Methylflavonolamine hydrochloride inhibits contractions induced by noradrenaline, calcium and potassium in rabbit isolated aortic strips

1. The effects of methylflavonolamine hydrochloride (4'-methyl-7-(2-hydroxy-3-isopropylamino-propoxy)-flavone hydrochloride, MFA) were investigated and compared with verapamil and papaverine on rabbit isolated aortic strips, which were contracted by noradrenaline, calcium and potassium. 2. Pre-incubation for 25 min with either MFA (0.03 to 0.2 mM) or papaverine (0.03 to 0.2 mM) induced non-parallel and concentration-dependent rightward displacements of the curves to noradrenaline (0.00001 to 0.1 mM) with the maximal response depressed. The calculated pD2' values (mean +/- s.d.) were 3.89 +/- 0.15 for MFA and 3.93 +/- 0.05 for papaverine, respectively. Verapamil (0.03 to 0.2 mM) inhibited the contraction induced by noradrenaline in a competitive manner with a pA2 value of 5.91 +/- 0.83. 3. In depolarized aortic strips of the rabbit, prior exposure to MFA (0.03 to 0.3 mM) and papaverine (0.03 to 0.2 mM) shifted the cumulative curves to Ca2+ (0.003 to 100 mM) parallel to the right with the maximal responses depressed, pD'2 values being 3.88 +/- 0.05 and 3.89 +/- 0.13, respectively. Verapamil produced comparable inhibition of the contraction at much lower concentrations (30 to 300 nM). 4. MFA (0.03 and 0.1 mM) inhibited the contraction elicited by graded depolarization at a constant Ca2+ concentration with a pD'2 value of 4.09 +/- 0.07. 5. The present results show that MFA has some actions consistent with a calcium antagonist. It resembles papaverine more closely than verapamil.

A comparison of the pharmacological actions of 4-aminopyridine and two of its derivatives in the monkey

The neuromuscular, cardiovascular and central nervous system stimulating effects of 4-aminopyridine (4-AP), 2,4-diaminopyridine (2,4-DAP) and LF-14 were investigated in the monkey. All these compounds were shown to reverse the stable neuromuscular blockade produced by the intravenous infusion of pancuronium bromide. The doses producing 50% antagonism (ED50) of the pancuronium-induced neuromuscular block were 0.50, 0.54 and 0.71 mg/kg for LF-14, 2,4-DAP and 4-AP respectively. The compounds had only slight cardiovascular effects. In contrast to 4-AP, LF-14 and 2,4-DAP did not reduce the duration of ketamine/diazepam-induced anesthesia, suggesting minimal if any central nervous system effects of these two compounds.

5-Hydroxytryptamine depresses depolarizing responses to GABA in the rat isolated vagus nerve

Axons of the rat isolated cervical vagus nerve responded to gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT) with a readily reversible dose-dependent depolarization. Prior application of 5-HT at 2 or 30 microM equally depressed the depolarizing responses to GABA, shifting the GABA dose-response curve to the right and depressing the maximum; by contrast, responses to 5-HT were affected only by large doses of GABA (greater than 300 microM), and to a lesser degree. In addition, the fade in responses to high doses of GABA was also reduced in the presence of 5-HT. It is suggested that such all-or-none depressive actions may explain the blockade of GABA-induced responses by 5-HT observed in other tissues such as the guinea-pig ileum.

An integrated model for the interaction of muscle relaxants with their antagonists

An integrated model describing the interaction of nondepolarizing neuromuscular blocking agents with reversible anticholinesterase agents is derived and compared with a naive model using experimental data obtained from four anesthetized dogs. Three consecutive but separate steady-state d-tubocurarine blocks (approximately 50, 70, and 90%) were induced in each of the four dogs and reversed by short edrophonium infusions. Edrophonium arterial concentrations and twitch tension of the anterior tibialis muscle were measured. Both the integrated and the naive model were fit to the twitch tension data using a model with a hypothetical "effect" compartment. The integrated model consistently fit the twitch tension data better than the naive model; the sum of squared deviations was lower by 46, 45, 87, and 69%, respectively, with the integrated model than with the naive model. Also, in contrast to the naive model, the integrated model is capable of describing the interaction of the anticholinesterase agent and the neuromuscular blocking agent when the concentration of either varies with time.

