Pharmacological Blockade of ERG K⁺ Channels and Ca²⁺ Influx through Store-Operated Channels Exerts Opposite Effects on Intracellular Ca²⁺ Oscillations in Pituitary GH₃ Cells

Abstract

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⁺]_ioscillations 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⁺]_iachieved 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.