A new method for excitation-contraction uncoupling in frog skeletal muscle.

The mechanical activity of frog sartorius muscle fibers can be uncoupled from the electrical activity of their surface membranes by immersing the preparation in Ringer solution containing either 1.5 or 2.0 M of formamide for 15--20 min. This uncoupling is not reversed when the muscle is transferred to normal frog Ringer solution. Formamide does not affect the electrical activity of the sciatic nerve branch, and both endplate potentials and miniature endplate potentials may be recorded from the uncoupled muscles. Prolonged exposure to formamide, beyond the time needed to paralyze, causes neuromuscular block.

Selective depolarization of the muscle membrane in frog nerve-muscle preparations by a chromatographically purified extract of the dinoflagellate Ostreopsis lenticularis.

1. The actions of a chromatographically identified extract of the marine dinoflagellate Ostreopsis lenticularis, named ostreotoxin-3 (OTX-3), were studied on frog isolated neuromuscular preparations. 2. OTX-3 (1-10 microg ml(-1)) applied to cutaneous pectoris nerve-muscle preparations depolarized skeletal muscle fibres and caused spontaneous contractions. The depolarization was neither reversed by prolonged washing nor by (+)-tubocurarine. 3. OTX-3 decreased the amplitude of miniature end plate potentials (m.e.p.ps) but did not affect their frequency. 4. Extracellular recording of compound action potentials revealed that OTX-3 affected neither excitability nor conduction along intramuscular nerve branches. 5. End-plate potentials (e.p.ps) elicited by nerve stimulation were reduced in amplitude by OTX-3 and even showed reversed polarity in junctions deeply depolarized by the toxin. 6. Membrane depolarization induced by OTX-3 was decreased about 70% in muscles pretreated for 30 min with 10 microM tetrodotoxin. In contrast, muscles pretreated with 5 microM mu-conotoxin GIIIA were completely insensitive to OTX-3-induced depolarization. 7. OTX-3 did not affect e.p.p. amplitude and the quantal content of e.p.ps in junctions in which muscle depolarization was abolished by mu-conotoxin GIIIA. 8. OTX-3 is a novel type of sodium-channel activating toxin that discriminates between nerve and skeletal muscle membranes.

Succinyl derivatives of N-tris (hydroxymethyl) methyl-2-aminoethane sulphonic acid: their effects on the frog neuromuscular junction.

Succinic anhydride (SA) dissolved in Ringer solution buffered with N-tris (hydroxymethyl) methyl-2-aminoethane sulphonic acid (SA-TES solution) potentiates the depolarizing action of acetylcholine (ACh, 10-40 microM) on frog muscle and the tension induced by bath application of this agonist. Applied from one side of a double-barrelled micropipette, SA-TES increases the amplitude of iontophoretically elicited ACh potentials. The potentiation of the effects of ACh by SA-TES does not involve changes in either the activity of the ACh esterase or the input resistance of the muscle membrane. For depolarizations of frog sartorius muscle, dose-response relationships obtained for ACh concentrations from 0.5 to 20 microM indicate that SA-TES increases the apparent affinity of ACh by a factor of 3. SA-TES exerts an "accelerating' effect on the responses elicited by bath-applied ACh; i.e., it increases the rate of depolarization when ACh is added to the bath and the rate of repolarization upon washing out. These effects are particularly marked in preparations treated with neostigmine (3 microM). SA-TES does not potentiate the depolarizing action of agonists which do not contain an ester group. Moreover, the time course of the responses elicited by these compounds is not influenced by SA-TES. SA-TES fails to influence significantly the effects of the neurally released transmitter. Only a 10% increase in the average amplitude of the endplate potentials was observed. SA hydrolyzes in about 30 min at room temperature; however the SA-TES solution retains its activity for several weeks. Succinate is inactive, and so is SA in Ringer buffered with phosphate. The SA-TES solution contains seven succinyl-TES derivatives, which were separated by ion-exchange chromatography and paper chromatography. At concentrations between 1 to 150 microM, these succinyl-TES derivatives affected the ACh-induced contraction of frog rectus abdominus muscle. The most abundant derivative potentiated the action of high doses of ACh, but was inhibitory at lower ones. The other derivatives were mostly inhibitory. These results are discussed in terms of two hypotheses. One postulates the presence of a diffusion barrier formed by groups that bind ACh and are saturated by SA-TES. The other assumes that SA-TES acts directly on the ACh receptor exerting its potentiating effect through a cooperative mechanism.

Lysis of red blood cells by extracts from benthic dinoflagellates.

