The electromotive action of acetylcholine at the motor end-plate

Calcium signalling mediated through α7 and non-α7 nAChR stimulation is differentially regulated in bovine chromaffin cells to induce catecholamine release

BACKGROUND AND PURPOSE

Ca(2+) signalling and exocytosis mediated by nicotinic receptor (nAChR) subtypes, especially the α7 nAChR, in bovine chromaffin cells are still matters of debate.

EXPERIMENTAL APPROACH

We have used chromaffin cell cultures loaded with Fluo-4 or transfected with aequorins directed to the cytosol or mitochondria, several nAChR agonists (nicotine, 5-iodo-A-85380, PNU282987 and choline), and the α7 nAChR allosteric modulator PNU120596.

KEY RESULTS

Minimal [Ca(2+) ](c) transients, induced by low concentrations of selective α7 nAChR agonists and nicotine, were markedly increased by the α7 nAChR allosteric modulator PNU120596. These potentiated responses were completely blocked by the α7 nAChR antagonist α-bungarotoxin (α7-modulated-response). Conversely, high concentrations of the α7 nAChR agonists, nicotine or 5-iodo-A-85380 induced larger [Ca(2+) ](c) transients, that were blocked by mecamylamine but were unaffected by α-bungarotoxin (non-α7 response). [Ca(2+) ](c) increases mediated by α7 nAChR were related to Ca(2+) entry through non-L-type Ca(2+) channels, whereas non-α7 nAChR-mediated signals were related to L-type Ca(2+) channels; Ca(2+) -induced Ca(2+) -release contributed to both responses. Mitochondrial involvement in the control of [Ca(2+) ](c) transients, mediated by either receptor, was minimal. Catecholamine release coupled to α7 nAChRs was more efficient in terms of catecholamine released/[Ca(2+) ](c) .

CONCLUSIONS AND IMPLICATIONS

[Ca(2+) ](c) and catecholamine release mediated by α7 nAChRs required an allosteric modulator and low doses of the agonist. At higher agonist concentrations, the α7 nAChR response was lost and the non-α7 nAChRs were activated. Catecholamine release might therefore be regulated by different nAChR subtypes, depending on agonist concentrations and the presence of allosteric modulators of α7 nAChRs.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

Deactivation and desensitization of mouse embryonic- and adult-type nicotinic receptor channel currents

Recombinant nicotinic acetylcholine receptor (nAChR) channels transiently expressed in HEK293 cells were investigated using the patch-clamp technique in the cell-attached and outside-out modes for single-channel analysis and ultra-fast agonist application to multiple channels. Deactivation (current decay after removal of agonist) and desensitization (current decay in the presence of agonist) were analyzed at embryonic- (gamma) and adult-type (epsilon) nAChR channels. Time constants of desensitization were similar for both receptor types (epsilon: 53.1+/-16.9 ms; gamma: 49.2+/-15.7 ms) and corresponded to the mean duration of clusters of single channel openings activated by pulses of 1 mM ACh. Deactivation showed distinct characteristics. Time constants were 1.76+/-0.16 ms for epsilon- and 3.19+/-0.18 ms for gamma-nAChR channels, corresponding to mean burst duration analyzed from single channels in the same preparation (epsilon: 1.85+/-1.2 ms, gamma: 3.85+/-2.1 ms). It is assumed that differences in deactivation are of functional relevance at the muscle endplate.

Paraldehyde and methylpentynol and ganglionic transmission

Paraldehyde and methylpentynol blocked transmission of nerve impulses through the superior cervical ganglion of the cat when the drugs were administered intra-arterially to the ganglion or intravenously using the nictitating membrane as an indicator. Electrical studies showed that concentrations of methylpentynol and paraldehyde which blocked transmission in the isolated rat superior cervical ganglion were without action on the preganglionic nerve fibre. In amounts which blocked transmission in the isolated rat ganglion, paraldehyde had no depolarizing activity directly on the ganglion cells and did not interfere with the depolarizing activity of added acetylcholine. The results suggest that the block in transmission of the impulse could be accounted for by a decrease in the release of acetylcholine from the preganglionic nerve terminals. In both species the block was reversible.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

The mode of neuromuscular blocking action of chlorpromazine

The inhibitory action of chlorpromazine on skeletal muscle has been studied with isolated preparations. In the nerve-muscle preparations of the frog sartorius and the rat diaphragm, the twitch responses to indirect stimulation are much more strongly depressed by chlorpromazine than those to direct stimulation. The conductivity of the nerve trunk is unaffected. The contractures of the frog rectus abdominis muscle caused by acetylcholine are depressed by chlorpromazine,but the contractures due to KCl are not influenced. Larger doses of chlorpromazine cause contracture by themselves, and this cannot be prevented by tubocurarine. In the sartorius muscle of the toad, the depolarization due to acetylcholine is reduced by chlorpromazine. The paralysing action of chlorpromazine adds to that of tubocurarine, and is antagonized to some extent by eserine or neostigmine. Muscles treated with chlorpromazine do not completely recover on washing. High concentrations of chlorpromazine depress the release of acetylcholine by motor-nerve stimulation, although they do not affect the enzymic synthesis of acetylcholine by acetone-dried powder of guinea-pig brain. The differences between the neuromuscular block produced by chlorpromazine and that by tubocurarine are discussed.

