Effect of the desensitization-potentiating agent SKF-525a on frog end-plate currents

The mechanisms of inhibition of frog endplate currents with homologous derivatives of the 1,1-dimethyl-3-oxybutyl phosphonic acid

The mode of inhibition of endplate currents by four esters of 1,1-dimethyl-3-oxybutyl phosphonic acid with different lipophilicities and molecule lengths were estimated by mathematical modeling based on previous electrophysiological data supplemented by several experiments with rhythmic stimulation. The aim was to discriminate between their receptor and non-receptor effects. It was shown that all esters have a two-component mechanism of depression: inhibition of the receptor open channel and allosteric modulation of the receptor-channel complex. The ratio of both functional components depends on the length and lipophilicity of the esters. Short and less lipophilic esters mostly act as open channel inhibitors and the rate of inhibition substantially depends on the rate of stimulation, i. e. probability of the receptor-channel opening. As the length of the ester radicals and their lipophilicity increased, these compounds were more active as allosteric receptor inhibitors, probably hindering the function of nAChRs from the lipid annulus.

Pre- and postsynaptic effects of the calcium channel blocker verapamil at neuromuscular junctions

Experiments on the frog sartorius muscle were used to study the effects of the L-type calcium channel blocker verapamil on endplate currents. Verapamil had no effect on the amplitudes of miniature and multiple-quantum endplate currents, the synchronicity of transmitter secretion, or repeat activity in nerve endings. Verapamil had no effect on the decay of miniature currents, but accelerated that of multiple-quantum currents. This effect was sharply increased after inhibition of cholinesterase activity. In conditions of inhibited cholinesterase activity, verapamil depressed currents during rhythmic stimulation. This depression was more marked in synapses with high quantal compositions and in conditions of membrane depolarization. Thus, the sensitivity of neuromuscular junction calcium channels to verapamil was unrelated to the release of transmitter from the motor nerve ending either at physiological levels of secretion or when secretion was potentiated by potassium channel blockers. At the postsynaptic level, the effect of verapamil was insignificant in relation to cholinoreceptors in the resting and active states, though verapamil could cooperatively enhance the transition of postsynaptic receptors into the desensitized state in conditions of prolonged transmitter action.

The role of desensitisation in decay time of miniature endplate currents in frogs Rana ridibunda and Rana temporaria

A new comparative characteristic of endplate microphysiology has been introduced. It is the feasibility of receptors to become desensitised as demonstrated on two frog species, Rana temporaria and Rana ridibunda: the decay times (tau(MEPC)) of single quantum miniature endplate currents (MEPCs) in the sartorius muscles of both species were about 1 ms and were not affected by the desensitisation-promoting agent proadifen when AChE was active. However, when the desensitisation was induced by anticholinesterase neostigmine and promoted by proadifen, the prolongation of tau(MEPC) from 1 ms was almost twice as great in Rana temporaria (tau(MEPC) = 4.4 ms) than in Rana ridibunda (tau(MEPC) = 3.1). This indicates that desensitisation reduces the number of available receptors and lowers the number of available ACh molecules for repetitive binding by trapping them by desensitised, high-affinity receptors significantly more in Rana ridibunda than in Rana temporaria. The application of proadifen, a promoter of desensitisation, decreased the prolongation of MEPCs in both species, but this shortening was more rapid in Rana ridibunda than in Rana temporaria. It is concluded that the desensitisation-induced reduction in the density, and the number of postsynaptic receptors is significantly higher at Rana ridibunda than in Rana temporaria endplates.

Desensitization of the post-synaptic membrane of neuromuscular synapses induced by spontaneous quantum secretion of mediator

Experiments on isolated frog neuromuscular junctions in voltage-clamped conditions demonstrated increases in the probability of spontaneous release of quanta of mediator, resulting in increases in the K+ ion concentration in the perfusion solution, and changes in the endplate miniature current amplitude induced by increased quantum generation were studied. Factors promoting the desensitization of the post-synaptic cholinergic membrane (inhibition of acetylcholinesterase and addition of proadifen) to levels greater than a certain (critical) frequency of endplate miniature current of the order of 50 spikes/sec were found to result in accumulation of activity and progressive reductions in the sensitivity of the post-synaptic membrane to the mediator acetylcholine.

