FIGURE 8.

Competition between Na⁺ and Ca²⁺ for transit through MDCK II PTJCs. (A) Diminished ¹⁴C-ethanolamine flux in the presence of concurrent Na⁺, Ca²⁺, and Mg²⁺ fluxes (n = 6). (B) Addition of Ca²⁺ did not affect paracellular Na⁺ permeability (n = 3). (C) Monovalent ion selectivity remained unaltered in the presence of 5 mM Ca²⁺. Two cell monolayers are plotted. (D) Increased Ca²⁺ and Mg²⁺ permeabilities in increasing ratios of basal Ca²⁺ to Na⁺ (n = 2). (E) Comparison of tetraionic and triionic (Ca²⁺, and Mg²⁺, and Cl⁻) flux showed increased Ca²⁺ and Mg²⁺ permeabilities in the absence of concurrent Na⁺ flux (n = 3). (F) Anomalous mole-fraction effect between Na⁺ and Ca²⁺ by TER measurements. Two monolayers are plotted.