Modulation of Gallbladder Contraction by Pirenzepine in Humans

Objectives : The mechanism (s) by which cholinergic innorvatiun modulates gallbladder contraction are not fully understood. To elucidate the role of muscarinic M, receptors in the mediation of gallbladder contraction, we investigated gallbladder volume reduction, plasma cholecystokinin (CCK). and pancreatic polypeptide (I'P) responses in humans during cephalic and intestinal phases of a meal under M, muscarinic receptor blockade with pirenzepine. Methods : In eight healthy subjects, intruduodenal meal- and in seven subjects, sham feeding-induced gallbladder volume reduction was measured by real-time ultrasonography during saline or piren/-epine administration. Plasma CCK and PP were measured by radioimmunoassay. Results : Pirenzepine partially inhibited gallbladder volume reduction in response to an intraduodenal fatty meal. The integrated gallbladder volume reduction over 120 min was 4462 ± 445%.min compared with 6879 ± 279%.min in the saline control group ( p < 0.01). Integrated plasma CCK and PP responses were unchanged in the presence of pirenzepine. Pirenzepine abolished sham feeding-induced gallbladder contraction and plasma PP response. Sham feeding with either isotonic saline or pirenzepine infusion did not modify fasting plasma CCK levels. Conclusion : M, muscarinic receptors play an important role in the intestinal and cephalic phases of gallbladder contraction. Plasma CCK response to intraduodenal meal is not influenced by M, muscarinic receptor blockade with pirenzepine.     

Inhibition of cholecystokinin-induced gallbladder contraction by atropine and pirenzepine in man

The effect of pirenzepine on cholecystokinin (CCK)-stimulated gallbladder contraction in man was compared with that of atropine. Following pretreatment with atropine, pirenzepine or physiological saline in a random order, 6 healthy male volunteers were infused for 75 min with CCK-octapeptide in doses that are reported to mimic physiological plasma concentrations. Gallbladder volumes were determined by real-time ultrasonography. Pirenzepine inhibited CCK-induced gallbladder volume reduction significantly up to 30 min (p less than 0.01) and the magnitude of this inhibition was similar to that of atropine. The results demonstrate that M1 neural muscarinic receptors are involved in the anticholinergic inhibition of CCK-induced gallbladder contraction in man.     

Effect of selective and nonselective muscarinic blockade on cholecystokinin-induced gallbladder emptying in man

In this study we investigated the effect of selective (M₁) and non-selective (M₁ and M₂) pharmacologic blockade of muscarinic receptors on cholecystokinin-induced gallbladder emptying. After validating the method of study, the gallbladder function was evaluated in 15 normal volunteers by quantitative biliary scintigraphy, and the effect of intravenous atropine (0.15 mg/10 kg) and pirenzepine (10 mg) was analyzed in each subject. Atropine significantly reduced the ejection period and the ejection fraction of gallbladder evacuation. Pirenzepine reduced the ejection period, but the ejection fraction remained unchanged. We conclude that the effect of cholecystokinin on gallbladder motility is mediated through muscarinic receptors. Our results suggest that M₂ receptors, but not M₁ receptors, are involved in this response.     

Role of cholecystokinin and the cholinergic system in intestinal stimulation of gallbladder contraction in man

