Heterogeneity of vascular muscarinic receptors

Innervation of the coronary arteries and its role in controlling microvascular resistance

The coronary circulation plays a crucial role in balancing myocardial perfusion and oxygen demand to prevent myocardial ischemia. Extravascular compressive forces, coronary perfusion pressure, and microvascular resistance are involved to regulate coronary blood flow throughout the cardiac cycle. Autoregulation of the coronary blood flow through dynamic adjustment of microvascular resistance is maintained by complex interactions among mechanical, endothelial, metabolic, neural, and hormonal mechanisms. This review focuses on the neural mechanism. Anatomy and physiology of the coronary arterial innervation have been extensively investigated using animal models. However, findings in the animal heart have limited applicability to the human heart as cardiac innervation is generally highly variable among species. So far, limited data are available on the human coronary artery innervation, rendering multiple questions unresolved. Recently, the clinical entity of ischemia with non-obstructive coronary arteries has been proposed, characterized by microvascular dysfunction involving abnormal vasoconstriction and impaired vasodilation. Thus, measurement of microvascular resistance has become a standard diagnostic for patients without significant stenosis in the epicardial coronary arteries. Neural mechanism is likely to play a pivotal role, supported by the efficacy of cardiac sympathetic denervation to control symptoms in patients with angina. Therefore, understanding the coronary artery innervation and control of microvascular resistance of the human heart is increasingly important for cardiologists for diagnosis and to select appropriate therapeutic options. Advancement in this field can lead to innovations in diagnostic and therapeutic approaches for coronary artery diseases.

Vasodilation and Blood Pressure-Lowering Effect of 3-Demethyl-2-Geranyl-4-Prenylbellidifoline, a Xanthone Obtained from Garcinia achachairu, in Hypertensive Rats

3-demethyl-2-geranyl-4-prenylbellidifoline (DGP), a natural xanthone isolated from Garcinia achachairu, has previously demonstrated remarkable diuretic and renal protective actions. The present study expands its actions on the cardiovascular system by evaluating its vasorelaxant and blood pressure-lowering effects in spontaneously hypertensive rats (SHRs). Aortic endothelium-intact (E+) preparations of SHRs pre-contracted by phenylephrine and exposed to cumulative concentrations of G. achachairu extract, fractions, and DGP exhibited a significant relaxation compared to vehicle-only exposed rings. The non-selective muscarinic receptor antagonist (atropine), the non-selective inhibitor of nitric oxide synthase (L-NAME), as well as the inhibitor of soluble guanylate cyclase (ODQ) altogether avoided DGP-induced relaxation. Tetraethylammonium (small conductance Ca2+-activated K+ channel blocker), 4-aminopyridine (a voltage-dependent K+ channel blocker), and barium chloride (an influx-rectifying K+ channel blocker) significantly reduced DGP capacity to induce relaxation without the interference of glibenclamide (an ATP-sensitive inward rectifier 6.1 and 6.2 K+ channel blocker). Additionally, administration of DGP, 1 mg/kg i.v., decreased the mean, systolic, and diastolic arterial pressures, and the heart rate of SHRs. The natural xanthone DGP showed promising potential as an endothelium-dependent vasorelaxant, operating through the nitric oxide pathway and potassium channels, ultimately significantly reducing blood pressure in hypertensive rats.

High Soluble Endoglin Levels Affect Aortic Vascular Function during Mice Aging

Endoglin is a 180 kDa transmembrane glycoprotein that was demonstrated to be present in two different endoglin forms, namely membrane endoglin (Eng) and soluble endoglin (sEng). Increased sEng levels in the circulation have been detected in atherosclerosis, arterial hypertension, and type II diabetes mellitus. Moreover, sEng was shown to aggravate endothelial dysfunction when combined with a high-fat diet, suggesting it might be a risk factor for the development of endothelial dysfunction in combination with other risk factors. Therefore, this study hypothesized that high sEng levels exposure for 12 months combined with aging (an essential risk factor of atherosclerosis development) would aggravate vascular function in mouse aorta. Male transgenic mice with high levels of human sEng in plasma (Sol-Eng⁺) and their age-matched male transgenic littermates that do not develop high soluble endoglin (Control) on a chow diet were used. The aging process was initiated to contribute to endothelial dysfunction/atherosclerosis development, and it lasted 12 months. Wire myograph analysis showed impairment contractility in the Sol-Eng⁺ group when compared to the control group after KCl and PGF_2α administration. Endothelium-dependent responsiveness to Ach was not significantly different between these groups. Western blot analysis revealed significantly decreased protein expression of Eng, p-eNOS, and ID1 expression in the Sol-Eng⁺ group compared to the control group suggesting reduced Eng signaling. In conclusion, we demonstrated for the first time that long-term exposure to high levels of sEng during aging results in alteration of vasoconstriction properties of the aorta, reduced eNOS phosphorylation, decreased Eng expression, and altered Eng signaling. These findings suggest that sEng can be considered a risk factor for the development of vascular dysfunction during aging and a potential therapeutical target for pharmacological intervention.

Effects of Anisodine Hydrobromide on the Cardiovascular and Respiratory Functions in Conscious Dogs

Purpose

Anisodine hydrobromide (Ani) is isolated from the medicinal plant Anisodus tanguticus (Maxim.) Pascher for clinical use. Although considerable research regarding Ani has been reported, the safety profiles of Ani are currently unknown. This study investigated the cardiorespiratory effects of Ani in conscious dogs to provide clinicians a detailed safety profile of Ani on the cardiorespiratory system.

Materials and Methods

Using the Latin square design, the study was divided into six phases, where in each phase, six telemetered beagle dogs received one dose of normal saline or sotalol hydrochloride or Ani (0.1, 0.4, 1.6, or 6.4 mg/kg). Electrocardiogram, blood pressure (BP) and respiratory parameters were collected before and after administration for 24 hours. Statistical comparisons were performed at scheduled time-points.

Results

The heart rate was significantly increased, PR and QT_CV intervals were significantly shortened in Ani 0.4, 1.6, 6.4 mg/kg treatment group after drug administration. Compared with the saline group, a significant increase in heart rate and shortening of PR, QT_CV intervals were observed in the Ani 1.6, 6.4 mg/kg treatment groups from 5 min to 4 h time-points. Diastolic and mean BP were significantly increased in Ani 1.6, 6.4 mg/kg from 1 h to 2 h time-points compared to those of the saline control. Accelerated breathing was observed in the first 20 min after Ani 0.4, 1.6, and 6.4 mg/kg treatment, although not statistically significant. Furthermore, no significant differences were observed in any of the corresponding indexes of Ani 0.1 mg/kg treatment group at different time-points compared to those of the saline group.

Conclusion

Ani may have adverse effects on the cardio-respiratory systems of dogs at doses above 0.4 mg/kg, whereas Ani 0.1 mg/kg was devoid of potentially deleterious effects on cardiorespiratory function.

Cardiovascular effects of prenatal stress-Are there implications for cerebrovascular, cognitive and mental health outcome?

Prenatal stress programs offspring cognitive and mental health outcome. We reviewed whether prenatal stress also programs cardiovascular dysfunction which potentially modulates cerebrovascular, cognitive and mental health disorders. We focused on maternal stress and prenatal glucocorticoid (GC) exposure which have different programming effects. While maternal stress induced cortisol is mostly inactivated by the placenta, synthetic GCs freely cross the placenta and have different receptor-binding characteristics. Maternal stress, particularly anxiety, but not GC exposure, has adverse effects on maternal-fetal circulation throughout pregnancy, probably by co-activation of the maternal sympathetic nervous system, and by raising fetal catecholamines. Both effects may impair neurodevelopment. Experimental data also suggest that severe maternal stress and GC exposure during early and mid-gestation may increase the risk for cardiovascular disorders. Human data are scarce and especially lacking for older age. Programming mechanisms include aberrations in cardiac and kidney development, and functional changes in the renin-angiotensin-aldosterone-system, stress axis and peripheral and coronary vasculature. Adequate experimental or human studies examining the consequences for cerebrovascular, cognitive and mental disorders are unavailable.

Distribution of muscarinic acetylcholine receptor subtypes in the murine small intestine

AIMS

Serotonin stimulates enterocyte turnover in the small intestine and studies suggest this is mediated by neuronal signaling via a cholinergic pathway. Distribution of the five known muscarinic receptor subtypes (mAChRs) in the small intestine has not been fully studied, and their role in intestinal growth is unknown. We hypothesized that mAChRs have distinct anatomic distributions within the bowel, and that mAChRs present within intestinal crypts mediate the effects of acetylcholine on the small intestinal mucosa.

