A pharmacological analysis of the responses of the gastrointestinal smooth muscle of the bat to transmural and periarterial nerve stimulation.

A comparative study of the responses of the gastrointestinal tract of the guinea-pig and of the fruit-eating bat Eidolon helvum to transmural nerve stimulation (TNS) was made. The stomach and rectum of the guinea-pig, the bat and the guinea-pig ileum contracted in response to TNS. These contractions were cholinergic in nature because atropine blocked and physostigmine potentiated them. Tetrodotoxin reversibly abolished these contractions suggesting that they were nerve-mediated. The bat isolated ileum usually responded to TNS with mixed motor and inhibitory components. In some cases, there were only motor or inhibitory components. The motor component was abolished by atropine and potentiated by physostigmine. However, the inhibitory component was non-adrenergic and non-cholinergic (NANC). Tetrodotoxin abolished the motor component without influencing the inhibitory components. Periarterial nerve stimulation of the bat ileum produced a relaxation that was blocked by bretylium, propranolol, phentolamine, reserpine and tetrodotoxin. It is concluded that the bat gastrointestinal smooth muscle, like the guinea-pig, has cholinergic excitatory innervation; however, the bat ileum has both a cholinergic excitatory innervation and a nonadrenergic and non-cholinergic inhibitory component.     

Role of extra- and intracellular calcium in the contractile action of agonists in the guinea-pig ileum

Summary The effects of Ca²⁺-channel blockers (nifedipine and verapamil), EGTA, caffeine or the removal of external Ca²⁺ on the contractile action of different agonists and transmural electrical stimulation were examined in isolated segments of the proximal and terminal part of the guinea-pig ileum. The effects of agonists and nerve stimulation on membrane potential were also studied by means of the sucrose gap method. Acetylcholine-elicited contractions in both parts and noradrenaline-as well as histamine-induced contractions in the terminal part of the ileum were composed of an initial phasic and a sustained tonic component. Single pulse transmural nerve stimulation elicited smooth muscle twitches, whereas addition of CaCl₂ to the tissue bath containing Ca²⁺-free and high-K⁺ medium elicited a sustained contraction. Both verapamil and nifedipine were more potent in inhibiting the tonic phase of the responses to the agonists or CaCl₂ than inhibiting the phasic contractions elicited by transmural nerve stimulation, acetylcholine or noradrenaline. The excitatory junction potentials (e j.p.s.) as well as smooth muscle twitches were reduced only by high nifedipine concentrations. The effects of acetylcholine on membrane potential and input membrane resistance were affected minimally by the omission of extracellular Ca²⁺, while the contractions gradually disappeared on repetitive agonist application in the absence of external Ca²⁺ and were blocked by caffeine preexposure. In Ca²⁺-free solution noradrenaline and histamine partially reduced each other's motor effect, while neither of them changed the contractile action of acetylcholine, yet the contraction induced by noradrenaline was prevented and that of histamine significantly reduced by preexposure to acetylcholine. These results suggest that the potency of acetylcholine to release Ca²⁺ from its caffeine-sensitive intracellular stores is much higher than that of histamine and noradrenaline. Send offprint requests to V. Bauer at the above address     

GABAA and GABAB receptor-mediated effects on the spontaneous activity of the longitudinal layer in cat terminal ileum.

1. GABA and GABAergic agonists-muscimol and (+/-)baclofen changed the spontaneous mechanical activity in isolated cat terminal ileum. 2. GABA at doses ranging from 5 microM to 2 mM produced concentration-dependent biphasic responses consisting of a transient relaxation followed by contractions with a tonic and a phasic components. 3. The GABA-induced relaxation was sensitive to bicuculline and picrotoxinin and was mimicked by muscimol, while the GABA-induced contractions were insensitive to bicuculline and picrotoxinin and were mimicked by (+/-)baclofen. Specific cross desensitization occurred between GABA and muscimol or GABA and (+/-)baclofen. 4. The bicuculline-sensitive relaxation induced by GABA and muscimol was abolished by atropine or tetrodotoxin (TTX), while the bicuculline-insensitive contractions induced by GABA and (+/-)baclofen were not antagonized by atropine or TTX, though they were slightly suppressed. 5. The GABA effects in the longitudinal layer of cat terminal ileum were mediated by the following receptors: -GABAA prejunctional receptors whose activation causes relaxation, probably through an inhibitory action on cholinergic neurons; -GABAB prejunctional receptors whose activation cause contractions; -GABAB postjunctional receptors located on the smooth muscle membrane whose activation induces tonic and phasic contractions.     

