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

Acetylcholine and local anesthetic binding to Torpedo nicotinic postsynaptic membranes after removal of nonreceptor peptides

After alkaline extraction, purified subsynaptic fragments isolated from Torpedo electric tissue exhibit on sodium dodecyl sulfate/polyacrylamide gel electrophoresis predominant peptides of apparent Mr 41,000, 50,000, and 65,000 (i.e., the peptides characteristic of the nicotinic receptor purified and isolated in detergent solutions). The peptide of Mr 43,000 that is also found in the isolated postsynaptic membranes is recovered in the supernatant after alkaline extraction. The alkaline-extracted membranes were functionally intact, as demonstrated by the following criteria. The kinetics of binding of [3H]acetylcholine in the presence and absence of 30 micron carbamoylcholine to occupy acetylcholine binding sites, [14C]-meproadifen [2-(diethylmethylaminoethyl)-2,2-diphenylvalerate iodide ] was bound with a dissociation constant, KD, of 0.3 +/- 0.1 micron to 0.3 +/- 0.1 site per [3H]alpha-toxin site. This binding was displaced by perhydrohistrionicotoxin. The carbamoylcholine-stimulated efflux of 22Na+ from the Torpedo vesicles were preserved after alkaline extraction. It is concluded that not only the acetylcholine binding site, but also the local anesthetic binding site, must be associated with the peptides of the cholinergic receptor itself and not that of Mr 43,000. Those peptides remaining after alkaline extraction are also sufficient for permeability control.

Tetraethylammonium: voltage-dependent action on endplate conductance and inhibition of ligand binding to postsynaptic proteins

Tetraethylammonium (Et(4)N(+)) ions depressed the amplitude and accelerated the decay rate of spontaneously occurring and nerve-evoked endplate currents (EPCs) in frog sartorius muscle. The relationship between peak EPC amplitude and membrane potential became nonlinear in the presence of 100 muM Et(4)N(+), and with drug concentrations of 250 muM or greater the current-voltage relationship exhibited negative conductance in the hyperpolarized region. Et(4)N(+) modified the exponential dependence of the EPC decay on membrane potential such that the decays between -150 and -50 mV were abbreviated and voltage independent but remained near control levels at more positive membrane potentials. The minimal effective concentration of Et(4)N(+) for altering the EPC time course was 10, and maximal effects were attained with 100 muM. Little additional shortening in the EPC decay phase was detected on raising the drug concentration to 1000 muM. Acetylcholine noise analysis revealed a voltage-dependent reduction in the mean channel open time, which was comparable in magnitude to the shortening in the EPC decay, and a depression of single-channel conductance. In concomitant biochemical studies, Et(4)N(+) was found to inhibit the binding of both [(3)H]acetylcholine and [(3)H]perhydrohistrionicotoxin to receptor-rich membranes from the electric organ of Torpedo ocellata with K(i) values of 200 muM and 280 muM, respectively. These results suggest that Et(4)N(+) interacts with both the acetylcholine receptor and its associated ionic channel. The voltage-dependent actions of Et(4)N(+) are attributed to blockade of the ionic channel in closed as well as open conformation.

Acetylcholine receptor and ionic channel of Torpedo electroplax: binding of perhydrohistrionicotoxin to membrane and solubilized preparations

