The effects of eserine and neostigmine on the guinea-pig ileum and on ileal longitudinal muscle strips

Effects of TAK-802, a novel acetylcholinesterase inhibitor, and various cholinomimetics on the urodynamic characteristics in anesthetized guinea pigs

In the present study, we investigated the effects of cholinomimetic drugs on the urodynamic characteristics in anesthetized guinea pigs. 8-[3-[1-[(3-fluorophenyl)methyl]-4-piperidinyl]-1-oxopropyl]-1,2,5,6-tetrahydro-4H-pyrrolo[3,2,1-ij]quinolin-4-one (TAK-802), a novel acetylcholinesterase inhibitor, (0.003-0.03 mg/kg, i.v.) increased the voided volume and the maximum flow rate without affecting either the intravesical pressure or the bladder compliance. Distigmine (0.03-0.3 mg/kg, i.v.) and neostigmine (0.01-0.1 mg/kg, i.v.), both carbamate acetylcholinesterase inhibitors, while not increasing the maximum flow rate, increased the intravesical pressure at the maximum flow rate. They also decreased the bladder compliance. Bethanechol (0.1-1 mg/kg, i.v.), a muscarinic receptor agonist, decreased the voided volume and the bladder compliance but did not affect the maximum flow rate. TAK-802 did not affect the intraurethral pressure at doses of up to 0.03 mg/kg in anesthetized guinea pigs. Distigmine increased the intraurethral pressure when administered at the dose of 0.3 mg/kg, and the effect was completely abolished by pretreatment with d-tubocurarine. These results suggest that TAK-802 reinforces the bladder-voiding functions by increasing the bladder contractility without decreasing the storage function. On the other hand, carbamate acetylcholinesterase inhibitors not only deteriorate the voiding function by inducing contraction of the external urethral sphincter muscle, resulting in increasing the urethral resistance, but also cause deterioration of the storage function. Bethanechol obviously decreased the bladder capacity, possibly due to a direct contractile effect on the detrusor smooth muscle. TAK-802 may therefore be a more useful drug than either carbamate acetylcholinesterase inhibitors or muscarinic receptor agonists in the treatment of voiding dysfunction associated with impaired detrusor contractility.

Bradycardia produced by pyridostigmine and physostigmine

PURPOSE

The bradycardia produced by pyridostigmine and physostigmine in an animal model of acute cardiac denervation was examined according to its relation to cholinesterase inhibition and sensitivity to block by cholinergic receptor antagonists.

METHODS

Cats were anaesthetised, vagotomised and propranolol-treated. Heart rate was continuously recorded. Erythrocyte cholinesterase activity of arterial blood was measured using a radiometric technique. Nicotinic and muscarinic M1 receptors were blocked with hexamethonium and pirenzepine, respectively. M2 receptors were blocked with gallamine, pancuronium and AFDX-116.

RESULTS

With pyridostigmine and physostigmine the dose-response relationship for the decrease in heart rate (ED50 1.05 +/- 0.25 and 0.198 +/- 0.03 mg.kg-1, respectively) was shifted to the right of that for the inhibition of cholinesterase activity (ED50 0.094 +/- 0.03 and 0.032 +/- 0.01 mg.kg-1, respectively). The decrease in cholinesterase activity reached a plateau at a cumulative dose of 0.56 +/- 0.08 and 0.32 +/- 0.08 mg.kg-1, respectively. In contrast, there did not appear to be a plateau in the bradycardic effect. The bradycardia produced by pyridostigmine and physostigmine was blocked by hexamethonium (ED50 10 +/- 1.3 and 15.3 +/- 2.4 mg.kg-1, respectively), pirenzepine (ED50 68 +/- 16 and 138 +/- 32 micrograms.kg-1, respectively), gallamine (56 +/- 11 and 67 +/- 17 micrograms.kg-1, respectively), pancuronium (32 +/- 10 and 30 +/- 4 micrograms.kg-1, respectively), and AFDX-116 (31 +/- 4 and 28 +/- 4 micrograms.kg-1, respectively).

