Tachykinin NK1 and NK2 receptor antagonists and atropine-resistant ascending excitatory reflex to the circular muscle of the guinea-pig ileum

Tachykinin NK2 antagonist for treatments of various disease states

Tachykinin NK₂ receptors are distributed in periphery, in the smooth muscle of the respiratory, gastrointestinal, genitourinary tract, and within the brain. Substance P (SP), neurokinin A (NKA), and neurokinin B (NKB) are endogenous ligands for NK₂ receptors and are active in the peripheral and central nervous systems. NK₂ antagonists have the potential to reduce airway motor responses and prevent hyperactivity by inhibiting NKA-induced bronchoconstriction in asthmatic patients. Due to its abundance, peripherally and centrally, tachykinin NK₂ receptor antagonists have high potential in treating various disease states ranging from asthma to irritable bowel syndrome, to detrusor hyperactivity, to anxiety. This review is an evaluation of NK₂ receptor antagonists as possible therapeutics for a myriad of pharmacological treatments.

Mechanism of ghrelin-induced gastric contractions in Suncus murinus (house musk shrew): involvement of intrinsic primary afferent neurons

Here, we have reported that motilin can induce contractions in a dose-dependent manner in isolated Suncus murinus (house musk shrew) stomach. We have also shown that after pretreatment with a low dose of motilin (10(-10) M), ghrelin also induces gastric contractions at levels of 10(-10) M to 10(-7) M. However, the neural mechanism of ghrelin action in the stomach has not been fully revealed. In the present study, we studied the mechanism of ghrelin-induced contraction in vitro using a pharmacological method. The responses to ghrelin in the stomach were almost completely abolished by hexamethonium and were significantly suppressed by the administration of phentolamine, prazosin, ondansetron, and naloxone. Additionally, N-nitro-l-arginine methylester significantly potentiated the contractions. Importantly, the mucosa is essential for ghrelin-induced, but not motilin-induced, gastric contractions. To evaluate the involvement of intrinsic primary afferent neurons (IPANs), which are multiaxonal neurons that pass signals from the mucosa to the myenteric plexus, we examined the effect of the IPAN-related pathway on ghrelin-induced contractions and found that pretreatment with adenosine and tachykinergic receptor 3 antagonists (SR142801) significantly eliminated the contractions and GR113808 (5-hydroxytryptamine receptor 4 antagonist) almost completely eliminated it. The results indicate that ghrelin stimulates and modulates suncus gastric contractions through cholinergic, adrenergic, serotonergic, opioidergic neurons and nitric oxide synthases in the myenteric plexus. The mucosa is also important for ghrelin-induced gastric contractions, and IPANs may be the important interneurons that pass the signal from the mucosa to the myenteric plexus.

Myenteric neural network activated by motilin in the stomach of Suncus murinus (house musk shrew)

BACKGROUND

It has been shown in human and canine studies that motilin, a gastroprokinetic hormone, induces gastric phase III contractions via the enteric nervous; however, the center of motilin action in the stomach has not been clearly revealed. In the present study, we investigated the neural pathway of motilin-induced gastric contraction by using Suncus murinus, a new animal model for motilin study.

METHODS

An isolated suncus stomach was used in vitro to determine the mechanism of motilin action through the myenteric plexus. Synthetic suncus motilin (10(-11) -10(-7) molL(-1) ) was added to an organ bath, and the spontaneous contraction response was expressed as a percent of ACh (10(-5) molL(-1) ) responses. Motilin-induced contractions were also studied by a pharmacological method using several receptor antagonists and enzyme inhibitor.

KEY RESULTS

Suncus motilin induced a concentration-dependent gastric contraction at concentrations from 10(-9) to 10(-7) molL(-1) . The responses to suncus motilin in the stomach were completely abolished by atropine and tetrodotoxin treatment and significantly suppressed by administration of hexamethonium, verapamil, phentolamine, yohimbine, ondansetron, and naloxone, whereas ritanserin, prazosin, timolol, and FK888 did not affect the action of motilin. Additionally, N-nitro l-arginine methylester slightly potentiated the contractions induced by motilin.

CONCLUSIONS & INFERENCES

The results indicate that motilin directly stimulates and modulates suncus gastric contraction through cholinergic, adrenergic, serotonergic, opioidergic, and NO neurons in the myenteric plexus.

Role for NK(1) and NK(2) receptors in the motor activity in mouse colon

The present study examined the effects induced by endogenous and exogenous activation of NK(1) and NK(2) receptors on the mechanical activity of mouse proximal colon. Experiments were performed in vitro recording the changes in intraluminal pressure from isolated colonic segments. Electrical field stimulation in the presence of atropine and guanethidine produced a small relaxation, followed by nonadrenergic noncholinergic (NANC) contraction. SR140333, NK(1) receptor antagonist, or SR48968, NK(2) receptor antagonist, significantly reduced the contraction, although SR48968 appeared more efficacious. The co-administration of SR140333 and SR48968 virtually abolished the NANC contraction. [Sar(9), Met(O(2))(11)]-substance P, selective NK(1) receptor agonist, induced a concentration-dependent biphasic effect, contraction followed by reduction of the mechanical spontaneous activity. Both effects were antagonized by SR140333, but not by SR48968. [beta-Ala(8)]-neurokinin A (4-10), selective NK(2) receptor agonist, evoked concentration-dependent contraction, which was antagonized by SR48968, but not by SR140333. The contraction induced by [Sar(9), Met(O(2))(11)]-substance P, but not by [beta-Ala(8)]-neurokinin A (4-10), was reduced by tetrodotoxin or atropine, and increased by N(omega)-nitro-L-arginine methyl ester (L-NAME), inhibitor of nitric oxide synthase. The inhibitory effects induced by [Sar(9), Met(O(2))(11)]-substance P were abolished by tetrodotoxin or L-NAME. The results of the present study suggest that in mouse colon both NK(1) and NK(2) receptors are junctionally activated by endogenous tachykinins to cause an additive response. NK(1) receptors appear to be located on cholinergic and on nitrergic neurons as well as on smooth muscle cells, whereas NK(2) receptors seem to be present exclusively on smooth muscle cells.