To determine the role of cholecystokinin and the cholinergic system in intestinal stimulation of gallbladder contraction, we studied the effects of atropine on plasma cholecystokinin and gallbladder contraction in six healthy volunteers (four men and two women aged 20 to 27 yr). Effects were noted after intraduodenal fat instillation and after dosage with exogenous cholecystokinin inducing plasma cholecystokinin concentrations similar to those after intraduodenal fat instillation. At regular intervals before and after administration of each stimulus, plasma cholecystokinin concentrations and gallbladder volumes were measured by radioimmunoassay and real-time ultrasonography, respectively. Intraduodenal infusion of 250 ml 20% Intralipid induced a peak plasma cholecystokinin increment of 10.2 +/- 1.6 pmol/L compared with 10.7 +/- 0.7 pmol/L during infusion of 1 Ivy dog unit per kilogram per hour of cholecystokinin. The increases in plasma cholecystokinin after fat and exogenous cholecystokinin administration were accompanied by similar decreases in gallbladder volume. Integrated gallbladder contraction after fat instillation was 3,939% +/- 288%.min compared with 3,301% +/- 359%.min during cholecystokinin infusion (NS). Atropine (0.015 mg/kg as bolus followed by 0.005 mg/kg/hr) did not change plasma cholecystokinin concentrations but induced similar inhibition of gallbladder contraction to 2,296% +/- 511%.min (p less than 0.05) after intraduodenal fat instillation and to 1,756% +/- 456%.min (p less than 0.05) during cholecystokinin infusion. We conclude that cholecystokinin is of major importance in intestinal stimulation of gallbladder contraction. Atropine inhibits the gallbladder response to intraduodenal fat. This inhibition is not due to a reduction in cholecystokinin secretion but to a diminished gallbladder response to cholecystokinin.     

An intrinsic neural pathway for long intestino-intestinal inhibitory reflexes

We studied the mechanisms of initiation and pathways for the propagation of intestino-intestinal inhibitory reflexes induced by close intraarterial injections of neostigmine in conscious dogs. Two or three T-shaped catheters were surgically implanted in the intestinal branches of the superior mesenteric artery to inject pharmacologic agents locally in 10-15-cm-long segments. Migrating myoelectric complexes were recorded by a set of 10 electrodes and strain-gauge transducers. Close intraarterial injection of neostigmine initiated strong contractions of long duration in the perfused segment that terminated phase III activity in progress 90-150 cm distal or proximal to the cannulated sites and stopped its further migration. Atropine or 4-diphenylmethoxy-N-methylpiperidine methiodide injected just before neostigmine administration through the same catheter blocked both the local contractile effects and the reflex inhibition of phase III activity. Pirenzepine or hexamethonium injected in a similar manner did not affect the local response to neostigmine but blocked the reflex inhibition of phase III activity. A transection and reanastomosis in the mid-small intestine blocked the reflex inhibition by close intraarterial injection of neostigmine beyond the transection site. Pirenzepine, atropine, or hexamethonium injected through a middle catheter also blocked the reflex inhibition of phase III activity beyond the site perfused with these cholinergic antagonists. Close intraarterial administration of 4-diphenylmethoxy-N-methylpiperidine methiodide at a middle site had no effect on reflex inhibition. We concluded that strong spasmodic contractions in the small intestine initiate an intestino-intestinal inhibitory reflex in both directions. This reflex is mediated through an intrinsic neural pathway involving nicotinic and M1 muscarinic receptors.     

The role of pirenzepine-sensitive (M1) muscarinic receptors in vagally mediated bronchoconstriction in humans

In a double-blind randomized study, we compared the effects of the M1-selective muscarinic receptor antagonists pirenzepine and the nonselective antagonist ipratropium bromide on bronchoconstriction induced by inhaled sulfur dioxide (SO2) and methacholine in atopic volunteers. Both inhaled pirenzepine (70 micrograms) and ipratropium bromide (7 micrograms) significantly inhibited vagally mediated bronchoconstriction by SO2 to the same extent (p less than 0.02). However, at this dose, pirenzepine had no effect on methacholine-induced bronchoconstriction, whereas ipratropium bromide gave significant protection (p less than 0.02). This indicates that vagally mediated bronchoconstriction in humans can be inhibited by blockade of pirenzepine-sensitive (M1) muscarinic receptors probably present on a different site from muscarinic receptors at the neuromuscular junction and presumably localized to parasympathetic ganglia. Pirenzepine may be useful in investigating ganglionic function and could be beneficial therapeutically in airway disease.     

