Time-dependent regulation of capacitative Ca²⁺ entry by IGF-1 in human embryonic kidney cells

Abstract.

In a wide variety of cells, mitogenic factors release Ca²⁺ from intracellular stores. The fall of the [Ca²⁺] within the lumen of the Ca²⁺-storing organelles triggers in many cells capacitative Ca²⁺ entry (CCE). The present study was performed to elucidate the effect of insulin-like growth factor (IGF-1) on CCE in human embryonic kidney (HEK 293) cells. After depletion of Ca²⁺ stores by thapsigargin, CCE was assessed by the increase in cytosolic free [Ca²⁺] (Fura-2 fluorescence imaging) when raising extracellular [Ca²⁺] from 0 to physiological concentrations. IGF-1 exposure (50 ng/ml) for 4 h in serum-free medium markedly enhanced CCE, while a 24-h exposure to IGF-1 depressed CCE profoundly. As some Ca²⁺ channels are highly sensitive to the cell membrane potential, and as IGF-1 has been reported to enhance K⁺ channel activity, the influence of K⁺ channel blockers on the IGF-1-dependent stimulation of CCE was also tested. TEA, charybdotoxin and margatoxin decreased CCE. Similar to the total capacitative calcium entry, the fraction of CCE that was sensitive to K⁺ channel blockers was increased after 4 h and decreased after 24 h exposure to IGF-1. Taken together, these data suggest that IGF-1 induces a transient increase followed by a decrease of CCE, and that these effects are at least partly dependent on IGF-1-induced K⁺ channel activity.