Ava[L-Pro9,N-MeLeu10] substance P(7-11) (GR 73632) and Sar9, Met(O2)11 increase distention-induced peristalsis through activation of neurokinin-1 receptors on smooth muscle and interstitial cells of cajal

Substance P is generally considered an excitatory neurotransmitter related to gut motor activity, although an inhibitory influence of neurokinin-1 (NK1) receptor activation on peristalsis has also been reported. With an optimized in vitro method to assess distention-induced peristalsis, our aim was to clarify the effect of NK1 receptor activation on peristaltic activity and to reveal the mechanisms by which NK1 activation alters peristalsis. Distention of the small intestine of the mouse and guinea pig induced periodic occurrence of rhythmic waves of propagating rings of circular muscle contraction, associated with slow waves and superimposed action potentials, that propelled intestinal contents aborally. Activation of NK1 receptors by Ava[l-Pro(9),N-MeLeu10] substance P(7-11) (GR 73632) and Sar(9), Met(O(2))(11) on smooth muscle cells resulted in prolongation of the activity periods and increased action potential generation occurring superimposed on the intestinal slow wave activity. Activation of NK1 receptors on interstitial cells of Cajal resulted in an increase in slow wave frequency. Slow wave amplitude increased, likely by increased cell-to-cell coupling. The NK1 antagonist (S)-1-(2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl)piperidin-3-yl]ethyl)-4-phenyl-1-azoniabicyclo[2.2.2]octane chloride (SR 140333) induced a decrease in the slow wave frequency and duration of the activity periods evoked by distention, which makes it likely that NK1 receptor activation plays a role in the normal physiological distention-induced generation of peristaltic motor patterns. In summary, NK1 receptors play a role in normal development of peristalsis and NK1 receptor activation markedly increases propulsive peristaltic contractile activity.

Cutaneous allergic contact dermatitis responses are diminished in mice deficient in neurokinin 1 receptors and augmented by neurokinin 2 receptor blockage

Sensory neuropeptides such as neurokinin A (NKA) or particularly substance P (SP) by neurokinin receptor (NK-R) activation modulate skin and immune cells functions during neurogenic inflammation. In this study, we examined the relative importance of SP/NK-1Rs or NKA/NK-2Rs in a murine model for allergic contact dermatitis (ACD) and tested if the functional absence of NK-Rs will impair inflammatory response in vivo. Mice lacking NK-1Rs (C57BL/6J-NK-1R-/-) displayed a significantly reduced ACD inflammatory ear swelling response to dinitrofluorobenzene (DNFB) with histological less edema and 50% fewer infiltrating leukocytes compared with the ACD response in wild-type (+/+) animals. In NK-1R+/+ mice, transient NK-1R inhibition impaired ACD sensitization. In vitro haptenized bone marrow-derived dendritic cells from NK-1R+/+ mice matured in the presence of an NK-1R antagonist displayed a reduced capability to induce T cell proliferation in vitro and ACD after adoptive transfer into naïve wild-type mice in vivo. By contrast, NK-2R inhibition significantly enhanced the ACD response in NK-1R null or in wild-type mice, whereas epicutaneous application of NK-2R agonists diminished the ACD inflammation. In conclusion, NK-1R and SP are required for antigen sensitization and a full inflammatory response to cutaneous allergens and NKA and the NK-2R mediate a contrasting anti-inflammatory role in ACD. Thus, SP, NKA, NK-1R, and NK-2R have important but differential roles in the regulation of cutaneous inflammatory responses.

Nicotinic transmission at functionally distinct synapses in descending reflex pathways of the rat colon

We examined descending reflex pathways in the rat colon using intracellular recording techniques. Inhibitory junction potentials (IJPs) were recorded from circular smooth muscle when descending pathways were excited by combined mucosal compression and distension. IJPs were reduced to 71% of control when synaptic transmission was blocked in the oral stimulation chamber of a divided organ bath suggesting that two reflex pathways exist, the one involving descending sensory neurones and the other involving descending interneurones. Hexamethonium (200 micromol L(-1)) in the recording chamber abolished reflexly evoked IJPs, while in the stimulation chamber, it was as effective as synaptic blockade. When hexamethonium was added to a chamber lying between the stimulation and recording chambers, it again sharply depressed IJPs to 27% of control; an extent similar to synaptic blockade. A P2 receptor antagonist did not reveal any purinergic neurotransmission. Either granisetron (5-HT3 receptor antagonist, 1 micromol L(-1)) or SB204070 (5-HT4 receptor antagonist, 1 micromol L(-1)) in the stimulation chamber significantly decreased IJPs; these decreases were not additive. We conclude that some sensory neurones and interneurones in rat colon have long anally projecting axons and that acetylcholine, acting via nicotinic receptors, is the primary neurotransmitter from sensory neurones, to inhibitory motor neurones and between interneurones.

Immunohistochemical demonstration of the NK(1) tachykinin receptor on muscle and epithelia in guinea pig intestine

BACKGROUND AND AIMS

Previous immunohistochemical studies failed to reveal neurokinin (NK)(1) tachykinin receptors on intestinal muscle, despite convincing pharmacologic data indicating their presence. This study aimed to apply optimal immunohistochemical methods to reveal the receptors.

METHODS

NK(1)-receptor immunoreactivity was examined by confocal microscopy in tissue incubated with or without 10(-7) mol/L substance P (SP), 10(-7) mol/L SP plus 10(-6) mol/L NK(1) receptor antagonist (CP99994), or with fluorescent cyanine 3.18 (Cy3) SP.