Muscarinic receptors control postprandial release of glucagon-like peptide-1: in vivo and in vitro studies in rats

Plasma levels of glucagon-like peptide-1 (GLP-1) rise rapidly after nutrient ingestion through an indirect mechanism triggered from the proximal intestine and involving the vagus nerve that stimulates the L cell in the distal gut. The role of muscarinic receptors in this pathway was thus investigated using the anesthetized rat and fetal rat intestinal cells (FRIC) in culture. GLP-1 secretion from the distal gut increased 5-fold after 3 ml corn oil were placed into the proximal duodenum (P < 0.001). Atropine (a nonspecific muscarinic receptor antagonist) completely inhibited fat-induced GLP-1 secretion in vivo (P < 0.01). Pirenzepine (an M1 muscarinic receptor antagonist) also inhibited fat-induced GLP-1 secretion in vivo, by 91 +/- 6% (P < 0.01). Gallamine (an M2 muscarinic receptor antagonist) and 4-diphenylacetoxy-N-methylpiperidine (an M3 muscarinic receptor antagonist) had no effect. Incubating FRIC cultures with bethanechol (a muscarinic receptor agonist) stimulated GLP-1 secretion to 200 +/- 22% of control (P < 0.01). Pirenzepine and gallamine significantly inhibited bethanechol-stimulated GLP-1 secretion, by 96 +/- 12% and 98 +/- 8%, respectively (P < 0.01). Unexpectedly, 4-diphenylacetoxy-N-methylpiperidine stimulated GLP-1 secretion by FRIC cells, to 324 +/- 52% of the control value (P < 0.01). Double immunofluorescent staining using GLP-1 and M1, M2, and M3 muscarinic receptor antibodies showed expression of the three subtypes of muscarinic receptors by the L cells in rat ileal sections and FRIC cultures. These results demonstrate the role of M1 muscarinic receptors expressed by L cells in the control of postprandial secretion of GLP-1. M2 muscarinic receptors also seem to play a role in controlling GLP-1 secretion by fetal, but not adult, L cells.     

Role of muscarinic receptor subtypes in the regulation of migrating myoelectric complex in the dog

The role played by muscarinic receptor subtypes in the regulation of the migrating myoelectric complex was investigated in 7 dogs chronically implanted with bipolar electrodes along the small intestine. Pirenzepine (3-300 micrograms/kg i.v.) and atropine (1-30 micrograms/kg i.v.) were used as selective and unselective antagonist, respectively. Atropine (30 micrograms/kg) significantly delayed the onset of the next complex. On the contrary, pirenzepine displayed a biphasic action: low doses (less than 100 micrograms/kg) shortened the cycle period, whereas at 300 micrograms/kg the drug behaved like atropine. Pirenzepine affected the cycle period in the low-dose range by reducing the length of phase I. Both atropine and pirenzepine impaired the migration of the ongoing complex, and significantly reduced the migration velocity of the following one. These findings suggest that the initiation of the migrating myoelectric complex in the dog is under an inhibitory influence mediated by the M1 muscarinic receptor subtype; on the other hand, M2 receptor activation is needed for the onset of the activity front. Finally, both receptor subtypes determine the normal migration of phase III.     

Analysis of muscarinic receptors on rat pancreatic acini by using 125I-quinuclidinyl benzilate and N-[3H]-methylscopolamine

To analyze muscarinic receptors on rat pancreatic acini, we studied the binding of 125I-quinuclidinyl benzilate (125I-QNB) and N-[3H]-methylscopolamine ([3H]-NMS) to these acini. Binding of 125I-QNB and [3H]-NMS to acini was specific and reversible. 125I-QNB bound to low affinity site, which was not recognised by [3H]-NMS. However, nonspecific binding of 125I-QNB to acini was very high (46%), so 125I-QNB may be inadequate to analyze muscarinic receptor on pancreatic acini. Muscarinic receptors are classified in two groups, M1 and M2, according to affinity of pirenzepine which binds to M1 receptor selectively. Pirenzepine was 530 times less potent than atropine in inhibiting the binding of 125I-QNB, and 250 times less potent than atropine in inhibiting the binding of [3H]-NMS. These results suggest that muscarinic receptors on pancreatic acini are mainly M2 receptors.     