The depolarizing action of 5-hydroxytryptamine on rabbit vagal primary afferent and sympathetic neurones and its selective blockade by MDL 72222

MDL 72222 (1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate) is a novel compound with potent and selective blocking actions at certain excitatory 5-hydroxytryptamine (5-HT) receptors on mammalian peripheral neurones. In the present study, the sucrose-gap technique has been used to record depolarizing responses to 5-HT from the cells of the rabbit nodose and superior cervical ganglia and to investigate the potency and selectivity of MDL 72222 as an antagonist of these responses. On nodose ganglia, responses to 5-HT were inhibited surmountably by MDL 72222 at concentrations up to 100 nmol/l. The threshold for antagonism was 2-10 nmol/l and the apparent pA2 value (Schild 1947) was 7.7 +/- 0.2, n = 10. Blockade was selective since responses to GABA and noradrenaline were unaffected by MDL 72222, 100 nmol/l. With concentrations of MDL 72222 higher than 100 nmol/l, antagonism was concentration-related but not in a manner consistent with simple competitive antagonism and even a concentration of 1 mumol/l failed to abolish the response to 5-HT. The results from the superior cervical ganglion were essentially similar to those obtained from the nodose ganglion. The threshold concentration of MDL 72222 for inhibition of 5-HT was 1-10 nmol/l and blockade was selective in that depolarizing responses to dimethylphenyl-piperazinium (DMPP) was unaffected by a concentration of MDL 72222 of 1 mumol/l. The data provide direct evidence that MDL 72222 is a potent and selective antagonist of the receptors for 5-HT which mediate depolarizing responses in vagal primary afferent cell bodies and in sympathetic ganglion cells.

Experimental and clinical evaluation of neuromuscular blocking agents

Neuromuscular blocking agents can be evaluated in anesthetized cats, dogs, or rhesus monkeys using dose-response relationships to assess the degree of separation between neuromuscular blocking activity and effects on cardiovascular and autonomic systems. Histamine release can be studied in anesthetized dogs by establishing whether any induced hypotension is reduced by pretreatment with H1 and H2 receptor antagonists. In anesthetized cats, the role of the liver in metabolism is investigated after administration of the drug by the hepatic portal vein and determining whether this "first pass" effect through the liver reduces potency. The involvement of the kidneys in the elimination of the drug can be assessed by ascertaining whether or not the potency and duration of action are enhanced after bilateral renal ligation. In anesthetized man, the simultaneous measurement of the tetanic (50 Hz) and single twitch contractions of the adductor pollicis muscle of the hand has proved a valuable technique for the quantitative assessment of skeletal muscle relaxants. Furthermore, the tetanic response overcomes the relative insensitivity of the single twitch, and tetanic fade provides a measure of the effects of these drugs at presynaptic sites. Cumulative effects may be evident as a prolonged recovery after repeated dosage or after an infusion. The effectiveness of neostigmine as an antagonist should be investigated both from partial and complete block, and recovery of respiration should be compared with that of peripheral neuromuscular blockade.

The neuromuscular effects of lignocaine

The local anesthetic drug lignocaine, in concentrations which completely block both nerve and muscle action potentials, produces concentration-dependent contractures in the vertebrate skeletal and smooth muscle fibres. The contractures produced by lignocaine were not related to membrane depolarization. Although lignocaine greatly reduced the depolarizations and contractures produced by depolarizing agents, such ACh or carbachol, it had a differential effect on the responses produced by tetraethylammonium (TEA) at the chick neuromuscular junction. Lignocaine reduced the TEA-induced depolarizations while it markedly enhanced the contracture responses. In the absence of membrane depolarization, the contractures produced by lignocaine may be attributed to an effect on release of intracellular calcium (Ca2+) from the sarcoplasmic reticulum (SR). The results suggested a multiple site of action of lignocaine at the vertebrate neuromuscular junction.