Secondary excitation of intestinal smooth muscle

1. A period of stimulation of intrinsic or extrinsic nerves to intestinal muscle is often followed by a secondary contraction. In the present work, the basis for such secondary contractions in the longitudinal muscle of the large bowel of guinea-pigs and rabbits was examined.2. In general, the action of cholinergic nerves did not contribute significantly to the secondary contractions. Concentrations of atropine or hyoscine, which completely blocked primary cholinergic contractions, potentiated or did not significantly reduce secondary contractions.3. Atropine resistant, nerve mediated primary contractions of the guinea-pig ileum were inhibited by anticholinesterases, although these drugs potentiated secondary contractions in other segments of the gut.4. The occurrence of a secondary contraction following inhibition of smooth muscle activity did not depend on the nature of the initial inhibition. Thus, secondary contractions were observed following the responses to both nonadrenergic and adrenergic inhibitory nerves, adenosine triphosphate, noradrenaline and brief periods of anoxia. The secondary contractions following hyperpolarizing drugs or anoxia were not prevented by tetrodotoxin in a concentration sufficient to paralyse all nerves.5. It is concluded that cholinergic nerves do not contribute significantly to secondary contractions except at frequencies of stimulation higher than about 50 Hz or after the inhibition of cholinesterases. In the segments of gut examined, the secondary contractions were principally myogenic.     

The role of prostacyclin in modulating cholinergic neurotransmission in guinea-pig ileum.

The mechanism of action of prostacyclin (PGI2) on isolated segments of guinea-pig terminal ileum was studied by recording the changes in isometric tension. In these preparations PGI2 (1 nM-1 microM) caused a concentration-dependent increase in muscle tension. This effect was rapid and short-lasting. PGI2-induced contractions were inhibited by atropine and potentiated by physostigmine. Hemicholinium-3 reduced the response to PGI2 and the inhibition was quantitatively comparable at any PGI2 concentration tested. Tetrodotoxin as well as low temperature (20 degrees C) abolished and beta-bungarotoxin reduced the effect of PGI2. Hexamethonium decreased the response to submaximal, but not to maximal PGI2 concentrations. PGI2 potentiated the twitch response of the ileum to electrical stimulation. In the presence of tetrodotoxin, PGI2 did not alter the effect of a sub-maximal concentration of acetylcholine (ACh). The present results give indirect evidence for the ability of PGI2 to facilitate ACh release from intramural nerves possibly by increasing the excitability of cholinergic cell bodies.     

Effect of selective phosphodiesterase inhibitors on synaptic transmission in the guinea-pig ileum

The effect of selective phosphodiesterase (PDE) inhibitors was studied on neural transmission within the enteric nervous system employing a two-compartment bath (containing the oral and the anal end of a segment of guinea-pig ileum, respectively). Ascending excitatory enteric nerve pathways were activated by electrical field stimulation (10 Hz for 2 s, 45 mA, 0.5 pulse duration) in the anal compartment and the resulting contraction of the intestinal circular muscle in the oral compartment was recorded. The partitioned bath enables PDE inhibitors and other drugs to be applied to enteric nerve pathways (in the anal compartment) without interfering with the recording of the smooth muscle contraction in the oral compartment. The PDE 4 inhibitors rolipram (0.01–10 μM) and Ro-20-1724 (0.01–10 μM) significantly (P<0.01) inhibited (10–91% and 9–83%, respectively) the nerve-mediated contractions. When both rolipram and Ro-20-1724 were tested after phentolamine (1 μM) or yohimbine (0.1 μM), they were significantly (P<0.01) less effective. By contrast prazosin (1 μM) was ineffective. Vinpocetine (50 μM), milrinone (30 μM) and zaprinast (100 μM), which inhibit PDE 1, 3 and 5, respectively, did not modify the nerve-mediated contractions. 8-Bromoadenosine 3’,5’-cyclic monophosphate (8-Br-cyclic AMP) or N⁶,2’-O-dibutyryladenosine 3’,5’ cyclic monophosphate (dibutyryl cyclic AMP), two analogues of cyclic AMP, at lower concentrations (0.1–1 μM) significantly (P<0.01) inhibited (15–73% and 5–49%, respectively) the nerve-mediated contractions, while at higher concentrations (10–100 μM) they caused a significant (P<0.01) potentiating (48–68% and 77–78%, respectively) effect. These results indicate that inhibition of PDE 4 (but not PDE 1, PDE 3 or PDE 5) produces a depression of neural transmission within the enteric nervous system, possibly by releasing noradrenaline acting at α₂-adrenoceptors on enteric neurons. Received: 21 November 1997 / Accepted: 11 March 1998     