MAIN METHODS

Small intestine from male C57BL/6 mice ages 2, 4, 6, and 8weeks were harvested. RNA was isolated and cDNA synthesized for PCR-amplification of subtype specific mAChRs. Ileum was fixed with Nakane, embedded in epon, and immunofluorescence microscopy performed using polyclonal antibodies specific to each mAChR1-5.

KEY FINDINGS

All five mAChR subtypes were present in the mouse duodenum, jejunum, and ileum at all ages by RT-PCR. Immunofluorescence microscopy suggested the presence of mAChR1-5 in association with mature enterocytes along the villus and within the myenteric plexus. Only mAChR2 clearly localized to the crypt stem cell compartment, specifically co-localizing with Paneth cells at crypt bases.

SIGNIFICANCE

Muscarinic receptors are widely distributed along the entire alimentary tract. mAChR2 appears to localize to the crypt stem cell compartment, suggesting it is a plausible regulator of stem cell activity. The location of mAChR2 to the crypt makes it a potential therapeutic target for treatment of intestinal disease such as short bowel syndrome. The exact cellular location and action of each mAChR requires further study.

Overview of the Anatomy, Physiology, and Pharmacology of the Autonomic Nervous System

Comprised of the sympathetic nervous system, parasympathetic nervous system, and enteric nervous system, the autonomic nervous system (ANS) provides the neural control of all parts of the body except for skeletal muscles. The ANS has the major responsibility to ensure that the physiological integrity of cells, tissues, and organs throughout the entire body is maintained (homeostasis) in the face of perturbations exerted by both the external and internal environments. Many commonly prescribed drugs, over-the-counter drugs, toxins, and toxicants function by altering transmission within the ANS. Autonomic dysfunction is a signature of many neurological diseases or disorders. Despite the physiological relevance of the ANS, most neuroscience textbooks offer very limited coverage of this portion of the nervous system. This review article provides both historical and current information about the anatomy, physiology, and pharmacology of the sympathetic and parasympathetic divisions of the ANS. The ultimate aim is for this article to be a valuable resource for those interested in learning the basics of these two components of the ANS and to appreciate its importance in both health and disease. Other resources should be consulted for a thorough understanding of the third division of the ANS, the enteric nervous system. © 2016 American Physiological Society. Compr Physiol 6:1239-1278, 2016.

Role of various subtypes of muscarinic cholinergic receptors in the development of posthemorrhagic abnormalities in systemic and portal circulation in rats

The experiments employing high-frequency ultrasonic technique and selective blockers of M1, M3, and M4 muscarinic cholinergic receptors pirenzepine, 4-DAMP, and tropicamide, respectively, revealed individual roles of these receptors in the development of severe posthemorrhagic hypotension in rats with low or high individual resistance to circulatory hypoxia. The study showed that M1 and M4 muscarinic receptors are involved in shock-limiting and shock-activating processes, respectively, while M3 receptors exert no effect on the course of posthemorrhagic abnormalities in systemic and hepatic portal circulation and on the posthemorrhagic lifespan. Poor resistance of the cardiovascular system to circulatory hypoxia during shock development is considered to be dysregulatory pathology.

Muscarinic receptor agonists and antagonists: effects on cardiovascular function

Muscarinic receptor activation plays an essential role in parasympathetic regulation of cardiovascular function. The primary effect of parasympathetic stimulation is to decrease cardiac output by inhibiting heart rate. However, pharmacologically, muscarinic agonists are actually capable of producing both inhibitory and stimulatory effects on the heart as well as vasculature. This reflects the fact that muscarinic receptors are expressed throughout the cardiovascular system, even though they are not always involved in mediating parasympathetic responses. In the heart, in addition to regulating heart rate by altering the electrical activity of the sinoatrial node, activation of M₂ receptors can affect conduction of electrical impulses through the atrioventricular node. These same receptors can also regulate the electrical and mechanical activity of the atria and ventricles. In the vasculature, activation of M₃ and M₅ receptors in epithelial cells can cause vasorelaxation, while activation of M₁ or M₃ receptors in vascular smooth muscle cells can cause vasoconstriction in the absence of endothelium. This review focuses on our current understanding of the signaling mechanisms involved in mediating these responses.

Effect of the effluent released from the canine internal mammary artery after intraluminal and extraluminal perfusion of acetylcholine and adenosine diphosphate

Segments of the canine internal mammary artery (35 mm in length) were suspended in vitro in an organ chamber containing physiological salt solution (95% O₂/5% CO₂, pH = 7.4, 37°C). Segments were individually cannulated and perfused at 5 ml/minute using a roller pump. Vasorelaxant activity of the effluent from the perfused internal mammary arteries was bioassayed by measuring the decrease in tension induced by the effluent of the coronary artery endothelium-free ring which had been contracted with prostaglandin F_2α (2 × 10^-6 M). Intraluminal perfusion of adenosine diphosphate (10^-5 M) induced significant increase in relaxant activity in the effluent from the perfused blood vessel. However, when adenosine diphosphate (10^-5 M) was added extraluminally to the internal mammary artery, no change in relaxant activity in the effluent was noted. In contrast, acetylcholine produced significant increase in the relaxant activity on the effluent of the perfused internal mammary artery with both intraluminal and extraluminal perfusion. The intraluminal and extraluminal release of endothelium-derived relaxing factor (EDRF) by acetylcholine (10^-5 M) can be inhibited by site-specific administration of atropine (10^-5 M). These experiments indicate that certain agonists can induce the release of EDRF only by binding to intravascular receptors while other agonists can induce endothelium-dependent vasodilatation by acting on neural side receptors.

Contribution of the M3 muscarinic receptors to the vasodilator response to acetylcholine in the human forearm vascular bed

AIMS

Acetylcholine (ACh) is a muscarinic agonist that causes receptor-mediated, endothelium-dependent vasodilatation in the forearm vasculature. Previous indirect evidence suggests this effect may be mediated by muscarinic M(3) receptors. Darifenacin is a recently developed antimuscarinic drug with greater M(3) selectivity, and our main objective was to investigate whether darifenacin affects dose-dependent vasodilatation to ACh in the forearm circulation.

METHODS

Healthy subjects were enrolled in two studies designed to assess the effects of atropine and darifenacin on the forearm blood flow (FBF) response to ACh.

RESULTS

In both studies ACh caused similar dose-dependent vasodilation in the forearm vasculature. In study I (5 subjects), the FBF response to ACh was largely attenuated by pretreatment with the nonselective muscarinic antagonist atropine. In study II (10 subjects), oral administration of darifenacin 15 mg for 1 week significantly reduced the FBF dose-dependent response to ACh 20 microg min(-1) (mean difference from placebo 5.8 [95% confidence interval (CI) 3.1, 8.7] ml min(-1) per 100 ml of forearm volume, P < 0.001) and to ACh 60 microg min(-1)[mean difference from placebo 5.9 (95% CI 3.1, 8.7) ml min(-1) per 100 ml of forearm volume, P < 0.001]. After darifenacin, the AUC of change in FBF from baseline was reduced by almost 50% compared with placebo.

CONCLUSIONS

These results suggest that, in the forearm vasculature, muscarinic M(3) receptors play a major role in ACh-induced endothelium-mediated vasodilatation.

