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Biomedical subjects

A T Eldefrawi

Publications and source records attributed to A T Eldefrawi.

At least 91 records · Page 5Linked to original sources
PubMed1978

Amantadine: neuromuscular blockade by suppression of ionic conductance of the acetylcholine receptor.

Amantadine hydrochloride decreases the sensitivity of denervated mammalian muscle to iontophoretically applied acetylcholine. The drug depresses the amplitude of the end-plate current and reverses the slope of the relation between half-decay time and membrane potential suggesting that it alters the ionic conductance that is mediated by the acetylcholine receptor. Binding studies confirm that amantadine acts on the ion conductance modulator rather than the acetylcholine receptor.

Acetylcholine↗
PubMed1978

Effects of alpha-bungarotoxin and reversible cholinergic ligands on normal and denervated mammalian skeletal muscle.

alpha-Bungarotoxin (BuTX; 5 micrograms/ml) completely blocked the endplate potential and extrajunctional acetylcholine (ACh) sensitivity of surface fibers in normal and chronically denervated mammalian muscles, respectively, in about 35 min. A 0.72 +/- 0.033 mV amplitude endplate potential returned in normal muscle fibers after 6.5 hr. of washout of alpha-BuTX, and an ACh sensitivity of 41.02 +/- 3.95 mV/nC was recorded in denervated muscle after 6.5 hr of wash (control being 1215 +/- 197 mV/nC). A two-step reaction of BuTX with binding sites which may allosterically interact is postulated. Several pharmacologic differences were noted between the ACh receptors at the normal endplate and those appearing extrajunctionally following denervation. In normal innervated muscles exposed to BuTX in the presence of 20 microM carbamylcholine or decamethonium, washout of both drugs restored twitch to control levels within 2 hr. Endplate potentials large enough to initiate action potentials were also recorded in most surface fibers. In contrast, these agents, in much higher concentrations (50 microM), were almost ineffective in preventing BuTX blockade of ACh sensitivity in denervated muscle. Hexamethonium (10 and 50 mM) depressed neuromuscular transmission and blocked the action of BuTX in normal muscle in a dose-dependent fashion. On the extrajunctional receptors, hexamethonium (50 mM) was ineffective in protecting against BuTX. We may conclude that at the normal endplate region there are two distinct populations of ACh receptors, both of which react with cholinergic ligands and BuTX, but that a small population (representing congruent to 1% of the total) reacts with BuTX reversibly. Our findings further suggest a clear distinction between ACh receptors located at the normal endplate region and those of the extrajunctional region of the chronically denervated mammalian muscle.

Acetylcholine↗
PubMed1977

Interactions of acetylcholine receptors with organic mercury compounds.

Micromolar concentrations of methylmercury and several organic mercury fungicides were found to block binding of [3H]acetylcholine (ACh) to the ACh-receptor of the electric organ of the electric ray, Torpedo ocellata. The same compounds had little or no effect on the catalytic activity of ACh-esterase of the same tissue. [14C]Methyl-mercury bound to the purified ACh-receptor with high affinity (Kd=7micrometer) and there were 6.5 +/- 0.5 binding sites for each ACh-binding site. Binding of methylmercury was highly cooperative with a Hill coefficient of 2.6. This binding was irreversible by redialysis in methylmercury - free medium, however, the bound [14C]methylmercury was easily displaced from the receptor protein with micrometer concentrations of BAL or penicillamine. Methylmercury also blocked binding of [3H] nicotine and [3H]pilocarpine to the nicotinic and muscarinic ACh-receptors of the rat brain, respectively. The data suggest that the ACh-receptor may be a target for methylmercury and other organic mercury compounds.

Acetylcholine↗
PubMed1977

Further toxicologic studies with commercial and candidate flame retardant chemicals. Part II.

A number of commercial and candidate flame retardants were studied with regard to their toxicity to goldfish, inhibition of cholinesterase, inhibition of acetyl choline binding to its receptor and insecticidal properties. Several of the flame retardants were notably toxic to fish. Some of the compounds showed modest inhibition of cholinesterase and/or microsomal oxidases, but none inhibited acetyl choline receptor binding. Whereas several of the flame retardants showed little or no insecticidal properties when added alone to a housefly diet, piperonyl butoxide greatly synergised their toxicity to houseflies.

Acetylcholine↗
PubMed1977

Perhydrohistrionicotoxin: a potential ligand for the ion conductance modulator of the acetylcholine receptor.

Histrionicotoxin from the Colombian frog Dendrobates histrionicus and its perhydro derivative reversibly block the acetylcholine-sensitive ion conductance system in frog neuromuscular preparations. The perhydro derivative and [3H]perhydrohistrionicotoxin, like histrionicotoxin, caused a significant decrease in the peak amplitude of the end-plate current and shortened its rise time and half-decay time. In membrane preparations from Torpedo electroplax, [3H]perhydrohistrionicotoxin bound reversibly to a limited number of high-affinity sites [dissociation constant, (KD) = 0.4 micronM]. The ratio of perhydrohistrionicotoxin to acetylcholine binding sites in these membrane preparations approached 2. Histrionicotoxins, local anesthetics, and certain cholinergic agonists inhibited binding of perhydrohistrionicotoxin. Binding of perhydrohistrionicotoxin to membranes was decreased by heat or treatment with proteases. Treatment of membranes with Triton X-100 solubilized acetylcholine binding proteins and apparently also perhydrohistrionicotoxin-binding proteins. However, the detergent Triton X-100 also bound [3H]perhydrohistrionicotoxin. This nonspecific binding was not saturable and complicated studies on the antagonism by drugs of binding of [3H]perhydrohistrionicotoxin. In solubilized preparations the binding protein for acetylcholine could be removed by affinity chromatography or immunoprecipitation without affecting binding of perhydrohistrionicotoxin. Sephadex chromatography also separated acetylcholine- from perhydrohistrionicotoxin-binding proteins. Perhydrohistrionicotoxin did not bind significantly to purified acetylcholine-receptor protein but presumably bound to an ion conductance modulator protein that was associated with the acetylcholine-receptor in intact membrane and readily separable from the receptor protein after solubilization.