RESULTS

Without incubation, NK(1)-receptor immunoreactivity was strong on muscle of the rectum and distal colon and weak in proximal colon and small intestine. NK(1) receptor was located on the surface of muscle cells in all gut regions. Exposure to SP increased the intensity of immunoreactivity, and the receptor moved into the cytoplasm. Mobilization of the receptor by SP was blocked by the NK(1)-receptor antagonist CP99994. Cy3-SP was internalized by muscle cells and colocalized with the receptor. NK(1)-receptor immunoreactivity occurred on crypt epithelial cells in the small intestine and the base of glands in the proximal colon.

CONCLUSIONS

The NK(1) receptor occurs on the external muscle throughout the small and large intestines. SP binds and triggers NK(1)-receptor aggregation and internalization in the muscle.

Tachykinin-dependent and -independent components of peristalsis in the guinea pig isolated distal colon

BACKGROUND & AIMS

In the intestine, tachykinins regulate motility by participating in neuromuscular and neuro-neuronal transmission. The aim of this study was to test the hypothesis that colonic propulsion is regulated by an interplay between tachykinergic and cholinergic transmission.

METHODS

Propulsion was elicited by intraluminal distention of a thin rubber balloon, which traveled from the oral to the anal end of guinea pig isolated distal colon segments. The overall contribution of endogenous tachykinins to colonic propulsion was examined by blocking NK1, NK2, and NK3 receptors simultaneously.

RESULTS

NK2-receptor blockade by MEN 11420 inhibited propulsion, whereas blockade of NK(1) by SR 140333 or of NK3 receptors by SR 142801 had minor effects on motility. Blockade of muscarinic or nicotinic receptors by hyoscine or hexamethonium decelerated peristalsis up to propulsion arrest. In the presence of partial muscarinic receptor blockade, the NK1-receptor antagonist SR 140333 and the NK2-receptor antagonist MEN 11420 markedly inhibited propulsion. Propulsion was also inhibited by the NK3-receptor antagonist SR 142801 in the presence of partial nicotinic receptor blockade. The simultaneous administration of the 3 tachykinin antagonists inhibited propulsion by 50%.

CONCLUSIONS

This study demonstrates the existence of an interplay between tachykinergic and cholinergic pathways during peristalsis and the importance of endogenous tachykinins acting at multiple receptor sites in the control of colonic propulsion.

Principles of tachykininergic co-transmission in the peripheral and enteric nervous system

The tachykinins substance P (SP) and neurokinin A (NKA) are synthesized and released from nerves in the peripheral and enteric nervous system (PNS and ENS). They act as nonadrenergic noncholinergic (NANC) excitatory transmitters in mammalian airways, and the genitourinary and gastrointestinal tract. At the postjunctional level, both NK(1) (SP-preferring) and NK(2) (NKA-preferring) receptors are often co-expressed by target cells innervated by TKergic nerves. Thus an issue of duplication seems to exists with regard to peripheral tachykininergic co-transmission, the duplication involving both messengers (the peptides) and effectors (the receptors). By using receptor selective antagonists it has been possible to dissect the relative contribution of different receptors to TKergic co-transmission: the available results indicate that multiple arrangements exist involving both summation, cooperation and specialization of different messengers/effectors in producing the overall response.

Tachykinins play a minor role in mediating the third phase of the contractile response to vagal nerve stimulation of the guinea-pig oesophagus

The aim of this study was to determine whether tachykinin receptors might be involved in the mediation of the atropine- and capsaicin-sensitive third phase of a triphasic contractile response to vagal nerve stimulation of the guinea-pig isolated oesophagus. The third phase was inhibited 23.3 +/- 1.7% (P< 0.001, n = 5) and 30. 8 +/- 9.0% (P< 0.05, n = 5) by the NK(3)receptor antagonist, SR 142 801 (0.1 and 1 microM respectively). SR 142 801 (0.1 and 1 microM) had no significant effect on the response to a submaximal concentration of acetylcholine (0.1 mM, n = 4). The third phase was not significantly affected by NK(1)or NK(2)receptor antagonists. Thus, in the guinea-pig oesophagus, it appears that while NK(1)and NK(2)receptors are not involved, NK(3)receptors play a minor role in mediating a contractile response when afferent neurones are excited by vagal nerve stimulation.

Hyperthermia prevents functional, histological and biochemical abnormalities induced during ileitis

Inflammatory bowel disease is associated with altered intestinal motility and epithelial damage. Hyperthermia induces heat shock protein expression, components of a basic cellular defence mechanism, and consequently prevents ischaemic damage. Here we investigate whether hyperthermia may prevent altered smooth muscle function as well as underlying inflammation in a model of inflammatory bowel disease. Ileal heat shock protein expression was induced in rats by hyperthermic shock (41.5 degrees C; 5 min). Two hours after heating or sham treatment, ileitis was evoked by TNBS. Ileal samples were taken 4 h later to determine the contractile response of circular muscle strips, and to measure heat shock protein expression, LTB4 generation and damage/inflammation. Ileitis was associated with an increase in the contractile response of circular muscle to substance P but not neurokinin A or nerve stimulation. Hyperthermia induced heat shock protein expression and also prevented this functional change as well as TNBS-induced LTB4 production, subsequent infiltration of neutrophils and epithelial damage. Thus, intestinal inflammation is associated with alterations in tachykinergic control of smooth muscle as well as inflammatory changes. Hyperthermia prevents these changes and induces heat shock protein expression. Pharmacological induction of these proteins may offer a novel clinical strategy in treating both of these aspects of disease.

Mechanism of tachykinin NK3 receptor-mediated colonic ion transport in the guinea pig

The guinea pig colon was used to elucidate the mechanism of tachykinin-induced secretion. Increased short-circuit current was observed in response to natural and synthetic tachykinins with rank orders of potency of substance P > neurokinin A = neuropeptide K>> neuropeptide gamma; and senktide (tachykinin NK3 receptor agonist)> Sar-substance P (tachykinin NK1 receptor agonist)> betaAlaneurokinin A (tachykinin NK2 receptor agonist)). A functional role of tachykinin NK1 receptors was confirmed as substance P and neurokinin A responsiveness was blocked by the tachykinin NK1 receptor antagonist GR82334. The tachykinin NK3 receptor antagonist SB222200 had no effect, leaving in doubt the identity of the natural tachykinin NK3 receptor ligand in the colon. The response to tachykinin NK3 receptor activation was abolished by tetrodotoxin and predominantly due to atropine sensitive cholinergic activation. The non-cholinergic component resulted from stimulation of tachykinin NK 1 and 5-HT receptors as the response to senktide was blocked by GR82334 and tropisetron. In conclusion, tachykinin NK3 receptor activation stimulates cholinergic and non-cholinergic (tachykinin NK1-receptor and serotonin-mediated) secretory pathways.

