The ion channel of muscle and electric organ acetylcholine receptors: differing affinities for noncompetitive inhibitors.
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.
Department of Biochemistry, Universidad Central del Caribe, Bayamón, Puerto Rico 00960.
V A
VA
Eterović
L
L
Li
P A
PA
Ferchmin
Y H
YH
Lee
R M
RM
Hann
A D
AD
Rodriguez
M G
MG
McNamee
D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D004224:Diterpenes / Q000302:isolation & purification / Q000494:pharmacology*; D004557:Electric Organ / Q000187:drug effects*; D015640:Ion Channel Gating / Q000187:drug effects; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000037:antagonists & inhibitors*; D009419:Nerve Tissue Proteins / Q000037:antagonists & inhibitors*; D018733:Nicotinic Antagonists; D009865:Oocytes; D009928:Organ Specificity; D010622:Phencyclidine / Q000494:pharmacology; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors; D017386:Sequence Homology, Amino Acid; D015222:Sodium Channels / Q000187:drug effects*; D013748:Tetracaine / Q000494:pharmacology; D014101:Torpedo; D014982:Xenopus laevis
D004224:Diterpenes; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D018733:Nicotinic Antagonists; D011994:Recombinant Proteins; D015222:Sodium Channels; D013748:Tetracaine; C082581:eupalmerin acetate; D008274:Magnesium; D010622:Phencyclidine