Presynaptic and postjunctional muscarinic receptors in dog ileum: binding studies

Characterization of pre- and postsynaptic muscarinic receptors in circular muscle of pig gastric fundus

1. This study investigated the subtype of muscarinic receptors on the cholinergic neurones and smooth muscle in the circular muscle of the pig gastric fundus. 2. Muscarinic antagonists, except MT-3, concentration-dependently inhibited the contractions induced by a given concentration of acetylcholine. Concentration-response curves by acetylcholine were shifted rightwards in a parallel manner without depression of the maximum by the muscarinic antagonists, except by MT-3 that induced a leftward shift. Correlation of the pIC(50) and pA(2) values with published pK(i) values for the five muscarinic receptor subtypes suggests that the muscarinic receptors on pig gastric fundus circular muscle belong to the M(3) subtype. 3. Electrically-evoked contractions (40 V, 4 Hz, 0.25 ms, 2 min) were concentration-dependently inhibited by the muscarinic antagonists except for methoctramine and AF-DX 116, that increased the amplitude of the electrically-induced contractions in lower concentrations. MT-3 tended to increase the electrically-induced contractions. 4. The antagonists, except MT-3, concentration-dependently increased the electrically-induced tritium outflow (40 V, 4 Hz, 0.25 ms, 2 min) after incubation of the tissues with [(3)H]-choline. MT-3 (3 x 10(-8) and 10(-7) M) decreased the electrically-induced tritium release. Correlation of the pIC(50) values with published pK(i) values for the different muscarinic receptor subtypes yielded a significant and comparable correlation for M(1), M(3), M(4) and M(5) receptors. 5. These results suggest that the postsynaptic receptors in circular muscle of the pig gastric fundus belong to the M(3) subtype. However, the presynaptic receptor could not be clearly defined, although it does certainly not belong to the M(2) subtype.

A possible approach to the suppression of side effects induced by PGE1

Prostaglandin E1 (PGE1) is known to possess various actions in vivo. Of these actions, the contraction of the ileum and inflammation are undesirable side effects. We previously proposed a hypothesis concerning the receptors for human blood platelet aggregation and its inhibition, and the contraction of the ileum and uterus based on a study of structure-activity relationships. If the same principle can be applied to contraction of the ileum and inflammation induced by PGE1, compounds that suppress the side effects of PGE1 can be developed. The antihistamine diphenylpyraline and the anticholinergic atropine antagonized PGE1-induced contraction of the ileum in guinea pigs. Papaverine, which is a smooth muscle relaxant, also acted as an antagonist. Gabexate mesylate (FOY), a non-peptide proteinase inhibitor, inhibits guinea pig ileum contraction induced by PGE1, but epsilon-guanidinocaproic acid (GCA), a metabolite of FOY, does not. Increased microvascular permeability of the abdominal skin in rats induced by the local injection of PGE1 and histamine was suppressed by atropine, papaverine and diphenylpyraline. FOY, not GCA, had a weak inhibitory action. We demonstrate the possibility of suppressing the side effects of PGE1 based on the results obtained in the present and previous studies.

Opposite modulation by muscarinic M1 and M3 receptors of acetylcholine release from guinea pig ileum as measured directly

Muscarinic receptor agonist and antagonist effects on acetylcholine release evoked by electrical or dimethylphenylpiperazinium stimulation from guinea pig ileum were evaluated by measuring acetylcholine with a high performance liquid chromatography-electrochemical detector system. AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine), a muscarinic M1 receptor agonist, increased markedly the evoked release of acetylcholine. In contrast, pirenzepine decreased the evoked acetylcholine release. 4-DAMP (4-diphenyl-acetoxy-N-methylpiperidine methiodide) and p-F-HHSiD (p-fluoro-hexahydrosiladifenidol), muscarinic M3 antagonists, increased the release of acetylcholine. Atropine enhanced acetylcholine release to a similar extent while bethanechol reduced the electrically evoked acetylcholine release. This reduction was virtually unaffected by methoctramine, but was antagonized by 4-DAMP or p-F-HHSiD. The results from direct determination of acetylcholine suggest that, in contrast to autoinhibition by stimulation of muscarinic M3 receptors, stimulation of presynaptic muscarinic M1 receptors is predominantly involved in enhancement of the acetylcholine release from guinea pig ileal nerves, and that AF102B functions as a muscarinic M1 agonist in this peripheral neuron.

