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

Human nicotinic acetylcholine receptor: the influence of second messengers on activation and desensitization

The amplitude and time course of acetylcholine(ACh)-induced membrane current were determined in cells of the human medulloblastoma cell line TE 671. ACh was applied and washed out very rapidly (about 50 ms) by a shift of a cell between two streams of solution, one of which contained the transmitter. ACh-induced current was recorded in the whole-cell mode of patch clamping. The time course of activation of the ACh-induced current could not be resolved because the method of ACh application was still too slow. Desensitization started immediately with ACh application; it could be described by two time constants. With an ACh concentration of 3 microM, the fast time constant was about 0.5 s and the slow time constant was about 3.9 s. When the ACh concentration was raised in steps to 100 mM, the peak amplitude of the current increased, reached a maximum at 1 mM and decreased again. The rate of desensitization was directly correlated with increasing ACh concentration. Current amplitude and desensitization time constants were not affected by intracellular application of cAMP or of a catalytic subunit of a cAMP-dependent protein kinase. When the intracellular calcium concentration was raised at a constant magnesium concentration, desensitization time constants remained unaffected, but the current amplitude decreased. This decrease is not caused by a decrease in single-channel conductance, therefore it may represent a decrease in the number of activatable channels.

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.

Two distinct kinetic phases of desensitization of acetylcholine receptors of clonal rat PC12 cells

1. The desensitization of nicotinic acetylcholine receptors on the PC12 sympathetic cell line was investigated by using a 22Na+ influx assay to measure receptor activation. 2. The rate of desensitization was dependent on temperature and at 4 degrees C two distinct kinetic phases were readily discernible: a rapid phase that was characterized by rate constants that were dependent on the chemical nature and concentration of the agonist, and a slower phase that was characterized by rate constants that were less dependent on these. 3. For acetylcholine, carbamylcholine and l-nicotine, the equilibrium desensitization parameter, Kdes, the concentration that produces half-maximal desensitization, was determined and compared with the corresponding value for Kact, the concentration that results in a half-maximal increase in the permeability response. For each agonist, the value of Kdes was found to be lower than Kact, a result to be expected if desensitization is associated with a higher-affinity state of the receptor than that associated with ion channel activation. Thus, extensive receptor desensitization can occur even at agonist concentrations that do not produce appreciable channel activation. Both activation and desensitization functions exhibited positive cooperativity so that each function occurs over a narrow range of agonist concentrations. 4. Following removal of the agonist, recovery from desensitization was reversible and occurred by two distinct kinetic phases characterized by rate constants that were independent of the chemical nature and concentration of the agonist that produced the desensitization. The relative contribution of each kinetic phase of recovery was, however, dependent on the duration of prior exposure to agonist. Following short incubation periods with agonist, most of the receptors were in a rapidly recovering state. With increasing duration of exposure, progressively more of the receptors were converted to a desensitized state that recovered more slowly. 5. The rate constants associated with the two kinetic phases of recovery were dependent on the recovery temperature. Following the initial rapid phase of desensitization, recovery at 4 degrees C was characterized by a time constant, t1/2, of 1.9 min, a value that was about 3-fold greater than that observed at 22 degrees C. The rate of recovery of the desensitized state achieved following equilibrium exposures to agonists was considerably more temperature dependent: recovery of this desensitized state was characterized at 4 degrees C by a t1/2 of 62 min that was about 37-fold greater than that at 22 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of cholinergic activation and desensitization at snake twitch and slow muscle fibre end-plates

Characteristics of receptor-channel activation and desensitization have been compared at voltage-clamped snake slow and twitch fibre end-plates maintained in an isotonic potassium propionate solution. Miniature end-plate current (m.e.p.c.) decay was slower and less voltage dependent at slow fibre end-plates than at twitch fibre end-plates. The peak m.e.p.c. amplitude versus voltage relationship and reversal potential were similar at the two end-plate types. Acetylcholine-induced noise and m.e.p.c.s were recorded at slow fibre end-plates. At most slow fibres the spectral density was not adequately fitted by a single Lorentzian function. Rather, the observed spectral density was greater at high frequencies than the values predicted using the m.e.p.c. decay rate. The noise could be well described by the sum of two Lorentzian functions, one of which corresponded to a single Lorentzian function with the corner frequency determined by the m.e.p.c. decay rate. The shape of the carbachol concentration-peak end-plate current relationship was similar at both slow and twitch fibre end-plates. However, for all concentrations tested, the peak carbachol-induced end-plate current (e.p.c.carb.) value was markedly less at slow fibre end-plates than at twitch fibre end-plates. The onset of desensitization was determined using two methods. The first concerned analysis of the time course of decay of the e.p.c.carb. from a peak value during the sustained application of agonist. The second involved a double-perfusion technique in which a 'desensitizing' dose was applied for varying intervals before the application of a second 'test' dose of carbachol. With both methods the development of desensitization at both end-plate types was dependent on carbachol concentration and duration of exposure. At each end-plate type the time course of desensitization onset often exhibited two components; one with a time constant of seconds and a slower component having time constants in the range of tens to hundreds of seconds. The slope of the relationship between carbachol concentration and equilibrium desensitization at slow and twitch fibre end-plates was close to two, suggesting that two molecules of agonist are probably bound during the development of desensitization. However, for all concentrations tested, desensitization developed more rapidly and to a greater extent at twitch fibre end-plates than at slow fibre end-plates. The voltage dependence of the 3 min steady-state desensitization produced by 108 microM-carbachol was very similar (approximately -0.0250 mV-1) at both fibre types.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of calcium on the binding of phencyclidine to acetylcholine receptor-rich membrane fragments from Torpedo californica electroplaque

