Effects of edrophonium on end-plate currents in frog skeletal muscle

The effects of edrophonium on end-plate currents measured at frog neuromuscular junctions were assessed using the two microelectrode voltage clamp technique. Low concentrations of edrophonium (10^?5 M and 2.5 × 10^?5 M) increased the peak amplitude and the kinetic parameters of end-plate currents measured in curare-Ringer and in 5 mM Ca²⁺–10 mM Mg²⁺-Ringer solutions. At the highest concentration tested (6 × 10^?5 M), edrophonium decreased the peak amplitude and maximum rate of rise but had little effect on the time to peak. Time to half decay was the only parameter studied which continued to increase as edrophonium concentration was increased from 10^?5 M to 6 × 10^?5 M. Normalized values of the parameters for both 10^?5 M and 2.5 × 10^?5 M edrophonium revealed no significant difference between end plates bathed in curare-Ringer and those bathed in 5 mM Ca²⁺–10 mM Mg²⁺-Ringer.It is suggested that the low concentration of edrophonium inhibit the postjunctional acetylcholinesterase thereby causing an increase in transmitter concentration in the end-plate region, but that the highest concentration (6 × 10^?5 M) has a direct desensitizing action as well as the anticholinesterase action on the end-plate membrane. This desensitizing action is present although no depolarization of the end-plate membrane is observable.     

A comparison of the effects of two inhibitors on brain cholinesterase

In the present paper various routes of administration (i.m., i.v. and i.c.v.) of physostigmine are compared and the effect of two drugs producing inhibition of cholinesterase, physostigmine and metrifonate, on the activity of cholinesterase in the brain of the rat and on levels of acetylcholine (ACh) and choline (Ch). After intramuscular administration of physostigmine (500 micrograms/kg), the activity of cholinesterase in brain was maximally inhibited (76%) at 5 min and recovered to 50% at 40 min. At 5 min, areas of the brain such as the striatum and medulla oblongata showed 49 and 67% inhibition, respectively. Levels of physostigmine in brain peaked at 5 min (1.28 nmol/g). With the exception of the cerebellum, there was a direct correlation between the concentration of physostigmine and inhibition of cholinesterase in a given area. With the intravenous route of administration (100 micrograms/kg), the activity of cholinesterase in brain was maximally inhibited (67%) at 3 min and recovered to 50% at 12 min. At 60 min, the activity of cholinesterase was 90% of control. Levels of physostigmine in brain peaked at 2 min (0.47 nmol/g). At 15 min, with intraventricular administration (4 micrograms), the activity of cholinesterase was 73% and 31% inhibited in the hippocampus and striatum, respectively. Other areas of brain showed intermediate values of inhibition. Levels of acetylcholine were increased 18 and 22% above control in the striatum and hippocampus, respectively and did not change in the medulla. After intramuscular administration of metrifonate (80 mg/kg), the activity of cholinesterase decreased to 26% at 30 min, recovered to 50% at 180 min and returned to 74% at 360 min. Levels of acetylcholine increased by 45% at 45 min, then returned to normal by 120 min. When metrifonate (2.5 mg) was given intraventricularly the activity of cholinesterase decreased in the left side injected at 30 min to 20% in hippocampus; 22% in the medulla; 50% in the cerebellum; 58% in the striatum and 72% in cortex. Levels of acetylcholine increased maximally at 45 min in hippocampus and cortex and peaked in the striatum at 60 min. The greatest increases were seen in the hippocampus and cortex with 60 and 55%, respectively. The results of this study reveal some major differences between the effects of the two substances in brain. Four major conclusions are apparent from this study. First, based on these results, it is concluded that metrifonate is more likely to produce a therapeutic effect in humans.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparison of the actions of saxitoxin and tetradotoxin on the motor end-plate of frog muscle

End-plate potentials (e.p.p.) and responses to ionophoretic application of acetylcholine (ACh potentials) have been recorded in frog muscle. Saxitoxin (STX) and tetrodotoxin (TTX) abolished the e.p.p. at concentrations of 10(-8) g/ml. In almost all cases there was no effect on the ACh potentials even when the concentrations were raised, STX to 1.2 x 10(-6) g/ml (3.2 x 1O(-6)M), or TTX to 5 x 10(-6) g/ml (1.5 x 10(-5)M). STX, 10(-6) g/ml (2.7 x 10(-6)M) often caused a slow rise in the frequency of miniature e.p.p., without significant effect on amplitude.     

Effect of palytoxin on the calcium current and the mechanical activity of frog heart muscle.