Evidence that tachykinins are the main NANC excitatory neurotransmitters in the guinea-pig common bile duct

Application of electrical field stimulation (EFS; trains of 10 Hz, 0.25 ms pulse width, supramaximal voltage for 60 s) to the guinea-pig isolated common bile duct pretreated with atropine (1 microM), produced a slowly-developing contraction ('on' response) followed by a quick phasic 'off' contraction ('off peak' response) and a tonic response ('off late' response), averaging 16+/-2, 73+/-3 and 20+/-4% of the maximal contraction to KCl (80 mM), n=20 each, respectively. Tetrodotoxin (1 microM; 15 min before) abolished the overall response to EFS (n 8). Neither in vitro capsaicin pretreatment (10 microM for 15 min), nor guanethidine (3 microM, 60 min before) affected the excitatory response to EFS (n 5 each), showing that neither primary sensory neurons, nor sympathetic nerves were involved. Nomega-nitro-L-arginine (L-NOARG, 100 microM, 60 min before) or naloxone (10 microM, 30 min before) significantly enhanced the 'on' response (294+/-56 and 205+/-25% increase, respectively; n=6-8, P<0.01) to EFS. The combined administration of L-NOARG and naloxone produced additive enhancing effects (655+/-90% increase of the 'on' component, n = 6, P<0.05). The tachykinin NK2 receptor-selective antagonist MEN 11420 (1 microM) almost abolished both the 'on' and 'off late' responses (P<0.01: n=5 each) to EFS, and reduced the 'off-peak' contraction by 55+/-8% (n=5, P<0.01). The subsequent administration of the tachykinin NK1 receptor-selective antagonist GR 82334 (1 microM) and of the tachykinin NK3 receptor-selective antagonist SR 142801 (30 nM), in the presence of MEN 11420 (1 microM), did not produce any further inhibition of the response to EFS (P>0.05; n=5 each). At 3 microM, GR 82334 significantly reduced (by 68+/-9%, P<0.05, n=6) the 'on' response to EFS. The contractile 'off peak' response to EFS observed in the presence of both MEN 11420 and GR 82334 (3 microM each) was abolished (P<0.01; n=6) by the administration of the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM). PPADS (30 microM) selectively blocked (75+/-9 and 50+/-7% inhibition, n = 4 each) the contractile responses produced by 100 and 300 microM ATP. Tachykinin-containing nerve fibres were detected by using immunohistochemical techniques in all parts of the bile duct, being distributed to the muscle layer and lamina propria of mucosa. In the terminal part of the duct (ampulla) some labelled ganglion cells were observed. In conclusion, this study shows that in the guinea-pig terminal biliary tract tachykinins, released from intrinsic neuronal elements, are the main NANC excitatory neurotransmitters, which act by stimulating tachykinin NK2 (and possibly NK1) receptors. ATP is also involved as excitatory neurotransmitter. Nitric oxide and opioids act as inhibitory mediators/modulators in this preparation.

Roles of neuronal NK1 and NK3 receptors in synaptic transmission during motility reflexes in the guinea-pig ileum

1. The role of NK1 and NK3 receptors in synaptic transmission between myenteric neurons during motility reflexes in the guinea-pig ileum was investigated by recording intracellularly the reflex responses of the circular muscle to distension or compression of the mucosal villi. Experiments were performed in a three-chambered organ bath that enabled drugs to be selectively applied to different sites along the reflex pathways. 2. When applied in the recording chamber, an NK1 receptor antagonist, SR140333 (100 nM), reduced by 40-50% the amplitudes of inhibitory junction potentials (i.j.ps) evoked in the circular muscle by activation of descending reflex pathways. This effect was abolished when synaptic transmission in the stimulus region was blocked with physiological saline containing 0.1 mM Ca2+ plus 10 mM Mg2+, leaving only the component of the descending reflex pathway conducted via long anally directed collaterals of intrinsic sensory neurons. 3. SR140333 (100 nM) had no effect on descending reflex i.j.ps when applied to the stimulus region. Ascending reflexes were also unaffected by SR140333 in the stimulus region or between the stimulus and recording sites. 4. Septide (10 nM), an NK1 receptor agonist, enhanced descending reflexes by 30-60% when in the recording chamber. [Sar9,Met(O2)11]substance P had no effect at 10 nM, but potentiated distension-evoked reflexes at 100 nM. 5. A selective NK3 receptor antagonist, SR142801 (100 nM), when applied to the stimulus region, reduced the amplitude of descending reflex responses to compression by 40%, but had no effect on responses to distension. SR142801 (100 nM) had no effect when applied to other regions of the descending reflex pathways. 6. SR142801 (100 nM) only inhibited ascending reflexes when applied at the recording site. However, after nicotinic transmission in the stimulus region was blocked, SR142801 (100 nM) at this site reduced responses to compression. 7. Contractions of the circular muscle of isolated rings of ileum evoked by low concentrations of septide, but not [Sar9,Met(O2)11]substance P, were potentiated by tetrodotoxin (300 nM). 8. Contractile responses evoked by an NK3 receptor agonist, senktide, were non-competitively inhibited by SR142801. After excitatory neuromuscular transmission was blocked, senktide produced inhibitory responses that were also antagonised by SR142801, but to a lesser extent and in an apparently competitive manner. 9. These results indicate that tachykinins acting via NK1 receptors partly mediate transmission to inhibitory motor neurons. NK3 receptors play a role in transmission from intrinsic sensory neurons and from ascending interneurons to excitatory motor neurons during motility reflexes.