Investigation of the mechanism for the relaxation of rat duodenum mediated via M1 muscarinic receptors

1 Relaxation responses of the rat isolated duodenum to the putative M1 muscarinic receptor agonist, McN-A-343, were examined to determine whether the response was due to the release of known non-adrenergic, non-cholinergic relaxant neurotransmitters and to establish the involvement of M1 muscarinic receptors. 2 The role of ATP was examined with the P2 receptor antagonist, suramin, which at 30 mum antagonized the relaxant responses to alpha,beta-methylene ATP. The same dose, however, failed to inhibit the relaxation by McN-A-343. 3 The role of nitric oxide (NO) was examined with the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 100 microm), which failed to inhibit the responses to McN-A-343. As NO mediates relaxation of the duodenum via cGMP generation through guanylyl cyclase, whether the relaxation by McN-A-343 was also via cGMP was examined with the guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The relaxation responses to the NO donor, S-nitroso-N-acetyl penicillamine, were inhibited in the presence of ODQ (3 microm), but not those by McN-A-343. 4 Release of gamma-aminobutyric acid (GABA) was examined with the GABAA receptor antagonist, bicuculline (10 microm), which shifted the concentration-response curves for the relaxation of the duodenum by GABA to the right. There was a similar degree of shift in the concentration-response curve for McN-A-343 by bicuculline indicating that release of GABA from enteric neurones of the duodenum could explain the relaxation response to McN-A-343. 5 To test whether the muscarinic receptors mediating the relaxation of the duodenum were of the M1 subtype, the susceptibility to the selective competitive antagonist, pirenzepine and the selective muscarinic toxin from green mamba, MT7, was examined. Pirenzepine (1 microm) shifted the concentration-response for McN-A-343 to the right in a parallel fashion with a dose ratio of 33.3 +/- 20.2. This yielded a pA2 value of 7.5, which concords with those for other responses reputed to be mediated via M1 muscarinic receptors. The toxin MT7 was used as an irreversible antagonist and following incubation with the duodenum was washed from the bath. An incubation time of 30 min with 100 nm of MT7 caused a significant parallel shift in the concentration-response to McN-A-343 confirming the involvement of M1 muscarinic receptors. 6 This study has confirmed that McN-A-343 relaxes the rat duodenum via muscarinic receptors of the M1 subtype and that these receptors are probably located on enteric neurones from which their stimulation releases GABA.

Methoctramine and gallamine inhibit PI hydrolysis in guinea-pig gallbladder

The present study aimed to determine the effect of two M2/M4-selective muscarinic receptor antagonists on blocking the hydrolysis of carbachol (CCh) stimulated phospho-inositide (PI) breakdown in order to address the possibility that a muscarinic receptor other than the M(3) receptor is involved in PI hydrolysis in this tissue. Gallbladder tissue slices labeled with myo-[2-3H] inositol were incubated with increasing concentrations of antagonists and agonist. After the reactions were terminated by the addition of chloroform/methanol, labeled inositol phosphates were separated using anion exchange chromatography. Muscarinic M2 antagonists methoctramine and gallamine both inhibited carbachol-induced PI breakdown at high concentrations, with log IC50 values of -5.145 and -6.049, respectively. Gallamine at 10(-5)M concentration failed to displace the dose-response curve for carbachol-induced accumulation of inositol triphosphate (IP3). Our data suggest that M(3) receptors play a major role in stimulation of PI hydrolysis in the guinea-pig gallbladder.

Neural connections between the hypothalamus and the liver

After receiving information from afferent nerves, the hypothalamus sends signals to peripheral organs, including the liver, to keep homeostasis. There are two ways for the hypothalamus to signal to the peripheral organs: by stimulating the autonomic nerves and by releasing hormones from the pituitary gland. In order to reveal the involvement of the autonomic nervous system in liver function, we focus in this study on autonomic nerves and neuroendocrine connections between the hypothalamus and the liver. The hypothalamus consists of three major areas: lateral, medial, and periventricular. Each area has some nuclei. There are two important nuclei and one area in the hypothalamus that send out the neural autonomic information to the peripheral organs: the ventromedial hypothalamic nucleus (VMH) in the medial area, the lateral hypothalamic area (LHA), and the periventricular hypothalamic nucleus (PVN) in the periventricular area. VMH sends sympathetic signals to the liver via the celiac ganglia, the LHA sends parasympathetic signals to the liver via the vagal nerve, and the PVN integrates information from other areas of the hypothalamus and sends both autonomic signals to the liver. As for the afferent nerves, there are two pathways: a vagal afferent and a dorsal afferent nerve pathway. Vagal afferent nerves are thought to play a role as sensors in the peripheral organs and to send signals to the brain, including the hypothalamus, via nodosa ganglia of the vagal nerve. On the other hand, dorsal afferent nerves are primary sensory nerves that send signals to the brain via lower thoracic dorsal root ganglia. In the liver, many nerves contain classical neurotransmitters (noradrenaline and acetylcholine) and neuropeptides (substance P, calcitonin gene-related peptide, neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, glucagon, glucagon-like peptide, neurotensin, serotonin, and galanin). Their distribution in the liver is species-dependent. Some of these nerves are thought to be involved in the regulation of hepatic function as well as of hemodynamics. In addition to direct neural connections, the hypothalamus can affect metabolic functions by neuroendocrine connections: the hypothalamus-pancreas axis, the hypothalamus-adrenal axis, and the hypothalamus-pituitary axis. In the hypothalamus-pancreas axis, autonomic nerves release glucagon and insulin, which directly enter the liver and affect liver metabolism. In the hypothalamus-adrenal axis, autonomic nerves release catecholamines such as adrenaline and noradrenaline from the adrenal medulla, which also affects liver metabolism. In the hypothalamus-pituitary axis, release of glucocorticoids and thyroid hormones is stimulated by pituitary hormones. Both groups of hormones modulate hepatic metabolism. Taken together, the hypothalamus controls liver functions by neural and neuroendocrine connections.

The cholinomimetic agent bethanechol activates IK(ACh) in feline atrial myocytes

The effect of the cholinomimetic agent, bethanechol on macroscopic membrane currents was studied in dispersed cat atrial myocytes, using the whole-cell patch-clamp technique. Bethanechol activated an inward rectifying potassium current similar to I(K(ACh)), and a delayed rectifying-like outward current, similar to I(KM3) activated by pilocarpine, choline, and tetramethylammonium, and I(KM4) activated by 4-aminopyridine. The relatively specific muscarinic receptors subtype antagonists methoctramine (M(2)), and tropicamide (M(4)) inhibited both current components induced by bethanechol, suggesting a lack of specificity of these antagonists on cat atrial myocytes. The specific antagonist of M(3) receptors, para-fluoro-hexahydro-siladifenidol did not significantly inhibit the bethanechol-induced currents. In addition, pretreatment with PTX prevented activation of the bethanechol-induced inward and outward currents, suggesting that M(3) receptors are probably not involved in the bethanechol action. The I(K(ACh)) specific blocker tertiapin inhibited both inward rectifying- and delayed rectifying-like currents. These results suggest that both current components result from activation of a single channel type, likely I(K(ACh)).

Acetylcholine affects rat liver metabolism via type 3 muscarinic receptors in hepatocytes

Although the role of acetylcholine (Ach) in hepatic glucose metabolism is well elucidated, it is still unclear if it influences gluconeogenesis, glycogenolysis and high-energy phosphate metabolism, and if it does what the mechanisms of this influence are. Therefore, using isolated perfused rat liver as a model, we have studied the effect of Ach on oxygen consumption, synthesis of glucose from lactate and pyruvate, glycogen formation, mitochondrial oxidative phosphorylation and ATP-synthesis. We have established that effects of Ach on oxygen consumption depend on its concentration. When used at a concentration of 10(-7) M, Ach exerts maximum stimulatory effect, while its infusion at 10(-6) M causes a decrease of oxygen consumption by the liver. Moreover, when used at a concentration of 10(-6) M or 10(-7) M, Ach increases rates of glucose production from the gluconeogenic substrates lactate and pyruvate, leading to enhanced glycogen content in perfused liver. It was also shown that Ach possesses a stimulating effect on alanine and aspartate aminotransferases. As detected by 31P NMR spectroscopy, continuous liver perfusion with pyruvate and lactate in the presence of Ach leads to a significant decrease of ATP level, implying enhanced energy requirements for gluconeogenesis under these conditions. Elimination of the described effects of Ach by atropine, the antagonist of muscarinic receptors, and identification of the type 3 muscarinic receptors (m3) in isolated hepatocytes as well as in whole liver, imply that Ach may exert its effect on liver metabolism through m3 receptors.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Inhibition of field stimulation-induced contractions of rabbit vas deferens by muscarinic receptor agonists: selectivity of McN-A-343 for M1 receptors