Muscarinic receptors control glucagon-like peptide 1 secretion by human endocrine L cells

Glucagon-like peptide 1 (GLP-1) released from distal intestinal endocrine L cells after food intake is a potent glucose-dependent stimulant of insulin secretion. Plasma levels of GLP-1 rise rapidly after nutrient ingestion through an indirect mechanism triggered from the proximal intestine and involving the vagus nerve. Our previous studies showed the involvement of M1 muscarinic receptors expressed by the L cells in the regulation of postprandial GLP-1 secretion in rats. The goal of this study was to explore the involvement of muscarinic receptors in the regulation of GLP-1 secretion by human L cells using a newly described human L cell line (NCI-H716). Phorbol 12-myristate 13-acetate (positive control) stimulated GLP-1 secretion to 252 +/- 38% of the control (P < 0.001). Bethanechol, a nonselective muscarinic agonist, significantly stimulated GLP-1 secretion to 187 +/- 20% of the control (P < 0.01, n = 8). Pirenzepine (M1 antagonist; 10-1000 microM) and gallamine (M2 antagonist; 10-1000 microM) completely inhibited bethanechol-induced GLP-1 secretion, whereas 4-diphenylacetoxy-N-methylpiperidine (M3 antagonist) had no effect on bethanechol-stimulated GLP-1 secretion. McN-A-343 (M1 muscarinic agonist) dose dependently stimulated GLP-1 secretion (to 252 +/- 50% of control at 1000 microM; P < 0.01), whereas oxotremorine (M3 agonist) had no effect. M1, M2, and M3 muscarinic receptors were shown to be expressed in NCI-H716 cells by Western blot, immunohystochemistry, and RT-PCR. Expression of the M1, M2, and M3 muscarinic receptor subtypes was also confirmed in paraffin-embedded human small intestine sections by double immunofluorescent staining. These results demonstrate the role of M1 and M2 muscarinic receptors expressed by human L cells in the control of GLP-1 secretion.     

Hyperglycemia modulates gallbladder motility and small intestinal transit time in man

The aim of the present study was to investigate the effect of acute hyperglycemia on (1) the intestinal phase of gallbladder contraction induced by the intraduodenal administration of emulsified fat, and (2) the small intestinal transit time measured by the lactulose breath hydrogen test. Six healthy volunteers were studied in random order during normoglycemia and hyperglycemia (blood glucose levels 15 mmol/liter). Gallbladder volumes were measured with ultrasonography. Administration of 1 and 2 g/hr of fat resulted in significant reductions in gallbladder volumes from 24±2 cm³ to 11±1 cm³ (P<0.05) and 8±1 cm³ (P<0.05), respectively during normoglycemia, and from 24±2 cm³ to 21±2 cm³ (P<0.05) and 16±2 cm³, respectively (P<0.05) during hyperglycemia. Compared to normoglycemia, the gallbladder contraction was significantly (P<0.05) reduced during hyperglycemia. No significant differences in CCK secretion were observed between experiments. Small intestinal transit time during hyperglycemia (101±12 min) was significantly (P<0.05) prolonged compared to normoglycemia (57±12 min). During hyperglycemia, basal PP levels and PP secretion in response to intraduodenal fat were significantly (P<0.05) reduced compared to normoglycemia. It is concluded that (1) low doses of intraduodenal emulsified fat result in significant gallbladder contraction and CCK secretion, (2) acute hyperglycemia inhibits intraduodenal fat induced gallbladder contraction, (3) acute hyperglycemia does not affect the intraduodenal fat induced CCK secretion, (4) small intestinal transit is significantly prolonged during acute hyperglycemia, and (5) acute hyperglycemia inhibits basal and stimulated plasma PP secretion, suggesting impaired vagal-cholinergic tone during hyperglycemia.     

