Characterization of muscarinic receptors mediating vasodilation in guinea-pig ileum submucosal arterioles by the use of computer-assisted videomicroscopy.

Muscarinic receptors of resistance vessels (submucosal arterioles, outside diameter 50-75 microns) from the guinea-pig small intestine were investigated in vitro using a computer-assisted videomicroscopy system (Diamtrak). The muscarinic receptor which mediates vasodilation of precontracted [U-46619 (300 nM) or (-)-noradrenaline (10 microM)] arterioles was characterized with several muscarinic agonists and subtype-selective antagonists. The following agonists all produced equivalent maximum vasodilation (given in rank order of potency): acetylcholine = arecaidine propargyl ester (APE) greater than oxotremorine = (+/-)-muscarine = (+/-)-methacholine greater than carbachol greater than 4-[[N-(4-chlorophenyl)carbamoyl]oxy]-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343). 4-[[N-(3-Chlorophenyl)-carbamoyl]oxy]-2-butynyltrimethylammonium chloride (McN-A-343) and N-ethyl-guvacine propargyl ester (NEN-APE) produced minimal or no arteriolar vasodilation. The muscarinic antagonists pirenzepine, (+-)-5,11-dihydro-11-[[[2-[2-((dipropylamino)methyl)-1-piperidinyl] ethyl]amino]-carbonyl]-6H-pyrido(2,3-b)(1,4)-benzodiazepin-6-one (AF-DX 384), 11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11-dihydro- 6H-pyrido(2,3-b)(1,4)-benzodiazepin-6-one (AQ-RA 741), p-fluorohexahydro-sila-difenidol (p-F-HHSiD), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and (R)- and (S)-hexahydro-difenidol [(R)-HHD, (S)-HHD] shifted the muscarine, methacholine or carbachol dose-response curve to the right in a competitive manner. Schild analysis of the data yielded pA2 values for pirenzepine (6.74/6.9), AF-DX 384 (6.72), AQ-RA 741 (6.58), p-F-HHSiD (7.53/7.57), 4-DAMP (9.06), (R)-HHD (7.88/8.32) and (S)-HHD (5.52/5.88). Thus, it can be concluded that submucosal arterioles possess only the M3 functional muscarinic receptor, the activation of which causes blood vessel dilation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vasoconstriction of guinea-pig submucosal arterioles following sympathetic nerve stimulation is mediated by the release of ATP.

1. The nature of the transmitter mediating vasoconstriction of guinea-pig submucosal arterioles following sympathetic nerve stimulation was studied. 2. Prazosin (0.1 microM) abolished the response to exogenously applied phenylephrine (1 microM) but had no effect on constrictions of submucosal arterioles evoked by nerve stimulation (100 pulses at 10 Hz). 3. Vasoconstrictions and excitatory junction potentials elicited by nerve stimulation were potentiated by idazoxan (0.1 microM). 4. Following reserpine treatment, catecholamine fluorescence was absent in submucosal arterioles but nerve-evoked vasoconstrictions were unaltered. 5. Vasoconstrictions and excitatory junction potentials recorded in response to sympathetic nerve stimulation, as well as constrictions evoked by exogenously applied ATP (3 microM), were abolished by the P2-purinoceptor antagonist, suramin (100 microM). Suramin had no effect on the vasoconstriction in response to noradrenaline (3 microM), or the nicotinic excitatory postsynaptic potentials (e.p.s.ps) and noradrenergic inhibitory postsynaptic potentials (i.p.s.ps) recorded from submucosal neurones. 6. We conclude that postjunctional responses of submucosal arterioles following sympathetic nerve stimulation are mediated solely through the activation of P2X-purinoceptors by ATP or a related purine nucleotide. The function of neurally released noradrenaline is to act through prejunctional alpha 2-adrenoceptors to depress transmitter release.     

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.     

