Physiological and molecular characterization of an IRK-type inward rectifier K+ channel in a tumour mast cell line

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Abstract
The basophilic leucaemia cell line RBL-2H3 exhibits a robust inwardly rectifying potassium current, I KIR, which is likely to be modulated by G proteins. We examined the physiological and molecular properties of this KIR conductance to define the nature of the underlying channel species. The macroscopic conductance revealed characteristics typical of classical K+ inward rectifiers of the IRK type. Channel gating was rapid, first order (τ ≈ 1 ms at −100 mV) and steeply voltage dependent. Both activation potential and slope conductance were dependent on extracellular K+ concentration ([K+]o) and inward rectification persisted in the absence of internal Mg2+. The current was susceptible to a concentration- and voltage-dependent block by extracellular Na+, Cs+ and Ba2+. Initial I KIR whole-cell amplitudes as well as current rundown were dependent on the presence of 1 mM internal ATP. Perfusion of intracellular guanosine 5′-Q-(3-thiotriphosphate) (GTP[γS]) suppressed I KIR with an average half-time of decline of approximately 400 s. It was demonstrated that the dominant IRK-type 25 pS conductance channel was indeed suppressed by 100 μM preloaded GTP[γS]. Reverse transcriptase-polymerase chain reactions (RT-PCR) with RBL cell poly(A)+ RNA identified a full length K+ inward rectifier with 94% base pair homology to the recently cloned mouse IRK1 channel. It is concluded that RBL cells express a classical voltage-dependent IRK-type K+ inward rectifier RBL-IRK1 which is negatively controlled by G proteins.