Desensitization of the neurokinin-1 receptor (NK1-R) in neurons: effects of substance P on the distribution of NK1-R, Galphaq/11, G-protein receptor kinase-2/3, and beta-arrestin-1/2

Observations in reconstituted systems and transfected cells indicate that G-protein receptor kinases (GRKs) and beta-arrestins mediate desensitization and endocytosis of G-protein-coupled receptors. Little is known about receptor regulation in neurons. Therefore, we examined the effects of the neurotransmitter substance P (SP) on desensitization of the neurokinin-1 receptor (NK1-R) and on the subcellular distribution of NK1-R, Galphaq/11, GRK-2 and -3, and beta-arrestin-1 and -2 in cultured myenteric neurons. NK1-R was coexpressed with immunoreactive Galphaq/11, GRK-2 and -3, and beta-arrestin-1 and -2 in a subpopulation of neurons. SP caused 1) rapid NK1-R-mediated increase in [Ca2+]i, which was transient and desensitized to repeated stimulation; 2) internalization of the NK1-R into early endosomes containing SP; and 3) rapid and transient redistribution of beta-arrestin-1 and -2 from the cytosol to the plasma membrane, followed by a striking redistribution of beta-arrestin-1 and -2 to endosomes containing the NK1-R and SP. In SP-treated neurons Galphaq/11 remained at the plasma membrane, and GRK-2 and -3 remained in centrally located and superficial vesicles. Thus, SP induces desensitization and endocytosis of the NK1-R in neurons that may be mediated by GRK-2 and -3 and beta-arrestin-1 and -2. This regulation will determine whether NK1-R-expressing neurons participate in functionally important reflexes.

Tachykinin NK1 and NK2 receptor-mediated control of peristaltic propulsion in the guinea-pig small intestine in vitro

The tachykinins substance P and neurokinin A are excitatory cotransmitters of cholinergic enteric neurons, their actions being mediated by NK1, NK2 and NK3 receptors. This study examined which of these receptors are part of the neural circuitry of peristalsis. Peristaltic propulsion in luminally perfused segments of the guinea-pig isolated ileum was elicited by a rise of the intraluminal pressure. The pressure threshold at which peristaltic contractions were triggered was used to quantify drug effects on peristalsis, inhibition of peristalsis being reflected by an increase in the pressure threshold. The NK1, NK2 and NK3 receptor antagonists SR-140333, SR-48968 and SR-142 801 (each at 0.1 microM), respectively, had little effect on peristaltic activity as long as cholinergic transmission was left intact. However, both the NK1 and NK2 receptor antagonist (each at 0.1 microM) abolished peristalsis after cholinergic transmission via muscarinic receptors had been blocked by atropine (1 microM) and peristalsis rescued by naloxone (0.5 microM). When cholinergic transmission via nicotinic receptors was suppressed by hexamethonium (100 microM) and peristalsis restored by naloxone (0.5 microM), only the NK2 receptor antagonist (0.1 microM) was able to attenuate peristaltic performance as deduced from a rise of the peristaltic pressure threshold by 106%. The NK3 receptor antagonist (0.1 microM) lacked a major influence on peristalsis under any experimental condition. It is concluded that tachykinins acting via NK1 and NK2 receptors sustain intestinal peristalsis when cholinergic neuroneuronal and neuromuscular transmission via muscarinic receptors has been suppressed. NK2 receptors help maintaining peristalsis once cholinergic neuroneuronal transmission via nicotinic receptors has been blocked, whereas NK3 receptors play little role in the neural pathways of peristalsis.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.

Nitric oxide synthase in neurons of the human gall-bladder and its colocalization with neuropeptides

The distributions of nerve cells and fibres that are immunoreactive for nitric oxide synthase (NOS) have been investigated in the human gall-bladder. In addition, the colocalization of NOS immunoreactivity (IR) with neuropeptide Y (NPY), pituitary adenylyl cyclase activating peptide (PACAP), somatostatin (SOM), substance P (SP), tyrosine hydroxylase (TH) and vasoactive intestinal peptide (VIP)-IR was determined. Nitric oxide synthase-IR nerve cell bodies comprised 13 and 30% of nerve cells in ganglia of the fibromuscular and subepithelial layers, respectively. To determine these percentages, neuron-specific enolase-IR was used as a marker for all nerve cells. Although SOM- and VIP-IR nerve cell bodies were found in both ganglia, they rarely contained NOS-IR. In the fibromuscular layer, NOS-IR nerve fibres were abundant and most PACAP-, SOM- and VIP-IR fibres and many NPY-IR fibres were also NOS positive. No colocalization was observed between NOS- and SP- or TH-IR. In the mucosal layer, moderate numbers of NOS-IR fibres were found and the degree of colocalization of NOS-IR with each of NPY-, PACAP-, SOM-, SP- and VIP-IR were as follows: PACAP and NPY > VIP > SOM and SP. Nitric oxide synthase and TH were not colocalized in mucosal fibres. These results suggest that nerve fibres in the fibromuscular layer in the human gall-bladder with the chemical coding NOS/NPY/PACAP/SOM/VIP are axons of inhibitory motor neurons. Nitric oxide synthase-IR fibres in the mucosal layer that contained NPY, PACAP, SOM, SP and VIP with various degrees of colocalization probably contribute to the control of epithelial secretion or absorption.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Investigation of 5-HT3 receptor-mediated contraction in guinea-pig distal colon

