Mechanisms of the inhibition by neostigmine of tetanic contraction in the mouse diaphragm

Neostigmine (0.5-2 microM) caused fade of tetanic contractions (Wedensky inhibition) evoked by repetitive nerve stimulation. The mechanism underlying this action was studied in intact and cut isolated phrenic nerve-diaphragm preparations of mice. The fade was brought about by failure to elicit muscle action potentials. During fade, the muscle was unable to conduct directly evoked action potentials across the central endplate zone. Recovery of excitability occurred in 5 s with continued stimulation. In the presence of neostigmine, the resting membrane potential at endplate areas during repetitive stimulation decreased from -80 mV to less than -50 mV within the first 10 pulses at 75-200 Hz and thereafter recovered gradually to about -60 mV in the following 5 s during continuous stimulation. The quantal content of endplate potentials evoked by single stimulation was not reduced by neostigmine whereas that evoked by high frequency stimuli (75 Hz) was reduced to about 1/3 in 10 pulses. It is concluded that the fade of tetanic contraction caused by inhibition of acetylcholinesterase is induced by the inactivation of sodium channels in the area surrounding the endplates and that the sustained fade is due to a decrease of transmitter release. Both effects are the result of acetylcholine accumulation.

A comparison of the antagonisms by neostigmine and diaminopyridine against the neuromuscular block caused by cobrotoxin and (+)-tubocurarine

Cobrotoxin was about 11-fold more potent than (+)-tubocurarine on a weight basis in blocking neuromuscular transmission in mouse isolated phrenic nerve-diaphragm preparations. Neostigmine and diaminopyridine increased the concentrations of cobrotoxin for 70% inhibition of indirect contraction by 290 and 320%, and increased those of (+)-tubocurarine by 180 and 230%, respectively. More than additive increases were obtained when neostigmine and diaminopyridine were used simultaneously. Cobrotoxin, however, was only 6-fold more toxic than (+)-tubocurarine after intraperitoneal injection in mice. The lethal dose of (+)-tubocurarine was increased by 80% when both antidotes were used together, but only by 15-20% when used alone. In contrast, the lethality of cobrotoxin was not decreased by these drugs. Unexpectedly, the time to death after treatment with cobrotoxin was shortened when mice were pretreated with these antidotes.

Pharmacological identification of saxitoxin-like toxins in the cultured purple clam, Hiatula diphos

An outbreak of poisoning from ingestion of purple clam (Hiatula diphos), obtained from a brackish fish-culture pond, occurred in southern Taiwan in early January 1986. The extract blocked the twitch responses and generation of the action potential of the mouse diaphragm without an effect on the resting membrane potential or the miniature endplate potential. The depolarizations and spontaneous firings of the diaphragm induced by veratridine and crotamine were effectively antagonized by the toxic clam extract, indicating the presence of saxitoxin-like substances, which may have amounted to 0.013% tetrodotoxin equivalent.

Electrophysiological studies of myotoxin a, isolated from prairie rattlesnake (Crotalus viridis viridis) venom, on murine skeletal muscles

Myotoxin a reduced the resting membrane potential of mouse and rat diaphragms from about -80 mV to -60 mV, induced spontaneous repetitive firing and enhanced the contractile force in response to single stimulations. The depolarizing effect was reversed noncompetitively by tetrodotoxin, local anesthetics or low Na+ solution, but was augmented by ouabain or low Cl-solution while being unaffected by high K+ solution or electrical stimulation of the muscle. The duration of muscle action potential was prolonged by only 20-30%, whereas the rate of rise (dV/dt) was unaffected. About a 40% increase of membrane conductance was observed, be abolished by the Na+-channel blocker tetrodotoxin. By contrast, K+ conductance was unaffected. Effects on caffeine-induced contracture, quantal release of neurotransmitter and the amplitude of miniature endplate potential were not appreciably affected. These effects of myotoxin a indicate that the toxin affects the muscle, but not the nerve, by acting specifically on the Na+-channel of the sarcolemma or T-tubule, like crotamine, rather than on the sarcoplasmic reticulum. The effects of sea anemone toxin II on the Na+-channel (marked depolarization and prolongation of action potential) could not be prevented by saturating the muscle with myotoxin a. On the other hand, the effect of veratridine, a member of another group of toxins acting on the Na+-channel, was enhanced. These results suggest that myotoxin a acts on the Na+-channel at a site which is discrete from those of tetrodotoxin, veratridine and sea anemone toxin II.