Cephalic stimulation of gallbladder contraction in humans: role of cholecystokinin and the cholinergic system

To determine the role of cholecystokinin and the cholinergic system in cephalic stimulation of gallbladder contraction and to compare the degree of gallbladder contraction by cephalic stimulation with postprandial gallbladder contraction, 8 healthy volunteers (4 males, 4 females, 20-65 years) underwent the following studies: sham feeding of an appetizing meal, sham feeding with intravenous atropine, and ingestion of the same meal. Gallbladder volume was measured by real-time ultrasonography and plasma cholecystokinin by a sensitive and specific radioimmunoassay using antibody T204. Gallbladder contraction in response to sham feeding, 30 +/- 4% (p = 0.0001 vs. basal), amounted to half of that seen after real feeding, 69 +/- 5% (p less than 0.0001 vs. basal). Significant dissociation between gallbladder response to sham feeding and real feeding was seen from 40 min (p less than 0.005-p = 0.0001). Atropine did not affect basal gallbladder volume but completely abolished gallbladder contraction in response to sham feeding. Neither sham feeding without nor sham feeding with atropine significantly affected plasma cholecystokinin levels. On the other hand, real feeding induced significant increases in plasma cholecystokinin from a basal level of 2.3 +/- 0.1 pM to a peak value of 5.9 +/- 0.4 pM at 40 min. It is concluded that an important cephalic phase of postprandial gallbladder contraction exists which is cholecystokinin-independent but dependent on a cholinergic mechanism.     

Effect of pirenzepine on aminopyrine uptake by isolated guinea pig parietal cells

The inhibitory action of pirenzepine on acid secretion of isolated guinea pig parietal cells was investigated by the aminopyrine method. Pirenzepine markedly inhibited acid secretion of isolated parietal cells induced by carbachol in a dose-dependent manner but showed no inhibition on acid secretion stimulated by histamine. These results may suggest a direct action of pirenzepine on muscarinic receptors in parietal cells.     

Characterization of muscarinic receptor subtypes inhibiting Ca2+ current and M current in rat sympathetic neurons.

Muscarinic receptors mediating suppression of Ca2+ current and of M-type K+ current in rat superior cervical ganglion neurons were subclassified pharmacologically by using the muscarinic receptor antagonists pirenzepine and himbacine. Our voltage clamp experiments previously distinguished fast and slow intracellular signaling pathways coupling muscarinic receptors to calcium channels. We now establish that the fast, pertussis toxin-sensitive suppression of Ca2+ current is mediated primarily by muscarinic receptors of the M4 subtype, whereas the slow, bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetate (BAPTA)-sensitive suppression of Ca2+ current is mediated primarily by muscarinic receptors of the M1 subtype. Both actions on Ca2+ current are blocked by guanosine 5'-[beta-thio]diphosphate. Muscarinic suppression of M current is slow, BAPTA-sensitive, and mediated by receptors of the M1 subtype. Hence the two muscarinic pathways use different receptors and different guanine nucleotide binding proteins to produce different actions on channels.     

Effect of a cholecystokinin receptor antagonist (loxiglumide) on gallbladder contractile function in guinea pigs

We evaluated the effect of the specific cholecystokinin (CCK) receptor antagonist, loxiglumide, on gallbladder contractile function in guinea pigs. Five mg/kg body weight (BW) of loxiglumide was administered orally to guinea pigs once a day for 3 days. We then investigated gallbladder contractile function and plasma CCK concentrations in the guinea pigs. Maximal gallbladder pressure induced by cerulein was significantly depressed on the 1st and 3rd days following loxiglumide administration. On the 1st day, the plasma CCK concentration was significantly increased compared with that of the control group during fasting and 15 min after the administration of an intraduodenal test meal. These results suggest that the disturbed gallbladder contraction is due to the competitive inhibition of CCK by loxiglumide. Gallbladder contractile function in guinea pigs is depressed by loxiglumide; however, this effect is reversible after short-term loxiglumide administration.     

Two types of muscarinic response to acetylcholine in mammalian cortical neurons.