We investigated the participation of cholinergic and tachykininergic mechanisms in 5-hydroxytryptamine (5-HT)-induced contraction via 5-HT3 receptors in longitudinal and circular muscle of guinea-pig isolated distal colon. 5-HT produced concentration-dependent contractile responses in longitudinal and circular muscle. The 5-HT3 receptor antagonists ramosetron (YM060) ((R)-5-[(1-methyl-3-indolyl) carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazole hydrochloride), YM114 (KAE-393) ((R)-5-[(2,3-dihydro-1-indolyl)carbonyl]-4,5,6,7-tetrahydro-1 H-benzimidazole hydrochloride), ondansetron and granisetron produced a concentration-dependent shift to the right of the 5-HT concentration-response curves in both muscle. However, methysergide and GR113808 had no effect on 5-HT-induced contraction. In the longitudinal muscle, atropine concentration-dependently inhibited 5-HT-induced contraction, and tetrodotoxin abolished it. (+/-)-CP96,345 attenuated the contractile response to 5-HT, but (+/-)-SR48,968 had no effect on it. In the presence of atropine, (+/-)-CP96,345 completely blocked 5-HT-induced contraction. In the circular muscle, atropine had no effect on the contractile response to 5-HT, whereas tetrodotoxin completely suppressed it. The contractile response elicited by 5-HT in the circular muscle was not inhibited by either (+/-)-CP96,345, (+/-)-SR48,968, devazepide, L-365,260 or indomethacin. It is suggested that 5-HT acts via 5-HT3 receptors to release acetylcholine and substance P, which in turn are responsible for contraction of the longitudinal muscle. In the circular muscle, as in the longitudinal muscle, 5-HT-induced contraction is mediated by the 5-HT3 receptor. Unlike the case in longitudinal muscle, however, this contraction involves neither cholinergic nor tachykininergic transmission. It is also suggested that neither cholecystokinin (CCK) nor prostaglandins participate in 5-HT3 receptor-mediated contraction in circular muscle.

Tachykinin receptors mediate atropine-resistant rat duodenal reflex contractions in vivo

The study aimed to establish the possible role of tachykinins as mediators of atropine-resistant reflex contractions evoked by balloon distension in the proximal duodenum of urethane-anesthetized, guanethidine (34 mumol/kg s.c.)-pretreated rats. Distension of the balloon with a small amount (0.2-0.3 ml) of saline induced the appearance of phasic rhythmic contractions (about 11 mmHg in amplitude) which were promptly suppressed by either atropine (3 mumol/kg i.v.) or hexamethonium (28 mumol/kg i.v.). Despite the continuous i.v. infusion of atropine (2 mumol/h), low-amplitude rhythmic phasic contractions recovered, which were promptly suppressed by hexamethonium, to indicate the involvement of an atropine-resistant excitatory reflex. The amplitude of these atropine-resistant contractions was increased to about 4-5 mmHg by further distension of the balloon (0.4-0.6 ml) : under these conditions, the atropine-resistant contractions undergo a progressive fading. The fading was prevented by i.v. administration of the nitric oxide (NO) synthase inhibitor, L-nitroarginine methyl ester (L-NAME, 55 mumol/h), to provide a suitable baseline (amplitude of contractions was 7-8 mmHg) for studying the effect of tachykinin receptor antagonists. I.v. administration of the selective tachykinin NK2 receptor antagonists, MEN 10,627 (10-100 nmol/kg) and SR 48968 (100-300 nmol/kg) or of the selective NK1 antagonist SR 140333 (100 nmol/kg), at doses which do not affect the duodenal contractions induced by acetylcholine (5.5 mumol/kg i.v.), produced a prompt and long lasting suppression of the atropine-resistant reflex duodenal contractions produced by balloon distension in urethane-anesthetized rats, whilst SR-48965 (300 nmol/kg), the enantiomer of SR-48968 devoid, of NK2 receptor blocking activity, was without effect. I.v. administration of the selective NK1 receptor agonists [Sar9] substance P sulfone and septide or of the NK2 receptor selective agonist, [beta Ala8] neurokinin A(4-10) produced dose-dependent contractions of the duodenum. SR 140333 (100 nmol/kg i.v.) selectively antagonized the duodenal contractions produced by [Sar9] substance P sulfone and septide without affecting those produced by [beta Ala8] neurokinin A(4-10). On the other hand, MEN 10,627 (30-100 nmol/kg i.v.) and SR 48968 (100-300 nmol/kg i.v.) but not SR 48965 (300 nmol/kg i.v.) antagonized, at a comparable extent, duodenal contractions induced by both the selective NK2 and NK1 receptor agonists. We conclude that endogenous tachykinins are involved in mediating atropine-resistant reflex contractions evoked by distension of the rat duodenum in vivo: both NK1 and NK2 receptors are activated by endogenous ligands to produce NANC contractions of rat duodenum in vivo. However, the contractile response to i.v. administered NK1 receptor agonists, [Sar9] substance P sulfone and septide, may involve the release of mediators producing smooth muscle contraction via NK2 receptors.

Evidence that tachykinin NK2 receptors modulate resting tone in the rat isolated small intestine