Inhibition of the field stimulation-induced twitch responses of the rabbit vas deferens by the muscarinic receptor agonist, McN-A-343, has been attributed to presynaptic muscarinic receptors of the M1 subtype located on noradrenergic nerve terminals. Stimulation of these receptors causes inhibition of transmitter release and inhibition of the contractile response. However, the selectivity of McN-A-343 for M1 receptors has been questioned and this throws doubt on whether the prejunctional receptors of the rabbit vas deferens are of the M1 subtype. In this study we have undertaken a comprehensive re-evaluation of the inhibition of prostatic and epididymal portions of the rabbit isolated field-stimulated vas deferens by several agonists, including McN-A-343, and quantified the antagonism by M1-selective antagonists, pirenzepine and telenzepine. Prostatic and epididymal portions of vasa deferentia from New Zealand White rabbits were immersed in a low Ca2+ Krebs solution at 32+/-0.5 degrees C gassed with 5% CO2 in oxygen. Yohimbine (1.0mM) was present throughout to block prejunctional alpha2-adrenoceptors. Field stimulation was applied by repeated application of single pulses (30 V, 0.05 Hz, 0.5 ms) and isometric contractions recorded. Carbachol and oxotremorine initially potentiated the epididymal contractions but at higher concentrations there was inhibition. In the prostatic portion, oxotremorine only inhibited. McN-A-343 produced inhibitory responses only in both epididymal and prostatic portions. Pirenzepine shifted the concentration-response curves forthe inhibitory responses to oxotremorine to the right. However, the potentiation of the twitches also became more apparent with the lower concentrations of oxotremorine. Schild plots for the antagonism by pirenzepine yielded pA2 values of 7.96+/-0.004 and 7.7+/-0.02 for the epididymal and prostatic portions, respectively. The concentration-response curves for the inhibition of twitches by McN-A-343 were displaced to the right in a parallel manner by pirenzepine in both prostatic and epididymal portions with no potentiation of the twitches. The Schild plot for this antagonism generated pA2 values of 7.68+/-0.01 and 8.07+/-0.01, respectively. Telenzepine caused parallel shifts of the McN-A-343 concentration-response curves to the right in prostatic portions, the pA2 value being 8.70+/-0.13. Telenzepine (10(-7) M) abolished the inhibitory effect of carbachol to reveal only concentration-dependent potentiation of the contractions. The Schild plot for antagonism of this contractile effect yielded a pA2 value (7.07+/-0.09) that was significantly less by almost two orders of magnitude (1.70) than the value for the antagonism by telenzepine of the McN-A-343-induced inhibitory response. The pA2 values of pirenzepine and telenzepine against the inhibitory responses of the rabbit vas deferens are consistent with the involvement of M1 receptors. This leads to the conclusion that McN-A-343 causes inhibition through this receptor type. The doubts concerning the selectivity of McN-A-343 for M1 receptors are therefore unfounded. The fact that McN-A-343 does not display a selective binding profile suggests that its selectivity does not arise from affinity differences but probably resides in its intrinsic efficacy.

Hemodynamic effects of MF 10058, a new cardioselective muscarinic M(2) receptor antagonist, in conscious dogs

The 5-¿4-[4-(diethylamino)butyl]-1-piperidinyl¿acetyl-5H-dibenz[b, f]-azepine (MF 10058) is a new potent and selective muscarinic M(2) receptor antagonist. The hemodynamic effects of MF 10058 were investigated in conscious freely moving dogs. Placebo and three doses of MF 10058 (2, 4 and 8 mg/kg) were orally administered according to a randomised four-way crossover design. Heart rate, cardiac conduction times, systolic and diastolic blood pressure were telemetrically recorded for 12-24 h after dosing. After placebo administration, a consistent reduction over time in heart rate was observed during the night-time period (-15%, P=0.019). MF 10058 administration antagonised the nocturnal bradycardia and shortened QT interval. The effect of the drug reached statistically significance, compared to placebo, with the highest dose of 8 mg/kg (+19% on heart rate, P=0.013; -4% on QT interval, P=0.049). The effect on heart rate lasted for the entire 24-h observation period (+16%, P=0.030). Nocturnal systolic and diastolic blood pressure were not significantly affected by MF 10058. No other signs of peripheral or central cholinergic block were observed at any dose. The results of this study demonstrated that oral administration of MF 10058 produces long-lasting hemodynamic effects in the conscious dog. The drug has a therapeutic potential for the treatment of bradycardic disorders.

Acetylcholinesterase-independent action of diisopropyl-flurophosphate in the rat aorta

Recent studies have shown that many organophosphates can bind competitively and noncompetitively to membrane muscarinic receptors. The present study investigated the responses of the rat aortic rings to diisopropyl-flurophosphate (DFP), an organophsophorus cholinesterase inhibitor, and the possible involvement of muscarinic receptors. DFP caused a concentration-dependent contraction when added cumulatively from 10(-8) to 10(-4) M. This contraction was inhibited in a noncompetitive manner by high concentrations of atropine (1.5 x 10(-6) and 1.8 x 10(-6) M) but was unaffected by similar concentrations of selective muscarinic receptor subtype antagonists, pirenzepine, 11-2[2-[(diethylamino)methyl]-1-piperidinyl]acetyl-5, 11-dihydro-6H-rido[2,3-b][1,4]benzodiazepin-6-one (AF-DX116) and 4-Diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP). Phentolamine, an alpha-adrenoceptor antagonist, was able to inhibit the DFP-induced contraction in a noncompetitive manner at a concentration of 10(-7) M. These findings suggested that the DFP-induced contraction in the rat aortic rings was mediated by norepinephrine that was released from sympathetic nerve terminals present in the aortic rings.

Further evidence for the heterogeneity of functional muscarinic receptors in guinea pig gallbladder

Previous studies have suggested the presence of multiple muscarinic receptor subtypes in guinea pig gallbladder smooth muscle, although the relative abundance and functional role of these subtypes remains an area of significant research efforts. The present study utilized both radioligand kinetic and functional experiments to further probe the nature of the muscarinic receptors in gallbladder smooth muscle and their mode of coupling to intra- and extra-cellular Ca(2+) sources. Dissociation kinetic studies using [3H]N-methylscopolamine ([3H]NMS) indicated that the binding profile in guinea pig gallbladder smooth muscle could not be reconciled with that expected for a single muscarinic receptor subtype, the latter determined in parallel experiments conducted on the cloned muscarinic M(1)-M(5) subtypes in Chinese hamster ovary (CHO) cells. Furthermore, comparison of the gallbladder data with the dissociation characteristics of [3H]NMS in guinea pig urinary bladder revealed a significantly different kinetic profile, with the urinary bladder, but not the gallbladder, demonstrating biphasic radioligand dissociation kinetics. In functional experiments, carbachol caused a concentration-dependent contraction of guinea pig gallbladder smooth muscle strips in Ca(2+)-free or 5 mM Sr(2+)-substituted physiological salt solutions (PSS) with amplitudes of the maximal contractions corresponding to 45.8+/-8.0% and 33.2+/-6.6% of control responses in normal PSS, respectively. Furthermore, the stimulus-response characteristics of carbachol-mediated contraction appeared significantly altered in Ca(2+)-free PSS relative to normal or Sr(2+)-substituted PSS. The antagonist, methoctramine (1x10(-7)-3x10(-5) M), exerted only a slight inhibition of carbachol (10(-5) M)-induced contractions in 5 mM Sr(2+)-substituted medium, whereas it was significantly more potent in antagonizing gallbladder contractions in response to 10(-5) M carbachol in the absence of extracellular Ca(2+). Both atropine and tripitramine were equipotent in antagonizing carbachol-induced contractions in Ca(2+)-free (pIC(50): 6.85+/-0.11 for atropine and 5.75+/-0.32 for tripitramine) and Sr(2+)-substituted media (pIC(50): 6.88+/-0.25 for atropine and 5.70+/-0.16 for tripitramine), and pirenzepine was only slightly more potent in Ca(2+)-free PSS (pIC(50): 5.66+/-0.23) than in Sr(2+)-substituted PSS (pIC(50): 5.33+/-0.21). Taken together, our data indicate that carbachol contracts guinea pig gallbladder by stimulating two distinct muscarinic receptor subtypes linked to extracellular Ca(2+) influx and intracellular Ca(2+) release. These two subtypes may represent the muscarinic M(3) and M(4) receptors, although the presence of the muscarinic M(2) receptor subtype is also suggested from the binding data.

