Pharmacological blockade of ERG K+ channels and Ca2+ influx through store-operated channels exerts opposite effects on intracellular Ca2+ oscillations in pituitary GH3 cells

In the present study, the effects on intracellular calcium concentration ([Ca²⁺](i)) oscillations of the blockade of ether-a-go-go-related gene (ERG) K⁺ channels and of Ca²⁺ influx through store-operated channels (SOC) activated by [Ca²⁺](i) store depletion have been studied in GH₃ cells by means of a combination of single-cell fura-2 microfluorimetry and whole-cell mode of the patch-clamp technique. Nanomolar concentrations (1-30 nM) of the piperidinic second-generation antihistamines terfenadine and astemizole and of the class III antiarrhythmic methanesulfonanilide dofetilide, by blocking ERG K⁺ channels, increased the frequency and the amplitude of [Ca²⁺](i) oscillations in resting oscillating GH₃ cells. These compounds also induced the appearance of an oscillatory pattern of [Ca²⁺](i) in a subpopulation of nonoscillating GH₃ cells. The effects of ERG K⁺ channel blockade on [Ca²⁺](i) oscillations appeared to be due to the activation of L-type Ca²⁺ channels, because they were prevented by 300 nM nimodipine. By contrast, the piperazinic second-generation antihistamine cetirizine (0.01-30 μM), which served as a negative control, failed to affect ERG K⁺ channels and did not interfere with [Ca²⁺](i) oscillations in GH₃ cells. Interestingly, micromolar concentrations of terfenadine and astemizole (0.3-30 μM), but not of dofetilide (10-100 μM), produced an inhibition of the spontaneous oscillatory pattern of [Ca²⁺](i) changes. This effect was possibly related to an inhibition of SOC, because these compounds inhibited the increase of [Ca²⁺](i) achieved by extracellular calcium reintroduction after intracellular calcium store depletion with the sarcoplasmic or endoplasmic reticulum calcium ATPase pump inhibitor thapsigargin (10 μM) in an extracellular calcium-free medium. The same inhibitory effect on [Ca²⁺](i) oscillations and SOC was observed with the first-generation antihistamine hydroxyzine (1-30 μM), the more hydrophobic metabolic precursor of cetirizine. Collectively, the results of the present study obtained with compounds that interfere in a different concentration range with ERG K⁺ channels or SOC suggest that 1) ERG K⁺ channels play a relevant role in controlling the oscillatory pattern of [Ca²⁺](i) in resting GH₃ cells and 2) the inhibition of SOC might induce an opposite effect, i.e., an inhibition of [Ca²⁺](i) oscillations.