Muscarinic receptor subtypes in human and rat colon smooth muscle

Muscarinic receptor subtypes in human and rat colon smooth muscle homogenates were characterized with [3H]N-methylscopolamine ([3H]NMS) by ligand binding studies. [3H]NMS saturation experiments show the existence of a homogeneous population of non-interacting binding sites with similar affinity (KD values of 1.38 +/- 0.20 nM in human colon smooth muscle and 1.48 +/- 0.47 nM in rat colon smooth muscle) and with Hill slopes close to unity in both samples of tissue. However, a significant (P less than 0.01) increase in muscarinic receptor density (Bmax) is found in human colon (29.9 +/- 2.9 fmol/mg protein) compared with rat colon (17.2 +/- 1.5 fmol/mg protein). Inhibition of [3H]NMS binding by non-labelled compounds shows the following order in human colon: atropine greater than AF-DX 116 greater than pirenzepine. Whereas in rat colon the rank order obtained is atropine greater than pirenzepine greater than AF-DX 116. Atropine and pirenzepine bind to a homogeneous population of binding sites, although pirenzepine shows higher affinity to bind to the sites present in rat colon (Ki = 1.08 +/- 0.08 microM) than those in human colon (Ki = 1.74 +/- 0.02 microM) (P less than 0.05). Similarly, IC50 values obtained in AF-DX 116 competition experiments were significantly different (P less than 0.01) in human colon (IC50 = 1.69 +/- 0.37 microM) than in rat colon (IC50 = 3.78 +/- 0.75 microM). Unlike atropine and pirenzepine, the inhibition of [3H]NMS binding by AF-DX 116 did not yield a simple mass-action binding curve (nH less than 1, P less than 0.01) suggesting the presence of more than one subtype of muscarinic receptor in both species. Computer analysis of these curves with a two binding site model suggests the presence of two populations of receptor. The apparent Ki1 value for the high affinity binding site is 0.49 +/- 0.07 microM for human colon smooth muscle and 0.33 +/- 0.05 microM for rat colon smooth muscle. The apparent Ki2 for the low affinity binding site is 8.01 +/- 1.0 microM for human samples and 6.07 +/- 1.1 microM for rat samples. These values are close enough to suggest that the first subtype of muscarinic receptor may be considered cardiac (M2) and the second subtype glandular (M3). The relative densities of the receptor subtypes are significantly different for both species. Human colon samples show the major densities of subtype M2, 22.62 +/- 1.11 fmol/mg protein, this represents 75.66 +/- 3.73% of the total receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic effects of methoctramine on release of acetylcholine

Stores of transmitter, labelled with [3H]choline, were used to study the negative feedback modulation of the release of acetylcholine by presynaptic M2 muscarinic receptors. The release of acetylcholine was measured by radioassaying the electrical stimulation-evoked release of [3H]acetylcholine from slices of cerebral cortex of the rat and from the Auerbach plexus of the guinea pig ileum. Experimental conditions (2 Hz, 240 shocks) were chosen where the negative-feedback modulation by endogenous acetylcholine was not significant, therefore the presynaptic affinity constant for antagonists was not underestimated. The M2 agonist oxotremorine inhibited the release of acetylcholine in a concentration-dependent manner in both preparations. The IC50 values for oxotremorine were 10.8 +/- 4.89 x 10(-6) M on the cortex and 5.89 +/- 3.85 x 10(-8) M on the Auerbach plexus (n = 4). The effect of oxotremorine was blocked by atropine, similarly to methoctramine, which is a cardioselective muscarinic receptor antagonist. The dose-ratio and dissociation constant were calculated by measuring the righward shift that methoctramine and other antagonists produced on the inhibitory dose-effect curve for oxotremorine. The antagonist equilibrium dissociation constants (pKB) of methoctramine were 5.69 +/- 0.27 and 5.51 +/- 0.37 on the cortical and the myenteric plexus preparations, respectively (n = 4). Postsynaptic antimuscarinic affinity (pA2) of methoctramine on the smooth muscle of the guinea pig ileum was found to be 6.68 +/- 0.11 (n = 4). These findings indicate that, although methoctramine is a cardioselective compound, unlike pancuronium, it may not be a useful tool for discriminating between different presynaptic muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)