Desensitization shortens the high-quantal-content endplate current time course in frog muscle with intact cholinesterase

1. The desensitization induced by bath-applied carbachol or acetylcholine (ACh) and potentiated by proadifen (SKF 525A) was studied in the frog sartorius with intact synaptic acetylcholinesterase (AChE). 2. The reduction in the density and number of postsynaptic receptors produced by desensitization lowered the amplitude of the endplate currents (EPCs) and shortened the EPC decay when the quantal content (m) of the EPC was about 170 and when multiple release of quanta at single active zones was highly probably. The shortening of high-quantal-content EPCs persisted for at least 15 min after the wash-out of agonists, at a time when the amplitude had recovered fully. 3. The decay times of the low-quantal-content EPCs recorded from preparations pretreated with 5 mM Mg2+ (m approximately 70) and single-quantum miniature endplate currents (MEPCs) were not affected by carbachol, ACh or proadifen. 4. The desensitization of ACh receptors potentiated by proadifen, prevented completely the 6- to 8-fold prolongation of EPC which was induced by neostigmine inhibition of synaptic AChE. 5. It is assumed that high-quantal-content EPCs increase the incidence of multiple quanta release at single active zones and the probability of repetitive binding of ACh molecules which leads to EPC prolongation. The shortening which persists after complete recovery of the amplitude during wash-out of the exogenous agonist is probably due to 'trapping' of ACh molecules onto rapidly desensitized receptors and the reduced density of functional AChRs during the quantum action.

Indirect inhibitory effect of succinylcholine on acetylcholine-activated channel activities and its modulation by external Ca2+ in mouse skeletal muscles

1. The effect of extracellular calcium on single acetylcholine (ACh)-activated channel activities when desensitizing concentrations of succinylcholine (SuCh) were applied to the surrounding endplate membrane was investigated by the cell-attached patch-clamp technique at endplates of single skeletal muscle (flexor digitorum brevis) fibres of adult mice. 2. Bath-applied SuCh (0.1-3 microM, in 2.5 mM Ca2+) increased in a concentration-dependent manner the mean open time of ACh-activated channel currents recorded at membrane potentials which cancelled the SuCh-induced depolarizations. 3. In the presence of 0.5 and 2.5 mM external Ca2+, SuCh (3 microM) applied outside the patch pipette prolonged the mean open time of ACh-activated channel currents in a time-dependent manner (by 45% and 52%, respectively), and simultaneously significantly decreased the single channel conductance (by 14% and 10%, respectively). These SuCh-induced effects did not occur in a nominally Ca(2+)-free extracellular medium. 4. Under the same conditions, SuCh (3 microM) augmented the time-dependent decline in the opening frequency of ACh-activated channel currents obtained in nominally Ca(2+)-free medium. 5. These results suggest that external calcium ions act to modulate nicotinic ACh receptor channel activity, and accelerate desensitization of the receptor.

Mechanisms of the synergistic interactions between organic calcium channel antagonists and various neuromuscular blocking agents

The effects of Mn2+, neomycin and four organic Ca2(+)-channel antagonists (OCA): nicardipine, nifedipine, diltiazem and verapamil on the neuromuscular blocking activities of tubocurarine, succinylcholine (SCh), decamethonium and neomycin were studied in isolated mouse phrenic nerve-diaphragm preparations. The effective concentration of SCh for 50% inhibition (IC50) of single indirect twitch responses were reduced markedly by more than 3-fold when the preparations were pretreated with OCA at 10 microM; the latter alone did not appreciably affect the indirect twitch response or the amplitude of miniature endplate potentials. The neuromuscular blocking effect of decamethonium was also enhanced synergistically by OCA to a similar extent. On the other hand, under the comparable condition. the combined uses of OCA plus tubocurarine or neomycin, neomycin plus tubocurarine or SCh, and Mn2+ plus tubocurarine, SCh or neomycin all resulted in insignificant potentiation. These results suggest that OCA have a specific effect to enhance the agonist effect of depolarizing agents on nicotinic acetylcholine receptors. Nicardipine at 2 microM non-competitively inhibited depolarizations of endplates elicited by SCh and decamethonium and abolished them completely at 10 microM nicardipine. The IC50's in inhibiting endplate potentials and miniature endplate potentials by SCh and decamethonium were also reduced 2 to 3.5-fold by nicardipine. It is inferred that OCA are endowed with a unique capability to allosterically affect the postsynaptic nicotinic acetylcholine receptor, promoting its desensitization liability, hence synergistic interaction with depolarizing agents. Presynaptic effects of OCA are probably not involved.