CONCLUSION

The bradycardia produced by reversible anticholinesterase drugs containing a carbamyl group is not clearly related to the degree of cholinesterase activity, and has a low sensitivity to nicotinic and muscarinic M1 and a high sensitivity to muscarinic M2 receptor antagonists.

Pardaxin produces postjunctional muscle contraction in guinea-pig intestinal smooth muscle

The action of pardaxin (PX), a toxin isolated from the secretion of the Red Sea flatfish, Pardachirus marmoratus, was studied on longitudinal muscle of guinea-pig ileum. Pardaxin contracted the ileum and subsequently abolished muscle contraction to 5-hydroxytryptamine (5-HT), but did not affect the responses to acetylcholine (ACh) and substance P(SP). Pardaxin-induced contraction was only partially suppressed by atropine and not affected by tetrodotoxin or morphine. Preparations desensitized to 5-HT or SP responded normally to pardaxin. Pardaxin-induced contractions were normal in K+-depolarizing Krebs Ringer solution and not affected by black widow spider venom. It is concluded that the pardaxin-induced muscle contractions are not mediated through the release of neurotransmitters and do not involve 5-HT, SP or ACh receptors, but are due to a direct action on the muscle contractile mechanism.

Contractures produced by carbamate anticholinesterases in bovine tracheal smooth muscle

1. In isolated strips of bovine tracheal muscle the carbamate anticholinesterases, neostigmine and eserine caused similar, slowly-developing, sustained spasms which were concentration-related in the range 10(-7)-10(-4) mol/l; these spasms could be abolished either by withdrawing the anticholinesterase or by addition of atropine (5 X 10(-7) mol/l). 2. Depletion of tissue stores of acetylcholine using hemicholinium-3 with or without electrical stimulation rendered the muscle unresponsive to neostigmine (10(-6) mol/l). Responses to acetylcholine itself were not impaired. 3. A low concentration of neostigmine (10(-8) mol/l) did not cause spasm but enhanced the contractile response of bovine trachealis to acetylcholine, carbachol and histamine. This concentration of neostigmine also increased the muscle's contraction upon exposure to a high-potassium solution, even in the presence of atropine (5 X 10(-7) mol/l). 4. It is concluded that neostigmine and eserine cause spasm not only by preventing breakdown of endogenously released acetylcholine but also by stimulating release of acetylcholine from nerve terminals and by a non-specific enhancement of muscle contraction.

The effects of paraoxon on blood pressure in the anaesthetized and in the conscious rat

1 Intravenous administration of paraoxon (150-825 mug/kg) to anaesthetized rats induced long-lasting, dose-dependent pressor effects. Only after injection of 825 mug/kg paraoxon was the pressor response followed by a depressor effect and a bradycardia that could be blocked by N-methylatropine. Intracerebroventricular injection of paraoxon into anaesthetized rats also induced pressor effects.2 In order to elucidate the mechanism of the pressor action rats were given dexetimide, N-methylatropine, mecamylamine, phentolamine, prazosin, yohimbine, atenolol and metoprolol. If treatment with these drugs resulted in a low initial blood pressure, vasopressin was infused to elevate blood pressure to normal levels. The influence of adrenalectomy, pretreatment with reserpine and midcollicular transection was also examined.3 The pressor effect of paraoxon was not influenced by N-methylatropine or mecamylamine. However, a combination of these drugs as well as dexetimide, phentolamine or prazosin combined with yohimbine, reduced or prevented the pressor effect.4 In conscious rats the effects of paraoxon and the action of antimuscarinic drugs upon the pressor response were similar to those observed in anaesthetized animals.5 Acetylcholinesterase activities were measured in various brain regions and in whole blood. Paraoxon concentrations within the CNS were also measured.6 It is concluded that the pressor effect of paraoxon in anaesthetized and conscious rats is mediated by a central mechanism, although a contribution of peripheral acetylcholinesterase inhibition in sympathetic ganglia to this pressor effect cannot be ruled out.