The electric organ of the ray, Torpedo ocellata, can serve as a source for both the acetylcholine (ACh) receptor and its ionic channel. The two entities were identified by their specific binding of [3H]ACh and [3H]perhydrohistrionicotoxin ([3H]H12-HTX), respectively. Binding of [3H]H12-HTX was inhibited by certain drugs and toxins, e.g., histrionicotoxin (HTX), amantadine, and tetraethylammonium (TEA) ions at concentrations that did not inhibit [3H]ACh binding. However, the specific carbamoylcholine-induced 22Na efflux from microsacs from the electric organ membranes was blocked by inhibitors of either the receptor or its ionic channel. The ionic channel had the properties of a protein as judged by heat sensitivity and the inhibition of [3H]H12-HTX binding, after incubation of the electric organ membranes with protein reagents such as p-chloromercuribenzenesulfonic acid (PCMBS) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The "binding" of [3H]H12-HTX at 4 X 10(-8) M to lipids in the microsacs was 12% of the total binding to intact microsacs and was nonsaturable and insensitive to heat or specific drugs. After solubilization with cholate, the [3H]H12-HTX binding subunits retained the same affinities for toxins and drugs. The Kd for [3H]H12-HTX was 3 X 10(-7) M. The majority of the ionic channel could be separated from the ACh receptors in the cholate extract by incubation with ACh-receptor affinity gel and ACh-receptor antibodies. The ACh receptor purified by this affinity gel contained only a few active ionic channel units as judged by low levels of high affinity binding of [3H]H12-HTX. On the other hand, after solubilization with Triton X-100, all the ionic channel molecules were either separated or denatured so that the purified ACh receptor did not exhibit high affinity binding for [3H]H12-HTX.

Voltage- and time-dependent action of histrionicotoxin on the endplate current of the frog muscle

Histrionicotoxin, a toxin isolated from skin secretions of a Colombian arrow poison frog, Dendrobates histrionicus, decreased the amplitude and time-course of the endplate current, and altered the voltage dependence of the half-decay time. In addition, the toxin produced a characteristic nonlinearity in the current-voltage relationship of the endplate current when 3-s voltage conditioning steps were used. Reduction in time of the conditioning steps to 10 ms made the current-voltage relationship linear. The decrease in peak amplitude of the endplate current (epc) produced by histrionicotoxin measured during long hyperpolarizing conditioning steps was fitted by a single exponential function. The calculated rate constants ranged from 0.03 to 0.14 s-1 and varied with membrane potential at hyperpolarizing levels. The voltage- and time-dependent action of histrionicotoxin does not require an initial activation of receptors by acetylcholine (ACh). The characteristic of the current-voltage relationship can be accounted for by the observed voltage and time dependency of the attenuation of the endplate current amplitude in the presence of histrionicotoxin during long conditioning steps. These effects of histrionicotoxin on the peak amplitude, and on the voltage and time dependence of the epc were concentration-dependent and slowly reversible upon washing out the toxin. Thus, the voltage- and time-dependent action of histrionicotoxin at the endplate is related to an increase in the affinity between the toxin and the ACh receptor-ionic channel complex. This increase in affinity is postulated to be due to a conformational change of the macromolecule in the presence of histrionicotoxin which is demonstrated to be relatively slow, i.e., on the order of tens of seconds.

Interaction of the acetylcholine (nicotinic) receptor protein from Torpedo marmorata electric organ with monolayers of pure lipids

Membrane fragments rich in cholinergic (nicotinic) receptor protein were purified from the electric organ of Torpedo marmorata. Their lipid composition is essentially characterized by the prominence of cholesterol, phosphatidylethanolamine and phosphatidylcholine, long-chain fatty acyl constituents, and the absence of sphingomyelin. Solubilised receptor was purified from these fragments and the concentration of sodium cholate lowered by dialysis to 0.01% (w/v). When this preparation was injected under a lipid monolayer, an increase of surface pressure developed, which was not observed with the detergent alone nor in the absence of lipid film. When covalently radiolabelled receptor preparations were injected at a constant surface pressure the radioactivity recovered with the film was proportional to the increase in area. It is concluded that the pressure or area increases are due to the penetration of the cholinergic receptor protein into the lipid film. Incorporation experiments into films formed from various pure lipids showed that the protein interacts more readily with cholesterol than with ergosterol, phosphatidylcholine, or other phospholipids. Its affinity is also higher for long-chain phosphatidylcholines than for short-chain ones. The degree of unsaturation and fluidity of the 3-sn-phosphatidylcholine (lecithin) films are of secondary importance. Parallel experiments with covalently and non-covalently labelled receptor preparations showed that part of the protein recovered with the film lost its alpha-toxin binding ability during the penetration. Similar data were obtained with the receptor purified from Electrophorus electricus electric organ.

