Amino acids and Ca2+ stimulate different patterns of Ca2+ oscillations through the Ca2+-sensing receptor

We determined the effect of aromatic amino acid stimulation of the human extracellular Ca2+-sensing receptor (CaR) on intracellular Ca2+ concentration ([Ca2+](i)) in single HEK-293 cells. Addition of L-phenylalanine or L-tryptophan (at 5 mM) induced [Ca2+](i) oscillations from a resting state that was quiescent at 1.8 mM extracellular Ca2+ concentration ([Ca2+](e)). Each [Ca2+](i) peak returned to baseline values, and the average oscillation frequency was similar to1 min(-1) at 37degreesC. Oscillations were not induced or sustained if the [Ca2+](e) was reduced to 0.5 mM, even in the continued presence of amino acid. Average oscillation frequency in response to an increase in [Ca2+](e) (from 1.8 to 2.5-5 mM) was much higher (similar to4 min(-1)) than that induced by aromatic amino acids. Oscillations in response to [Ca2+](e) were sinusoidal whereas those induced by amino acids were transient. Thus both amino acids and Ca2+, acting through the same CaR, produce oscillatory increases in [Ca2+](i), but the resultant oscillation pattern and frequency allow the cell to discriminate which agonist is bound to the receptor.