Muscarinic M(2) receptors are not primarily involved in the contraction of guinea-pig gallbladder smooth muscle

The presence of M(1)-M(4) receptors in guinea-pig gallbladder smooth muscle cells has been reported recently. The majority of these receptors are said to be of M(2) subtype. However, there are controversial reports about the functional muscarinic receptors that mediate contraction in this tissue. Similar to gallbladder, it was claimed that M(4) receptors mediate guinea-pig uterine contractions, but these receptors have appeared to be of M(2) subtypes later. Therefore, the antagonistic affinities of three M(2)-selective muscarinic antagonists were determined in contraction and radioligand binding experiments in guinea-pig gallbladder in the present study. The antagonistic affinity values (p K(i)) of gallamine, tripitramine and imperialine were as follows, respectively: 6.28+/-0.15, 8.65+/-0.10 and 6.55+/-0.07 against 0.250 n m [(3)H]QNB binding. All three antagonists displaced the concentration- response curves to carbachol to the right in parallel without affecting the maximum responses. The p A(2) values obtained from constrained Schild plots (-log K(B)) were 4.14+/-0.18 for gallamine, 6.79+/-0.09 for tripitramine, and 7.02+/-0.09 for imperialine. The antagonistic affinity values of gallamine, tripitramine and imperialine for M(2) receptors are reported to be 6. 3, 9.6, 7.7, respectively. The p A(2) values obtained in this study clearly indicate that the primary muscarinic receptors involved in carbachol-induced guinea-pig gallbladder contraction are not of M(2) subtype. The poor correlation between the antagonistic affinity values of these antagonists obtained at radioligand binding (p K(i)) and contraction (p A(2)) experiments also support the conclusion that the majority of muscarinic receptors which have been reported to be of M(2) do not mediate the contractile responses.

Exercise training increases basal nitric oxide production from the forearm in hypercholesterolemic patients

The objective of this study was to investigate the effects of cycle training on basal nitric oxide (NO) production and endothelium-dependent dilator capacity in hypercholesterolemic patients in whom acetylcholine responsiveness is impaired. Nine sedentary hypercholesterolemic volunteers (total plasma cholesterol >6.0 mmol/L; 2 female) aged 44+/-3 years (mean+/-SEM) participated in the study. Subjects remained sedentary for 4 weeks and performed 4 weeks of home-based cycle training (3 x 30 minutes/week at 65% maximum oxygen consumption [VO(2)max]) in a randomized order. Arteriovenous nitrate/nitrite (NO(x)) gradient was assessed and plethysmography was used to measure the forearm blood flow responses to arterial infusions of acetylcholine, sodium nitroprusside, and N(G)mono methyl L-arginine. Training increased VO(2)max from 30.4+/-1.9 to 34.3+/-1.4 mL x kg(-1) x min(-1) (P=0.01). Intrabrachial diastolic blood pressure was reduced from 70+/-3 to 68+/-3 mm Hg (P=0.02) with training, whereas systolic pressure did not change. Plasma triglycerides and total, LDL, and HDL cholesterol were not different between interventions. In the sedentary state, there was a positive forearm arteriovenous difference in plasma NO(x) indicating net extraction (6.8+/-4.0 nmol x 100 mL(-1) x min(-1)), whereas in the trained state this difference was negative, indicating net production (-5.8+/-5.8 nmol x 100 mL(-1) x min(-1); P=0.03). N(G)mono methyl L-arginine, at a dose of 4 micromol/min, caused a greater vasoconstriction after training (79.6+/-3.4% versus 69.9+/-6.8%; P=0.05). Acetylcholine and sodium nitroprusside induced dose-dependent elevations in forearm blood flow that were unaffected by training. These data suggest that basal release of endothelium-derived NO is increased with 4 weeks of home based training in hypercholesterolemic patients, independently of lipid profile modification. This may contribute to the cardiovascular protective effects of exercise training, including reduced blood pressure.

Comparison of anti-M2-muscarinic effect of AF-DX 116 on atrioventricular nodal conduction with those of pirenzepine and atropine as antibradyarrhythmic drugs

Selectivity of antimuscarinic actions of AF-DX 116 (AF-DX) on the atrioventricular (AV) nodal conduction was compared with those of pirenzepine and atropine by using the canine isolated, blood-perfused AV node preparation and the open-chest in situ dog heart. In the isolated AV node preparation, dose-response curves for negative dromotropic effects (prolongation of Atrio-His interval) of carbachol (CCh) injected into the posterior septal artery were shifted to the right in parallel by AF-DX, pirenzepine, and atropine with apparent pA2-values of 13, 27.5, and 0.45 microg, respectively, and slopes of the modified Schild plot of nearly unity. Meanwhile, dose-response curves for coronary vasodilator effects of CCh were shifted to the right by AF-DX, pirenzepine, and atropine with the apparent pA2 values of 68, 12.5, and 0.55 microg, respectively, but the slopes were far from unity. In the in situ open-chest heart, dose-response curves for negative dromotropic effects (prolongation of AV conduction time) of CCh given intravenously were shifted to the right in parallel by AF-DX, pirenzepine, and atropine with apparent pA2 values of 36, 32, and 1.25 microg/kg, respectively, and the slope of nearly unity, whereas dose-response curves for hypotensive effects of CCh were shifted to the right by AF-DX, pirenzepine, and atropine with apparent pA2 values of 105, 15, and 0.65 microg/kg, respectively, but the slopes of AF-DX and pirenzepine were far from unity. In addition, prolongations of AV conduction time by electrical stimulation of the left vagus nerve in the in situ heart were suppressed by AF-DX, pirenzepine, and atropine with the ID50, dose for 50% suppression, of 40, 35, and 1.9 microg/kg, respectively. These results suggest that (a) the potency of antimuscarinic actions of AF-DX on the CCh-induced negative dromotropic effects was almost equal to that of pirenzepine, and approximately 30 times less potent than atropine; (b) the M2-subtype selectivity of AF-DX was considerably higher in comparison with pirenzepine and atropine; and (c) the muscarinic receptor subtype on the canine AV node is entirely of the M2-type, but only sparsely developed in the coronary vascular beds.

Ocular effects of antimuscarinic compounds: is clinical effect determined by binding affinity for muscarinic receptors or melanin pigment?

Although antimuscarinic drugs are being used with increasing frequency in clinical practice for the purposes of mydriasis and cycloplegia, the extent of their actions varies considerably between different compounds. Investigation of the binding characteristics of these agents revealed that as their reported clinical potency increased, so did their specific binding affinity for muscarinic receptors in the iris sphincter and ciliary muscle and their nonspecific binding affinity for melanin pigment. However, the affinity of each drug for melanin pigment was much lower than for the muscarinic receptors. Therefore, although binding to melanin can significantly influence the overall response, differences in the clinical effect of various compounds appear to be primarily due to their differences in specific affinity for muscarinic binding sites.

Direct pharmacological comparison of the muscarinic receptors mediating relaxation and contraction in the rabbit thoracic aorta

The purpose of the present studies was to directly compare the pharmacology of the muscarinic cholinergic receptors coupled to carbachol-induced relaxation and contraction of the intact and the endothelium-denuded rabbit thoracic aorta, respectively. The order of potencies of agonists for producing relaxation in the intact aorta was similar to that for producing contraction in the denuded aorta. In both preparations, the partial agonists pilocarpine, McN-A-343, and RS86 functioned as antagonists, indicating a lack of receptor reserve in both preparations. Further, the pA2 values for antagonists in both tissues were virtually identical and were consistent with the pharmacology of M3 receptors.

Pilocarpine modulates the cellular electrical properties of mammalian hearts by activating a cardiac M3 receptor and a K+ current

1. Pilocarpine, a muscarinic acetylcholine receptor (mAChR) agonist, is widely used for treatment of xerostomia and glaucoma. It can also cause many other cellular responses by activating different subtypes of mAChRs in different tissues. However, the potential role of pilocarpine in modulating cardiac function remained unstudied. 2. We found that pilocarpine produced concentration-dependent (0.1-10 microM) decrease in sinus rhythm and action potential duration, and hyperpolarization of membrane potential in guinea-pig hearts. The effects were nearly completely reversed by 1 microM atropine or 2 nM 4DAMP methiodide (an M3-selective antagonist). 3. Patch-clamp recordings in dispersed myocytes from guinea-pig and canine atria revealed that pilocarpine induces a novel K+ current with delayed rectifying properties. The current was suppressed by low concentrations of M3-selective antagonists 4DAMP methiodide (2-10 nM), 4DAMP mustard (4-20 nM, an ackylating agent) and p-F-HHSiD (20-200 nM). Antagonists towards other subtypes (M1, M2 or M4) all failed to alter the current. 4. The affinity of pilocarpine (KD) at mAChRs derived from displacement binding of [3H]-NMS in the homogenates from dog atria was 2.2 microM (65% of the total binding) and that of 4DAMP methiodide was 2.8 nM (70% of total binding), consistent with the concentration of pilocarpine needed for the current induction and for the modulation of the cardiac electrical activity and the concentration of 4DAMP to block pilocarpine effects. 5. Our data indicate, for the first time, that pilocarpine modulates the cellular electrical properties of the hearts, likely by activating a K+ current mediated by M3 receptors.

