Ascorbate Transport in Pig Coronary Artery Smooth Muscle: Na ⁺ Removal and Oxidative Stress Increase Loss of Accumulated Cellular Ascorbate

Abstract

Pig deendothelialized coronary artery rings and smooth muscle cells cultured from them accumulated ascorbate from medium containing Na⁺. The accumulated material was determined to be ascorbate using high-performance liquid chromatography. We further characterized ascorbate uptake in the cultured cells. The data fitted best with a Hill coefficient of 1 for ascorbate (K_asc = 22 ± 2 μ M) and 2 for Na⁺ (K_Na = 84 ± 10 m M). The anion transport inhibitors sulfinpyrazone and 4,4′-diisothiocyanatostilbene-2,2′-disulfonate (DIDS) inhibited the uptake. Transferring cultured cells loaded with ¹⁴C-ascorbate into an ascorbate-free solution resulted in a biphasic loss of radioactivity – an initial sulfinpyrazone-insensitive faster phase and a late sulfinpyrazone-sensitive slower phase. Transferring loaded cells into a Na⁺-free medium increased the loss in the initial phase in a sulfinpyrazone-sensitive manner, suggesting that the ascorbate transporter is bidirectional. Including peroxide or superoxide in the solution increased the loss of radioactivity. Thus, ascorbate accumulated in coronary artery smooth muscle cells by a Na⁺-dependent transporter was lost in an ascorbate-free solution, and the loss was increased by removing Na⁺ from the medium or by oxidative stress.