Conditions affecting the interaction of cholinergic agents on the longitudinal muscle of the guinea-pig ileum--unexpected effects with hexamethonium

A simple method for measuring the effects of drugs on intestinal longitudinal and circular muscle

A tissue preparation is described which makes it possible to examine the effect of drugs on the nerves that control both longitudinal and circular muscles of the small intestine. The preparation consists of a segment of rat ileum set up to measure the tension developed in the longitudinal muscle and intraluminal pressure developed by the circular muscle in response to transmural electrical stimulation. The method showed that both muscles differed in their patterns of responses to transmural stimulation from the threshold of 2.5-60 Hz. All responses were tetrodotoxin-sensitive, the fast longitudinal muscle contraction and the intraluminal pressure response being caused by both pre- and postganglionic cholinergic nerves. The responses differed in their sensitivities to the alpha 2-adrenoceptor agonists clonidine and UK 14,304, whereby the cholinergic responses of the circular but not longitudinal muscle were inhibited by the agonists. Comparison of results with the guinea-pig ileum set up in the same way revealed a marked species variation in responses, in that the intraluminal pressure response of the guinea-pig ileum was not sensitive to alpha 2-adrenoceptor agonists. This clear species difference makes it possible to use the circular muscle of the rat ileum, as measured indirectly via intraluminal pressure, to assess compounds for activity at alpha 2-adrenoceptors over a wide range of frequencies.

The interaction of hexamethonium with muscarinic receptor subtypes in vitro

1. The action of hexamethonium has been studied at a range of muscarinic receptors in vitro by use of both functional and radioligand binding studies. 2. In functional studies, hexamethonium exhibited little or no significant (P less than 0.05) antagonism of contractile responses to carbachol at muscarinic receptors in the guinea-pig ileum, oesophageal muscularis mucosae, urinary bladder and trachea. However, antagonism was observed at muscarinic receptors in the guinea-pig left atria mediating negative inotropic responses and the calculated pKB value was 3.80. Hexamethonium also antagonized contractile responses to carbachol in the canine saphenous vein. The pKB value at these receptors was 3.75. 3. In the presence of 3.2 mM hexamethonium, the pA2 value for methoctramine at atrial muscarinic receptors was reduced by approximately 10 fold (control pA2 value was 7.81 +/- 0.05; pA2 value in hexamethonium was 6.73 +/- 0.04). In contrast at tracheal muscarinic receptors, the pA2 values for methoctramine were unaffected in the presence of 3.2 mM hexamethonium (control pA2 = 5.58 +/- 0.07; pA2 value in hexamethonium was 5.63 +/- 0.12). All values quoted are mean +/- s.e. mean, n = 8. 4. In competition radioligand binding studies, hexamethonium exhibited a higher affinity for cardiac M2 receptors (pKi = 3.68) than for cerebrocortical M1 receptors (pKi = 3.28) or for submaxillary gland M3 receptors (pKi = 2.61). At M2 receptors hexamethonium at concentrations of 0.1-10 mM, increased the half life of the dissociation rate of [3H]-N-methylscopolamine 1.6-4.3 fold. This was observed at M3 receptors only at 10 mM, when the half life was increased 1.7 fold. 5. We conclude that hexamethonium, in addition to its well characterized nicotinic antagonist properties, can act as a weak muscarinic antagonist and differentiates between cardiac M2 receptors and glandular/smooth muscle M3 receptors. However, hexamethonium differentiates less clearly between M1 and M2 receptors. The selectivity between M2 and M3 receptors observed in the present study with hexamethonium is comparable to other M2 selective antagonists such as AF-DX 116 and himbacine. 6. Caution should be exercised with regard to the inclusion of hexamethonium in functionsal studies of M2 muscarinic receptor subtypes at concentrations of 0.1 mm and above.

Characterization of cholinoreceptors in the rat urinary bladder by the use of agonists and antagonists of the cholinergic system

The effects of stimulatory and inhibitory compounds acting on both nicotinic and muscarinic receptors have been evaluated in the isolated urinary bladder from adult and immature (14-18 days old) rats. Acetylcholine and bethanechol were found to induce concentration-dependent contractions which were inhibited by atropine and pirenzepine; compound McN-A-343 had a negligible contractile activity whereas DMPP had no effect at all. Responses to electrical field stimulation were abolished by tetrodotoxin (3 X 10(-8) M), enhanced by eserine (10(-8) M) and scarcely affected by hexamethonium (10(-3) M), trimethaphan (10(-3) M) and d-tubocurarine (10(-3) M). Atropine, pirenzepine and DMPP induced only a partial inhibition (50%) of the twitch response, whereas compound McN-A-343 caused a concentration-dependent inhibitory effect which was maximum (100% inhibition) at 10(-2) M. No significant differences were found between results obtained in immature and adult animals as regards either the stimulatory or the inhibitory compounds tested. It was concluded that postjunctional muscarinic receptors were responsible for the stimulatory responses observed, whereas an additional involvement of unknown mechanisms, probably not related to the cholinergic system, was suggested by the peculiar results obtained with DMPP and compound McN-A-343 on electrically-stimulated urinary bladder. Whatever the mechanisms involved, however, they are already present at the first stage of postnatal development.

Modification by hexamethonium of the muscarinic receptors blocking activity of pancuronium and homatropine in isolated tissues of the guinea-pig

In the guinea-pig left atrium, hexamethonium (C6) (0.1-3 mM) caused a parallel rightward shift of concentration-response curves for the negative inotropic response to carbachol (CCh) and oxotremorine (Oxo), the dose ratios obtained with the latter agonist being significantly greater than those for CCh at all concentrations of C6 investigated. In the presence of C6, the muscarinic receptor blocking activity of homatropine (20 microM) or of pancuronium (0.3-2.7 microM) appears to be reduced but if the effect of hexamethonium on concentration-response curves to the agonists is taken into consideration, the dose ratios produced by the combination of C6 with either homatropine or pancuronium were essentially as predicted for the combination of 2 competitive antagonists. The difference in the affinity of pancuronium for cardiac muscarinic receptors and ileal muscarinic receptors was also reduced in the presence of hexamethonium (0.3 mM).

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.