Subtypes of muscarinic receptors in rat duodenum: a comparison with rabbit vas deferens, rat atria, guinea-pig ileum and gallbladder by using imperialine

The specific binding of [3H]QNB to rat duodenum smooth muscle membranes was a saturable process and Scatchard transformation of the saturation curves indicated a linear plot (nH = 1.017+/-0.071). The K(D) and Bmax values were 0.168+/-0.025 nM and 46.7+/-8.6 fmol/mg protein, respectively. Analyses of competition curves using pirenzepine and guanylpirenzepine indicated more than one class of binding site. A minor population of muscarinic binding sites showed high affinity (M1) for both pirenzepine (19.3+/-1.2%; pKi = 8.29+/-0.36) and guanylpirenzepine (29.4+/-2.0%; pKi = 7.28+/-0.11). The antagonistic affinity values of pirenzepine and guanylpirenzepine for the remaining low affinity binding sites, and that of methoctramine indicated the presence of both M2 and M3 subtypes. McN-A-343 produced relaxations in rat duodenum and inhibited twitch contractions of rabbit vas deferens induced by electrical stimulation in a concentration dependent manner. Carbachol (Cch) exerted concentration-dependent negative inotropic effect in rat atria and contractile effects in guinea-pig gallbladder and ileum longitudinal muscle-myenteric plexus preparation. Imperaline displaced the concentration-response curves to McN-A-343 and Cch to the right in parallel, without affecting the maximum responses in all tissues studied. The rank order of the pA2 values was rabbit vas deferens > rat atria > guinea-pig gallbladder = guinea-pig ileum > rat duodenum. The presynaptic muscarinic receptors at the rat duodenum and rabbit vas deferens were concluded to be of M1 and M4 subtypes, respectively.

Mechanisms underlying ACh induced modulation of neurogenic and applied ATP constrictions in the submucosal arterioles of the guinea-pig small intestine

1. Role of the vascular endothelium in acetylcholine (ACh) induced modulation of neurogenic and applied ATP (adenosine 5'-triphosphate) constrictions of intestinal submucosal arterioles was investigated. 2. Arteriole constrictions, induced either by exogenous ATP or evoked by perivascular nerve stimulation, were attenuated in the presence of ACh. 100 nM ACh almost completely abolished neurogenic constrictions whereas up to 10 microM ACh reduced constrictions to exogenous ATP by only about 60%. 3. Treatment of the arterioles with 100 microM Nomega-nitro-L-arginine (NOLA) and 5 microM indomethacin, to block respectively nitric oxide (NO) and prostanoid release from the endothelium, had no effect on the ACh induced inhibition of neurogenic constrictions but significantly attenuated the inhibitory effects of ACh on constrictions to exogenous ATP. 4. Disruption of the vascular endothelium had no effect on the ACh induced inhibition of neurogenic constrictions but attenuated the inhibitory effects of ACh on applied ATP constrictions to the same extent as after treatment with NOLA and indomethacin. In comparison, endothelial disruption completely abolished the inhibitory effect of substance P (SP) on exogenously applied ATP constrictions. 5. 50 nM ACh significantly attenuated the amplitude of neurally evoked excitatory junction potentials (ejps) recorded from the vascular smooth muscle without altering the time constant of decay (taudecay) of the ejps. 6. It is concluded that ACh inhibits neurogenic constrictions by prejunctional modulation of transmitter release from the perivascular sympathetic nerves with no major role for endothelial paracrine factors. 7. Endothelial NO and/or prostanoids mediate some of the ACh induced inhibition of constrictions to exogenous ATP whereas the endothelium independent inhibitory effects of ACh are attributed to a direct action of ACh on the vascular smooth muscle. However, an indirect effect resulting from activation of vasodilator nerves cannot be ruled out.

Muscarinic receptor subtypes in the porcine lung during postnatal development

The responsiveness of the pulmonary circulation to acetylcholine changes in the newborn piglet. Therefore muscarinic receptors have been studied in the developing porcine lung from birth to adulthood using ligand binding, Northern blotting and in situ hybridisation. Maximal binding capacity of [N-methyl-3H] scopolamine and the affinity of the receptor in lung membranes increased between birth and 16 days (p < 0.05). Subtype affinity changed with age, but always M3, > M1 > M2. Northern blots of porcine muscarinic receptor subtypes showed m1, m2 and m3 mRNA present in lung membranes. m2 mRNA was present at all ages and decreased with age. m1 mRNA was absent at birth and m3 mRNA was absent at 3 days. Autoradiographic localisation showed ligand binding to the parenchyma and airway smooth muscle and nerves, there was no binding to intrapulmonary vessels. In situ hybridisation localised mRNA of all three subtypes to the smooth muscle cells of both vessels and airways. Changes in the receptor subtypes may explain the pharmacological changes during postnatal adaptation.

Impaired endothelium-dependent relaxation in large, but not small arteries in rats after coronary ligation

Vascular responses were studied in both large and small arteries of rats following 8 weeks of heart failure produced by coronary ligation. Responses to noradrenaline, acetylcholine and sodium nitroprusside were studied in isolated thoracic aorta and mesenteric arteries. In the aorta, concentration-response curves for noradrenaline were similar between heart failure and sham animals and unaffected by the nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NOARG). Relaxation by acetylcholine was impaired in heart failure rats (EC50-6.79 log M heart failure vs. -7.15 log M sham). In the presence of L-NOARG, relaxation by acetylcholine was completely abolished in rings from sham rats, whereas constriction was observed in rings from heart failure rats. Relaxation by sodium nitroprusside was not different between sham and heart failure rats. In mesenteric arteries, responses to noradrenaline, acetylcholine and sodium nitroprusside were not different between heart failure and sham rats. L-NOARG reduced the maximum response to acetylcholine in both heart failure (82% to 50%) and shams (89% to 49%) by a similar magnitude, with no effect on relaxation to sodium nitroprusside. These data suggest that acetylcholine-induced relaxation is impaired in the aorta, but not mesenteric arteries in rats with heart failure. The mechanism is not solely due to impaired nitric oxide release and may be due to acetylcholine-induced contraction.

The interaction between methylene blue and the cholinergic system

1. The inhibitory effects of methylene blue (MB) on different types of cholinesterases and [3H]-N-methylscopolamine ([3H]-NMS) binding to muscarinic receptors were studied. 2. Human plasma from young healthy male volunteers, purified human pseudocholinesterase and purified bovine true acetylcholinesterase were incubated with acetylcholine and increasing concentrations of MB (0.1-100 mumol l-1) in the presence of the pH-indicator m-nitrophenol for 30 min at 25 degrees C. The amount of acetic acid produced by the enzymatic hydrolysis of acetylcholine was determined photometrically. 3. Rat cardiac left ventricle homogenate was incubated with [3H]-NMS and with increasing concentrations of MB (0.1 mmol l-1 mumol l-1) at 37 degrees C for 20 min. THe binding of [3H]-NMS to the homogenate was quantified by a standard liquid scintillation technique. 4. MB inhibited the esterase activity of human plasma, human pseudocholinesterase and bovine acetylcholinesterase concentration-dependently with IC50 values of 1.05 +/- 0.05 mumol l-1, 5.32 +/- 0.36 mumol l-1 and 0.42 +/- 0.09 mumol l-1, respectively. MB induced complete inhibition of the esterase activity of human plasma and human pseudocholinesterase, whereas bovine acetylcholinesterase was maximally inhibited by 73 +/- 3.3%. 5. MB was able to inhibit specific [3H]-NMS binding to rat cardiac left ventricle homogenate completely with an IC50 value of 0.77 +/- 0.03 mumol l-1, which resulted in a Ki value for MB of 0.58 +/- 0.02 mumol l-1. 6. In conclusion, MB may be considered as a cholinesterase inhibitor with additional, relevant affinity for muscarinic binding sites at concentrations at which MB is used for investigations into the endothelial system. In our opinion these interactions between MB and the cholinergic system invalidate the use of MB as a tool for the investigation of the L-arginine-NO-pathway, in particular when muscarinic receptor stimulation is involved.

