The ion channel of muscle and electric organ acetylcholine receptors: Differing affinities for noncompetitive inhibitors

Abstract

1. Muscle and electric organ acetylcholine receptors (AChR's) were expressed in Xenopus laevis oocytes and differential effects of noncompetitive blockers on each type of receptor were analyzed using a two-electrode voltage clamp. 2. The positively charged channel blockers, phencyclidine (PCP) and tetracaine, displayed a much lower potency on muscle receptor than on the electric organ receptor. The IC50 for both blockers at the electrocyte receptor was close to 1 microM at -60 mV and even lower at more hyperpolarized voltages. In contrast, with muscle receptor IC50's were 20 to 40 microM at -60 or -80 mV. 3. Eupalmerin acetate, an uncharged noncompetitive inhibitor that displaces [3H]PCP from its high-affinity binding site, inhibited both receptors with a similar potency: IC50 of 4.9 and 6.4 microM for electrocyte and muscle receptors, respectively. However, eupalmerin acetate affected the desensitization process in each receptor type differently and triggered an unusual biphasic response in the muscle receptor. 4. These results are discussed with respect to differences in the amino acid sequences of the M2 regions of the two receptors. 5. A third type of noncompetitive inhibitor, Mg2+, was also examined and it inhibited both receptors with a similar potency (IC50, 0.5-1.0 mM). However, Mg2+ appeared to act at sites other than the PCP site.

M2 mutations of the nicotinic acetylcholine receptor increase the potency of the non-competitive inhibitor phencyclidine

“…Since PCP potency is not decreased by membrane depolarization, the binding site of PCP is not in the ion channel of BC 3 H-1 receptors. This contrasts with previous observations of voltage dependence of PCP with the electric organ receptor (Eterovic, et al, 1993b) suggesting that PCP may bind to different sites in these two receptors.…”

“…Using single-channel recordings from muscle cells (Changeux et al, 1986;Oswald, 1986, 1989) and two-electrode voltage-clamp from oocytes expressing BC 3 H-1 nAChRs (Eterovic et al, 1993b), it has been observed previously that PCP inhibits ACh-induced currents in a bimodal maner; low and high, but not intermediate concentrations of PCP inhibit the receptor. Furthermore, PCP inhibits the AChR from Torpedo electric organ with higher potency than mouse muscle AChR.…”

Antagonism of nicotinic acetylcholine receptors by inhibitors of monoamine uptake

“…The different results may be due to molecular differences in the M2 region between frog and mouse muscle AChRs, 34 similar to the difference in affinity to phencyclidine between AChRs from Torpedo electric organ and mouse muscle; which is based on a difference in three amino acids within the ionic channel. 35 It has been reported that ACh potentials in rat soleus muscle fibers are potentiated with 1 M imipramine. 25 However, under our experimental conditions even smaller concentrations of imipramine exerted an inhibitory effect on the ACh-current.…”

Inhibitory Effect of Clonidine on Cholinergic Transmission

“…Considering this feature, we used MINDO3 method to compute equilibrium geometry and electronic structure of clonidine molecule and molecules of three typical LA: 2-(ethylpropylamino)-2',6'-butyroxylidide (etidocaine), procaine, and tetracaine. A certain similarity in the spatial structure of the examined molecules was revealed.In addition to the blocking action on the potentialoperated ionic channels, many LA modify firing of neurons and skeletal and cardiac muscle cells by inhibiting the nicotinic receptor-channel complex [5,6]. This inhibitory effect (IC 50 ) varied form 1 to 70 µM [9].…”

“…In addition to the blocking action on the potentialoperated ionic channels, many LA modify firing of neurons and skeletal and cardiac muscle cells by inhibiting the nicotinic receptor-channel complex [5,6]. This inhibitory effect (IC 50 ) varied form 1 to 70 µM [9].…”