The interaction between caffeine and calcium in the desensitization of muscle postjunctional membrane receptors

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.

Ruthenium red reduces acetylcholine sensitivity and increases desensitization at the frog neuromuscular junction

The actions of Ruthenium Red on the synaptic membrane were studied at the neuromuscular junction of the frog Rana pipiens. Ruthenium Red blocks mitochondrial calcium transport and thus is expected to elevate the intracellular calcium level [Alnaes and Rahamimoff (1975) J. Physiol., Lond. 248, 285-306]. The postjunctional membrane sensitivity was reduced by Ruthenium Red as determined by (a) the amplitude of the miniature endplate potentials, (b) iontophoretic application of carbachol, (c) microperfusion of carbachol. Desensitization was assayed by measuring the decline in the amplitude of the postsynaptic depolarization produced during repetitive iontophoretic application of carbachol. Following Ruthenium Red treatment desensitization was increased. This action of Ruthenium Red was enhanced by raising the extracellular calcium concentration from 1.8 to 10 mM.

Response of isolated guinea pig myocardium to insulin therapy during normothermia and graded hypothermia

The discovery of insulin in 1922 aroused immediate clinical interest in its use in heart disease. In severe heart failure, insulin release is suppressed by the combined effect of poor pancreatic perfusion and by increased sympathetic activity. In these circumstances, myocardial metabolism of glucose may break down through the deficiency of insulin. Because of this, glucose, insulin and potassium solution (GIK solution) has been used in cardiopulmonary resuscitation. However, its mechanism is not yet fully known. This study was designed to determine the effect of insulin on cardiac muscle at various temperatures. The mechanical response of papillary muscle isolated from guinea pig ventricle was observed under various thermal conditions (23-37 degrees C). Twitch tension was increased by the administration of 0.2 I.U./ml insulin under each thermal condition. In all circumstances, the increase in contractile force was noted about 2 min after the administration of insulin. The effect of insulin on 20 preparations demonstrated the mean maximum contractile force was 226% ( +/- 34 S.D., n = 5) in 37 degrees C, 194% ( +/- 36 S.D., n = 5) in 30 degrees C, 190% ( +/- 30 S.D., n = 5) in 27 degrees C and 200% ( +/- 36 S.D., n = 5) in in 23 degrees C. The differences between different temperatures was not significant. The effect of insulin during depression Na-K pump by high concentration of ouabain (g-strophanthin, 10(-5) M) was also observed. Insulin (0.2 I.U./ml) was administered when the papillary muscle showed no response to electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of sodium and calcium ions and membrane potential on glutamate receptor desensitization

1. The effects of Na, Ca and membrane potential on desensitization of postjunctional glutamate receptors on locust muscle were investigated. 2. The kinetics of desensitization were measured ionophoretically. 3. Replacement of Na by equimolar concentrations of the permeant cations Li, NH4 and guanidine and the impermeant cation choline accelerated desensitization onset, increased the steady-state leve of desensitization and reduced the rate of recovery from desensitization. 4. Desensitization onset rates and steady-state levels of desensitization were not significantly altered either by changing the extracellular Ca concentration or by changing the membrane potential of voltage clamped muscle fibres.

Desensitization of the acetylcholine receptor of denervated rat soleus muscle and the effect of calcium

1 Desensitization of the extrajunctional acetylcholine receptors of denervated rat soleus muscle has been studied by evoking iontophoretic acetylcholine potentials at a high frequency (10 Hz). 2 Desensitization occurred with a slow and at least one fast component. 3 Increasing the dose of acetylcholine decreased the half time of onset of desensitization, but did not alter recovery half time. 4 Alteration of external calcium between 0 and 50 mM, or perfusion with agents which increase intracellular calcium such as 2,4-dinitrophenol and the calcium ionophores, X537A and A23187, did not alter desensitization half time. 5 The cholinoceptor agonists acetylcholine, acetylthiocholine, tetramethylammonium and phenyltrimethylammonium all caused desensitization at the same rate.