Functional characterization of peripheral muscarinic subtypes in anesthetized cats

This investigation was undertaken to characterize the muscarinic receptor subtypes involved in methacholine-induced vasodilation, vagal bradycardia, neurally-evoked sudomotor responses and sympathetic muscarinic ganglionic transmission in anesthetized cats. Dose-response curves were constructed using the putatively selective antagonists pirenzepine (M1), gallamine (M2) and 4-DAMP (M3: 4-diphenyl-acetoxy-N-methylpiperidine) and compared with the non-selective blocker, atropine. Methacholine hypotension and evoked sudomotor responses exhibited an M3 muscarinic receptor profile with the following potency relationships: atropine > or = 4-DAMP > pirenzepine >> gallamine. Vagal bradycardia (M2) was antagonized by gallamine and exhibited a lower relative sensitivity to 4-DAMP when corrected for atropine effect. Pirenzepine was inactive in inhibition of bradycardia but was highly potent against transmission in the sympathetic ganglion (M1) with the following potency relationships: atropine > or = pirenzepine > 4-DAMP >> gallamine. In comparison with atropine, 4-DAMP exhibited a significantly lower potency for M1 and M2 muscarinic receptors as compared to its effect on the M3 muscarinic receptor subtypes.

Comparison of cholinergic vasodilator responses to acetylcholine and methacholine in the human forearm

In order to study the contribution of the nitric oxide (NO)-pathway to cholinergic vasodilatation in the resistance vessels of the human forearm, we infused acetylcholine (ACh; 0.1 1000 ng/kg/min) or methacholine (MCh; 0.1 A 100 ng/kg/min) in the presence of saline, the NO-scavenger and guanylate cyclase inhibitor methylene blue (MB; 1000 ng/kg/min), or the NO-synthase inhibitor NG-monomethyl-L-arginine (L-NMMA; 30 micrograms/kg/min) into the brachial artery of normotensive volunteers (n = 32), using venous occlusion plethysmography. We calculated the plasma concentrations of the infused compounds to obtain EC50-values (-log mol/l). ACh and MCh both caused concentration-dependent vasodilatation (EC50-values of 6.43 +/- 0.05 and 7.24 +/- 0.08, respectively). MB (13 mumol/l) did not change basal forearm blood flow (FBF) when administered alone, but it markedly potentiated the vasodilator response to ACh, shifting the concentration-response curve (CRC) leftwards by 1.5 log-step (p < 0.001). MB did not affect MCh-induced vasodilatation. L-NMMA (1 mmol/l) alone caused dose-dependent vasoconstriction that was subject to tachyphylaxis. In addition, L-NMMA caused a steepening of the slopes of the CRCs of ACh, and MCh L-NMMA attenuated the ACh-/MCh-induced vasodilator responses in the lowest concentration ranges (p < 0.05) only, but did not alter the response at higher concentrations. The 10-fold higher potency of MCh compared to ACh can be explained by the more rapid degradation of ACh by cholinesterases. The observation that high concentrations of L-NMMA only affect vasodilation mediated by low concentrations of ACh or MCh, suggests a second mechanism in cholinergic vasodilatation, such as a direct effect on smooth muscle cells or the release of a relaxing factor other than NO.

Pharmacological basis for functional selectivity of partial muscarinic receptor agonists

Muscarinic receptor agonists activate phosphoinositide hydrolysis and adenylate cyclase in Chinese hamster ovary cells transfected with cDNAs encoding the human muscarinic ml and m3 receptors. Whereas carbachol activates similarly both receptor subtypes, 4-[3-chlorophenyl-carbamoyloxy]-2-butynyltrimethyl ammonium chloride (McN-A-343) preferentially activates the m1 subtype over m3, in regard to both phosphoinositide hydrolysis and adenylate cyclase activity. On the other hand, oxotremorine activates phosphoinositide hydrolysis to a similar extent in both cell lines, but it activates preferentially adenylate cyclase in m1 versus m3 receptor expressing cells. Relative to carbachol, both McN-A-343 and oxotremorine activate preferentially phosphoinositide hydrolysis over adenylate cyclase in both cell lines. Prolonged incubation of cells with either carbachol, McN-A-343, or oxotremorine down-regulated the m1 receptors. This was accompanied by a parallel decrease in adenylate cyclase activity, whereas phosphoinositide hydrolysis remained relatively high. Inactivation of the receptors by alkylation with acetylethylcholine mustard, or by blocking with atropine, reduced carbachol-stimulated adenylate cyclase activity more effectively than carbachol-induced phosphoinositide hydrolysis in both m1 and m3 receptor expressing cells. These findings imply that the receptor reserve in these cell lines is greater for phosphoinositide hydrolysis response than for adenylate cyclase response. Yet, the receptor reserve for each of these responses is similar in both m1 and m3 receptor expressing cells. Since the binding affinities of McN-A-343 and of oxotremorine to m1 and m3 receptors are very similar, and since both cell lines contain similar amounts of spare receptors, we propose that the preferential activation of muscarinic m1 over m3 receptor by partial agonists is related to differences in the abilities of the two receptor subtypes to undergo conformational changes following agonist binding. This hypothesis is supported by results showing that the muscarinic m1 but not m3 receptor exhibits two affinity states in a competition binding assay.

Selective muscarinic receptor agonists and antagonists

Muscarinic receptors are composed of a family of four subtypes each of which can be distinguished pharmacologically and structurally. The physiological role of each subtype in the central and peripheral nervous systems remains to be clarified, due, in part, to a lack of agonists and antagonists with adequate subtype selectivity. Nonetheless, several agonists with functional selectivity for M1 receptors are now in advanced clinical evaluation for Alzheimer's disease, while selective M1/M3 antagonists may prove useful in the treatment of disorders of smooth muscle function. These novel compounds thus provide an advance over earlier therapeutics with which the clinical efficacy was compromised by the side effect profile. This review attempts to assess novel, selective agonists and antagonists, both in terms of their use in defining muscarinic receptor subtypes and their potential clinical utility.

Actions of vasodilator nerves on arteriolar smooth muscle and neurotransmitter release from sympathetic nerves in the guinea-pig small intestine

1. Brief constrictions of arterioles of the isolated submucosa of the guinea-pig small intestine were evoked by stimulation of the perivascular sympathetic nerves. Prior stimulation of vasodilator neurones in the submucosal nerve plexus greatly reduced the constrictor response to sympathetic stimulation. 2. Vasodilator nerve stimulation reduced both the amplitude and rate of decay of the excitatory junction potential (EJP) evoked in the arteriolar smooth muscle by sympathetic nerve stimulation. 3. Computer simulation of the effect of membrane resistance changes on the EJP amplitude indicated that the change in amplitude could not be explained by the fall in membrane resistance alone, suggesting that vasodilator nerve activity reduced neurotransmitter release from the sympathetic nerves.

In vivo and in vitro effects of muscarinic receptor antagonists on contractions and release of [3H]acetylcholine in the rabbit urinary bladder

The functional effects of muscarinic receptor antagonists were examined in vivo and in vitro on the rabbit urinary bladder. Inhibitory effects on carbachol-evoked contractions of detrusor strips were pronounced for 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; -logIC50: 8.64), p-fluoro-hexahydro-sila-diphenidol (pFHHSiD; 7.84) and atropine (8.27), while they were less pronounced for pirenzepine (6.62) and methoctramine (5.36). 4-DAMP and methoctramine increased 3H overflow from [3H]choline-labelled strips in response to electrical stimulation, contrary to pirenzepine, which decreased the overflow. Concomitant contractions were markedly reduced by 4-DAMP and by pirenzepine, but not by methoctramine. The -logIC50 estimations for atropine-sensitive electrically evoked contractions revealed methoctramine (4.85) to be less potent on nerve-evoked contractions than on carbachol-evoked contractions, in contrast to pirenzepine (7.15) and 4-DAMP (9.15). The effects of the antagonists in anaesthetized rabbits resembled those in vitro. Thus, muscarinic receptors in the rabbit urinary bladder are heterogeneous; prejunctional facilitatory (M1) and inhibitory (M2) for acetylcholine release, and postjunctional muscarinic M3 receptors mediating contractile responses.

