Extracellular calcium concentration controls the frequency of intracellular calcium spiking independently of inositol 1,4,5-trisphosphate production in HeLa cells

Stimulation of single HeLa cells with histamine evoked repetitive increases of the intracellular calcium ion concentration (Ca2+ spikes). The frequency of Ca2+ spiking increased as the extracellular hormone concentration was elevated. In addition, the frequency of Ca2+ spiking could be accelerated by increasing the extracellular Ca2+ concentration ([Ca2+](o)) in the presence of a constant hormone concentration. The range of [Ca2+](o) over which the spiking frequency could be titrated was nominally-zero to 10 mM, being half-maximally effective at approx. 1 and 2.5 mM for 37 and 22 degrees C respectively. The effect of [Ca2+](o) on inositol phosphates production was also examined. Changes of[Ca2+](o) over a range which had been found to affect the frequency of Ca2+ spiking did not have any effect on the rate of myo-inositol 1,4,5-trisphosphate (InsP(3)) production, although an increase in inositol phosphates production was observed as [Ca2+](o) was increased from zero to values giving less than half-maximal Ca2+ spike frequency. These data suggest that at low Ca2+ spike frequency, Ca2+-stimulated activation of phospholipase C may contribute to Ca2+ spiking in HeLa cells, but under some conditions the availability of Ca2+ to the intracellular stores, rather than changes in the rate of InsP(3) production, determines the Ca2+ spike frequency.