Open time of channels activated by binding of two distinct agonists

Abstract

At the frog endplate, acetylcholine (ACh) causes depolarization by opening synaptic channels, and it is generally assumed that more than one molecule must bind to open one channel. This interpretation, proposed to explain the sigmoidicity of the dose–response curve^1,2, also accounts for the potentiating effect of pipette-released ACh on the response to nerve-released ACh³. We have described a similar cross-potentiation between ACh and carbachol (CCh)^4,5. The response to simultaneous application of these two agonists is larger than the sum of the responses to separate application, and we have assumed that this reflects the binding of both ACh and carbachol to individual receptor molecules. One approach to an understanding of the molecular basis of transmitter action is to study the mean time a channel remains open in its presence. Because this mean ‘Open time’ depends on the nature of the agonist used to activate a given receptor, we have measured this parameter in order to understand better the mechanism underlying the cross-potentiation. We compared the ‘Open times’ observed when the two agonists were applied separately with those obtained when both were applied together. In each case, when a ‘faster’ agonist (one inducing ‘short’ open times) and a ‘slower’ agonist (one inducing ‘long’ open times) are applied together, the channels opened by combined activation have a short mean open time, as if these were only controlled by the faster agonist.