Transport and metabolism of pyridoxine in rabbit iris--ciliary body

Isolated rabbit iris--ciliary body preparations were found to accumulate 3H-pyridoxine by a mechanism that was time- and temperature-dependent, saturable in part, but not altered by omission of oxygen or specific ions. Tissue accumulation was only partially energy-dependent, and metabolic inhibitors had only small effects. Other B6 vitamers markedly blocked accumulation. Metabolism of 3H-pyridoxine within the tissue was extensive. After 60 min incubation with 3H-pyridoxine, about 65% of the radioactivity in the iris--ciliary body was associated with phosphorylated vitamers which did not efflux from the tissue as readily as did the nonphosphorylated forms. Unaltered pyridoxine accounted for only 16% of intracellular 3H-B6, and this did not represent accumulation against a concentration gradient. A similar saturable uptake process occurred in vivo. These data were consistent with the hypothesis that pyridoxine was accumulated by facilitated diffusion with intracellular trapping of phosphorylated metabolites.