Organic calcium channel antagonists provoke acetylcholine receptor autodesensitization on train stimulation of motor nerve

The effects of nicardipine and other organic Ca2(+)-channel antagonists on the responses induced by indirect train stimulation (3 s, 50-100 Hz) were studied in mouse phrenic nerve diaphragm preparations. Nicardipine at 1-10 microM, which alone did not affect single or tetanic contractions or the amplitude of evoked endplate potentials and spontaneous miniature endplate potentials, caused tetanic contraction to fade completely in the presence of 0.3 microM neostigmine or 50 microM diisopropylfluorophosphate. In combination with these anticholinesterases, nicardipine caused a severe run-down and shortening of endplate potentials in 1-2 s. This effect on endplate potentials was dependent on stimulus frequency and on extracellular Ca2+. The effect was accelerated by intracellular injection of Ca2+, but retarded by injection of EGTA. The amplitudes of miniature endplate potentials and the evoked endplate depolarization were also depressed during repetitive stimulation. On termination of repetitive stimulation, all postsynaptic responses, including evoked endplate potentials, miniature endplate potentials and single twitches, recovered to pre-train level in 3-10 s. These results suggest that the postsynaptic nicotinic receptors had lost the functional activity during repetitive stimulation. The time-courses of the aforementioned changes initiated by repetitive stimulation were similar to the fast phase of desensitization induced by acetylcholine. The irreversible action of alpha-bungarotoxin on acetylcholine receptor was attenuated in the presence of nicardipine and neostigmine if repetitive stimulation was applied. The same effects were observed with other organic Ca2(+)-channel antagonists (diltiazem, verapamil and nifedipine) as well as agonist (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine- 5-carboxylate, BAY K8644), but not with Mn2+, theophylline or caffeine. It is inferred that organic Ca2(+)-channel antagonists interact directly with acetylcholine receptor ion channel, enhance its autodesensitization liability and thus cause extinction of endplate potentials on repetitive stimulation.

Development of desensitization during repetitive end-plate activity and single end-plate currents in frog muscle

1. The amplitudes of end-plate currents (EPCs) in short trains of fifteen to seventeen EPCs at 10 Hz were depressed in the presence of 10 microM-proadifen when acetylcholinesterase (AChE) was inhibited. 2. The proadifen-induced EPC depression was voltage-dependent and the effect was more pronounced at negative membrane potentials. 3. In the presence of proadifen, the mean amplitude of miniature end-plate currents (MEPCs) was reduced by 36% 5 s after the EPC train as compared with MEPCs before the train. 4. Without proadifen, but with inhibited AChE, an increase of temperature from 20 to 26 degrees C and elevation of external Ca2+ from 1.8 to 2.5 mM led to EPC amplitude depression in the train, which was also potential-dependent. 5. After AChE inhibition, proadifen (10 microM) progressively shortened MEPC decay without significant reduction of amplitude up to 40 min of exposition. MEPCs were not affected by proadifen when AChE was active. 6. It is concluded that these postsynaptic effects of proadifen can be explained neither by its action on the resting acetylcholine receptors (AChR) nor on open ion channels but are due to its desensitization-promoting action.

Stimulation of vascular prostacyclin by SKF 525-A (proadifen) and related compounds

SKF 525-A (proadifen), a well-known inhibitor of drug metabolism and cytochrome P-450 activity, stimulated the release of prostacyclin (PGI2) from the rabbit aorta in vitro. The threshold concentration producing a detectable effect was 20 microM; the time course of SKF 525-A action exhibited particular features--progressive onset, long duration and slow reversibility--distinct from those of other stimuli (ADP, ionophore A23187 f.i.). The PGI2-stimulating activity of SKF 525-A was characterized by specific structural requirements: activity was abolished by the deletion of the terminal propyl group and increased by its elongation into an isobutyl group; chlorination of the phenyl groups increased the potency. SKF 525-A increased the production of PGI2 by cultured endothelial cells from bovine aorta and human umbilical vein, but had no effect on cultured smooth muscle from the bovine aortic media. Stimulation of PGI2 release could be explained by an increased availability of free arachidonic acid, which was probably independent from cytochrome P-450 inhibition. In human platelets, SKF 525-A inhibited prostaglandin and thromboxane production induced by A23187, thrombin and ADP. Simultaneous stimulation of endothelial PGI2 and inhibition of platelet TxA2 represents an original pharmacological profile: SKF 525-A might thus constitute the prototype of a new class of antiplatelet drugs.

