FIG. 2.

Time course of chloride uptake by intestinal BBM purified from noninfected rabbits: effect of H⁺, electrical, and/or Cl⁻ gradients. Cl⁻ uptake was determined with 15 mM cis ³⁶Cl as substrate. The extra- and intravesicular spaces contained a 20 mM HEPES-40 mM citric acid buffer adjusted with Tris base to give at zero time a pHo/pHi gradient of either 7.5/7.5 (□, ⋄, Δ) or 5.0/7.5 (▪) and was supplemented with appropriate mixtures of either Tris gluconate, K⁺ gluconate, or KCl to obtain the following ionic concentration gradients: [K⁺]o/[K⁺]i = 0/0 mM and [Cl⁻]o/[Cl⁻]i = 15/0 mM (▪, □); [K⁺]o/[K⁺]i = 100/0 mM and [Cl⁻]o/[Cl⁻]i = 15/0 mM (⋄); and [K⁺]o/[K⁺]i = 100/100 mM and [Cl⁻]o/[Cl⁻]i = 15/100 mM (Δ). When K⁺ was present, valinomycin was added at 10 μg · mg protein⁻¹. Results are expressed as absolute uptake values in nanomoles · milligrams of protein⁻¹ ± SD, with n = 16 to 26 determinations per point derived from four different membrane preparations. Because the uptake values at equilibrium (2 h) were identical, data have been pooled (n = 18). So as not to overload the figure, the uptake values at 4 s, 60 s, and equilibrium were used.