1. The effect of palytoxin (PTX) on the Ca current (ICa) and the mechanical activity of frog atrial fibres was studied by use of the double sucrose gap voltage clamp technique. 2. In normal Ringer solution, PTX transiently increased the electrically-evoked peak tension which then decreased while a major contracture developed. PTX slowed the time course of the relaxation phase of the evoked tension. 3. Evidence is presented which suggests that the toxin also increased the entry of Ca and Sr via the Na-Ca exchange mechanism. It also induced the development of a Ca-dependent outward current which was inhibited by Sr. 4. In Na-free solution, PTX increased ICa and shifted the reversal potential for Ca towards more negative membrane potentials, thus suggesting that the internal Ca concentration had increased. Current-voltage, tension-voltage, time to peak-voltage and inactivation time constant-membrane potential curves were all shifted towards more negative membrane potentials in the presence of PTX. 5. These effects of PTX are similar to those caused by the increase in internal Ca concentration induced by Na ionophores by way of voltage-dependent Ca influx of the Na-Ca exchange mechanism.     

Psychopharmacologic effects of some cholinesterase inhibitors in comparison with imipramine

Imipramine and physostigmine, neostigmine and dyflos inhibited the catalepsy elicited by tetrabenazine, chlorpromazine and bulbocapnine respectively. This inhibition was demonstrable only at a given dose interval in certain cases.Both imipramine and cholinesterase inhibitors tested enhanced the hypermotility caused by amphetamine in mice, but not in conditioned rats.The effect exerted by chlorpromazine on conditioned reflex activity was inhibited by neostigmine and physostigmine.The results were discussed in relation to the effect produced by cholinesterase inhibitors on the permeability of the blood-brain barrier, and in relation to the possible significance of the cholinergic affinity of imipramine on its pharmacological effects.     

The voltage-dependent blocking effect of phalloidin on the delayed potassium current of voltage-clamped frog skeletal muscle fibres

The effects of phalloidin (10(-14)-10(-6) M) were tested on voltage-clamped isolated frog muscle fibres. The toxin reversibly blocked the potassium current similarly in both detubulated and intact fibres. Neither the reversal potential nor the activation curve of the current were affected by the toxin (10(-8) M). The inactivation curve was shifted toward negative values at holding potentials more than +20 mV from the reference potential. This shift enhanced the potential-dependent facilitation of the current block observed between -40 and +40 mV from the reference holding potential (the higher the depolarization, the greater the blocking effect, which reached 100% at +40 mV). Contrary to what was seen with the current, hyperpolarization did not relieve the mechanical block. The effect of phalloidin did not seem to be frequency-dependent.     

Effects of sulphydryl inhibitors on frog sartorius muscle: N-ethylmaleimide

1. The characteristic action of the -SH inhibitor, N-ethylmaleimide (NEM), is muscle rigour. The dose-response curve indicates a biphasic effect with maximum rigour tension produced by 1.0 mM NEM; beyond 1.0 mM there was an inverse relationship between dose and response.2. NEM produces a membrane depolarization unrelated to rigour development.3. NEM causes a sustained increase in (45)Ca efflux from whole muscle. Pretreatment of a muscle with ethylenediamine tetra-acetic acid (EDTA, 5 mM) to remove membrane calcium does not alter the NEM induced (45)Ca efflux.4. It is suggested that the primary site of NEM action is inhibition of calcium uptake by the sarcoplasmic reticulum thereby producing rigour. At concentrations above 1.0 mM, NEM may affect the myofilaments.     

The effect of potassium on frog stomach muscle.

1 KCl relaxed strips of frog stomach muscle. Usually, the effect was biphasic, i.e. contraction followed by relaxation at a concentration of 137 mM. 2 The effect was mimicked by K2SO4. 3 Ouabain (5 and 10 micrograms/ml) blocked the relaxations and reversed them to contractions. The tension of strips was not affected. 4 Phenoxybenzamine (1 microgram/ml) and procaine (20 micrograms/ml) inhibited the relaxations, potentiated the contractile component and lowered the tone of the muscle. 5 The relaxation and the tone were inhibited by papaverine (5 micrograms/ml) but the contractile component was unaffected. 6 It is suggested that potassium-induced relaxations are mediated through the activation of the sodium pump.     

The effect of quinidine on membrane electrical activity in frog auricular fibres studied by current and voltage clamp.

1 The action of quinidine sulphate 50 muM has been investigated on frog auricular trabeculae transmembrane currents recorded with a double sucrose gap apparatus. Results were obtained either in current or in voltage clamp conditions. 2 Quinidine modified the time course of repetitive activity elicited by long lasting depolarizing currents and reduced the range of current over which repetitive activity could be triggered, eventually abolishing repetitive responses altogether. 3 Several authors have emphasized the limitations of the voltage clamp method. Taking into account these limitations, the numerical values of the parameters obtained in the present work must not be considered as exact values but may be interpreted as indicators of the variations of the parameters. 4 The results are in agreement with previous findings that the main features of the action of quinidine are to produce (a) a reduced maximum rate of depolarization (MRD), (b) a reduced total amplitude of action potential, (c) a flattening of the plateau of the action potential, (d) a slight prolongation of the tail of the action potential, (e) an increased effective refractory period without greatly prolonging action potential duration, (f) no change of resting potential and of 50% repolarization time. 5 The analysis of ionic conductances has provided explanations for the above effects. 6 Quinidine reduced the reactivation kinetics of the sodium inward current, and decreased sodium conductance and the steady state of activation. These effects account for (a) and (b). 7 Quinidine increased the activation and inactivation time constants of sodium conductances, which account in part for (e). 8 Quinidine delayed reactivation of slow inward current, reduced calcium conductance, and decreased the steady state of activation of calcium conductance. These effects could account for (c). 9 The amplitudes of the two components of the delayed conductances responsible for repolarization were decreased by quinidine, and the time constant of activation for the faster of the two was slowed. These effects could account for (d) and in part for (e).     