Desensitization and recovery at the frog neuromuscular junction

The time course of carbachol-induced desensitization onset and recovery of sensitivity after desenitization have been compared at the frog neuromuscular junction. The activation-desensitization sequence was determined from input conductance measurements using potassium-depolarized muscle preparations. Both desensitization onset and recovery from desensitization could be adequately described by single time constant expressions, with tauonset being considerably shorter than taurecovery. In nine experiments, tauonset was 13+/-1.3 s and taurecovery was 424+/-51 s with 1 mM carbachol. Elevating the external calcium or carbachol concentration accelerated desensitization onset without changing the recovery of sensitivity after equilibrium desensitization. Desensitization onset was accelerated by a prior activation-desensitization sequence to an extent determined by the recovery interval that followed the initial carbachol application. The time course of return of tauonset was closely parallel to, but slower than the time course of recovery of sensitivity. These results are consistent with a cyclic model in which intracellular calcium is a factor controlling the rate of development of desensitization.

Permeability of the endplate membrane activated by acetylcholine to some organic cations

The ability of various organic cations to depolarize the ACh-activated endplate membrane in the absence of Na ions was examined on frog sartorius muscle by measuring the endplate potential on the muscle surface with the moving electrode technique. The ACh-activated endplate membrane was very permeable to ammonium and its methyl and hydroxy derivatives, and moderately permeable to guanidine derivatives and Tris (hydroxymethyl) aminomethane. The permeability of alkylol derivatives of ammonium diminished progressively with increase in molecular size. The present results suggested that the endplate ionic channels can be represented by a pore of about 6.4 A in diameter.

Effect of ionophore X-537A on desensitization rate and tension development in potassium-depolarized muscle fibres

The effects of the ionophore X-537A were studied on carbamylcholine (carbachol)-induced densitization and on tension development in relaxed potassium-depolarized frog sartorius muscles. 2 X-537A accelerated carbachol-induced desensitization in Ca2+-deficient solutions without having any effect on the conductance of the membrane in the absence of carbachol or on the extent of the carbachol-induced increase in conductance. 3 In Ca2+-deficient solution, the acceleration of desensitization by the ionophore was concentration-dependent. No effect was observed with concentrations less than 5 muM and maximal acceleration was evident with 10 muM. 4 The influence of X-537A on desensitization was time-dependent. At 20 muM X-537A, there was a marked acceleration of desensitization by the end of 5 min exposure. An additional gradual acceleration occurred during a 5 to 30 min treatment. No acceleration of desensitization was evident when X-537A was simultaneously applied with carbachol to the end-plate region without prior exposure to the ionophore. 5 Desensitization also was accelerated by 30 min exposure to 20 muM X-537A in solutions containing Ca2+ or deficient in both Mg2+ and Ca2+; the rate being increased 2.8-fold in Ca2+-containing solutions, 2.9-fold in Ca2+-deficient solutions containing Mg2+, and 2.5-fold in divalent cation-deficient solutions. 6 Tension development gradually occurred in relaxed potassium-depolarized muscle preparations exposed to 20 muM X-537A. The onset of tension development occurred only after approximately 25 min of exposure both in preparations kept in Ca2+-deficient or Ca2+-containing solutions. By the end of 90 min in the ionophore, the tension developed was approximately 12% and 23% of the initial potassium contracture in those preparations maintained in the Ca2+-deficient or Ca2+-containing solutions, respectively. 7 We assume that the increase in desensitization rate following exposure to X-537A results from an elevation of the intracellular Ca2+ concentration. That muscle tension gradually increased during exposure to the ionophore supports this conclusion. The acceleration of densitization by X-537A in the absence of external Ca2+ supports the view that the site of calcium acceleration is not on the external surface of the end-plate membrane either at or near the agonist-recognition site but rather on the inner surface.

Puromycin's suppression of memory in mice as affected by caffeine

It has previously been shown that expression of maze-learning in mice is blocked for long periods of time by puromycin injected intracerebrally one or more days after the training experience. Treatment with caffeine after training has now been found to reduce greatly the amnestic effects of puromycin. With a high dose of caffeine (200 mg/kg) this reduction is evident 6 days after treatment with puromycin. With a lower dose of caffeine (25 mg/kg) the effect becomes evident only after a more extended period of time. In view of control experiments, we suggest that caffeine modifies factors necessary for the expression of memory and that this alteration makes puromycin relatively ineffective in blocking memory.