Hyoscine-resistant peristalsis in guinea-pig ileum

The effect of hyoscine on the peristaltic activity of the proximal and distal ileum of the guinea-pig was studied. Hyoscine strongly impaired peristalsis as indicated by the elevation of the threshold pressure and by the increased number of incomplete peristalses and blockades. Functional activity of the circular musculature was more markedly impaired. However, particularly in the distal ileum, complete peristalses occurred even after 70 min exposure to hyoscine at a concentration of 10^?6 g/ml. A tenfold increase in hyoscine concentration failed to producedfurther impairment of peristaltic activity and of the oral reflex contraction. The activity which remained in the presence of hyoscine was blocked by methysergide and by d-tubocurarine. The hypothesis is advanced that once the muscarinic receptors have been blocked, increased radial stretch of the circular coat results in activation of a separate, tetrodotoxin sensitive, excitatory nervous pathway, which is sufficient to maintain a discrete degree of peristaltic activity.     

Dihydropyridines on guinea-pig ileum

1. The effects of three dihydropyridine derivates were studied and compared with those of nifedipine in guinea-pig ileum. 2. Nifedipine, nimodipine, nitrendipine (25-200 nM) and nisoldipine (10-80 nM) produced a dose-dependent inhibition of the contractile responses induced by single pulse stimulation, acetylcholine (1 microM) and high-K (50 mM). 3. From these experiments it is concluded that nimodipine and nitrendipine, like nifedipine, produced a similar and potent inhibitory effect of the contractile responses induced in the guinea-pig ileum. 4. Nisoldipine resulted to be the most active agent.     

Clonidine dependence in the guinea-pig isolated ileum

1 Compared with the response of preparations incubated in solutions without clonidine, a three to four fold increase in the magnitude of the contracture of the longitudinal muscle to challenge with phentolamine (1.0 μm) was induced by incubating the guinea-pig isolated ileum at 22°C for 24 h with clonidine (1.0 μm) in Krebs solution containing hexamethonium (70 μm). Incubation of the ileum with clondine (1.0 μm) for 0.5 h at 37°C did not increase responsiveness to phentolamine.2 The increase in responsiveness to phentolamine was directly related to the clonidine concentration in the incubation fluid over the range 0.01 to 1.0 μm.3 The magnitude of the contracture to phentolamine of ilea incubated with clonidine (1.0 μm) (withdrawal contracture) was directly related to the challenge dose of phentolamine over the range 0.3 to 1.0 μm.4 Yohimbine (1.0 μm) or piperoxane (1.0 μm) elicited a response comparable to that elicited by phentolamine but propranolol (1.0 μm) was inactive.5 Addition of phentolamine (1.0 μm) to clonidine (1.0 μm) in the incubation fluid abolished the increased response of the preparation to subsequent challenge with phentolamine.6 Addition of hyoscine (0.5 μm) immediately after challenge with phentolamine restored the tension of the withdrawal contracture to its resting level.7 Tetrodotoxin (3.0 μm) given before challenge, prevented phentolamine from eliciting a withdrawal contracture.8 Ileal segments incubated with clonidine (1.0 μm) were unresponsive to challenge with naloxone (100 nm); and segments incubated with normorphine (1.0 μm) were unresponsive to phentolamine (1.0 μm), although responsive to naloxone.9 Normorphine (1.0 μm) restored to resting level the tension of the clonidine withdrawal contracture; and clonidine (0.1 μm) restored to resting level the tension of the contracture to naloxone in ileal segments incubated with normorphine.10 These experiments indicate that incubation with clonidine induces, in the final cholinergic motor neurones of the myenteric plexus of the isolated ileum, a dependence the withdrawal from which is expressed as a contracture in response to α-adrenoceptor antagonists.11 Although opiate receptors are not involved in clonidine dependence nor α-adrenoceptors in opiate dependence, the findings that normorphine suppresses the clonidine withdrawal-contracture and that clonidine suppresses the contracture of opiate-dependent ileum to naloxone, suggest that the withdrawal effect studied in both clonidine and normorphine dependence in this preparation is mediated by release of acetylcholine from the final motor neurone.     

