Fig. 3.

Effect of external Na⁺reduction on the decay phase of Ca²⁺ transients.A, Two Ca²⁺ transients from the same cell were superimposed. The traces indicated with NaCl and LiCl were obtained in normal Tyrode's solution (145 mm[Na⁺]_o) and in low Na⁺ condition (20 mm[Na⁺]_o and 125 mm[Li⁺]_o), respectively. The K pipette solution (for its composition, see Materials and Methods) containing 50 μm fura-2 was used for the internal solution. Calibration: 2 sec, 200 nm. The decay phases of the traces indicated with NaCl and LiCl were fitted with the following bi-exponential equations: 0.20 μm · exp (−0.19t) + 1.18 μm · exp (−1.42t) and 0.19 μm · exp(−0.13t) + 1.02 μm · exp(−0.48t), respectively. Inset, The plot of the time derivatives (−d[Ca²⁺]/dt) of the decay phases of the two Ca²⁺ transients as a function of Δ[Ca²⁺]_i (white dots, NaCl; black dots, LiCl). B, The instantaneous γ values at various Δ[Ca²⁺] were calculated by dividing the time derivatives (insetin A) by Δ[Ca²⁺](t) and were fitted with fourth-order polynomial functions (f_Na, a fit to γ in the normal condition; f_Li, a fit to γ in the low [Na⁺]_o condition). The difference between the two polynomial fits was plotted as asolid line (marked byf_Na −f_Li). The fraction of γ inhibited by the reduction of [Na⁺]_o, (f_Na −f_Li)/f_Na, was plotted as gray open circles (the right ordinate).