Concentration-dependent effect of UDP on I_SC and [Ca²⁺]_i.

(A) Epithelia were initially bathed in normal K-H solution while changes in I_SC were recorded. A serosal-to-mucosal-directed Cl⁻ gradient was applied across the monolayers by changing the apical K-H solution to one with reduced Cl⁻ concentration to facilitate Cl⁻ secretion (not shown). Different concentrations of UDP were added to the apical side as indicated. Data presented are representative of at least four independent experiments. (B) UDP-evoked increases in [Ca²⁺]_i were monitored by a microfluorimetric technique using the calcium-sensitive fluorescent dye Fura-2. Cells grown on glass coverslips were initially superfused with normal K-H solution. The K-H solution was then changed to a nominally Ca²⁺-free K-H solution, and the cells were stimulated with different concentrations of UDP as indicated. Data presented are representative of at least four independent experiments. (C) Concentration-response relationship for the effect of UDP upon ΔI_SC in 16HBE14o- epithelia. The changes in I_SC for the first and second peaks were quantified and plotted against the concentration of UDP used. Each data point represents the mean ± S.E. (n = 4–5). (D) Concentration-response relationship for the effect of UDP upon changes in [Ca²⁺]_i represented by Δratio. Each data point represents the mean ± S.E. (n = 4–5). (E) Application of CysLT₁ receptor agonists (10 µM), LTC₄ or LTD₄, did not cause any discernible increase in I_SC and did not affect the subsequent 100-µM UDP-evoked I_SC responses. Data presented are representative of at least four independent experiments.