Capacitative calcium entry supports calcium oscillations in human embryonic kidney cells

Treatment of human epithelial kidney (HEK293) cells with low concentrations of the muscarinic agonist methacholine results in the activation of complex and repetitive cycling of intracellular calcium (Ca²⁺]_i), known as Ca²⁺]_i oscillations. These oscillations occur with a frequency that depends on the concentration of methacholine, whereas the magnitude of the Ca²⁺]_i spikes does not. The oscillations do not persist in the absence of extracellular Ca²⁺, leading to the conclusion that entry of Ca²⁺ across the plasma membrane plays a significant role in either their initiation or maintenance. However, treatment of cells with high concentrations of GdCl₃, a condition which limits the flux of calcium ions across the plasma membrane in both directions, allows sustained Ca²⁺]_i oscillations to occur. This suggests that the mechanisms that both initiate and regenerate Ca²⁺]_i oscillations are intrinsic to the intracellular milieu and do not require entry of extracellular Ca²⁺. This would additionally suggest that, under normal conditions, the role of calcium entry is to sustain Ca²⁺]_i oscillations. By utilizing relatively specific pharmacological manoeuvres we provide evidence that the Ca²⁺ entry that supports Ca²⁺ oscillations occurs through the store-operated or capacitative calcium entry pathway. However, by artificial introduction of a non-store-operated pathway into the cells (TRPC3 channels), we find that other Ca²⁺ entry mechanisms can influence oscillation frequency in addition to the store-operated channels.