Release of endothelium-derived hyperpolarizing factor (EDHF) by M3 receptor stimulation in guinea-pig coronary artery

1. The muscarinic receptor subtype(s) involved in the release of endothelium-derived hyperpolarizing factor (EDHF) were studied in the guinea-pig coronary artery by recording the membrane potential in the smooth muscle cells with intracellular microelectrodes. 2. Acetylcholine (ACh, pD2 6.68) was 10 times more potent than the M2 agonist, oxotremorine (pD2 5.65) and 500 fold more potent than the M1 agonist, McN-A-343 (pD2 3.95) in evoking the EDHF hyperpolarization. 3. The M3 muscarinic antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) was the most potent (pA2 9.5) in inhibiting the release of EDHF evoked by ACh, being more potent than pirenzepine (pA2 6.7), and AFDX-116 (pA2 6.1) which preferentially block M1 and M2 receptors, respectively. 4. These results suggest that EDHF is released from the endothelium of the guinea-pig coronary artery upon the activation of the muscarinic M3 receptor subtype.

In vivo characterization of muscarinic receptor subtypes that mediate vasodilatation in patients with essential hypertension

Attenuated cholinergic vasodilatation has been suggested as an endothelium-related mechanism involved in essential hypertension. We investigated the role of muscarinic (M) receptor subtypes in the forearm resistance vasculature. In eight white men with essential hypertension and eight matched normotensive control subjects (age of both groups, 47 +/- 4 years; mean +/- SEM), we infused the nonselective agonist methacholine in the presence of saline and the antagonists atropine (nonselective), pirenzepine (M1-selective), and AF-DX 116 (M2-selective) into the brachial artery and measured forearm blood flow and forearm vascular resistance using venous occlusion plethysmography. Affinity constants (pKb values) were determined from calculated plasma concentrations of the infused compounds and EC50 values. Sodium nitroprusside was given as an endothelium-independent control, and minimal forearm vascular resistance after 10 minutes of ischemia was used as a marker of structural vascular changes. Hypertensive patients showed higher minimal forearm vascular resistance, indicating structural vascular changes. However, sodium nitro-prusside- and methacholine-induced vasodilatation was similar in both groups, with apparent EC50 values (log moles per liter; mean +/- SEM) of -7.32 +/- 0.13 and -7.51 +/- 0.21 in hypertensive patients and -7.37 +/- 0.13 and -7.45 +/- 0.02 in control subjects, respectively. Atropine, pirenzepine, and AF-DX 116 caused a shift to the right of the concentration-response curve of methacholine, with apparent pKb values of 8.63 +/- 0.08, 6.81 +/- 0.13, and 5.51 +/- 0.29 in hypertensive individuals and 8.62 +/- 0.10, 6.98 +/- 0.08, and 5.49 +/- 0.09 in control subjects, respectively. Again, there were no statistically significant differences in these pharmacological parameters between hypertensive patients and normotensive subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial metabolic and functional responses to acetylcholine are altered in thyroxine-induced cardiac hypertrophy

We tested the hypothesis that acetylcholine would reduce myocardial O2 consumption and function, and that thyroxine (T4, 0.5 mg/kg for 16 days) induced cardiac hypertrophy would change this relationship. Anesthetized open-chest New Zealand white rabbits were divided into four groups: control-vehicle (CV, n = 8), control-acetylcholine (CA, n = 10), T4-vehicle (T4V, n = 9), and T4-acetylcholine (T4A, n = 10). Either vehicle or acetylcholine (10(-3) M) was topically applied to the left ventricular surface. Coronary blood flow (radioactive microspheres) and O2 extraction (microspectrophotometry) were used to determine O2 consumption, and muscarinic receptor density and affinity were also determined. T4 increased the heart weight/body weight ratio from 2.6 +/- 0.1 to 3.4 +/- 0.1. T4-treated animals had higher heart rates, blood pressures, and left ventricular dP/dtmax than control rabbits. Topical acetylcholine depressed hemodynamic parameters with a greater decrement in pressures and cardiac output in the T4A group (CA, -25%, T4A, -40%). Myocardial O2 consumption and coronary blood flow were higher in the T4-treated hearts. Myocardial O2 consumption significantly declined in both groups during acetylcholine, but the reduction was greater in the T4-treated hearts (CV 7.9 +/- 0.4 to CA 5.8 +/- 0.6 and T4V 18.8 +/- 3.0 to T4A 7.3 +/- 1.0 mL O2.min-1.100 g-1). Muscarinic receptor density (Bmax) was elevated by 41% in the T4-treated hearts, but affinity (Kd) was not altered. Thus, the T4-treated hearts responded to acetylcholine to a greater extent than control hearts in terms of functional and O2 consumption decrements.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic receptors and drugs in cardiovascular medicine

The parasympathetic system and its associated muscarinic receptors have been the subject of a renaissance of interest for the following two main reasons: (1) the association of endothelial muscarinic receptors and the nitric oxide (NO) pathway; (2) the discovery of several muscarinic receptor subtypes and drugs interacting with them. In the present survey modern insights into the subdivision of muscarinic receptors have been dealt with as the basis for a description of the muscarinic receptor agonists and antagonists thus far known. There are at least four pharmacologically defined M receptors (M1, M2, M3, M4) in primary tissues, and five muscarinic receptors have been cloned (m1, m2, m3, m4, m5). Selective agonists for M-receptor subtypes hardly exist, and all classical agonists (acetylcholine, carbachol, etc.) are clearly nonselective. A few selective antagonists for M1 (pirenzepine) and M2 receptors (AF-DX 116) have been introduced, although selective M3 receptors are hardly available. Finally, the potential therapeutic use of M-receptor agonists (myocardial ischemia, hypertension) and muscarinic antagonists (certain forms of bradycardia, coronary spasm) has been critically discussed. Although only in a preliminary stage, this development appears to be promising and at least of great fundamental interest.

Hypotensive effect of an M2-selective muscarinic antagonist in anaesthetized guinea-pigs

1. In order to determine an involvement of muscarinic M2 receptors in the regulation of systemic arterial blood pressure, we investigated the cardiovascular effects of the M2-selective antagonist methoctramine and other agents in anaesthetized guinea-pigs. 2. Intravenous injection of methoctramine, atropine, pirenzepine (an M1-selective muscarinic antagonist) or 4-DAMP (an M3-selective muscarinic antagonist) each significantly increased heart rate in comparison to vehicle controls. 3. Methoctramine produced significant, dose-dependent decreases in mean arterial blood pressure, with an ED50 of 0.1 mg kg-1. Atropine decreased blood pressure only at high doses. Pirenzepine and 4-DAMP did not alter blood pressure, indicating that M1 or M3 receptor antagonism was not responsible for the cardiovascular effects of methoctramine. 4. The hypotensive effect of methoctramine was unaltered by indomethacin pretreatment, ruling out an alteration in arachidonic acid metabolism as the mechanism of action. 5. In contrast to methoctramine, mecamylamine (a nicotinic ganglionic receptor antagonist) greatly decreased heart rate and slightly decreased blood pressure, suggesting that ganglionic blockade was not the mechanism for the cardiovascular effects of methoctramine. 6. Methoctramine (0.3 mg kg-1) pretreatment did not alter the hypertensive effect of intravenous noradrenaline, demonstrating that methoctramine did not directly inhibit vascular reactivity and indicating an indirect hypotensive of action of methoctramine. 7. In summary, the results suggest that the hypotensive action of methoctramine resulted from selective M2 receptor antagonism. Therefore, muscarinic M2 receptors appear to play a role in the regulation of systemic arterial blood pressure in guinea-pigs. However, the anatomical site(s) of action of methoctramine remains to be determined.

Vasodilatation and smooth muscle membrane potential changes in arterioles from the guinea-pig small intestine

1. Dilatation of arterioles isolated from the guinea-pig small intestine was evoked by stimulation of a submucous ganglion and the application of acetylcholine, vasoactive intestinal peptide, galanin or dynorphin A. Changes in arteriole diameter and smooth muscle membrane potential were recorded simultaneously. 2. Ganglion stimulation caused vasodilatation and smooth muscle hyperpolarization that varied in both amplitude and time course from one arteriole to another. Vasodilatation could occur without hyperpolarization. 3. Vasodilatation caused by acetylcholine was accompanied by a rapidly developing hyperpolarization that began to decline before the maximum vasodilator effect had developed. 4. Vasoactive intestinal peptide caused dilatation without any change in smooth muscle membrane potential. 5. Galanin and dynorphin caused dilatation and a hyperpolarization of similar time course to the dilatation. 6. In 48% of arterioles tested the dilatation appeared to be mediated solely by acetylcholine. In 31% there was a cholinergic component, but no evidence for the involvement of acetylcholine in the remaining 21%. When the non-cholinergic dilatation occurred without a hyperpolarization we conclude that it was due to vasoactive intestinal peptide; otherwise it may have been due to either galanin or dynorphin.