Histamine-mediated acetylcholine release in the guinea-pig ileum

The isometric contraction induced by histamine in guinea-pig ileum longitudinal muscle was biphasic in Krebs ([Ca]: 3.36 mM) but monophasic in Tyrode solution ([Ca]: 1.80 mM). The late phase (histamine: 20-400 nM) was common to the two solutions but the early phase (histamine: 2-20 nM) was observed only in Krebs solution. This early phase could be blocked with atropine (0.01-0.2 microM), morphine (0.1, 1 microM), adenosine (5, 20 microM) and tetrodotoxin (0.3 microM) without affecting the late phase. Washout of morphine or adenosine was fast. Neostigmine (100 nM) greatly potentiated the effect of histamine (4 nM) in the early phase, the muscle undergoing almost maximum contraction but also reversible desensitization to doses of histamine less than or equal to 20 nM for as long as 40 min after washout. Beyond this concentration, the preparation responded to increasing doses of histamine as observed in the late phase. It is concluded that low concentrations of histamine that have no observable direct effect on muscle contractility release acetylcholine in the presence of [Ca] 3.36 mM, the early phase being entirely due to release of endogenous acetylcholine.     

Carbamate-induced rhythmic activity in the guinea-pig ileum

The action of newly synthesised basic esters of alkoxy-substituted derivatives of phenylcarbamic acid was compared with that of procaine and trimecaine in the guinea-pig small intestine. Some of the carbamate local anaesthetics, in concentrations high enough to markedly suppress cholinergic twitches, elicited regular rhythmic activity of the guinea-pig ileum. This effect of the carbamates was dependent on the position of the alkoxysubstituent and was independent of the basic group (pyrrolidine, piperidine or perhydroazepine) present. Only the ortho-substituted derivatives were active. It was also found that the rhythmic activity evoked was of myogenic origin, produced by rhythmic undulations of the sodium-pump activity, accompanied by a potential-dependent change in Ca2+ permeability. It is suggested that the mechanism by which carbamates reveal this endogenous rhythm in the guinea-pig ileum could be a selective block of noncholinergic, nonadrenergic innervation or the TEA-like activity of carbamates.     

The effects of LSD in the guinea-pig ileum

The effects of lysergic acid diethylamide (LSD) on acetylcholine release and on smooth muscle tone were studied in the myenteric plexus-longitudinal muscle preparation of the guinea pig. LSD (0.01-10 microM) depressed in a concentration-dependent manner the electrically-evoked [3H]-acetylcholine outflow from strips preincubated with [3H]-choline. The maximal effect was a 45% inhibition by 1 microM LSD. The spontaneous outflow was not affected. Metitepine competitively antagonized (pA2 8.0) the LSD-induced reduction of the evoked outflow. Tolazoline and mepyramine did not affect the inhibitory action of LSD. The contractions in response to electrical stimulation were enhanced by 34% in the presence of 0.1 microM LSD. Other concentrations of LSD did not affect the twitches. LSD caused an increase in muscle tone which was antagonized non-competitively by mepyramine, metitepine and ketanserin. Ketanserin was a competitive antagonist against the histamine-induced contractions of the longitudinal muscle (pA2 8.49). The results suggest that LSD stimulates presynaptically located 5-HT receptors and thereby decreases the evoked acetylcholine release. In addition, LSD increases smooth muscle tone either directly through stimulation of H1 receptors or indirectly via histamine release.     

