human needs to be considered.