The effect of ergot alkaloids ergosinine, dihydroergosine and dihydroergotamine on neurotransmission and contractility of the isolated ileum of the guinea-pig

The effects of ergosinine (ESNN), dihydroergosine (DHESN) and dihydroergotamine (DHE) on contractions of the isolated terminal and middle segments of the guinea-pig ileum were studied in-vitro. Responses to cholinergic (3 Hz) and adrenergic stimulation (30 Hz in the presence of atropine) were inhibited, albeit at high concentrations of all three alkaloids (1-30 micrograms ml-1). Cholinergic neurotransmission was surprisingly more affected than adrenergic transmission. Noradrenaline (NA) contractions, however, were inhibited at very low concentrations (1-30 ng ml-1) with the following order of potency: DHESN = DHE greater than ESNN. Prazosin was equally as potent as DHESN in inhibiting NA contractions and similarly potent in inhibiting responses to adrenergic stimulation. ESNN, DHESN and DHE when used at concentrations from 1-30 micrograms ml-1 were also found to inhibit 5-hydroxytryptamine greater than histamine greater than acetylcholine greater than KCl contractions. The results suggest that the principal pharmacological action of ESNN, DHESN and DHE on the guinea-pig isolated ileum is the antagonism to NA on the postsynaptic and extrajunctional population of alpha-adrenoceptors. The neurotransmission, adrenergic as well as cholinergic, appeared to be inhibited via a non-specific presynaptic mechanism presumably regulating the transmitter release. Anti-5-hydroxytryptamine, anti-acetylcholine and antihistamine actions were obtained at similar and relatively high concentrations, thus pointing to a non-specific depressant action upon a common mechanism regulating the contractility of the smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Suppression by NG-nitro-L-arginine of relaxations induced by non-adrenergic, non-cholinergic nerve stimulation in dog duodenal longitudinal muscle.

In dog duodenal longitudinal muscle strips, transmural electrical stimulation (10 Hz, 15 sec) elicited a transient contraction, which was abolished by tetrodotoxin and atropine but potentiated by treatment with NG-nitro-L-arginine (L-NA), a nitric oxide (NO) synthesis inhibitor. The potentiation was reversed by L-arginine but not by its D-enantiomer. Acetylcholine-induced contractions were not influenced by L-NA. After treatment with atropine, the electrical neural stimulation relaxed the muscle strips partially contracted with bradykinin, the relaxation being abolished by tetrodotoxin and suppressed by L-, but not D-, NA. L-arginine reversed the L-NA-induced inhibition. Oxyhemoglobin abolished the relaxation caused by nerve stimulation and NO. The neurally-induced relaxation was not attenuated by adrenoceptor antagonists and indomethacin. It is concluded that electrical stimulation of non-adrenergic, non-cholinergic nerves relaxes dog duodenal smooth muscle, due possibly to NO produced upon neural excitation, and potentiation by L-NA of the contractile response to cholinergic nerve stimulation, would be derived from elimination of the neurally-induced relaxation.     

Effects of histaminergic drugs on the contractile activity of smooth muscles from imipramine-treated rats

The effects of histaminergic drugs and their interaction with adrenergic neurotransmission in smooth muscles from imipramine-treated animals were studied. The experiments were performed on isolated preparations (vasa deferentia and segments from terminal ileum and proximal colon) from male Wistar rats which were given imipramine, 10 mg/kg orally, twice daily for 2 weeks. The effects of histamine (HIST) and diphenhydramine (DPH) on the tone and phasic contractions of ileum and colon were investigated. The action of the histaminergic drugs on the vas deferens contractions evoked by low-frequency electrical stimulation (LFES) was also studied. In imipramine-treated animals LFES elicited much stronger contractions than in the controls. HIST modulatory action on the LFES-induced contractions was not markedly changed. The potentiation of the vas deferens contractions by DPH was strongly decreased. Imipramine reduced the potentiating action of both histaminergic agents on the ileum phasic contractions as well as the enhanced effects of HIST applied 10 min after DPH. Imipramine had opposite effects on the potentiating action of HIST and DPH on the colon phasic contractions: the HIST effects were slightly reduced and those of DPH were enhanced. After H1-blockade HIST exerted a stronger potentiation of the colon spontaneous contractions as compared to the controls. Present data show that imipramine could change the modifying action of histaminergic agents on adrenergic neurotransmission and also their interaction with histaminergic receptors in smooth muscles.     