Biphasic effects of endothelin in the guinea-pig ileum

The intestinal effects of porcine endothelin (ET-1), a potent endothelium-derived vasoconstrictor, were studied on the guinea-pig isolated ileum. ET-1 (3 x 10(-10)-3 x 10(-7) M) caused biphasic responses on spontaneous smooth muscle tone, an initial relaxation followed by a late contraction. The contractile response was elicited in a concentration-dependent manner. Both the relaxing and contractile phases were not affected by pretreatment with tetrodotoxin, phentolamine, tolazoline, propranolol, guanethidine, 8-phenyl-theophylline, naloxone, methylsergide, [D-Pro4,D-Try7,9]SP-(4-11), diphenylhydralamine and indomethacin. Atropine (3 x 10(-7) M) also had no effect on the ET-1-induced contraction. The ET-1-induced contraction, however, was markedly inhibited by verapamil (greater than 10(-8) M) and H-7 (3 x 10(-5) M). These results suggest that, in intestinal smooth muscle, ET-1-induced contraction can be attributed to the direct effect on muscle and appears to be mediated by increased Ca2+ influx through voltage-dependent Ca2+ channels as well as protein kinase C activation. On the other hand, ET-1-induced relaxation is due neither to the indirectly evoked release of inhibitory neurotransmitters nor to the direct activation of adrenoceptors, and purine and opiate receptors. The exact mechanism responsible for ET-1-induced relaxation needs to be further studied.     

Characterization of kappa-opioid receptors in the guinea-pig ileum

Equilibrium binding saturation studies with [3H]bremazocine, under mu- and delta-suppressed conditions and [3H]U69593 have demonstrated that both radioligands bind with high affinity to an apparently homogeneous population of binding sites in the guinea-pig ileum longitudinal muscle-myenteric plexus preparation. In competition studies, the absolute affinities and slopes of the inhibition curves for several unlabelled ligands against [3H]bremazocine were not significantly different to those against [3H]U69593 and were consistent with binding to the kappa-opioid binding site. In the intestinal layers underlying the longitudinal muscle-myenteric plexus [3H]bremazocine, under kappa-selective conditions, recognized both a high and low affinity site. In contrast, [3H]U69593 bound to a homogeneous population of binding sites. The [3H]U69593 binding site and the [3H]bremazocine high affinity site demonstrated comparable characteristics to the single, kappa site identified in the longitudinal muscle layer. The nature of the low affinity site was not investigated due to difficulties associated with low specific binding, and its significance therefore remains to be investigated.     

Carbachol-induced nonspecific desensitization in guinea-pig ileum

Summary The effects of repeated stimulation by carbachol on force development have been examined in smooth muscle of the longitudinal layer of the guinea-pig ileum. Carbachol was applied at 20°C for 5 min. Each application was followed by a 25-min washout period and the desensitization was expressed by the decline of the maximal force development. Three hours after the first carbachol-induced contraction the peak amplitude was about 40% of the initial value. Increasing the frequency of application, thereby decreasing the washout time, enhanced the desensitization, while the presence of the competitive blocker atropine reduced the phenomenon. At 35°C no desensitization could be observed. Blocking the Na⁺/K⁺ pump by ouabain or by K⁺-free solution reduced the force development to less than 20%. Increasing [K⁺]₀ in the washout solution at 20°C reduced the desensitization phenomenon, while decreasing [K⁺]₀ resulted in an enhanced desensitization as expressed by a decline of the force development.The total cellular Na⁺ content after various stimulation sequences was determined at 20° and 35°C from the ²²Na⁺ effluxes. At 35°C the cellular Na⁺ content did not change significantly during stimulation for 10 min with 10^–4 mol/l carbachol. At 20°C the resting Na⁺ content was significantly increased, and it doubled during carbachol stimulation for 10 min. Furthermore, the recovery of the cellular Na⁺ content after washout proceeded extremely slowly at that temperature. The appearance of desensitization was increased by 10 mol/l ryanodine, while it was reduced by adding the Ca²⁺ agonist Bay K 8644. Also the presence of pertussis toxin reduced the desensitization. The desensitizing effects of other agonists as histamine and substance P were less pronounced but also frequency dependent. The nonspecific nature of the carbachol desensitization was confirmed by the reduced response to histamine after stimulations with carbachol.We conclude that nonspecific desensitization is an agonist-, time- and temperature-dependent phenomenon. This desensitization could be the result of a G-protein-mediated inactivation of voltage-gated Ca²⁺ currents, which would thereby reduce the amount of [Ca²⁺]_i mobilized during repeated stimulation. Send offprint requests to: B. Himpens at the above address     

Model of opiate dependence in the guinea-pig isolated ileum

1 Segments of ileum, incubated for 2-24 h at 22°C with normorphine (0.01 - 1.0 μM), in the presence of hexamethonium, contracted when challenged with naloxone (0.03 μM). No response to this dose of naloxone was induced either by incubation in control solution without opiate for 2-24 h or by exposure of the preparation to opiate for 30 min at 37°C.