Applications of acetylcholine (AcCho) to pyramidal cells of guinea pig cingulate cortical slices maintained in vitro result in a short latency inhibition, followed by a prolonged increase in excitability. Cholinergic inhibition is mediated through the rapid excitation of interneurons that utilize the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). This rapid excitation of interneurons is associated with a membrane depolarization and a decrease in neuronal input resistance. In contrast, AcCho-induced excitation of pyramidal cells is due to a direct action that produces a voltage-dependent increase in input resistance. In the experiments reported here, we investigated the possibility that these two responses are mediated by different subclasses of cholinergic receptors. The inhibitory and slow excitatory responses of pyramidal neurons were blocked by muscarinic but not by nicotinic antagonists. Pirenzepine was more effective in blocking the AcCho-induced slow depolarization than in blocking the hyperpolarization of pyramidal neurons. The two responses also varied in their sensitivity to various cholinergic agonists, making it possible to selectively activate either. These data suggest that AcCho may produce two physiologically and pharmacologically distinct muscarinic responses on neocortical neurons: slowly developing voltage-dependent depolarizations associated with an increase in input resistance in pyramidal cells and short-latency depolarizations associated with a decrease in input resistance in presumed GABAergic interneurons.     

Effects of neuraminidase on equine isolated bronchi

It has been demonstrated in mammals that the airway hyper-responsiveness, which accompanies viral infections, is the result of increased reflex bronchoconstriction due to inhibition of muscarinic prejunctional receptors, which belong to M2 subtypes. Multiple mechanisms account for virus-induced M2 receptor dysfunction. Viral neuraminidase may deglycosylate the M2 receptor, decreasing acetylcholine affinity. Equine influenza remains a common viral respiratory disease of horses worldwide, which results in loss to the equine industry, by decreasing performance, convalescence time and loss of peak performance due to chronic sequelae, such as airway hyper-responsiveness. The purpose of this study was to evaluate the effect of neuraminidase on equine isolated bronchi, assessed in equine bronchial smooth muscle rings, derived from five healthy equine male lungs. A pretreatment with vehicle did not modify contraction induced by electrical field stimulation (EFS) studies at each frequency tested. A pretreatment with pilocarpine (1-100 microM) significantly reduced, while methoctramine (1-100 microM) significantly increased contraction induced by EFS. Finally, neuraminidase (0.5 UI) significantly increased contraction induced by EFS. These results suggest that airway hyper-responsiveness that follows a viral influenza infection might be related to a dysfunction of muscarinic prejunctional receptors.     

Mechanisms mediating cholinergic antral circular smooth muscle contraction in rats

AIM: To investigate the pathway (s) mediating rat antral circular smooth muscle contractile responses to the cholinomimetic agent, bebhanechol and the subtypes of muscarinic receptors mediating the cholinergic contraction. METHODS: Circular smooth muscle strips from the antrum of Sprague-Dawley rats were mounted in muscle baths in Krebs buffer. Isometric tension was recorded. Cumulative concentration-response curves were obtained for (+)-cisdioxolane (cD), a nonspecific muscarinic agonist, at 10^-8-10^-4mol/L, in the presence of tetrodotoxin (TTX, 10^-7mol/L). Results were normalized to cross sectional area. A repeat concentration-response curve was obtained after incubation of the muscle for 90min wibh antagonists for M1(pirenzepine), M2(methoctramine) and M3(darifenacin) muscarinic receptor subtypes. The sensitivity to PTX was tested by the ip injection of 100mg/kg of PTX 5d before the experiment. The antral circular smooth muscles were removed from PTX-treated and non-treated rats as strips and dispersed smooth muscle cells to identify whether PTX-linked pathway mediated the contractility to bethanechol. RESULTS: A dose-dependent contractile response observed with bethanechol, was not affected by TTX. The pretreatment of rats with pertussis toxin decreased the contraction induced by bethanechol. Lack of calcium as well as the presence of the L-type calcium channel blocker, nifedipine, also inhibited the cholinergic contraction, with a reduction in responsefrom 2.5&#177;0.4g/mm^2 to 1.2&#177;0.4g/mm^2(P&lt;0.05), The doseresponse curves were shifted to the right by muscarinic antagonists in the following order of affinity: darifenacin (M3)&gt;methocramine (M2)&gt;pirenzepine (M1). CONCLUSION: The muscarinic receptors-dependent contraction of rat antral circular smooth muscles was linked to the signal transduction pabhway(s) involving pertussis-toxin sensitive GTP-binding proteins and to extracellular calcium via L-type voltage gated calcium channels. The presence of the residual contractile response after the treatment with nifedipine, suggests that an additional pathway could mediate the cholinergic contraction. The involvement of more than one muscarinic receptor (functionally predominant type 3 over type 2) also suggests more than one pathway mediating the cholinergic contraction in rat antrum.     

