Fig. 1.

A: addition of l-proline (Pro) or l-phenylalanine (Phe) induced an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in STC-1 cells. Three separate experiments with l-proline, l-phenylalanine, or l-tryptophan (Trp) are overlaid. At the time marked by the downward pointing arrow, amino acid is injected into the cuvette (final concentration 5 mM, injection time + mixing time < 3 s). The concentration of Ca²⁺ in HBSS was 1.8 mM. B: increase in [Ca²⁺]_i (defined as the difference between the peak [Ca²⁺]_i of the transient response and the baseline value) after addition of l-proline (n = 13), l-phenylalanine (n = 4), or l-tryptophan (n = 5). C: dose-response curves of peak [Ca²⁺]_i as a function of l-proline or l-phenylalanine concentration in STC-1 cells incubated in HBSS containing 1.8 mM Ca²⁺. D: single-cell imaging shows the majority of STC-1 cells respond to 5 mM l-proline. Top: cluster of 20–30 STC-1 cells at rest. Cells are pseudocolored to show varying levels of [Ca²⁺]_i. Scale bar shows colors change from blue to green to red to white with increasing [Ca²⁺]_i. Bottom: 30 s after addition of 5 mM l-proline to the HBSS bathing the cells. Virtually all cells showed increased [Ca²⁺]_i. E: response of human embryonic kidney (HEK)-293 cells expressing human Ca²⁺-sensing receptor (CaR) to the same amino acids (i.e., l-proline, l-phenylalanine, or l-tryptophan), as applied to the STC-1 cells in A. Amino acids were added to the cuvette at the times marked by the downward arrow.