2 When segments were incubated for 24 h, the size of the response to naloxone was directly related both to the normorphine concentration in the incubation fluid (0.01 to 0.1 μM), and to the concentration of naloxone applied (0.03 to 0.1 μM).

3 A spontaneous withdrawal contracture was elicited in ilea that had been incubated with normorphine (1.0 μM), when the normorphine-containing bathing fluid was exchanged for one without opiate.

4 Normorphine restored to resting level the tension of the withdrawal contracture, whether it had been elicited spontaneously or by naloxone challenge.

5 Addition of naloxone (1.0 μM) to normorphine (1.0 μM) in the incubation fluid abolished the withdrawal contracture to subsequent challenge with naloxone.

6 Naloxone elicited a contracture from segments incubated for 24 h at 22°C with levorphanol (0.1 μM) but not from those incubated with dextrorphan.

7 Application of (+)-naloxone (0.03 μM) to segments previously incubated with normorphine (0.1 μM) did not elicit a contracture.

8 The contracture elicited by naloxone in preparations incubated with morphine (10 μM) was associated with a reduction in sensitivity to the acute inhibitory effect of morphine on the electrically-evoked response.

9 Addition of hyoscine (0.5 μM) immediately after challenge with naloxone restored the tension of the withdrawal contracture to resting level.

10 Tetrodotoxin (3.0 μM) given before challenge, prevented naloxone from eliciting a withdrawal contracture.

11 The inclusion of 5-hydroxytryptamine (10 μM) with morphine (10 μM) inhibited the induction of tolerance to morphine.

12 These experiments, together with those described earlier, indicate that incubation with opiate induces a dependence in the final cholinergic motor neurones of the myenteric plexus, manifested as a contracture of the longitudinal muscle on removal of opiate or administration of an antagonist. This dependence is associated with tolerance, expressed as a decrease in sensitivity to inhibition by morphine of the electrically-evoked contracture. Tolerance and dependence are induced and withdrawal precipitated through specific and stereospecific opiate receptors.

     

Contractile effects of cysteamine on the guinea-pig ileum

Cysteamine (beta-mercaptoethylamine HCl) (1.0-40.0 mM) induced a concentration-dependent increase in tonic and phasic contractions of segments of guinea-pig ileum in vitro. Myenteric plexus-longitudinal muscle (MPLM) preparations also responded with an increase in tonic contractions but phasic contractions were either greatly reduced or absent, indicating that these were a response of the circular muscle. Atropine (5 microM) inhibited the cysteamine-induced contractions, whereas hexamethonium and guanethidine had no effect, suggesting that cysteamine was acting at least partly via a cholinergic mechanism involving muscarinic receptors. Tetrodotoxin increased the phasic contractions of ileal segments, but had no effect on the tonic component. Treatment of MPLM preparations with morphine (1 microM) resulted in a small reduction in responsiveness to cysteamine, and blocked electrically-induced contractions by at least 90%. Since morphine acts by inhibiting acetylcholine release via hyperpolarization of intrinsic neurones, a small but significant part of the cysteamine-induced contractions probably resulted from stimulation of acetylcholine release from intrinsic neurones. Following a response to high cysteamine concentrations (greater than 15 mM) tissues were refractory to subsequent cysteamine administration. Cross-desensitization between cysteamine and acetylcholine also occurred, as short-term (1-3 min) incubation of MPLM preparations with high concentrations of either compound (1-10 microM acetylcholine or 20 mM cysteamine) resulted in a reduced responsiveness to both. A reduced sensitivity to acetylcholine or cysteamine was obtained following long-term (45 min) incubation with acetylcholine (1 microM). Removal of Na+ from the incubation medium negated this effect. In contrast, the refractoriness to acetylcholine or cysteamine following long-term (45 min) incubation with cysteamine (20 mM) was accentuated in low Na+ medium. It is suggested that cysteamine induces a contraction of both the circular and longitudinal muscle of the guinea-pig ileum by stimulating the release of acetylcholine from intrinsic neurones, by an action at the level of the smooth muscle muscarinic receptor, and possibly by a non-cholinergic mechanism. However, the mechanisms by which acetylcholine and cysteamine induce tissue refractoriness probably differ.     