Centenary of the synapse: from Sherrington to the molecular biology of the synapse and beyond

Few concepts have meant more to neuroscience than the synapse, commonly understood to mean the junction between two excitable cells. The term was introduced by Charles Sherrington in 1897. The centenary of this event is an appropriate time to review the term's origins and utility. There are some surprises. The term didn't actually come from him. His concept was more functional than structural. The pioneering physiological and structural studies in the 1950s in fact did not lead to a rigorous definition. There is still confusion on how to define neurotransmitters. As molecular biological approaches are increasingly refining the concept of a fundamental synaptic unit, many types of neuronal interactions are appearing that do not fit with the synaptic concept. Are the neural circuits underlying behaviour strictly synaptic? In dealing with these questions, a longer perspective is useful for understanding how the term arose, how it has evolved to the present, and what kinds of challenges may be coming in the future.

Prolactin enables normal development of ACh-stimulated current in cultured larval bullfrog skin

The response to acetylcholine (ACh) can be used as a marker for larval-type bullfrog skin because apically applied ACh induces an increase in short-circuit current (SCC) in larval-type but not adult-type skin. EDTA-treated larval skin, which contains only basal cells and does not respond to ACh, was used as the starting material for our culture. ACh, carbamylcholine, and choline stimulated SCC in skin that had been cultured with aldosterone (5 x 10(-7) M) supplemented with prolactin (PRL; 2 micrograms/ml). Atropine and d-tubocurarine each inhibited the ACh-induced stimulation of SCC in skin so cultured. Eserine, an inhibitor of acetylcholinesterase, also inhibited the ACh response. Amiloride stimulated SCC itself, but it reduced the ACh response. All of these results are quite similar to those seen in intact larval skin, suggesting that a larval-skin had differentiated from the basal cells used as the starting point for our culture. This is the first physiological report that PRL induces differentiation in vitro into a true larval-type bullfrog skin.

Sherrington's legacy: evolution of the synapse concept, 1890s-1990s

The history of the synapse concept is traced from its inception in the 1890s to the present day. Three major periods are highlighted: the 1890s, the mid-twentieth century and modern times. The dependence of progress on the development of techniques is emphasised. The significance of the external context, the metaphors and general theory within which work proceeds, is also stressed. The development of the synapse concept can be seen to depend on a complex interplay between these internal and external factors. The reticularist/neuronist controversy provides a continuing leitmotiv.

A molecular scheme for the reaction between acetylcholine and nicotinic channels

In outside-out patches of mouse-muscle membrane, embryonic-like channels were activated by pulses of acetylcholine (ACh). On increasing the ACh concentration, the rate of desensitization, 1/tau d, increased linearly with the peak open probability, indicating desensitization from the open state. Desensitization had only one time constant tau d at each ACh concentration. Recovery from desensitization was only approximately 10 times slower than desensitization, whereas the probability of steady-state channel opening, declined to < 0.01 with > 10(-6) M ACh. The peak probability of opening in > 10(-4) M ACh pulse was close to 1. A linear reaction scheme was not compatible with these results. The scheme had to be expanded resulting in a circular scheme with two additional ACh binding steps to desensitized channel states. The approximate rate constants of all reaction steps in the circular scheme could be determined using computer simulations. The model predicted that clusters of channel opening had the average duration tau d at the respective ACh concentration. In cell-attached patches on intact muscle fibers, similar average cluster durations were observed at the respective ACh concentration. This indicates that tau d in the intact muscle fibers has similar values as in outside-out patches.