1. In the progress of experiments aimed at evaluating the role of tachykinins as enteric nonadrenergic noncholinergic (NANC) transmitters, we noted that certain tachykinin receptor antagonists produce a relaxation of circular muscle strips in the rat small intestine. This study aimed to assess the nature of this response and to determine the receptor type involved. The majority of the experiments were performed in capsaicin- (10 microM for 15 min) pretreated mucosa-free circular muscle strips from the rat small intestine, in the presence of atropine (1 microM), guanethidine (3 microM) and indomethacin (10 microM). 2. Under isometric recording of mechanical activity, the tachykinin NK1 receptor antagonist SR 140,333 (0.1 microM) had no effect on resting tone or spontaneous activity in duodenal or ileal circular muscle strips. The NK2 receptor antagonists, MEN 10,627 (0.1 microM) and GR 94,800 (0.1 microM) produced, after a delay of 10-15 min, a relaxation which averaged 61 +/- 3 and 57 +/- 6% (n = 6 and 4, respectively) of the maximal response (Emax) to isoprenaline (1 microM). The effect of maximal concentrations of MEN 10,627 and GR 94,800 when applied together was non-additive. The relaxant effect of MEN 10,627 (0.1 microM) was similar in the absence and presence of apamin (0.3 microM) and L-nitroarginine (100 microM). 3. Under isotonic recording of mechanical activity, MEN 10,627 (10 nM-1 microM) produced a concentration- and time-related relaxation of duodenal strips. The maximal relaxation averaged 72 +/- 4 and 69 +/- 4% (n = 5 each) of Emax to isoprenaline (1 microM) and was achieved 15-20 or 20-30 min after application of 1.0 or 0.1 microM MEN 10,627, respectively. 4. Duodenal strips were relaxed by other NK2 receptor selective antagonists (values in parentheses are % of Emax to isoprenaline at the given concentration of antagonist) GR 94,800 (69 +/- 3% at 1 microM, n = 4), SR 48,968 (60 +/- 3% at 1 microM, n = 4) and MDL 29,913 (66 +/- 4% at 1 microM, n = 4). SR 48,965 (1 microM), the inactive enantiomer of SR 48,968, was without effect. The NK1 receptor selective antagonists, SR 140,333 (0.1 microM), FK 888 (10 microM) RP 67,580 (1 microM) and GR 82,334 (10 microM) were also without effect (n = 4-5). 5. A cocktail of peptidase inhibitors, thiorphan, bestatin and captopril (1 microM each) had no significant effect on tone or spontaneous activity of duodenal strips. In the presence of peptidase inhibitors, MEN 10,627 (1 microM) produced a relaxation of duodenal strips (72 +/- 6% of Emax to isoprenaline, n = 5), whilst GR 82,334 (10 microM, n = 6) had no significant effect. 6. The relaxant response to MEN 10,627 was preserved in mucosa-free strips not pre-exposed to capsaicin. Tetrodotoxin (1 microM), saxitoxin (1 microM), hexamethonium (100 microM) and omega-conotoxin (0.1 microM) had no significant effect on the resting tone of duodenal strips nor did they affect the relaxation to MEN 10,627. L-Nitroarginine (100 microM) increased the tone of the strips but did not affect the response to MEN 10,627. Nifedipine (1 microM) relaxed the strips by 62 +/- 4% (n = 4), but in its presence a small relaxant effect to MEN 10,627 (26 +/- 5%, n = 4) was still evident. 7. Under isotonic recording of mechanical activity along the longitudinal axis, MEN 10,627 (1 microM) produced a slowly developing relaxation (39 +/- 3% of Emax to isoprenaline; n = 6) of whole segments of rat duodenum. When similar experiments were performed on whole segments of rat proximal colon MEN 10,627 had no effect. 8. The present findings document the observation that tachykinin NK2 receptors contribute to the maintenance of resting tone of the rat isolated small intestine. We found no evidence to suggest that this effect follows the blockade of the contractile effect of spontaneously released endogenous tachykinins. The present findings raise the possibility that constitutively active NK2 receptors account for the relaxant effect produced by NK2 receptor ant

Role of tachykinins as excitatory mediators of NANC contraction in the circular muscle of rat small intestine

1. The aim of this study was to assess the role of tachykinins, acting via NK1 and NK2 receptors, in mediating nonadrenergic noncholinergic (NANC) contractions produced by electrical field stimulation (EFS) in the circular muscle of the rat small intestine. 2. In the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (10 microM), apamin (0.3 microM) and L-nitroarginine (L-NOARG, 100 microM) and after in vitro capsaicin (10 microM for 15 min) pretreatment, EFS (0.25 ms pulse width, 100 V, 1-30 Hz for 5 s) produced a frequency-dependent NANC contraction of mucosa-free circular muscle strips from the rat proximal duodenum and terminal ileum. In the duodenum, the NANC contraction was preceded by a transient NANC relaxation. All responses to EFS were abolished by 1 microM tetrodotoxin. 3. The NK1 receptor selective antagonist, SR 140,333 (0.1 microM for 60 min) and the NK2 receptor selective antagonist, MEN 10,627 (0.1 microM for 60 min), both produced a partial inhibition of the contractile response to EFS. The co-administration of SR 140,333 and MEN 10,627 produced a profound inhibition of the response to EFS in the duodenum, larger than that produced by each antagonist alone; a fraction (about 25% of the response at 30 Hz) of the NANC contraction of the duodenum persisted in the presence of the two antagonists. This residual response was however abolished after co-administration of the NK1 and NK2 receptor antagonists, GR 94,800 (1 microM) and GR 82,334 (10 microM). The co-administration of SR 140,333 and MEN 10,627 nearly abolished the NANC contraction to EFS in the ileum. 4. Nifedipine (1 microM) induced a profound depression of the NANC contraction to EFS in both duodenal and ileal strips. A fraction of the response to EFS (about 25 and 5-10% of the response at 30 Hz in the duodenum and ileum, respectively) was nifedipine-resistant. SR 140,333 (0.1 microM) had little effect on the nifedipine-resistant response to EFS in the duodenum although it reduced by about 50% the response in the ileum. MEN 10,627 (0.1 microM) produced a partial inhibitory effect of the nifedipine-resistant response in both regions. The co-administration of SR 140,333 and MEN 10,627 nearly abolished the nifedipine-resistant response in the ileum while a small fraction (about 20% of control) of the response persisted in the duodenum.

Tachykinin receptors in gastrointestinal motility

For a long time research on the action of TKs on gastrointestinal tissue has been demonstrating the importance of the TKs as non-cholinergic stimulators of motility in most parts of the mammalian gastrointestinal tract. The past years witnessed the development of TK agonists and antagonists selective for the various receptor types, which prompted a wealth of new insight into the pharmacology and molecular biology of the TK receptors. This knowledge now allows a more specific elucidation of the role of TKs and their receptors in the various aspects of gastrointestinal motility, not only in normal tissue but also under pathological conditions.

