Activation and Inactivation of Postjunctional Membrane Receptors*

Abstract

In skeletal muscle, it is now widely accepted that neuromuscular transmission is mediated by acetylcholine. On arrival of a nerve impulse at the motor neuron terminals, acetylcholine is released and it diffuses across the junctional cleft to reach and react with the postjunctional membrane (PJM). As a result of this interaction, the permeability of the PJM to Na and K ions is increased.'At the molecular level, it is commonly visualized that there are receptors sites on the PJM and that the permeability increase is brought about when these sites combine with acetylcholine (ACh). In our laboratory we have been interested in learning more about the nature of the receptive sites and the molecular mechanisms which control the ionic permeability of the PJM. In this paper, we wish to summarize some of our own work and that of other investigators whose publications bear on this subject. Location of Receptor SitesAn important point concerns the location of the receptor sites in skeletal muscle. The work of del Castillo an$. KatzZ has shown that these sites are found on the outer but not on the inner surface of the PJM. In addition, they can be found, more sparsely distributed, on the outer surface of the extrajunctional membrane of the muscle fiber. Extrajunctional receptors are easy to detect in chronically denervated muscle where the entire muscle fiber membrane becomes sensitive to acetylcholine. If we extend this thinking further, it is reasonable to speculate that chemosensitive receptor sites may also be located in the walls of the transverse tubules of each sarcomere because these tubules have been shown to be an inward extension of the plasma membrane. However, at the moment there is no direct evidence which indicates what kind of chemosensitivity exists in the transverse tubules. Activation of Receptor SitesSince the nature of membrane receptor sites is unknown we cannot directly measure their rate of reaction with an activating ion such as ACh. However, we can estimate the speed of onset of this interaction by measuring the changes in the PJM potential produced by the close range application of ACh. Several years ago there was originated in our laboratory an electrophoretic technique for applying ACh and similar ions to discrete sites at high speed.3 A diagrammatic representation of this technique is shown in FIGURE 1. With this arrangement, ACh can be rapidly applied to the PJM thereby causing it to undergo a short enduring depolarization. Increasing the amount of ACh applied increases the amplitude of the PJM depolarization and eventually an action potential can be initiated (FIGURE 2). If a still greater amount of ACh is ejected a train of action potentials is produced.

Desensitization in the Innervated and in the Chronically Denervated Soleus Muscle of the Mouse

“…This hypothesis is consistent with the Katz & Thesleff (1957) greater proportion of them into the desensitized configuration. Experimental evidence for greater desensitization at higher agonist concentrations has been obtained by Katz & Thesleff (1957), Miledi (1960a), Katz & Miledi (1964), Nastuk et al (1966 and Harvey & Dryden (1974a …”

“…The mean control contraction produced by 5.5 x 10-2M acetylcholine in 10 preparations was 0.14 ± 0.01 g and on repetition (Katz & Thesleff, 1957;Nastuk, Manthey & Gissen, 1966) in the chick biventer cervicis muscle and leech dorsal muscle (Rang & Ritter, 1970) and cultured skeletal muscle (Harvey & Dryden, 1974a). This conclusion is supported by the observation that following a contraction to KCI, the sensitivity to acetylcholine was not reduced, and therefore this type of desensitization does not follow the action of all agonists.…”

Innervation of heart cells in culture by an endogenous source of cholinergic neurons.

“…We used carbamylcholine to study cholinergic interactions due to its virtual resistance to hydrolysis by serum and endogenous esterases. Carbamylcholine is a cholinergic agonist with apparent dissociation constants ranging from 0.1 to 20,AM in nicotinic (25,26) and muscarinic (27,28) systems. We did not use acetylcholine in conjunction with esterase inhibitors (e.g., physostigmine) because of atropine-like In contrast, action potentials from heart cells in the same dish last several hundred milliseconds.…”

Further Studies of the Effect of Calcium on the Time Course of Action of Carbamylcholine at the Neuromuscular Junction

“…This gradual disappearance of the depolarizing action of acetylcholine is one aspect of a general decrease in chemosensitivity which occurs when cholinoceptive tissues are exposed to this agent for long periods. The phenomenon has been referred to by different authors as "desensitization" (1), "inactivation" (2), or "fade" (3), and it may also be related to the "second phase block" produced by decamethonium at the neuromuscular junction (2,4) and nicotine in autonomic ganglia (5). Since desensitization occurs during the action of several different cholinergic compounds (6) and in different tissues (3,(7)(8)(9)(10), it may be due to some general and fundamental feature of their action at cholinoceptive sites.…”