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⁺ (IK_Ca) channel. Cell-attached recordings confirmed that histamine activated a K⁺ channel with properties congruent with the IK_Ca 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 IK_Ca channel activity, which depended on a La²⁺ (10 μM)-sensitive Ca²⁺ influx. Intracellular Ca²⁺ measurements revealed that the sustained IK_Ca 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⁺ (BK_Ca) channel, with a unitary conductance of ca. 230 pS in symmetrical 140 mM K⁺, and a Ca²⁺ dissociation constant [ K_D(Ca)] of ca. 3 μM, at −40 mV. Notably in no instance was the BK_Ca channel activated by histamine under physiological conditions. The most parsimonious explanation based on the different K_D(Ca) for the two K_Ca channels is provided.