The effects of nerve terminal activity on non-quantal release of acetylcholine at the mouse neuromuscular junction

1. Local endplate depolarization induced by anticholinesterase application to mouse nerve-diaphragm preparations was taken as a measure of non-quantal release of acetylcholine. 2. Non-quantal acetylcholine release occurred within 20-60 s after anticholinesterase application, either spontaneously or evoked by nerve stimulation. Non-quantal release declined with time and disappeared after 3-5 min. 3. The amplitude of stimulation-evoked non-quantal release increased with the frequency of stimulation and was maximal at frequencies above 50 Hz. Two stimuli were sufficient to evoke the maximal effect. 4. Micromolar concentrations of atropine, pirenzepine and vesamicol reduced the amplitude and shortened the duration of non-quantal release. Oxotremorine (10(-8) M) enhanced the amplitude and ouabain (10(-4) M) prolonged the duration of non-quantal release. 5. Our results support the idea that the non-quantal release is due to the vesicular acetylcholine transport system which becomes transiently a part of the nerve terminal during exocytotic release of quantal acetylcholine.

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

Acetylcholine sensitivity of the spine-test articular capsule of the sea urchin Eucidaris tribuloides

1. The changes in the consistence of the spine-test articular capsule, or ligament, of the primary spines of Eucidaris tribuloides induced by acetylcholine (ACh) have been studied. Two complementary techniques were used: (a) "forced-vibration", which detects variations in the stiffness of the ligament along a single diametral plane; and (b) "forced-rotation" which records the spatial distribution of those changes. 2. ACh (1 microM to 1 mM) caused a rapid increase in the resistive force opposed by the ligament to passive stretching. Similar effects were elicited by several monoquaternary, N-substituted derivatives of trimethylammonium. 3. The opposite effect, i.e. softening, was induced by decamethonium, dimethylphenylpiperazine, and 2-ketoamyltrimethylammonium. 4. The involvement in these effects of ACh-binding groups with pharmacological properties similar to those of the "anionic sites" of nicotinic ACh receptors is suggested.

Agonist and blocking effects of choline at the neuromuscular junction

The effects of choline chloride were studied at the voltage-clamped frog neuromuscular junction by measuring miniature endplate currents and equilibrium dose response curves for acetylcholine applied by microionophoresis. Choline reduced the amplitude and shortened the time constant of miniature endplate currents in a dose dependent manner. Dose response curves carried out in the presence of low doses of choline (200 microM) were shifted to the right and the apparent dissociation constant for ACh was increased without affecting the Hill coefficient or the maximum conductance at the endplate. Higher doses of choline shifted the curve even further to the right but reduced the Hill coefficient and maximum conductance. Choline ionophoretic dose response curves were carried out but the conductance response was only about 1% of the response to comparable concentrations of ACh. In the presence of ethanol, which reduces the agonist dissociation constant, choline responses were increased and the dose response curve analysis revealed that the efficacy of choline was about 17% in comparison to ACh. Similar effects were measured at rat endplates. Rat nerve-muscle preparations were used to investigate the effects of choline upon neuromuscular transmission.

Agonist and inhibitory effects of pyridostigmine at the neuromuscular junction

When all of the AChE at the endplate is irreversibly inhibited by phospholine iodide the ionophoretically induced ACh endplate currents are increased more than 10-fold in amplitude. The reversible AChE inhibitor pyridostigmine only increases the current to about half this value because its effects are obscured by receptor blocking. It was found that pyridostigmine can activate the receptor ion channels when released by ionophoresis at the endplate, thus suggesting that agonist-like desensitization could contribute to the blocking effects.

Succinyl derivatives of N-tris (hydroxymethyl) methyl-2-aminoethane sulphonic acid: their effects on the frog neuromuscular junction

Succinic anhydride (SA) dissolved in Ringer solution buffered with N-tris (hydroxymethyl) methyl-2-aminoethane sulphonic acid (SA-TES solution) potentiates the depolarizing action of acetylcholine (ACh, 10-40 microM) on frog muscle and the tension induced by bath application of this agonist. Applied from one side of a double-barrelled micropipette, SA-TES increases the amplitude of iontophoretically elicited ACh potentials. The potentiation of the effects of ACh by SA-TES does not involve changes in either the activity of the ACh esterase or the input resistance of the muscle membrane. For depolarizations of frog sartorius muscle, dose-response relationships obtained for ACh concentrations from 0.5 to 20 microM indicate that SA-TES increases the apparent affinity of ACh by a factor of 3. SA-TES exerts an "accelerating' effect on the responses elicited by bath-applied ACh; i.e., it increases the rate of depolarization when ACh is added to the bath and the rate of repolarization upon washing out. These effects are particularly marked in preparations treated with neostigmine (3 microM). SA-TES does not potentiate the depolarizing action of agonists which do not contain an ester group. Moreover, the time course of the responses elicited by these compounds is not influenced by SA-TES. SA-TES fails to influence significantly the effects of the neurally released transmitter. Only a 10% increase in the average amplitude of the endplate potentials was observed. SA hydrolyzes in about 30 min at room temperature; however the SA-TES solution retains its activity for several weeks. Succinate is inactive, and so is SA in Ringer buffered with phosphate. The SA-TES solution contains seven succinyl-TES derivatives, which were separated by ion-exchange chromatography and paper chromatography. At concentrations between 1 to 150 microM, these succinyl-TES derivatives affected the ACh-induced contraction of frog rectus abdominus muscle. The most abundant derivative potentiated the action of high doses of ACh, but was inhibitory at lower ones. The other derivatives were mostly inhibitory. These results are discussed in terms of two hypotheses. One postulates the presence of a diffusion barrier formed by groups that bind ACh and are saturated by SA-TES. The other assumes that SA-TES acts directly on the ACh receptor exerting its potentiating effect through a cooperative mechanism.