The influence of calcium on the binding of phencyclidine (PCP) to acetylcholine (ACh) receptor-rich membrane fragments was investigated. Calcium decreased the equilibrium affinity for PCP in the presence, but not in the absence, of the cholinergic agonist carbamylcholine. The effect of calcium was rapidly reversible by EGTA, indicating that it was not attributable to a calcium-activated protease or a phospholipase. Following detergent solubilization of the nicotinic ACh receptor, the calcium effect on PCP remained, suggesting that calcium may interact directly with the receptor to exert its effect. Other divalent cations (Mn2+, La2+, Co2+, Mg2+) had similar effects. A correlate of "desensitization" of the ACh receptor can be observed using PCP binding, and a two-step "desensitization" process can be observed. Calcium seemed to increase the amplitude of a rapid component of receptor "desensitization." The results presented in this paper suggest that calcium may play a role in the modulation of the nicotinic ACh receptor.

Two-component desensitization at the neuromuscular junction of the frog

Desensitization to ionophoretically applied ACh has been studied at voltage-clamped neuromuscular junctions of Rana pipiens. The time courses of both the onset of and recovery from desensitization displayed two components. The application of metabolic inhibitors to the preparation decreased the degree of recovery and also decreased both time constants of onset. The effect on onset was more pronounced on the slow component while no effect selective for either of the two components of recovery was seen. Intracellular injection of EGTA into the muscle cell increased both time constants of desensitization onset. The effect was much more pronounced on the slow component. Increasing the dose of ACh selectively decreased the fast time constant of desensitization onset. The effect was more selective in low-calcium than normal Ringer solution. These observations suggest that there are at least two independent mechanisms in the process of desensitization at the neuromuscular junction. These mechanisms differ in time course, degree of dependency on [Ca]i, and sensitivity to acetylcholine dose.

Magnesium increases rate of onset of desensitization in frog muscle

The effect of changes in extracellular and intracellular magnesium content on desensitization rate in frog (Rana pipiens) sartorius muscle was studied using measurements of input conductance of single fibers during local superperfusion of the postjunctional region with carbamylcholine chloride (0.27-2.7 mM). Two intracellular KCl-filled glass capillary microelectrodes were used for current injection and recording in fibers equilibrated in high-K+ media (33-165 mM). In low-ionic strength solutions (33 mM K), time to half-decline of conductance during desensitization to carbamylcholine (T1/2) was decreased by increasing extracellular Mg2+ concentration throughout range 0-100 mM. Equivalent effects on T1/2 were produced by increased extracellular Ca2+ in lower concentrations (0-10 mM). Increase in K+ concentration or decrease of carbamylcholine concentration resulted in increased T1/2 in both Mg2+ and Ca2+ media. Increase of intracellular Mg2+ by soaking in high-Mg2+ solutions (100 mM) or by intracellular iontophoresis caused decrease in T1/2. It is concluded that Mg2+, when introduced either in the extracellular or intracellular phase, can promote increased desensitization rate and that this action is similar to but weaker than that of calcium.

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.

Voltage dependence of desensitization. Influence of calcium and activation kinetics

The voltage dependence of carbachol-induced desensitization has been analyzed in potassium-depolarized frog sartorius muscle preparations with voltage clamp techniques over a wide voltage range (-120 to +40 mV). Desensitization developed exponentially at all voltages with tau, the time constant of desensitization onset, varying as a logarithmic function of membrane voltage. The voltage dependence of tau remained in calcium-deficient solutions and was not altered by elevating either the level of extracellular or intracellular calcium. We have analyzed our results according to a simple sequential kinetic scheme in which the rate-limiting step in the development of desensitization is a transition of the receptor channel complex from the activated conducting state to a desensitized, nonconducting state. We conclude (a) that the observed voltage sensitivity of desensitization primarily resides in the voltage dependence of this transition, and (b) the kinetics of activation appear to have a greater influence on the observed rate of desensitization than on its voltage dependence. The magnitude of the voltage dependence suggests that a greater change in free energy is required for the transition to the desensitized state than for the transition between the open and closed states of the receptor channel complex.

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.