All-or-nothing Ca²⁺ mobilization from the intracellular stores of single histamine-stimulated HeLa cells

1. Histamine-stimulated mobilization of intracellular Ca2+ stores was monitored in intact and permeabilized populations of HeLa cells using both the fluorescent Ca2+ indicator Fura-2 and 45Ca2+ measurements. Digital video imaging of Fura-2-loaded cells was used to measure the intracellular calcium concentration ([Ca2+]i) of single cells. 2. In populations of HeLa cells, histamine caused a concentration-dependent increase in cytoplasmic [Ca2+]. The initial transient increase was independent of extracellular Ca2+ (Ca2+o) and was followed by a sustained increase that was abolished by removal of Ca2+o. 3. In Ca2+-free medium ([Ca2+]o 2+]i, and a subsequent challenge with histamine failed to evoke a further response indicating that the inositol 1,4,5-trisphosphate (InsP3)-sensitive Ca2+ stores had been completely emptied. Lower concentrations of histamine (0.5-10 μM) caused smaller, concentration-dependent increases in [Ca2+]i that were also transient. After exposure to these low histamine concentrations, where [Ca2+]i returned to baseline within 2 min, addition of a higher histamine concentration evoked a further increase in [Ca2+]i. The second increase in [Ca2+]i was inversely proportional to the increase caused by the first exposure to histamine, indicating that Ca2+ released in the initial response was not substantially resequestered into histamine-sensitive stores. 4. Single HeLa cells challenged with low concentrations of histamine in Ca2+-free medium responded with transient increases in [Ca2+]i, but individual cells differed in their sensitivity with 51% of cells responding to 1 μM, and 98% responding to 25 μM-histamine. 5. When single cells in Ca2+-free medium were challenged with stepwise increases in histamine concentration, they responded to each step with a transient [Ca2+]i increase after which [Ca2+]i returned to baseline within 1 min. Prolonging the interval between histamine additions by up to 25 min did not affect the [Ca2+]i increase evoked by a subsequent histamine addition. 6. Unidirectional 45Ca2+ efflux from saponin-permeabilized HeLa cells showed that, under conditions that prevented Ca2+ resequestration, submaximal concentrations of InsP3 rapidly emptied only a fraction of the InsP3-sensitive Ca2+ stores. The failure of low InsP3 concentrations to fully mobilize the InsP3-sensitive Ca2+ stores was not a consequence of InsP3 degradation. 7. We conclude that within single HeLa cells, intracellular Ca2+ stores are heterogeneous in their sensitivity to InsP3, and the fraction of Ca2+ stores mobilized by InsP3 increases as the InsP3 concentration increases