Interaction of agonists and selective antagonists with gastric smooth muscle muscarinic receptors

The control of gut motility

Gut motility in non-mammalian vertebrates as in mammals is controlled by the presence of food, by autonomic nerves and by hormones. Feeding and the presence of food initiates contractions of the stomach wall and subsequently gastric emptying, peristalsis, migrating motor complexes and other patterns of motility follow. This overview will give examples of similarities and differences in control systems between species. Gastric receptive relaxation occurs in fish and is an enteric reflex. Cholecystokinin reduces the rate of gastric emptying in fish as in mammals. Inhibitory control of peristalsis is exerted, e.g. by VIP, PACAP, NO in fish and amphibians, while excitatory stimuli arise from nerves releasing tachykinins, acetylcholine or serotonin (5-HT). In crocodiles, we have found the presence of the same nerve types, although the effects on peristalsis have not been studied. Recent studies on signal transduction in the gut smooth muscle of fish and amphibians suggest that external Ca2+ is of great importance, but not the only source of Ca2+ recruitment in tachykinin-, acetylcholine- or serotonin-induced contractions of rainbow trout and Xenopus gastrointestinal smooth muscle. The effect of acetylcholine involves reduction of cAMP-levels in the smooth muscle cells. It is concluded that, in general, the control systems in non-mammalian vertebrates are amazingly similar between species and animal groups and in comparison with mammals.

Effects of GTP analogs and metal ions on the binding of neurotensin to porcine brain membranes

Using 125I-labeled neurotensin (NT), porcine brain membranes were found to contain two types of high-affinity receptors, one class (approximately 1/3 of total) with an apparent Kd of 0.12 nM and another with an apparent Kd of 1.4 nM. Nonhydrolyzable analogs of GTP inhibited NT binding in a dose-dependent manner. In the presence of 60 microM guanosine 5'-(3-thio) 5'-(beta, gamma-imino) triphosphate. NT binding was decreased by 35% with an associated decrease in the number of binding sites and little change in the Kd. Cross-linking of 125I-labeled NT to brain membranes using disuccinimidyl suberate was found to specifically label two substances of approximately 120 kDa and approximately 160 kDa, which could represent different binding proteins or complexes. For a series of NT analogs, there was close agreement between the IC50 in the binding assay and the ED50 in a bioassay based on ability to contract the guinea pig ileum. In addition, metal ions inhibited NT binding and the contractile action of NT with the same order of potency (Hg++ > Zn++ > Cu++ > Mn++ > Mg++ > Li++). There was a linear relationship between the standard reduction potential for these ions and the logarithm of the IC50 in the binding assay. The results suggest that porcine brain contains high-affinity, G-protein-linked receptors for NT, the functioning of which depends upon group(s), perhaps sulfhydryl(s), which can interact strongly with certain heavy metal ions.

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.