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 approximately 1 min(-1) at 37 degrees C. 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 (approximately 4 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.