Mechanism of inhibition of connexin channels by the quinine derivative N-benzylquininium

The anti-malarial drug quinine and its quaternary derivative N-benzylquininium (BQ+) have been shown to inhibit gap junction (GJ) channels with specificity for Cx50 over its closely related homologue Cx46. Here, we examined the mechanism of BQ+ action using undocked Cx46 and Cx50 hemichannels, which are more amenable to analyses at the single-channel level. We found that BQ+ (300 µM–1 mM) robustly inhibited Cx50, but not Cx46, hemichannel currents, indicating that the Cx selectivity of BQ+ is preserved in both hemichannel and GJ channel configurations. BQ+ reduced Cx50 hemichannel open probability (Po) without appreciably altering unitary conductance of the fully open state and was effective when added from either extracellular or cytoplasmic sides. The reductions in Po were dependent on BQ+ concentration with a Hill coefficient of 1.8, suggesting binding of at least two BQ+ molecules. Inhibition by BQ+ was voltage dependent, promoted by hyperpolarization from the extracellular side and conversely by depolarization from the cytoplasmic side. These results are consistent with binding of BQ+ in the pore. Substitution of the N-terminal (NT) domain of Cx46 into Cx50 significantly impaired inhibition by BQ+. The NT domain contributes to the formation of the wide cytoplasmic vestibule of the pore and, thus, may contribute to the binding of BQ+. Single-channel analyses showed that BQ+ induced transitions that did not resemble pore block, but rather transitions indistinguishable from the intrinsic gating events ascribed to loop gating, one of two mechanisms that gate Cx channels. Moreover, BQ+ decreased mean open time and increased mean closed time, indicating that inhibition consists of an increase in hemichannel closing rate as well as a stabilization of the closed state. Collectively, these data suggest a mechanism of action for BQ+ that involves modulation loop gating rather than channel block as a result of binding in the NT domain.

A Review of the Treatment of Opioid-induced Constipation with Methylnaltrexone Bromide

Objectives Review and summarize the mechanism of action of methylnaltrexone bromide (methylnaltrexone) and its effectiveness in the treatment of opioid-induced constipation. Data Source A multi-database search was conducted using PubMed and MEDLINE databases, in addition to electronic links to related articles and references. Background Opioids are effective medications for the management of moderate to severe pain, but they are associated with a number of side effects, especially within the gastrointestinal system. Constipation is a very common adverse reaction in patients with late-stage, adverse illness, who require long term administration of opioids on a chronic basis to help alleviate pain. In April 2008, the Food and Drug Administration approved the use of methylnaltrexone, a quaternary derivative of naltrexone which does not cross the blood brain barrier, for the management of patients with opioid-induced constipation. Methylnaltrexone acts as a selective peripheral Mu-receptor antagonist, without affecting the effects of opioids on central analgesia. Conclusions Studies have been shown that methylnaltrexone can be used safely in the treatment of opioid-induced constipation without either interfering with opioid effects on central anesthesia or precipitating opioid withdrawal.

Intracellular QX-314 blocks the hyperpolarization-activated inward current Iq in hippocampal CA1 pyramidal cells

1. Whole cell voltage-clamp recordings (access resistance < or = 12 M omega) from CA1 pyramidal cells in the guinea pig hippocampal slice revealed a hyperpolarization-activated inward current with an inward tail upon repolarization. The current activation range extended from approximately -50 mV to -130 mV, with half-activation at -86 mV. This current was identified as the q current (Iq). 2. Intracellular QX-314 (5 or 10 mM), a quaternary derivative of lidocaine, blocked Iq completely throughout its activation range. 3. There is a growing realization that Iq may be responsible for the pacemaker depolarization in cells that display rhythmic calcium spikes. Because QX-314 blocks Iq completely, it could be used to test whether Iq is essential to this oscillatory activity.