Fig. 6.

A: STC-1 cells respond to 10 mM α-methyl-amino-isobutyric acid (MeAIB) with an increase in [Ca²⁺]_i. In this and subsequent panels, injection times are marked by downward arrows. Resting external Ca²⁺ was 1.8 mM, pH 7.4. B: lowering pH from 7.4 to 7.0 reduces the response of STC-1 cells to MeAIB (solid bars), but did not alter Ca²⁺ signaling in response to bombesin (open bars). The addition of 10 mM MeAIB to STC-1 cells was followed after 120 s with 5 nM bombesin. C: the response of STC-1 cells to 5 mM l-proline required the presence of Na⁺ in the medium. When Na⁺ was substituted by NMDG, l-proline-induced Ca²⁺ signaling was abolished. The response to 5 mM l-proline in Na⁺-containing saline was not blocked by tetrodotoxin (TTX; 1 μM). D: STC-1 cells responded to l-proline when Na⁺ was replaced by Li⁺ in the medium. E: addition of 10 mM l-proline to STC-1 cells produces membrane depolarization, as indicated by the voltage-sensitive dye di-8-aminonaphthylethenylpyridinium (di-8-ANEPPS). Depolarization is measured as percent change (Δ) in the ratio of emissions at 560 nm and 620 nm (R) over resting levels (R_rest). Solid bar, l-proline; open bars, KCl at different concentrations. F: individual traces showing responses to 10 mM l-proline and 50 mM KCl.