The effect of omega conotoxin GVIA,a peptide modulator of the N-type voltage sensitive calcium channels,on motor responses produced by activation of efferent and sensory nerves in mammalian smooth muscle

1. The effect of omega-conotoxin (CTX) GVIA, a peptide which blocks neuronal calcium channels, were investigated on nerve-mediated motor responses in a variety of isolated smooth muscle preparations from rats and guinea-pigs. 2. In the rat or guinea-pig isolated vas deferens CTX (1 nM-1 microM) produced a concentration and time-related inhibition of the response to field stimulation, while the responses to KCl, noradrenaline or adenosine triphosphate were unaffected. In the presence of CTX a series of tetrodotoxin-resistant contractions could be elicited by field stimulation by increasing pulse width and/or voltage. 3. In the rat or guinea-pig isolated urinary bladder, CTX produced a concentration and time-dependent inhibition of twitch responses to field stimulation without affecting the response to exogenous acetylcholine. In the rat bladder the maximal effect did not exceed 25% inhibition while a much larger fraction of the response (about 70%) was inhibited in the guinea-pig bladder. The CTX-resistant response was abolished, in both tissues, by tetrodotoxin. 4. The effects of CTX in the rat bladder were also studied with a whole range of frequencies of field stimulation (0.1-50 Hz). Maximal inhibition was observed toward contractions elicited at frequencies of 2-5 Hz. At low frequencies the inhibitory effects of CTX and atropine were almost additive while at high frequencies of stimulation a large component of the atropine-sensitive response was CTX-resistant. 5. In the rat isolated proximal duodenum, field stimulation in the presence of atropine and guanethidine produced a primary relaxation followed by a rebound contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of nerve-mediated contractions in isolated guinea-pig ileum by 1-methylisoguanosine, a novel purine from a sponge

1-Methylisoguanosine, a novel purine isolated from the sponge Tedania digitata (Schmidt) selectively inhibited contractions produced by nerve stimulation in the guinea-pig ileum but was without effect on contractions produced by acetylcholine or histamine. The ED50 for inhibition of nicotine responses or responses to submaximal transmural stimulation was 1.1 mumoles/l. The inhibition of nerve-mediated contractions appeared to be due to inhibition of transmitter release from nerve endings in the ileum, as has been suggested for the action of adenosine. Theophylline antagonized the action of 1-methylisoguanosine and overall the results suggest that 1-methyl-isoguanosine acts at an adenosine receptor in the guinea-pig ileum, but is approximately ten times more potent than adenosine itself. A series of related purines which were resistant to the action of adenosine deaminase were also tested for their effect on the nerve-mediated contractions of guinea-pig ileum and the results compared with the in vivo effect on muscle relaxation in mice. All active purines tested produced results qualitatively similar to those of 1-methylisoguanosine itself.     

Evidence that sensory neurons participate in the non-cholinergic, non-adrenergic contractile response of the guinea-pig ileum

The possible role of non-cholinergic, non-adrenergic (NCNA) nerves in responses of the guinea-pig terminal ileum to transmural nerve stimulation (TNS) and that of sensory nerves in NCNA responses were investigated. The action of acetylcholine was almost abolished in the presence of histamine, whereas the contractions elicited by TNS were changed to frequency-dependent contraction followed by a secondary relaxation. Guanethidine did not alter the contractions or secondary relaxations. Atropine abolished the action of acetylcholine and transiently suppressed the responses to low (up to 2 Hz) and attenuated (by about 50%) those to high (4 to 20 Hz) frequency stimulation. The remaining complex NCNA response was the sum of the excitatory and inhibitory responses. During desensitization to capsaicin, and in its presence, the NCNA contractions were reduced, whereas the relaxations were not significantly enlarged. The present results suggest that besides the cholinergic innervation, the excitatory and inhibitory NCNA innervation also participates in the responses of the guinea-pig ileum to TNS even without suppression of cholinergic and adrenergic transmission, and that the sensory nerves are, at least to some extent, involved in the NCNA excitatory response.     