Samples of the cultured benthic dinoflagellates Gambierdiscus toxicus and Ostreopsis lenticularis, both isolated from a shallow back reef habitat in southwestern Puerto Rico, were extracted in methanol, dried and resuspended in distilled water. After centrifugation, aliquots of the supernatant, or dilutions thereof, were added to suspensions of washed human and mouse red blood cells and incubated at different temperatures for different time periods. Further spectrophotometrical examinations of the samples showed a hemolytic activity against mouse and human red blood cells. The hemolytic activity of G. toxicus extract was 3 to 4 times greater than that of O. lenticularis and was less temperature-dependent. Such findings suggest that these two dinoflagellates produce chemically different hemolysins.

Modulation of acetylcholine receptor channel by a polar component isolated from toxic Ostreopsis lenticularis extracts.

Methanol extracts obtained from O. lenticularis clones are toxic to mice and inhibit acetylcholine-induced contractions in frog skeletal muscle. Chromatographic fractionation of extracts produced two major fractions with different retention times. Single channel recordings in myocyte membrane patches exposed to more polar fraction showed the appearance of acetylcholine-activated channels whose mean current amplitude was nearly half that of the controls. Channel open times under control and experimental conditions were similar. Thus, this dinoflagellate fraction reduces the ionic conductance of nicotinic receptor channels without altering their lifetime.

Improved method for the isolation, characterization and examination of neuromuscular and toxic properties of selected polypeptide fractions from the crude venom of the Taiwan cobra Naja naja atra.

An improved chromatographic method was developed to isolate and purify polypeptides and proteins from the crude venom of the Taiwan cobra Naja naja atra. The procedure devised is simple, easy to reproduce, and enables large scale isolation of almost all polypeptides and proteins in this cobra venom. Six pure polypeptide fractions of the venom were isolated and characterized using gel filtration on Sephadex G50 (medium), ion exchange chromatography on SP-Sephadex C25, desalting on Sephadex G25 (fine) and preparative HPLC on a RPC 18 column. The neuromuscular activity of these fractions was tested on the chick biventer cervicis nerve-muscle preparation and their toxicity (LD(50)) was determined after i.v. administration in mice. Their antinociceptive activity was tested in the mouse abdominal test by i.v. application. Two of these polypeptide samples had major physiological effects: one acted as a cardiotoxin causing reversible myocardial contractures with no effect on muscle twitches elicited by nerve stimulation (NS); another was a neurotoxin that blocked muscle contractions in response to NS and exogenously added acetylcholine. The cardiotoxic fraction was identified as CTX I, a well-known cardiotoxin present in this venom, and the neurotoxin was identified as neurotoxin-α with an LD50 in mice of 0.075 mg/kg.

Localization of neuropeptides in the nervous system of the marine annelid Sabellastarte magnifica.

Immunohistochemical studies of the nervous system of Sabellastarte magnifica, a sedentary polychaete, showed the presence of neuropeptide expressing cells and fibers within the double ventral nerve cord. Immunoreactivity to cholecystokinin, neuropeptide Y, enkephalins, substance P, and FMRFamide was found to be present in specific populations of cells, identifiable by their location and by the neuropeptide they expressed. Fibers expressing the various neuropeptides were also observed in particular locations within the nerve cord. This characteristic distribution of the various neuron subgroups and fiber pathways may represent functional circuits within the nervous system of this annelid.

Anti-muscarinic activity of a family of C11N5 compounds isolated from Agelas sponges.

In a search for potential target sites for C11N5 compounds obtained from marine sponges of the genus Agelas we evaluated their interaction with muscarinic acetylcholine receptors from rat brain membranes. In competition experiments with 3H-QNB these compounds displayed the following rank order of potency: sceptrin greater than oroidin greater than or equal to dibromosceptrin greater than or equal to clathrodin. Sceptrin (50 microM) was shown to be a competitive inhibitor of 3H-QNB binding as revealed by Scatchard analysis. The results demonstrate the ability of these compounds to interact with multiple target molecules in the micromolar range.

Positive modulators of muscle acetylcholine receptor.

A solution of succinic anhydride (SA) in buffer N-tris(hydroxymethyl)methyl-2-aminoethane sulfonic acid (TES), the SATES solution, potentiates the effect of carbonyl containing agonists on frog muscle (del Castillo et al., Br. J. Pharmac. 84: 275-288, 1985). Here we report that the main compound in the SATES solution is a monosuccinyl ester of TES (MST). This compound is not an agonist of the acetylcholine (ACh) receptor nor is it an inhibitor of ACh esterase, yet MST potentiates the ACh-induced tension and depolarization in frog muscle to a new maximum. It increases the amplitude of miniature endplate potentials but has no effect on the time course of miniature endplate currents. The acetylated analogue of MST (mono-acetyl-TES: MAT) had similar but more pronounced effects on frog muscle. Neither MST nor MAT affect [3H]Ach binding to receptor-rich membranes from the electric organ of Torpedo californica, or the affinity state transition induced by agonists. Radiolabeled MST does not bind to these membranes and MAT does not alter agonist-induced ion flux. Therefore, these compounds seem to act as positive modulators of muscle ACh receptor but are inactive in Torpedo vesicles.