Vol. 58, Issue 5, 1115-1128, November 2000

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

Agnese Secondo, Maurizio Taglialatela, Mauro Cataldi, Giovanna Giorgio, Monica Valore, Gianfranco Di Renzo, and Lucio Annunziato

Unit of Pharmacology, Department of Neuroscience, School of Medicine, University of Naples Federico II, Naples, Italy (A.S., M.T., M.C., G.G., M.V., L.A.); and School of Pharmacy, University of Catanzaro, Catanzaro, Italy (G.D.R.)

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.