Mechanism of rhythmic contractions induced by uranyl ion in the ileal longitudinal muscle of guinea-pig

The uranyl ion (UO2(2+)) produces rhythmic contractions of the longitudinal muscle of the ileum, similar to those induced by repetitive transmural stimulation. Hexamethonium inhibited the action of UO2(2+), indicating a preganglionic site of action of UO2(2+) and interneurons possibly being involved in the ACh-releasing effect of UO2(2+). In addition, the action of UO2(2+) was enhanced by physostigmine but antagonized by atropine, ATP, adrenaline and morphine suggesting multiple sites of action of UO2(2+). The effects of Ba2+ were studied simultaneously in order to compare them with those of UO2(2+). Atropine and hexamethonium had no effect on the rhythmic contractions with Ba2+ but physostigmine enhanced contractions induced by low concentration of Ba2+ indicating that Ba2+ induces contractions mainly by acting on the smooth muscle and releases ACh from the nerve to a lesser extent.     

Transmission to the longitudinal muscle of the guinea-pig vas deferens: The effect of pretreatment with guanethidine

1 Tissue was taken from guinea-pigs that had been injected with guanethidine (100 mg/kg, i.p.) 24 h before they were killed, and from paired control animals.2 Pretreatment with guanethidine caused a significant, substantial, and sometimes complete reduction of the nerve-mediated contractions of the vas deferens. There were no significant changes in the responses of the ileum to stimulation of cholinergic nerves or of the distal colon to stimulation of intrinsic (non-adrenergic) inhibitory nerves. Responses of the vas deferens and ileum to acetylcholine were unchanged, but contractions of the vas deferens elicited by exogenous noradrenaline were enhanced.3 The nerve-mediated contractions of the vas deferens were restored by exposing it to (+)-amphetamine followed by noradrenaline in vitro.4 It is concluded that noradrenaline is the transmitter released from motor nerves to the longitudinal muscle of the guinea-pig vas deferens. Possible explanations for the ineffectiveness of receptor blocking agents in antagonizing transmission are discussed.     

Receptors for substance P. I. The pharmacological preparations

The undecapeptide substance P (SP) is both a potent stimulant of intestinal (guinea pig ileum), vascular venous (rabbit mesenteric vein) and tracheal (guinea pig trachea) smooth muscles, and a relaxant of large arteries (dog common carotid artery, rabbit aorta). SP also decreases the peripheral vascular resistance in the coronary vessels of rabbit isolated hearts. The contractions of the guinea pig ileum in response to SP are partially antagonised by atropine and significantly depressed by indomethacin, while the contractions of the guinea pig trachea are strongly potentiated by indomethacin. Large arteries, but not the veins, are insensitive to SP when the endothelium is removed, and slightly less responsive to the peptide when this is applied in the presence of mepacrine, eicosatetraynoid acid or BW 755C. These results indicate that all myotropic actions of SP, except that on venous smooth muscle, are partially or completely indirect and most of them are due to the release of endogenous agents: these are acetylcholine, prostaglandins and possibly histamine in the guinea pig ileum, prostaglandins and possibly a leukotriene in the guinea pig trachea and a still unidentified factor of endothelial origin in dog and rabbit large arteries. The effect of substance P on the rabbit isolated anterior mesenteric vein appears to be a direct one.     

Anticholinergic properties of progesterone in the isolated ileum of the guinea-pig

To determine if the cholinergic system is involved in the intestinal smooth muscle relaxant action of progesterone, segments of guinea-pig ileum were mounted in 20 ml chambers containing Krebs solution. In one group of experiments, we studied the effect of various doses of progesterone and atropine on the twitch-like contraction of the ileum caused by electrical stimulation. In the second group of experiments, dose-response curves were constructed for acetylcholine, histamine, and barium chloride alone and in the presence of progesterone or atropine. Progesterone, like atropine, decreased in a dose-dependent manner the phasic contractions and tone of the unstimulated intestinal segments, and exerted a rapid, reversible, and dose-dependent inhibitory action on intestinal responses to electrical stimulation. In addition, in the presence of the hormone or atropine, the dose-response curve to acetylcholine was clearly shifted to the right in a parallel manner with no change in maximal response. At the concentrations tested, neither progesterone nor atropine blocked the contractile effect of barium chloride on the ileum and only slightly reduced the spasmogenic action of histamine. These results suggest that the relaxant action of progesterone on the intestinal smooth muscle may be mediated through muscarinic receptors, indicating that progesterone is an endogenous product with anticholinergic activity. © 1996 Wiley-Liss, Inc.     

