Histamine hyperpolarizes human glioblastoma cells by activating the intermediate-conductance Ca2+-activated K+ channel

Abstract

The effects of histamine on the membrane potential and currents of human glioblastoma (GL-15) cells were investigated. In perforated whole cell configuration, short (3 s) applications of histamine (100 μM) hyperpolarized the membrane by activating a K+-selective current. The response involved the activation of the pyrilamine-sensitive H1 receptor and Ca2+ release from thapsigargin-sensitive intracellular stores. The histamine-activated current was insensitive to tetraethylammonium (3 mM), iberiotoxin (100 nM), and d-tubocurarine (100 μM) but was markedly inhibited by charybdotoxin (100 nM), clotrimazole (1 μM), and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34, 1 μM), a pharmacological profile congruent with the intermediate conductance Ca2+-activated K+ (IKCa) channel. Cell-attached recordings confirmed that histamine activated a K+ channel with properties congruent with the IKCa channel (voltage independence, 22 pS unitary conductance and slight inward rectification in symmetrical 140 mM K+). More prolonged histamine applications (2–3 min) often evoked a sustained IKCa channel activity, which depended on a La2+ (10 μM)-sensitive Ca2+ influx. Intracellular Ca2+ measurements revealed that the sustained IKCa channel activity enhanced the histamine-induced Ca2+ signal, most likely by a hyperpolarization-induced increase in the driving force for Ca2+ influx. In virtually all cells examined we also observed the expression of the large conductance Ca2+-activated K+ (BKCa) channel, with a unitary conductance of ca. 230 pS in symmetrical 140 mM K+, and a Ca2+ dissociation constant [ KD(Ca)] of ca. 3 μM, at −40 mV. Notably in no instance was the BKCa channel activated by histamine under physiological conditions. The most parsimonious explanation based on the different KD(Ca) for the two KCa channels is provided.