The inotropic and chronotropic effects of physostigmine and neostigmine on guinea-pig isolated atria

1 Physostigmine (0.1-1 mM) induced dose-dependent negative and neostigmine (0.1-1 mM) positive inotropic effects on driven guinea-pig isolated atria. Physostigmine decreased and neostigmine increased cardiac frequency of spontaneously beating atria. 2 The depression of amplitude of contraction by physostigmine cannot be attributed to its inhibitory effect on tissue cholinesterase. The negative inotropic action of physostigmine is not influenced by preincubation with atropine. 3 The positive inotropic effect of neostigmine is not caused by a nicotine-like action of the drug. Neostigmine (1 mM) counteracts the negative inotropic effect of acetylcholine. The possible mechanisms of action are discussed.

Antibiotic-associated colitis: an in vitro investigation of the effects of antibiotics on intestinal motility

1 Nine antibiotic compounds in common use were studied to determine their ability to affect intestinal motility in vitro, in the guinea-pig ileum and rabbit colon. 2 Ampicillin, doxycycline, mecillinam and metronidazole were without effect over a concentration range which included typical serum levels found when these drugs are used therapeutically. 3 Clindamycin, gentamicin, kanamycin, pivmecillinam and trimethoprim were all found to inhibit evoked and reflex responses of the guinea-pig ileum but only clindamycin and trimethoprim also affected evoked responses of the rabbit colon. 4 Kanamycin and gentamicin appeared to have a predominantly pre-junctional action, pivmecillinam and trimethoprim a predominantly post-junctional action. Clindamycin had a pre-junctional action at low concentrations and long exposure times, and a post-junctional action at high concentrations and short exposure times. 5 The concentration of each antibiotic required to inhibit the peristaltic reflex of the guinea-pig ileum was less than that required to inhibit its responses to electrical stimulation or exogenous acetylcholine or histamine but greater than the serum levels associated with their respective use in therapeutic doses. 6 A sequence of events whereby antibiotic-induced alterations in gastro-intestinal motility could lead to the development of pseudomembranous colitis is proposed.

Actions of various muscarinic agonists on membrane potential, potassium efflux, and contraction of longitudinal muscle of guinea-pig intestine

1 Depolarizations were recorded intracellularly in smooth muscle from the taenia of the guinea-pig caecum in response to the iontophoretic application of acetylcholine, carbachol, oxotremorine-M, methylfurmethide, hexyl trimethylammonium and tetramethylammonium (TMA). 2 No differences between the iontophoretic responses to agonists stable to cholinesterase were detected. 3 The latency and time to peak of acetylcholine-induced depolarizations were less than those to stable agonists, and the response was briefer and less complex in shape. These differences were reduced, or disappeared, upon inhibition of cholinesterase. 4 The rate of loss of 42K and changes in length were measured in superfused strips weighing about 10 mg of separated longitudinal muscle of guinea-pig ileum. 5 Acetylcholine, carbachol, methylfurmethide, butyltrimethylammonium and TMA contracted the muscle and increased the rate of loss of 42K. 6 Contrary to previous reports, no evidence of a selective action of any of these agonists on 42K loss was detected. TMA appeared to be a partial agonist in evoking 42K loss, although it produced a maximum contraction. 7 The maximum 42K efflux produced by acetylcholine was about 40% of the maximum evoked by application of carbachol or methylfurmethide. If cholinesterase was inhibited, the 42K efflux evoked by maximally effective concentrations of acetylcholine was comparable to that evoked by a stable agonist. 8 These results are consistent with the idea that the muscarinic agonists used interact in an essentially similar way with muscarinic receptors to produce their effects on membrane potential, permeability, and tension.

Inhibition of 5-hydroxytryptamine contraction of guinea-pig smooth muscle treated with black widow spider venom

Black widow spider venom (BWSV) elicited a prolonged tonic contraction of guinea-pig ileum which abolished the nicotine-induced contraction; however, the subsequent acetylcholine stimulation was found to be unchanged. These findings indicate a depletion of nerve terminal transmitters caused by BWSV without a disruption of the muscle contractile mechanism. Following the BWSV-induced contraction, the stimulation to 5-hydroxytryptamine (5-HT) was found to be abolished. This abolishment indicates a mediation of 5-HT-induced smooth muscle contraction in the nerve rather than the muscle.