Effects of nicorandil on electromechanical activity of frog atrial muscle

The effects of the antianginal drug nicorandil on electrical and mechanical activity of bullfrog atria were examined. The application of nicorandil at concentrations from 0.3 to 10 mM caused a negative inotropic effect without any appreciable change in the resting membrane potential. The amplitude of action potentials was attenuated. In the voltage clamp experiments, the slow inward current was slightly inhibited, but the background inward current and delayed outward current were not affected in the presence of nicorandil. The inward rectifying properties of time-independent outward current were slightly inhibited by 10 mM nicorandil. Nicorandil inhibited the caffeine-induced contractures elicited in the depolarized preparation, but had no effect on the contracture mediated through the Na-Ca exchange mechanism. These results suggest that the negative inotropic action of nicorandil is caused by inhibition of Ca influx and intracellular mobilization of Ca. Nicorandil had little effect on the potassium conductance in this preparation.     

Potentiation by cholinesterase inhibitors of cholinergic activity in rat isolated stomach and colon

Acetylcholinesterase (AChE) inhibitors stimulate gastrointestinal (GI) motility and are potential treatments of conditions associated with inadequate GI motility. The ability of itopride to facilitate neuronally (predominantly cholinergic) mediated contractions of rat isolated stomach, evoked by electrical field stimulation (EFS), has been compared with other cholinesterase inhibitors and with tegaserod, a clinically effective prokinetic and non-selective 5-HT(4) receptor agonist which also facilitates GI cholinergic function. Neostigmine greatly increased EFS-evoked contractions over a narrow concentration range (0.01-1 microM; 754+/-337% facilitation at 1 microM); higher concentrations (1, 3 microM) also increased muscle tension. Donepezil increased EFS-evoked contractions gradually over the full range of concentrations (0.01-10 microM; maximum increase 516+/-20% at 10 microM). Itopride increased the contractions even more gradually, rising to 188+/-84% at 10 microM. The butyrylcholinesterase inhibitor iso-OMPA 0.01-10 microM also increased EFS-evoked contractions, to a maximum of 36+/-5.0% at 10 microM, similar to that caused by tegaserod (35+/-5.2% increase at 1 microM). The effects of tegaserod, but not itopride were inhibited by the 5-HT(4) receptor antagonist SB-204070A 0.3 microM. In rat isolated colon, neostigmine was again the most efficacious, causing a defined maximum increase in EFS-evoked contractions (343+/-82% at 10 microM), without changing muscle tension. Maximum increases caused by donepezil and itopride were, respectively, 57.6+/-20 and 43+/-15% at 10 microM. These data indicate that the abilities of different AChE inhibitors to increase GI cholinergic activity differ markedly. Understanding the reasons is essential if AChE inhibitors are to be optimally developed as GI prokinetics.     

Effects of piperazine derivatives on the activity of frog skeletal muscle fibers

• 1.1. This study was undertaken to characterize the effects of some piperazine derivatives on excitable cell membranes. Three original Bulgarian compounds with favorable effects on cardiovascular and nervous system—piperazine derivatives with code names P-II (N¹-[3-oxo-3-phenyl-2-methyl-propyl]-N⁴-[trans-3-hydroxy-1,2,3,4-tetrahydro-2-naphthyl]-piperazine dihydrochloride), AS₂ (N¹-benzhydryl-N⁴-allyl piperazine dihydrochloride) and 35-M (Schiff's base of N¹-benzhydryl-N⁴-aminopiperazine with triacetonamine, dioxalate salt) were tested in experiments with conventional microelectrode technique on isolated frog muscle fibers.
• 2.2. After 30-min treatment with tested drugs at concentrations of 10–100 μM the recorded intra-(ICAP) and extracellular action potentials (ECAPs) showed an amplitude decrease and duration increase. The total ionic current (I_i) decreased as the outward phase was almost abolished by P-11. The propagation velocity (PV) of excitation and the twitch amplitude also decreased. These changes were agent- and concentration-dependent.
• 3.3. The effect potency of the agents diminished in the following order: P-11 > AS₂ > 35-M.
• 4.4. Concentrations higher than 100 μM for all agents completely, but reversibly, inhibited membrane excitability.
• 5.5. The results demonstrate compound- and concentration-induced modulation of Ca²⁺ current with blockade of Ca²⁺-dependent K⁺ and Cl⁻ membrane channels of muscle fiber treated with the compound tested.