Amphibian Venoms↗
PubMed1975

Tryptophan and cystein residues of the acetylcholine receptors of Torpedo species. Relationship to binding of cholinergic ligands.

Several methods were used to analyze for tryptophan in the acetylcholine (ACh) receptors purified from the electric organs of the electric rays, Torpedo californica and Torpedo marmorata. The best value of tryptophan was 2.4 mol %. When excited at 290 nm, both receptors fluoresced with a maximum at 336, but there was no change in the fluorescence emission spectra upon binding of carbamylcholine, d-tubocurarine, ACh, or decamethonium. The free SH content of the Torpedo receptors varied in different preparations, and was highest in that purified from fresh T. californica using deaerated solutions and dialysis under nitrogen, and lowest in that prepared from the aged lyophilized membranes of T. marmorata. The maximum free SH content was 20 nmol/mg of protein or 0.22 mol %, equal to at most 18% of the total cysteic acid residues. Reaction of either 33% or of all the SH residues with p-chloromercuribenzoate reduced maximum ACh binding to the pure receptor prepared from fresh T. californica by only 23%.

Acetylcholine↗
PubMed1972

Characterization and partial purification of the acetylcholine receptor from Torpedo electroplax.

Binding of acetylcholine in the concentration range 1 nM-1 muM was measured by equilibrium dialysis to a particulate preparation of Torpedo electroplax, without or with prior treatment of the tissue with one of three chemical modifying reagents. Significant reduction in binding of acetylcholine resulted after treatment with 1,4-dithiothreitol, p-chloromercuribenzoate, or p-(trimethylammonium)-benzenediazonium fluoroborate. Partial reversal of the reduction in binding occurred when dialysis was performed in the presence of 5,5'-dithiobis-(2-nitrobenzoic acid) or potassium ferricyanide (in the case of treatment with dithiothreitol), and 2-mercaptoethanol (in the case of treatment with p-chloromercuribenzoate). It is concluded that the functional acetylcholine-receptor macromolecule of Torpedo electroplax has disulfide bond(s), sulfhydryl group(s), and one or more of the amino acids vulnerable to diazotization by p-(trimethylammonium)-benzenediazonium fluoroborate. This, plus the effect of phospholipase C (EC 3.1.4.3) in elimination of detectable binding of acetylcholine after electrofocusing, is additional evidence that the functional acetylcholine receptor is a phospholipoprotein or a phospholipid-protein complex, which has a low isoelectric point of 4.5 +/- 0.2, yet is denatured by exposure to low pH for 24 hr. Due to this adverse effect, recovery of binding of acetylcholine after electrofocusing, as detected by equilibrium dialysis or ultrafiltration, is only 23% and, so far, only 6.3-fold purification of functional acetylcholine receptors by this technique is possible. Three or two forms of acetylcholinesterase (EC 3.1.1.7), whose peaks have isoelectric points ranging from 4.3 to 7.2, appear after electrofocusing of Torpedo extracted with 1% Triton X-100 or Lubrol, respectively. The major peak in either preparation has an isoelectric point of 5. Although the peaks of the functional acetylcholine receptors and of acetylcholinesterase of Torpedo electroplax are separable by electrofocusing, it has not been possible to isolate fractions that contain functional receptors but that are free of acetylcholinesterase. The opposite is possible.

Acetylcholine↗
PubMed1971

Acetylcholine binding to Torpedo electroplax: relationship to acetylcholine receptors.

Binding of [(3)H]acetylcholine to a particulate fraction of Torpedo electroplax was measured by equilibrium dialysis. Two high-affinity sites present on phospholipoproteins bound acetylcholine reversibly, and binding was blocked by nicotinic drugs. Characteristics of this binding suggest that these phospholipoproteins may be acetylcholine receptors.

Acetylcholine↗
PubMed1971

Binding sites for cholinergic ligands in a particulate fraction of Electrophorus electroplax.

The binding of four radioactive cholinergic ligands to a particulate fraction of Electrophorus electricus electroplax was investigated by equilibrium dialysis and found to be reversible. A single site (concentration, 0.02-0.03 nmol/g electroplax) that bound muscarone and nicotine was found to be on a phospholipoprotein; it is suggested that this is the acetylcholine receptor. Four binding sites for decamethonium were found; the highest-affinity site (0.03 nmol/g) may be the same one that binds muscarone and nicotine, both because of the chemical nature of the binding macromolecule and because decamethonium competes with muscarone and nicotine in binding. Dimethylcurare bound to three sites. The lower-affinity sites appeared to be on different macromolecules.When binding of the four cholinergic ligands (at 0.01 muM) to 28 arbitrarily selected proteins was determined, it was concluded that muscarone would be the most specific for the acetylcholine receptor, in that it bound to none of the 28 proteins tested; nicotine bound to one, dimethylcurare to two, and decamethonium to ten of the proteins. Nicotine or muscarone appear to be the ligands of choice in studying binding to the acetylcholine receptor; decamethonium appears to have inadequate specificity.

Acetylcholine↗