Changes in regional myocardial perfusion by muscarinic receptor subtypes in dogs

To determine the muscarinic receptor subtype (M1 or M2 or both) responsible for cholinergic coronary vasodilatation and alterations in the transmural distribution (endocardial to epicardial blood flow ratio) of coronary blood flow, systemic and coronary haemodynamic indices and regional myocardial blood flow (radioactive microspheres) were measured in anaesthetised dogs. Submaximal vasodilative doses of the mixed muscarinic agonist acetylcholine were given by intracoronary infusion to avoid peripheral haemodynamic effects. Acetylcholine produced significant increases in myocardial perfusion and selectively redistributed flow to the subendocardium (increased endocardial to epicardial blood flow ratio). Pirenzepine (160 nmol X kg-1 iv), a selective M1 antagonist, produced no change in endocardial to epicardial blood flow ratio or myocardial blood flow but blocked the increase in endocardial to epicardial blood flow ratio and attenuated the transmural increase in myocardial perfusion during acetylcholine infusion. 4-Diphenylacetoxy-N-methylpiperidine methyl bromide (4-DAMP) (17 nmol X kg-1 iv), a selective M2 antagonist, also attenuated the transmural increase in myocardial blood flow but had little effect on the increase in endocardial to epicardial blood flow ratio produced by acetylcholine. These results support the hypothesis that M1 muscarinic coronary receptors are responsible for the redistribution of blood flow to the subendocardium (increased endocardial to epicardial blood flow ratio) during cholinergic coronary vasodilatation, whereas both M1 and M2 receptors are involved in increasing myocardial perfusion.     

Aging-related alterations in the contractile responses to acetylcholine, muscarinic cholinoceptors and cholinesterase activities in jejunum and colon of the male Fischer 344 rats

In an attempt to examine whether the muscarinic receptor-activated intestinal function is altered by aging, we studied the changes in (1) contractile responses to acetylcholine (Ach), (2) muscarinic cholinoceptors and (3) cholinesterase (ChE) activities, in jejunum and colon of the young (2-3 months) and aged (24-28 months) Fischer 344 rats. In the physiological contraction experiments of jejunum and colon, Ach concentration-dependently increased the force of contraction, and the contractile responses to Ach were not affected by aging. In addition, the true- and pseudo-ChE activities were not significantly changed by aging. The Ach-induced contraction was competitively inhibited by muscarinic M3-selective antagonist hexahydro-sila-difenidolhydrochloride p-fluoroanalog (p-F-HHSiD), suggesting that the contractile responses in the rat jejunum and colon were mediated through M3-cholinoceptor. Age-related changes in muscarinic cholinoceptors of jejunum and colon were determined with the use of specific muscarinic radioligand [3H]-quinuclidinylbenzilate (QNB). The [3H]QNB saturation binding experiments revealed that the maximal binding (B(max)) was increased only in aged jejunum without changes in K(D) values. These results suggest that aging may not attenuate the Ach-induced intestinal contraction via muscarinic M3 receptor, although the expression of muscarinic cholinoceptor is differentially modulated in jejunum and colon.