Determination of dose-response curves by quantitative ionophoresis at the frog neuromuscular junction.
1. Quantitative ionophoresis at the neuromuscular junction is possible when (a) the drug is released from appropriate distances (15--20 micrometer for most drugs), (b) the topology of receptors is known and (c) high resistance drug pipettes (100--200 M omega) are sued. 2. With this method, drug concentration-endplate conductance relations were determined in voltage-clamped end-plates of the frog for the agonists ACh, carbamylcholine (CCh) and suberyldicholine (SubCh). 3. Based on the co-operative and independent model, theoretical dose-response curves were computed using as parameters the Hill coefficient nH, maximum conductance gmax., and apparent dissociation constant K. It was found that the co-operative model fitted the data much better than the independent model. 4. Based on the co-operative model, the mean maximum conductance for ACh was gmax. = 169 nS/micrometer, equivalent to 9000 ionic channels/micrometer length of a nerve terminal which can be opened at high drug concentrations. 5. The maximum conductance for CCh at--80 mV membrane potential was, on the average, 78% of that for ACh measured at the same end-plates. This value is termed the relative efficacy of CCh. 6. The mean values for the apparent dissociation constant K were 27.8 micrometer for ACh, 336 micrometer for CCh and 18 micrometer for SubCh. 7. The inhibition of the acetylcholinesterase activity by edrophonium (3--10 micrometer) affected only the local ACh concentration at the receptor sites, but not gmax. and nH. 8. Dose-response curves measured before and after removal of single nerve terminals in collagenase-treated muscle fibres showed no change in the nH, gmax. and K. A slight increase in gmax. to a value of 218 nS/micrometer observed comparing collagenase-treated and untreated end-plate. 9. Desensitization of receptors may occur in the range of several tens of milli-seconds.
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001001:Anura; D002217:Carbachol / Q000494:pharmacology; D002794:Choline / Q000031:analogs & derivatives / Q000494:pharmacology; D003998:Dicarboxylic Acids / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D007478:Iontophoresis; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008954:Models, Biological; D009045:Motor Endplate / Q000187:drug effects / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D011893:Rana esculenta; D011955:Receptors, Drug / Q000502:physiology*
D003998:Dicarboxylic Acids; D011955:Receptors, Drug; D002217:Carbachol; D002794:Choline; D000109:Acetylcholine