Muscarinic receptor differentiation

Oxotremorine-m potentiation of glucose-induced insulin release from rat islets involves M3 muscarinic receptors

cDNAs encoding for M1 and M3 muscarinic acetylcholine (ACh) receptors were detected in rat pancreatic islet cells by polymerase chain reaction (PCR) amplification techniques. A new cholinergic agonist, oxotremorine-m (oxo-m), in the presence of glucose (5.6 mM), produced a dose-dependent potentiation of insulin secretion saturating at approximately 5 microM. This effect was suppressed by the L-type Ca2+ channel blocker nifedipine. Higher doses of oxo-m (50 microM) induced a biphasic insulin response both at low (5.6 mM) or high (16.7 mM) glucose concentrations. In a Ca(2+)-deficient medium containing glucose (5.6 mM), oxo-m evoked only a reduced first phase of insulin secretion. The potentiating effects of oxo-m were inhibited by the muscarinic receptor antagonists 4-diphenylacetoxy-N-methylpiperidine methiodide (M3), hexahydro-sila-difenidol hydrochloride, p-fluoro analogue (M3 > M1 > M2), and pirenzepine (M1) in a dose-dependent manner; half-maximal inhibitory concentration values were approximately 5, 20, and 340 nM, respectively. The PCR results demonstrate the presence of M1 and M3 muscarinic ACh receptors in the islet tissue, and the secretion data strongly suggest that the potentiation of glucose-induced insulin release evoked by oxo-m depends on the activation of a muscarinic M3-subtype receptor present in the beta-cell membrane.

Different muscarinic receptor subtypes mediating the phasic activity and basal tone of pig isolated intravesical ureter

1. We have studied the effects of muscarinic cholinoceptor agonists and specific antagonists on both phasic activity and basal tone of the isolated intravesical ureter of the pig by means of isometric techniques in vitro. 2. Acetylcholine in the presence and absence of physostigmine increased both phasic activity and basal tone of ureteral strips in a concentration-dependent manner. Moreover carbachol, methacholine and oxotremorine-M increased both contractile parameters while bethanechol and McN-A-343 evoked only increases in tone without affecting the frequency of the phasic contractions. 3. The nicotinic receptor blocker, hexamethonium (10(-6)-10(-4) M), failed to modify the contractions evoked by a single dose of carbachol (10(-5) M), whilst the muscarinic antagonist, atropine inhibited both phasic and tonic responses. 4. The muscarinic M1 (pirenzepine), M2 (AF-DX 116 and methoctramine), M3 (4-DAMP, HHSiD and p-F-HHSiD), and putative M4 receptor (tropicamide) antagonists significantly reversed increases in both frequency of phasic activity and baseline tone induced by a submaximal dose of carbachol (10(-5) M). The pIC50 values for inhibition of the induced phasic activity were: atropine (10.16) > 4-DAMP (9.12) > HHSiD (8.22) = methoctramine (7.98) = p-F-HHSiD (7.88 > tropicamide (7.62) = pirenzepine (7.53) = AF-DX 116 (7.45) and for inhibition of basal tone were: atropine (10.73) > 4-DAMP (9.32) > HHSiD (8.65) = pirenzepine (8.43) = p-F-HHSiD (8.38) > methoctramine (7.79) > tropicamide (7.53) > AF-DX 116 (7.04). 5. The antagonist profile indicates that an M1 receptor mediates the tonic response while the phasic activity could involve either both M2 and M3 or an M4 muscarinic receptor. These results suggest that different muscarinic receptor subtypes mediate the phasic and tonic contractile activity induced by a submaximal concentration of carbachol in the porcine intravesical ureter.

Pharmacological characterization of muscarinic receptor subtypes in rabbit isolated tissue preparations

1 The affinity of some muscarinic antagonists for muscarinic receptors was determined in functional isolated tissue studies in order to compare the muscarinic receptor subtypes in the rabbit. 2 Our attention was specially focused on the question of whether the muscarinic receptors mediating vasodilatation in the aorta resemble or not the ones present on the jejunum of the gastrointestinal tract. 3 Isolated aorta, jejunum, stimulated left atrium and vas deferens preparations of rabbit were investigated with the following muscarinic antagonists: atropine, pirenzepine, methoctramine (N,N'-bis[6-92-methoxybenzyl)amino hexyl]-1,8-octane-diamine tetrahydrochloride) and 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide). 4 The results demonstrate that the receptors on aorta are unlike those on the other rabbit tissues: pirenzepine pA2 was 6.4 on aorta but 8.1 on vas deferens; methoctramine pA2 was 5.9 on aorta but 7.1 on heart; 4-DAMP pA2 was 8.7 on aorta and 8.0 on jejunum. This raises the question: what subtype might be involved?

Characterization of muscarinic receptor subtype of rat eccrine sweat gland by autoradiography

The muscarinic cholinergic receptor of rat eccrine sweat gland was characterized using quantitative autoradiography and [3H]QNB as radioligand. The distribution of radioligand was maximal in the secretory coil. Autoradiographic competition binding studies were performed using selective antagonists to M1 (pirenzepine), M2 (AF-DX 116), and M3 (4-DAMP) and the classical nonselective antagonist atropine. pKi for pirenzepine, AF-DX 116, 4-DAMP, and atropine was 6.58, 5.47, 8.50, and 8.66 respectively indicating that the eccrine sweat gland muscarinic receptor was predominantly M3.

p-Fluoro-hexahydro-sila-difenidol exhibits poor selectivity between M3 and M1 muscarinic receptors

We investigated the potential ability of p-fluoro-hexahydro-sila-difenidol (p-F-HHSiD) to discriminate between M1 and M3 muscarinic receptor subtypes using Chinese hamster ovary cells stably transfected with the genes encoding the two receptors. Both radioligand binding and functional assays were utilized for this purpose. In contrast to initial reports of a 14-fold selectivity of this antagonist for M3 versus M1 receptors, we have detected a qualitatively similar selectivity that was markedly smaller in magnitude.