Tetrahydrobiopterin (BH4) was released from PC 12 cells to the extracellular fluid. We found that the BH4 outward flow from the cells placed in Hanks medium was increased when NaCl concentration of the medium was decreased. Increase in the BH4 out flow was not observed when NaCl in the Hanks medium was substituted with sodium glutamate (0.14 M), choline chloride (0.14 M) or sucrosc (0.25 M). HgCl2, an inhibitor of water channel, aquaporin, prevented the increase in BH4 out flow caused by the hypotonic medium. The results suggested the presence of a site in BH4 transport which was stimulated by osmotic pressure and/or water influx.
