Effects Of Muscarinic Receptor Antagonists Research Articles

M3 muscarinic receptors mediate pepsinogen secretion via polyphosphoinositide hydrolysis in guinea pig gastric chief cells

The muscarinic receptor system involved in pepsinogen secretion from isolated guinea pig gastric chief cells was investigated by evaluating the effect of muscarinic receptor antagonists on carbamylcholine (CCh)-stimulated chief cell responses. CCh stimulated the hydrolysis of polyphosphoinositide in chief cells at the same concentrations as it stimulated pepsinogen secretion. Each of five different muscarinic receptor antagonists, atropine, pirenzepine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), AF-DX116 and scopolamine, inhibited both pepsinogen secretion and inositol phosphate accumulation stimulated by graded concentrations of CCh. The pA2 values of the antagonists calculated from data on inositol phosphate accumulation and pepsinogen secretion (atropine = scopolamine = 4-DAMP greater than pirenzepine greater than AF-DX116) suggest that the muscarinic acetylcholine receptor in gastric chief cells is the M3 subtype. On the other hand, CCh did not affect the adenylate cyclase/cAMP signaling pathway in gastric chief cells. All pA2 values of the antagonists were also in agreement with the Ki values determined by [3H]NMS binding to chief cells. Furthermore, GTP gamma S reduced [3H]acetylcholine binding to chief cell membranes in a concentration-dependent manner. The present study, therefore, suggests that muscarinic M3 receptors, which may be coupled to a G protein, mediate pepsinogen secretion, probably by activation of the polyphosphoinositide second messenger system in guinea pig gastric chief cells.

The M3 muscarinic receptor links to three different transduction mechanisms with different efficacies in circular muscle of guinea-pig stomach.

1. In a previous publication, we showed that 10 microM carbachol induced contraction by activating three independent transduction mechanisms in circular smooth muscle of guinea-pig gastric fundus (Parekh & Brading, 1991). These were: inositol trisphosphate-mediated intracellular Ca2+ release, Ca2+ influx through a nifedipine-sensitive route and Ca2+ influx through a receptor operated nifedipine-insensitive pathway. The former two processes contribute to the phasic contraction and the latter two to the tonic contraction. In this paper, we have studied the effects of muscarinic receptor antagonists with known selectivity for different muscarinic receptor subtypes, on the contraction evoked by 10 microM carbachol. 2. Low concentrations of pirenzepine (M1 selective) had little effect on the contraction initiated by carbachol. Higher concentrations (greater than 1 microM) reduced only the phasic component. This concentration of pirenzepine greatly reduced the contraction evoked by 10 microM carbachol in Ca(2+)-free solution, indicating inhibition of intracellular Ca2+ release. 3. In the presence of 10 microM nifedipine, the tonic contraction evoked by 10 microM carbachol (reflecting the receptor-operated nifedipine-insensitive route) was abolished by 10 microM pirenzepine. In the absence of nifedipine pretreatment, however, 10 microM pirenzepine did not abolish the contraction to 10 microM carbachol. This contraction was subsequently abolished by nifedipine. 4. Only high concentrations (greater than 10 microM) of the M2-selective antagonist, gallamine, inhibited the contraction to 10 microM carbachol. Like pirenzepine, gallamine preferentially inhibited the phasic component of the contraction, indicating an effect on intracellular Ca2+ release. 5. The non-selective muscarinic receptor antagonist, atropine, abolished all components of the contraction. At low concentrations, atropine also reduced the phasic component without affecting the tonic one, indicating preferential inhibition of intracellular Ca2+ release.6. It is concluded that (i) the different transduction mechanisms have different sensitivities to the antagonists used and (ii) an M3 receptor activates the three transduction mechanisms with different efficacies.

Different muscarinic receptors mediate autoinhibition of acetylcholine release and vagally-induced vasoconstriction in the rat isolated perfused heart

Experiments were carried out on rat isolated perfused hearts with both vagus nerves attached. The acetylcholine stores were labelled with [14C]-choline. The effects of muscarinic receptor antagonists on the [14C]-overflow and increase in perfusion pressure evoked by vagus nerve stimulation (10 Hz, 4-10 mA) were studied in order to determine the muscarinic receptor type involved in autoinhibition of acetylcholine release and vagally-induced vasoconstriction in the rat heart. Stimulation of the vagus nerves (1200 pulses) caused an increase in [14C]-overflow and in perfusion pressure which was significantly reduced by hexamethonium 500 mumol/l and abolished by tetrodotoxin 0.3 mumol/l or perfusion with Ca2(+)-free solution. The fractional rate of evoked [14C]-overflow per pulse upon stimulation at 10 Hz (720 pulses) was doubled in the presence of the non-selective antagonist atropine (0.01-1 mumol/l) as well as in that of the M2-selective compounds methoctramine (0.1 mumol/l) and AF-DX 116 (0.1-1 mumol/l), but remained unaffected by the M3-selective hexahydrosiladifenidol (0.1 mumol/l). The increase in perfusion pressure upon nerve stimulation was reduced by atropine (0.01 mumol/l) or hexahydrosiladifenidol (0.1 mumol/l) to approximately 50% and increased by about 50% in the presence of AF-DX 116 (0.1 mumol/l). The results show that the autoinhibition of acetylcholine release in the rat heart is mediated by M2 receptors. On the other hand, the increase in perfusion pressure upon vagus nerve stimulation is caused by a different muscarinic receptor, more sensitive to hexahydrosiladifenidol than to M2-selective antagonists.

P-226 - Comparisons of effects of muscarinic receptor antagonists in isolated, blood-perfused canine atrial preparations

P-226 - Comparisons of effects of muscarinic receptor antagonists in isolated, blood-perfused canine atrial preparations