Figure 4.

Modulatory effect of l-Phe on Ca²⁺ mobilization induced by Ca²⁺_o and spermine. Fura-2-loaded HEK-293 cells that stably expressed the CaR (HEK-5001 cells) were exposed to physiological saline in the absence or presence of l-Phe or d-Phe before being loaded into the fluorometer. Baseline Ca²⁺_o was 0.5 mM. The vertical bars in a and b indicate the fluorescence ratios (340/380). (a) Effects of 1-mM increments of Ca²⁺_o (open arrows) on Ca²⁺ mobilization in the presence or absence of 10 mM l-Phe. (b) Effects of 0.25-mM increments of spermine (open arrows) on Ca²⁺ mobilization in the presence or absence of 10 mM l-Phe. (c) Effects of various concentrations of l-Phe on the fractional response of the CaR to Ca²⁺_o. l-Phe concentrations were as follows: ○, zero; ▵, 1.0 mM; □, 3.0 mM; ●, 10 mM; ▴, 30 mM; ■, 100 mM. (d) Concentration dependence of the stereoselective decrease in the EC₅₀ for Ca²⁺_o induced by l-Phe. The following parameters were obtained by curve fitting for l-Phe: EC₅₀ = 3.5 mM; Hill coefficient = 1.4. The following parameters were obtained by curve fitting for d-Phe: EC₅₀ = 7.4 mM; Hill coefficient = 1.9.