Differential effects of perhydrohistrionicotoxin on neurally and iontophoretically evoked endplate currents

Perhydrohistrionicotoxin, at concentrations of 10(-12)-10(-7) M, depressed the current generated by iontophoretic application of acetylcholine to endplate regions of soleus and extensor digitorum longus muscles of rats. However, no changes in the amplitude or time course of spontaneous miniature endplate potentials or currents were seen with these concentrations of toxin. Evoked endplate currents were also unaffected by the toxin. Similarly, the responses to iontophoretic acetylcholine were depressed by these concentrations of perhydrohistrionicotoxin in chronically denervated muscles. Depression of responses in both normal and chronically denervated muscles developed gradually, was greater at higher concentrations, and was reversible. The different effects of the toxin on neurally evoked currents and currents produced by iontophoretic application of acetylcholine raise the possibility of the existence of two different population of receptor complexes.

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.

Distinct protein components from Torpedo marmorata membranes carry the acetylcholine receptor site and the binding site for local anesthetics and histrionicotoxin

Highly purified subsynaptic membrane fragments prepared from Torpedo marmorata electric organ (specific activity, greater than 4 mumol of Naja nigricollis alpha-[3H]toxin per mg of protein) exhibit, on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, two major protein bands of apparent molecular weight 40,000 and 43,000, respectively. Dissolution of these membranes by the nondenaturing detergents Triton X-100 and Berol 043 followed by standard fractionation yielded (i) the 9S acetylcholine-receptor protein which still binds the alpha-[3H]toxin and after further purification yielded, in the presence of sodium dodecyl sulfate, the 40,000-dalton component, covalently labeled by the affinity reagent 4-(N-maleimido)phenyl[3H]trimethylammonium; only serine was found as the NH2-terminal amino acid of this protein; and (ii) a high molecular weight aggregate named 43,000 protein which was resolved in denaturing gels almost exclusively as the 43,000-dalton band, In the absence of detergents, the 43,000 protein binds compounds known to interact with the acetylcholine ionophore: a fluorescent local anesthetic quinacrine and histrionicotoxin (apparent dissociation constant, 7 +/- 1 X 10(-7) M). The regulation of quinacrine fluorescennce by carbamylcholine, observed in the intact membrane, no longer occurs with the isolated 43,000 component.

Histrionicotoxin enhances agonist-induced desensitization of acetylcholine receptor

Dihydroisohistrionicotoxin inhibits acetylcholine receptor-dependent 22Na+ uptake of cultured chick muscle cells with a KI of 0.2 micrometer. The inhibition is noncompetitive with respect to agonists. The toxin enhances desensitization of the receptor by agonists which is accompanied by a 10-fold increase in receptor affinity for agonists. Dihydroisohistrionicotoxin increases the affinity of the desensitized form of the receptor for agonists but not antagonists. The results suggest that dihydroisohistrionicotoxin inhibits the acetylcholine receptor by causing an increase in the affinity of the desensitized form of the receptor for agonists and thereby stabilizing the desensitized state.

Membrane adenosinetriphosphatase: a digitalis receptor?

The enzyme Na+,K+-ATPase is a good model for receptor studies because of its known functional correlates. The binding of digitalis to the enzyme observed in vitro satisfied the criteria for receptor binding. Studies of the relationship between the digitalis binding and the drug action reveal an impressive correlation between these events but fail to provide proof of a causal relationship. Studies with other Na+,K+-ATPase inhibitors and agents that affect transmembrane Na+ movements (steps that would follow Na+,K+-ATPase inhibition) provide further supportive evidence that sodium pump inhibition and the resulting enhancement of intracellular Na+ transients cause the inotropic action of digitalis.