Activation of delta-type opioid receptors modulates the responses of cat terminal ileum to field electrical stimulation

1. The effects of (D-Ala2, D-Leu5) enkephalin amide (DADLE) on the responses of the cat terminal ileum to field electrical stimulation (pulse duration of 0.5 msec, train duration of 10 sec, 30 V) were evaluated by the changes in the contractile or the relaxatory responses of longitudinal and circular strips to electrical stimuli with a frequency of 2, 10 or 30 Hz. 2. Stimulation with a frequency of 2, 10 or 30 Hz elicited contractile responses from the longitudinal strips while in the circular strips 2 Hz stimulation induced contractions and 10 or 30 Hz stimulation caused relaxation. Tetrodotoxin (TTX) (0.1 mumol/l) abolished the electrically-induced responses in both longitudinal and circular strips. 3. DADLE (1 nmol/l) significantly inhibited the cholinergic contractile responses of the longitudinal strips to 2, 10 or 30 Hz stimulation and the contractile responses of the circular strips to 2 Hz stimulation. The relaxatory responses of the circular strips to 10 or 30 Hz stimulation were insignificantly increased by DADLE. 4. On the background of guanetidine (10 mumol/l) and atropine (3 mumol/l) DADLE significantly decreased the nonadrenergic, noncholinergic relaxatory responses of the circular strips to 2, 10 or 30 Hz stimulation. 5. DADLE did not change the maximum effects and the EC50 values of acetylcholine and noradrenaline in both longitudinal and circular strips. 6. It is suggested that in the cat terminal ileum activation of delta-type opioid receptors modulates the mechanical activity suppressing the cholinergic responses in the longitudinal and circular layers as well as the adrenergic and nonadrenergic, noncholinergic responses in the circular layer.     

Pharmacological studies on Zingiber mioga (ZM) (1). General pharmacological effects of water extracts (author's transl)

In the central nervous system, ZM decreased locomotor activity and potentiated hypnosis of hexobarbital-Na in mice. ZM had little hypothermic action and there were no anticonvulsive effects on chemoconvulsion and electroconvulsion shock. ZM, 3 mg/kg i.v. produced a sleep-like pattern in the spontaneous EEG activity of cat; from 20 to 30 min. after injection, spindle burst-like waves (12-13 Hz) appeared in the cortex and subcortex. These EEG activities were antagonized by atropine sulfate. In the respiratory and cardiovascular system, ZM, 1 mg/kg or over produced a fall in blood pressure and stimulated respiration in dogs. This hypotensive action was antagonized by atropine sulfate and diphenhydramine hydrochloride, and tachyphylaxis was observed in blood pressure. This compound inhibited cardiomotility in isolated toad heart, had little effect on peripheral blood flow, and produced contractions of isolated guinea pig ileum which were inhibited by atropine by sulfate. Regarding inflammatory response, ZM showed inhibitory effects on the acute edema induced serotonin and dextran. These results indicate that water extracts of ZM have cholinergic actions and in peripheral tissues, histaminergic-like actions.     

The mode of action of bisacodyl on the smooth muscle of the small and the large intestine of the guinea pig]

4,4'-Diacetoxy-diphenyl-(pyridyl-2)-methan (Bisacodyl, Dulcolax, 12--240 mug/ml) was shown to initiate dose-dependently contracile responses in the guinea pig isolated terminal ileum and taenia coli (a preparation of pure longitudinal muscle fibers). In contrast to muscle contractions induced by 1 mug/ml acetylcholine (ACh) and 1 mug/ml histamine, respectively bis-acodyl-induced contractile responses were antagonized neither by the anticholinergic agent atropine (6 mug/ml) nor by the antihistaminic compound pheniramine (6 mug/ml). On the other hand, bisacodyl inhibited contractile responses induced by ACh or histamine in a dose-dependent manner. The possibility that bisacodyl-induced contractions were due to a fall in cyclic 3,5-AMP level was excluded by estimation of endogenous cyclo-AMP. Since exogenous cyclic 3,5-AMP (which should possess calcium-antagonistic properties in smooth muscle) as well as verapamil (a calcium inhibitor) inhibited bisacodyl-induced contractions, a site of action on the calcium-dependent contractile system of the smooth muscle cell was discussed. Furthermore, bisacodyl should diminish the sensibility of the contractile system to other contractile compounds (ACh, histamine).