Evidence that tachykinin NK1 and NK2 receptors mediate non-adrenergic non-cholinergic excitation and contraction in the circular muscle of guinea-pig duodenum

1. In the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM), apamin (0.1 microM) and L-nitroarginine (L-NOARG, 30 microM), electrical field simulation (EFS) produced a nonadrenergic, noncholinergic (NANC) excitatory junctional potential (e.j.p.), action potentials and contraction of the circular muscle of the guinea-pig proximal duodenum, recorded by the single sucrose gap technique. 2. The selective tachykinin (TK) NK1 receptor antagonist, GR 82,334 (30 nM-3 microM) produced a concentration-dependent inhibition of the EFS-evoked NANC e.j.p. and contraction. Similarly, the selective NK2 receptor antagonists, MEN 10,627 (30 nM-3 microM) and GR 94,800 (100 nM-10 microM), both produced a concentration-dependent inhibition of the EFS-evoked NANC e.j.p. and contraction. GR 82,334 inhibited the electrical and mechanical NANC responses to EFS in an almost parallel manner, while MEN 10,627 and GR 94,800 were more effective in inhibiting the mechanical than the electrical response to EFS. 3. Activation of the NK1 or NK2 receptor by the selective agonists, [Sar9]substance P (SP) sulphone and [beta Ala8]neurokinin A (NKA) (4-10), respectively (0.3 microM each), produced depolarization, action potentials and contractions. GR 82,334 selectively inhibited the responses to [Sar9]SP sulphone, without affecting the responses to [beta Ala8]NKA (4-10). MEN 10,627 and GR 94,800 inhibited or abolished the responses to [beta Ala8]NKA (4-10), without affecting the responses to [Sar9]SP sulphone. 4. Nifedipine (1 microM) abolished the action potentials and contraction produced either by EFS or by the TK receptor agonists [Sar9]SP sulphone or [beta Ala8]NKA (4-10). 5. In the presence of nifedipine, the NANC e.j.p. produced by EFS was biphasic: in the majority of strips tested (21 out of 29) an early fast phase of depolarization was followed by a second slow component. The combined administration of GR 82,334 and GR 94,800 (3 microM each) reduced both components, the slow phase being inhibited to a greater extent than the fast phase. 6. The P2 purinoreceptor antagonist, suramin (100 microM) reduced the fast phase of the e.j.p. produced by EFS in the presence of nifedipine, without affecting the slow phase. The combined administration of suramin, GR 82,334 and GR 94,800 produced a nearly complete blockade of the e.j.p. produced by EFS in the presence of nifedipine. 7. When tested in the absence of apamin and L-NOARG, EFS induced a NANC inhibitory junction potential (i.j.p.) followed by an e.j.p., and the selective P2Y receptor agonist, adenosine-5'-O-(2-thiodiphosphate) (ADP beta S, 10 microM), produced membrane hyperpolarization. After addition of apamin and L-NOARG, the ij.p. was blocked, and EFS produced a pure NANC e.j.p.; ADPPS produced depolarization, action potentials and contraction.8. Suramin (100 microM) blocked the depolarization, action potentials and contractions produced by ADP beta S in the presence of apamin and L-NOARG, without affecting the responses produced by the NK1receptor agonist, [Sar9}SP sulphone.9. We conclude that NK1 and NK2 receptors cooperate in producing NANC excitation and contraction of the circular muscle in the guinea-pig proximal duodenum. Activation of either TK receptor produces membrane depolarization and both receptors contribute to generate action potentials which are essential for producing muscle contraction, via nifedipine-sensitive calcium channels. It appears that endogenous ATP chiefly acts as an inhibitory transmitter but, after blockade of NANC inhibitory mechanism(s),ATP may act as a fast signalling excitatory transmitter.

Specialization of tachykinin NK1 and NK2 receptors in producing fast and slow atropine-resistant neurotransmission to the circular muscle of the guinea-pig colon

We studied the relative contribution of tachykinin NK1 and NK2 receptors in producing nonadrenergic noncholinergic excitation of the circular muscle of the guinea-pig proximal colon in response to electrical field stimulation. All experiments were performed in the presence of atropine, guanethidine, indomethacin, apamin and L-nitroarginine. In organ bath experiments, electrical stimulation produced a tetrodotoxin-sensitive frequency-dependent contraction. The NK1 receptor antagonists, FK 888 (1-10 microM) and GR 82,334 (0.3-3 microM) markedly reduced but did not abolish the nonadrenergic noncholinergic response. The NK2 receptor antagonist, GR 94,800 (0.3-3 microM) was partly effective at 3 microM. The combined administration of FK 888 (10 microM) and GR 94,800 (3 microM) or GR 82,334 and GR 94,800 abolished the nonadrenergic noncholinergic contraction. The response to a prolonged period of stimulation (3 Hz for 5 min) was evenly depressed by FK 888 or GR 82,334, while GR 94,800 was more effective in inhibiting the late (87% inhibition) than the peak response (25% inhibition). In the presence of nifedipine (1 microM) a marked inhibition of the nonadrenergic noncholinergic contraction was observed and a time lag was evident between stimulus application and onset of contraction, which showed slow onset and offset kinetics. The nifedipine-resistant nonadrenergic noncholinergic contraction was unaffected by FK 888 or GR 82,334 but was suppressed by GR 94,800. Submaximally effective (1-3 nM) concentrations of substance P and neurokinin A produced distinct patterns of contraction: the response to substance P was fast and declined rapidly toward baseline; the response to neurokinin A was slow and sustained. In the presence of nifedipine, the response to substance P was greatly depressed and became slower in onset; nifedipine did not affect the contraction to neurokinin A but slowed its time-course. In sucrose gap experiments, either a short (10 Hz for 1 s) or a prolonged period of electrical stimulation (3 Hz for 3 min) evoked membrane depolarization, action potentials and contraction: in response to the "prolonged" stimulation, distinct phasic and tonic component of contraction were observed. Nifedipine abolished action potentials and the phasic contraction produced by a short period of stimulation, reduced by about 50% the maximal contraction developed during the prolonged stimulation without affecting the amplitude of the tonic response. In the presence of nifedipine, GR 82,334 (3 microM) blocked the membrane depolarization but did not affect contraction; GR 94,800 (0.1 microM) did not affect depolarization but abolished contraction.(ABSTRACT TRUNCATED AT 400 WORDS)