Inhibition of G Protein-Activated Inwardly Rectifying K+ Channels by Ifenprodil

Abstract

G protein-activated inwardly rectifying K + channels (GIRK, also known as Kir3) are regulated by various G-protein-coupled receptors. Activation of GIRK channels plays an important role in reducing neuronal excitability in most brain regions and the heart rate. Ifenprodil, which is a clinically used cerebral vasodilator, interacts with several receptors, such as a 1 adrenergic, N-methyl-D-aspartate, serotonin and s receptors. However, the molecular mechanisms underlying the various clinically related effects of ifenprodil remain to be clarified. Here, we examined the effects of ifenprodil on GIRK channels by using Xenopus oocyte expression assays. In oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, ifenprodil reversibly reduced inward currents through the basal GIRK activity. The inhibition was concentration-dependent, but voltage-and time-independent, suggesting that ifenprodil may not act as an open channel blocker of the channels. In contrast, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to ifenprodil. Furthermore, GIRK current responses activated by the cloned k-opioid receptor were similarly inhibited by ifenprodil. The inhibitory effects of ifenprodil were not observed when ifenprodil was applied intracellularly, and were not affected by extracellular pH, which changed the proportion of the uncharged to protonated ifenprodil, suggesting its action from the extracellular side. The GIRK currents induced by ethanol were also attenuated in the presence of ifenprodil. Our results suggest that direct inhibition of GIRK channels by ifenprodil, at submicromolar concentrations or more, may contribute to some of its therapeutic effects and adverse side effects.

Mapping the Binding of GluN2B-SelectiveN-Methyl-d-aspartate Receptor Negative Allosteric Modulators

“…In general, the optimal linker length is often between 9 and 11 Å (Chenard et al, 1991;Marinelli et al, 2007;Tahirovic et al, 2008;Mosley et al, 2009). The piperidine ring of Ro 25-6981 and ifenprodil, with a pK a of 9.05 (Kobayashi et al, 2006), is protonated under physiological pH and forms a hydrogen bond with the oxygen of Gln110 (Karakas et al, 2011). Mutation of GluN2B Gln110 to Ala reduced ifenprodil potency by 10-fold (Table 2), consistent with the idea that this interactions helps to stabilize ligand orientation in the binding site.…”

“…The overall system was neutralized at pH 7.0 using an ion concentration of 0.15 NaCl. The protonation state of ifenprodil used was as described by Kobayashi et al (2006). The system was first relaxed with the use of the Desmond relaxation model.…”

Inhibition of G Protein-Activated Inwardly Rectifying K+ Channels by the Antidepressant Paroxetine

“…Because Kir channels allow K + ions to enter the cells much more readily than does K + permeation in the outward direction (34), the magnitude of Kir currents is enhanced by utilizing the driving force for potassium by raising extracellular K + to 96 mM. In the hK solution used to readily analyze Kir currents, the K + equilibrium potential was close to 0 mV, and inward K + current flow through Kir channels was observed at negative holding potentials, as previously shown (12,33,34,36). For examining the effect of intracellular paroxetine, 23 nl of 5 mM paroxetine dissolved in distilled water was injected into an oocyte by using a Nanoliter injector (World Precision Instruments, Sarasota, FL, USA) as described previously (8), and the oocyte currents were continuously recorded for approximately 30 -40 min.…”

σ2-Receptor Ligand-Mediated Inhibition of Inwardly Rectifying K+ Channels in the Heart

“…It was suggested by Jeanjean et al (1993) that a link between 2 receptors and the K ϩ channels targeted by class III antiarrhythmic drugs could exist (Jeanjean et al, 1993). Moreover, the classic 2 -receptor agonist, ifenprodil, was recently described as a blocker of G protein-activated inwardly rectifying potassium channels (Kobayashi et al, 2006). Taken together, these data suggest a link between 2 receptors and K ϩ channels; however, the mechanisms and the consequences of this link have never been investigated.…”