PRE-STEADY-STATE AND STEADY-STATE FUNCTION OF THE ILEAL BRUSH-BORDER SO42--OH- EXCHANGER

Rapid-sampling analysis of the detailed time course of (SO42-)-S-35 up take under pH-gradient (pH(in) = 7.5; pH(out) = 5.5) conditions conver ged to a model of an initial burstlike pre-steady-state with relaxatio n to linear steady-state uptake across pig ileal brush border vesicles . A model of low affinity transport (K-m = 7.7 mM) plus an unsaturable component described the steady-state component of (SO42-)-S-35 uptake . Steady-state transport was maximal under pH-gradient conditions and sensitive to inhibition by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Significant steady-state (SO42-)-S-35 transport sensitive to 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid was found under acidic (pH(in) = pH(out) = 5.5) and neutral (pH(in) = pH(out) = 7.5) pH-equil ibrated conditions. Varying conditions from pH gradient to neutral pH equilibrated had no effect on the amplitude of the pre-steady-state bu rst or on the time constant for relaxation from pre-steady-state to st eady-state conditions. However, maximal amplitude was obtained under a cidic pH-equilibrated conditions. These results are incorporated into a model for the low affinity ileal brush border SO42--OH- exchanger wh ereby, under 0-trans (0 internal substrate concentration) conditions, translocation of inner-facing to outer-facing conformations defines th e overall rate-limiting step of the transporter. Provision for slippag e of the unloaded carrier could account for (SO42-)-S-35 transport und er acidic pH-equilibrated conditions.