Kinetics of morphine-sensitive [3H]-acetylcholine release from the guinea-pig myenteric plexus

1 Longitudinal muscle-myenteric plexus preparations from the guinea-pig ileum were superfused at a constant rate while isotonic contractions were monitored. 2 The preparations were superfused with [3H]-choline while stimulated supramaximally at 0.1 Hz followed by washout in the presence of hemicholinium-3. The evoked release of the label due to a second 0.1 Hz stimulation in the absence of an anticholinesterase was measured. 3 Evoked efflux of the label was initially fast followed by a slower phase. 4 Morphine reduced the size of the pool and the rate of the initial fast efflux and the size of the pool but not the rate of the slow efflux evoked by supramaximal stimulation. 5 Submaximal stimulation reduced only the size of the pools from which the fast and slow efflux originated. 6 Naloxone reversed the depression of contractions and evoked release produced by morphine. 7 Results suggest that 0.1 Hz stimulation releases [3H]-acetylcholine simultaneous from two pools. The fast release may originate from spontaneously firing units whose rate of discharge is depressed by morphine, while the slow release originates from neurones which do not fire spontaneously and whose threshold to field stimulation is increased by morphine.

The effect of cholinesterase inhibitors on the antimuscarinic effect of hemicholinium-3 (HC-3) in the rat

The effect of hemicholinium-3 (HC-3) on responses of the rat isolated bladder and ileum to acetylcholine and carbachol was investigated in the absence and presence of a number of anticholinesterases. Responses of the bladder to acetylcholine were potentiated by DFP, edrophonium, BW284C51 and physostigmine but were unaffected by the specific butyrylcholinesterase inhibitor iso-OMPA. Responses to carbachol were not potentiated by the anticholinesterases. HC-3 (1.7 X 10(-4) M) inhibited responses to carbachol without affecting those to acetylcholine. In the presence of physostigmine or DFP responses to acetylcholine were inhibited by HC-3 but no such inhibition was observed in the presence of BW284C51, edrophonium or iso-OMPA or a combination of the latter two anticholinesterases. Responses to carbachol were also inhibited to a greater extent in the presence of DFP. In the ileum, responses to acetylcholine were increased in the presence of DFP, edrophonium and physostigmine but were unaffected by iso-Ompa. responses to carbachol were not increased by any of the anticholinesterases. HC-3 (2.8 X 10(-4) M) inhibited responses to both acetylcholine and carbachol in the ileum and the degree of inhibition was not significantly altered by the presence of any of the anticholinesterases used. Although a weak anticholinesterase, HC-3 was also found to decrease the inhibitory action of physostigmine on the hydrolysis of acetylcholine by homogenates of rat ileum. A similar effect was noted with DFP but not with edrophonium. The results obtained do not support a prejunctional action for HC-3 in antagonizing responses to carbachol. It is concluded that in addition to an inhibitory action on the post-junctional muscarinic receptor HC-3 may interfere with the anticholinesterase activity of some cholinesterase inhibitors such as physostigmine and DFP but not edrophonium.

The irreversible binding of acetylcholine mustard to muscarinic receptors in intestinal smooth muscle of the guinea-pig

1. Acetylcholine mustard (N-2-chloroethyl-N-methyl-2-acetoxyethylamine), a potent muscarinic agonist, binds virtually irreversibly to muscarinic receptors in longitudinal muscle strips from guinea-pig small intesting, as shown by the inhibition of the binding of E13-H]-propylbenzilycholine mustard ([3-H-PrBCM), an affinity label for the muscarinin receptor. 2. A value for the apparent binding affinity of acetylcholine mustard and a value for the rate constant for the receptor alkylation reaction have been deduced from the rate of onset of the inhibition of [3-H]-PrBCM binding. 3. The kinetic constants obtained may refer largely to the interaction between acetylcholine mustard and the desensitized receptor. 4. At high concentrations acetylcholine mustard practically abolishes the contractile response to carbachol. At the concentrations acetylcholine mustard appears to have multiple actions on the tissue.