Quisqualic acid excitation of cortical neurones is selectively antagonized by streptomycin

Although evidence exists for at least 3 kinds of excitatory amino acid receptor in the CNS, responding to N-methyl-D-aspartic acid, kainic acid and quisqualic acid, respectively, only antagonists at the former two sites are currently available. It is now reported that when applied by microiontophoresis to neurones in the rat cerebral cortex, excitatory responses to quisqualic acid can be selectively reduced by streptomycin.

Tetramethylenedisulphotetramine: an inhibitor of gamma-aminobutyric acid induced depolarization of the isolated superior cervical ganglion of the rat

Tetramethylenedisulphotetramine (TETS), a potent convulsant, antagonized the depolarizing action gamma-aminobutyric acid (GABA) in the isolated superior cervical ganglion of the rat. No antagonism of responses to the cholinomimetic agent carbachol was observed. TETS appeared to act in a non-competitive manner and was reversible. Its activity profile was comparable to that of bicuculline in the same tissue except that the latter appears to act in a competitive manner.

The actions of some anticholinesterase drugs on skeletal muscle in culture

The actions of the anticholinesterase drugs, physostigmine, neostigmine and diisopropylfluorophosphate (DFP) on chick embryonic skeletal muscle in culture were studied. None of the anticholinesterases potentiated depolarization responses to acetylcholine. In high concentrations neostigmine and physostigmine produced depolarization. The neostigmine-induced, but not the physostigmine-induced, depolarization was antagonized by tubocurarine. DFP caused an increase in the rate of repolarization during the presence of a cholinomimetic. It is concluded that the cholinesterase present in cultured muscle fibres does not have a physiological role in hydrolysing acetylcholine and that physostigmine and DFP have an action at the ionic channels that are linked to the cholinoreceptor.

Inhibitory action of Ruthenium red on neuromuscular transmission

The effect of Ruthenium Red on synaptic transmission was examined at isolated junctions of the frog, by conventional methods for stimulation and intracellular recording. Ruthenium Red (2.5-10.0 muM) reduces the synaptic potential to subthreshold levels. An analysis of this phenomenon shows that the main action of Ruthenium Red is on the presynaptic nerve terminal where it decreases the number of quanta of transmitter liberated by the nerve impulse. It has the following additional effects: a reduction in the amplitude of the spontaneous miniature end plate potentials; an increase in their frequency; and an increase in delayed release of transmitter after a nerve impulse. Some of these results are discussed in terms of the known inhibitory action of Ruthenium Red on calcium transport across mitochondrial membranes.

Pentobarbital: selective depression of excitatory postsynaptic potentials

The effects of pentobarbital (Nembutal) on synaptic transmission and postsynaptic potentials were studied by the use of several invertebrate preparations. Pentobarbital selectively and reversibly depressed both excitatory postsynaptic potentials and sodium-dependent postsynaptic responses to putative excitatory transmitters without affecting either inhibitory postsynaptic potentials or chloride- and potassium-dependent postsynaptic responses to putative transmitters. A selective depression of postsynaptic excitatory events was also observed with other central nervous system depressants (ethanol, chloroform, chloralose, diphenylhydantoin, and urethane). The results suggest that central and peripheral depression observed during general anesthesia is due to a selective depression of excitatory synaptic events.

Gamma-aminobutyric acid antagonism and presynaptic inhibition in the frog spinal cord

The convulsant alkaloid bicuculline blocked presynaptic inhibition, dorsal root potentials, primary afferent depolarization, and depolarizing effects of gamma-aminobutyric acid on dorsal root terminals of the amphibian spinal cord, but did not block effects of other putative amino acid transmitters. These actions of bicuculline suggest that gamma-aminobutyric acid may be the transmitter involved in spinal presynaptic inhibition.