TI-233 as a glutamate channel blocker at the crayfish neuromuscular junction

Effects of TI-233 (4-isopropyl-1-[N2-(5,6-dimethyl-aminonaphthalene-1-sulphonyl)-L-arginyl ]- piperidine) on glutamate-induced responses and nerve-evoked synaptic responses were compared at the crayfish neuromuscular junction. Intracellularly recorded excitatory junctional potentials (e.j.ps) were markedly augmented by TI-233 when they were evoked at long intervals, whereas the unit size of extracellular e.j.ps was hardly affected by TI-233 and, at that stage, the glutamate-induced current was markedly reduced by TI-233. The decay rate of extracellular e.j.ps was slightly increased 3 min after the addition of TI-233 at concentrations higher than 0.05 mM. Repetitive stimulation of the excitatory axon at a high frequency caused a gradual decrease in the amplitudes of extracellular e.j.ps in the presence of TI-233. After prolonged application of TI-233 with repetitive nerve stimulation, the glutamate-induced response became significantly smaller than the control. TI-233 increased the input resistance of the crayfish muscle fibre and facilitated transmitter release at the excitatory neuromuscular junction. These two effects would entirely explain the augmentation of intracellular e.j.ps by TI-233. TI-233 (greater than 3 microM) reduced the amplitude of current responses to trains of glutamate pulses in a dose-dependent manner, but this reduction by TI-233 was time- and activity-dependent. The effect of TI-233 on glutamate-induced responses was voltage-dependent and hyperpolarization increased this effect. Pretreatment of the muscle fibre with concanavalin A did not affect the gradual decline, caused by TI-233, of the successive currents evoked by a train of glutamate pulses. The apparent differences between the glutamate-induced current and nerve-evoked synaptic response revealed by TI-233 can be explained by open-channel block of the glutamate-activated ion-channel, and do not confute the hypothesis that glutamate is the natural transmitter substance at this junction.

Trifluoperazine and calcium antagonists accelerate cholinergic desensitization in Aplysia neurons

The onset of desensitization of excitatory acetylcholine responses in isolated, voltage clamped Aplysia RB neurons to microperfused agonist was accelerated by trifluoperazine and the calcium antagonists SKF-525A and D-600. In the absence of these drugs the time course of desensitization is described as the sum of two exponentials plus a constant. All 3 compounds accelerated the fast exponential component of desensitization, but had opposite effects on the slow component: trifluoperazine and SKF-525A accelerated the slow component, whereas this phase was greatly slowed by D-600. These results provide further evidence that the two components of cholinergic desensitization may be independent and influenced by agents which may affect calcium binding to the acetylcholine receptor.

End-plate currents evoked by paired stimuli in frog muscle fibres

Using voltage-clamp techniques spontaneously occurring miniature end-plate currents (mepc) and nerve-evoked end-plate currents (epc) were recorded in frog glycerol-treated or cut muscle preparations. Epcs were induced by pairs of stimuli (the delay of the 2nd stimulus, delta t being 6 ms-30 s; one pair was delivered every 60-90 s). The decay time constant of the epc (tau epc) was longer, the larger its quantal content despite the presence of active acetylcholinesterase (AChE). After treatment with anticholinesterases (prostigmine or armin, an irreversible inhibitor) this increase in tau epc became more pronounced. When AChE was fully active the decay of the 1st epc (tau 1) was slightly faster than the decay of the 2nd epc (tau 2) only when the interstimulus interval was rather short (delta t less than 20 ms). Following treatment with anticholinesterases this difference between tau 2 and tau 1 could be determined even when delta t was as long as 30 s. In anticholinesterase-treated preparations delta tau was found to be inversely proportional to log delta t: a 50% increase in the decay time-constant of the 2nd epc occurred with delta t = 120 ms. During continuous stimulation (10 impulses/s) tau epc increased from the 1st to the 5-6th responses, but then decreased in parallel with the fall in the epc amplitude. The phenomenon of postsynaptic potentiation we observed could be readily abolished when quantal content was decreased by the presence of magnesium ions, but it was relatively unaffected when the receptor density was decreased by alpha-bungarotoxin (alpha-BuTX).(ABSTRACT TRUNCATED AT 250 WORDS)