Potentiation between neostigmine and 3,4-diaminopyridine in antagonizing the neuromuscular block induced by D-tubocurarine

The antagonisms to the d-tubocurarine-induced neuromuscular blockade by neostigmine and 3,4-diaminopyridine (DAP) were studied quantitatively in the isolated phrenic nerve-diaphragm preparation of mice in vitro and in lethality of mice in vivo by assaying concentrations of d-tubocurarine needed to produce 70% block of indirect muscle contraction and LD50, respectively. The "antagonist efficacies", defined as the ratio of d-tubocurarine concentration (dose) after pretreatment with antagonistic agents over that of control, were 1.87, 2.24 and 14.7, respectively, for neostigmine, DAP and both agents combined when the stimulus pulses were at 0.1 Hz. Under 50 Hz train stimulation, the antagonist efficacies were lower, being 1.85, 1.64 and 5.33, respectively. For the lethality to d-tubocurarine, the values were still lower, being only 1.21, 1.33 and 1.84, respectively. The synergism between neostigmine and DAP, as evident from the marked increase of antagonist efficacy in vitro, is more than expected from the possible interaction of the major pharmacological actions of these two agents.

A study on the membrane depolarization of skeletal muscles caused by a scorpion toxin, sea anemone toxin II and crotamine and the interaction between toxins

Quinquestriatus toxin (QTX) isolated from the venom of a scorpion (Leiurus quinquestriatus) and sea anemone (Anemonia sulcata) toxin II enhanced the twitch response of the rat and mouse diaphragms and like crotamine (isolated from the venom of Crotalus durissus terrificus) caused spontaneous fasciculation of the muscle. Trains of action potentials in muscles at 70-250 Hz, which could not be antagonized by (+)-tubocurarine, were triggered by single stimulation or occurred spontaneously after treatment with these toxins. QTX and toxin II prolonged the rat muscle action potential 3 to 4 fold whereas crotamine prolonged the action potential by only 30%. The membrane potential was depolarized from about -82 mV to -55 mV by crotamine 2 micrograms ml-1, -41 mV by toxin II 5 micrograms ml-1 and to -50 mV by QTX 1 microgram ml-1. The concentrations to induce 50% maximal depolarization (K0.5) were 0.07, 0.15 and greater than 0.4 microgram ml-1, respectively, for QTX, crotamine and toxin II, whereas the rates of depolarization were in the order toxin II greater than or equal to crotamine greater than QTX. The depolarizing effects of crotamine and QTX, but not of toxin II, were saturable. The depolarizing effects of all three toxins were irreversible whereas the membrane potential could be restored by tetrodotoxin non-competitively. Simultaneous treatment with crotamine and QTX or crotamine and toxin II at concentrations below K0.5 caused only additive effects on depolarization. When the muscle was depolarized by pretreating with a saturating concentration of crotamine, the onset of depolarization by QTX was greatly retarded whereas that by toxin II was unaffected. Action potentials were further prolonged in both cases. 8 It is inferred that all three peptide toxins act at sites on the sodium channel and the binding sites for QTX and crotamine overlap to a considerable extent. On the other hand, the site for toxin II appears not to overlap with that of crotamine but may overlap with that of QTX.

Potentiation by crotamine of the depolarizing effects of batrachotoxin, protoveratrine A and grayanotoxin I on the rat diaphragm

The interactions between crotamine and tetrodotoxin and group II sodium channel toxins, including batrachotoxin, protoveratrine A and grayanotoxin I, were studied on the rat diaphragm muscle. When the diaphragm was pretreated with 0.1 micrograms crotamine/ml for 45 min (a condition known to depolarize the muscle by less than 3 mV, which is only 20% of the maximal depolarization induced by a saturating concentration of crotamine), the rate of depolarization by group II toxins was markedly enhanced and the time to reach the steady state depolarization was greatly shortened. The maximal depolarizations induced by each of the group II toxins, however, were not increased. Pretreatment with saturating concentrations of crotamine also caused no change of the steady state depolarization induced by batrachotoxin or grayanotoxin I. Moreover, pretreatment of the diaphragm with a high concentration of grayanotoxin I, whose effect is reversible, did not impede the depolarizing effect of crotamine. Tetrodotoxin restored the membrane potential, depolarized by crotamine, with 50% restoration at a concentration of 16 ng/ml, no matter whether a high (20 micrograms/ml) or a low (2 micrograms/ml) concentration of crotamine were used. The above results indicate that there is no competition between crotamine and group II toxins or between crotamine and tetrodotoxin. However, crotamine may affect the binding of group II toxins allosterically, increasing their affinity although the intrinsic activity may not be changed.

Cystadenocarcinoma of the ovary in a 4-year-old: benign transformation during therapy

Ovarian tumors comprise only 1% of all tumors in females below the age of 17 years. Of these, the epithelial tumors are the least common, being particularly rare before puberty and then increasing in frequency with advancing age after puberty. The youngest girl previously reported in the literature with an epithelial ovarian tumor was a 9-year-old child with an ovarian papillary cystadenocarcinoma. This is the only case thus far reported of a malignant ovarian epithelial tumor occurring before puberty. Only histologically benign appearing glands were detected in the resected tumor masses, on a second-look laparotomy following the use of multiagent chemotherapy.