Action of bretylium on the isolated guinea-pig ileum

Bretylium inhibits the contractions of the longitudinal muscle caused by histamine and, to a lesser extent, those by acetylcholine, carbachol and nicotine. It is a very weak antagonist of 5-hydroxytryptamine and in concentrations of up to 190 μg/ml. does not inhibit the actions of bradykinin and substance P on the longitudinal muscle. The inhibitory effect of bretylium on the emptying phase of the peristaltic reflex is similar to that of ganglion blocking agents, while the much less pronounced effect on the contraction of the longitudinal muscle during the preparatory phase is assumed to be due to a weak atropine-like action.     

Capsaicin-induced, capsazepine-insensitive relaxation of the guinea-pig ileum

The mechanisms underlying transient receptor potential vanilloid receptor type 1 (TRPV1)-independent relaxation elicited by capsaicin were studied by measuring isometric force and phosphorylation of 20-kDa regulatory light chain subunit of myosin (MLC(20)) in ileum longitudinal smooth muscles of guinea-pigs. In acetylcholine-stimulated tissues, capsaicin (1-100 microM) and resiniferatoxin (10 nM-1 microM) produced a concentration-dependent relaxation. The relaxant response was attenuated by 4-aminopyridine and high-KCl solution, but not by capsazepine, tetraethylammonium, Ba(2+), glibenclamide, charybdotoxin plus apamin nor antagonists of cannabinoid receptor type 1 and calcitonin-gene related peptide. A RhoA kinase inhibitor reduced the relaxant effect of capsaicin at 30 microM. Capsaicin and resiniferatoxin reduced acetylcholine- and caffeine-induced transient contractions in a Ca(2+)-free, EGTA solution. Capsaicin at 30 microM for 20 min did not alter basal levels of MLC(20) phosphorylation, but abolished an increase by acetylcholine in MLC(20) phosphorylation. It is suggested that the relaxant effect of capsaicin at concentrations used is not mediated by TRPV1, but by 4-aminopyridine-sensitive K(+) channels, and that capsaicin inhibits contractile mechanisms involving Ca(2+) release from intracellular storage sites. The relaxation could be explained by a decrease in phosphorylation of MLC(20).     

Dual action of nitroparaffins on the guinea-pig ileum

Nitroparaffins act on the guinea-pig ileum by several mechanisms. They produce contraction partly by ganglionic stimulation and partly by direct liberation of transmitter from the nerve endings. They also inhibit the response to acetylcholine and nicotine, but interfere less with the action of histamine, serotonin or bradykinin. The two opposite effects of the nitro compounds are independent of each other. Within the homologous series of normal nitroparaffins, the excitatory action diminishes and the inhibitory effect increases with the lengthening of the carbon chain.     

Spasmolytic action of adenosine on the guinea-pig ileum

Adenosine (10^?5M) reduces the contractile response of the guinea-pig isolated ileum to 5-hydroxytryptamine or barium more than the responses to acetylcholine or histamine. Adenosine also inhibits the contractile response of the guinea-pig ileum to a calcium-free medium. These results suggest that adenosine effects a blockade of the indirect responses to barium or 5-hydroxytryptamine mediated through intrinsic nerves and has a weak direct inhibitory effect on the smooth muscle cells of this tissue.     

Antihistaminic activity of pulegone on the guinea-pig ileum

Pulegone, a natural monoterpene compound, has an antihistamine effect on guinea-pig ileum. Its antagonism is of the competitive type (PA2 = 6.35) like that of mepyramine and dexchlorpheniramine, two H1-antagonists, with PA2 values of 10.15 and 8.74, respectively.