Figure 6.

Analysis of Na⁺/Ca²⁺ exchange in single S₁, S₂, and S₃ cells. To assess Na⁺/Ca²⁺ exchange activity, proximal tubule cells were loaded with the Ca²⁺-sensitive dye fura-2 and single cells were assayed by measuring [Ca²⁺]_i upon reversal of the electrochemical gradient for Na⁺ by isosmotic replacement of extracellular Na⁺ in the presence or absence of external Ca²⁺ in paired analysis on the same cell. DCT cells served as the positive controls. (A) Representative traces are shown for each cell type analyzed. Slash marks in the traces indicate a ten minute wash with control buffer. All proximal cells showed similar rises of [Ca²⁺]_i in the presence and absence of external Ca^2+. However, the DCT cell exhibited a large rise of [Ca²⁺]_i in the presence of external Ca²⁺ but only a minimal rise in the absence of external Ca²⁺. (B) The difference between the peak and plateau [Ca²⁺]_i in the presence or absence of external Ca²⁺ was not significantly different for all proximal lines. In contrast, the control DCT cell showed a large rise of [Ca²⁺]_i in the presence of external Ca²⁺, and a rise similar to that of proximal cells in the absence of external Ca²⁺ (P < 0.01). Each bar represents the mean ± SEM of three independent experiments performed on each cell type.