Development and regeneration of motor systems under the influence of ACTH peptides

ACTH peptides exert quantitative and qualitative influences on the formation and maturation of motor units in developing and regenerating neuromuscular systems. ACTH 4-10, administered daily (10 micrograms/kg. s.c.) from the day of birth, accelerated the rate at which muscle strength developed in the immature rat, the effect of this peptide being most marked in animals 11-15 days old. A similar increase in grasping time occurred in ACTH 4-10 treated animals, indicating that the peptide affects neuronal maturation at a time in development when organization and maturation of the neuromuscular system is most active. The synthetic analogue of ACTH 4-9 (Org 2766), administered in the same dosage, had little effect on these parameters, indicating a differential sensitivity to these similar peptides. Elevated circulating titers of ACTH, whether exogenous (0.2 U ACTH 1-39 IP daily), or endogenous (adrenalectomy), stimulated the formation of more functional motor units, as indicated by increased amplitude of muscle action potentials and tetanic tension following nerve stimulation. ACTH appears to favor the recovery of high threshold, small-size motor units. Fine control of muscle function in peptide-treated animals is partially restored, as indicated by the return of stepwise recruitment to an extent not seen in the reinnervated, saline-treated controls.

Hyperpolarization produced by methohexitone in the chick biventer cervicis skeletal muscle

The effect of methohexitone on endplate potential and on depolarizations produced by acetylcholine (ACh) and tetraethylammonium (TEA) was studied in the isolated chick biventer cervicis skeletal muscle using a sucrose-gap recording technique. Methohexitone (5.5-180 uM) produced concentration-dependent hyperpolarization in the chick skeletal muscle without producing an initial contracture. Methohexitone had a differential effect on the depolarizations produced by ACh (5.5 uM- 11 mM) and TEA (0.95- 48 mM); it greatly reduced the ACh whereas it increased the TEA-induced responses. It was suggested that methohexitone had a dual postjunctional action, but a presynaptic effect was not ruled out.

Effect of lignocaine on the responses produced by depolarizing agents at the chick neuromuscular junction

The effect of lignocaine on the depolarization and contracture responses produced by acetylcholine (ACh) or tetraethylammonium (TEA) was studied in the isolated chick biventer cervicis (BVC) nerve-muscle preparation using the moving fluid electrode technique. Lignocaine (37 mumol X 1-1-1.8 mmol X 1-1) produced concentration-dependent contractures in the chick BVC muscle. The contractures produced by lignocaine were not accompanied by membrane depolarizations. ACh (5.5 mumol X 1-1-11.0 mmol X 1-1) and TEA (0.48-24.0 mmol X 1-1) produced concentration-dependent depolarizations and contractures in the chick BVC muscle. Lignocaine (0.93 mmol X 1-1) greatly reduced the responses produced by ACh, while it markedly increased the contracture responses produced by TEA. The depolarizations produced by TEA were reduced in lignocaine (0.93 mmol X 1-1). It was concluded that the local anaesthetic lignocaine produces contractures in the chick BVC muscle. Lignocaine potentiates the contractures produced by TEA, while it greatly reduces those produced by ACh.

The effects and interactions of caffeine, lignocaine and carbachol at the chick neuromuscular junction

The effects and interactions of lignocaine, carbachol and caffeine on the contractile response and endplate potential were investigated in the isolated chick biventer cervicis nerve-muscle preparation using a moving fluid electrode technique. Lignocaine (37 microM-1.8 mM) produced a dose-dependent contracture response that was not related to an endplate depolarization and appeared to inhibit non-competitively the usual depolarization and contractile response to carbachol (5.5 microM-0.8 mM). In contrast, lignocaine enhanced the contractile response to caffeine (0.54-10.8 mM) possibly by lowering the threshold and mainly by increasing the maximal response. The peak effects occurred at the same caffeine concentration regardless of the presence of lignocaine. The possibility that both lignocaine and caffeine may produce contractures in the chick muscle by acting on intracellular calcium concentration is discussed.