Transport of organic substrates via a volume-activated channel

We have investigated the volume-activated transport of organic solutes in flounder erythrocytes. Osmotic swelling of cells suspended in a Na⁺-free medium led to increased membrane transport of taurine, glucose, and uridine. For each compound there was a significant lag period (1-2 min at 10°C) between cell swelling and activation of the flux. The volume-activated fluxes of each of the substrates increased in parallel with increasing cell volume, and those of taurine and uridine increased linearly with concentration (up to 19 mM). The volume-activated fluxes of each of the three compounds showed similar sensitivities to a number of anion-selective channel blockers (5-nitro-2-(3-phenylpropylamino)benzoic acid > 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid ≈ MK-196 > niflumic acid > furosemide); the IC₅₀ for the inhibition of the volume-activated fluxes by NPPB was around 12 μM. The results are consistent with the hypothesis that the volume-activated transport of organic osmolytes is via a pathway with the characteristics of a volume-activated "chloride channel." This raises the question of whether the transport of organic substrates might represent a physiological role for such channels in other cell types.