A modified single sucrose gap. Junction potentials and electrotonic potentials in gastrointestinal smooth muscles

Modular double sucrose gap apparatus for improved recording of compound action potentials from rat and mouse spinal cord white matter preparations

Compound action potential (CAP) recording is a powerful tool for studying the conduction properties and pharmacology of axons in multi-axonal preparations. The sucrose gap technique improves CAP recording by replacing the extracellular solution between the recording electrodes with a non-conductive sucrose solution to minimize extracellular shunting. The double sucrose gap (DSG), conferring similar advantages at the stimulation site, has been extensively used on guinea pig spinal cord white matter (WM) in vitro. Establishing the DSG methodology for WM preparations from smaller animals such as rats and mice is appealing due to their extensive use in basic and translationally oriented research. Here we describe a versatile modular DSG apparatus with rubber membrane separation of the compartments, suitable for WM strips from rat and mouse spinal cord. The small volumes of compartments (<0.1 ml) and the air-tight design allow perfusion rates of 0.5-1 ml/min with faster refreshment rates compared to commonly used 2-3 ml/min and larger compartments, providing economical usage of expensive pharmacological drugs. Our improved DSG design is particularly efficient for uncovering slower conducting, higher threshold CAP components, as demonstrated by recordings of C-wave (non-myelinated axons) in rat dorsal WM. In myelin-deficient Shiverer mice with genetically dysmyelinated axons, our DSG apparatus recordings revealed a multi-peak C-wave without preceding faster components. The improved stimulation and recording with our DSG apparatus, lowering the range of required stimulus intensities and reducing the artifact interference with recorded CAPs provide for critical technical advantages that allow for more detailed analysis of CAPs in relatively short preparations.

Involvement of purinergic nerves in the NANC inhibitory junction potentials in pigeon oesophageal smooth muscle

1. Electrical field stimulation (EFS) (0.5 ms in train of 2-32 Hz for 300 ms) in smooth muscle of pigeon oesophagus, in the presence of atropine (1 microm) and guanethidine (1 microm), elicited an inhibitory response consisting of a transient hyperpolarization (inhibitory junction potential, IJP) associated with muscle relaxation. 2. Sodium nitroprusside (SNP, 100 microm) induced hyperpolarization correlated to mechanical relaxation. 3. The nitric oxide (NO) synthase inhibitor N(omega)-nitro-l-arginine (from 0.1 to 100 microm) caused a concentration-dependent reduction of electromechanical response to EFS indicating a role for NO in this response. 4. Apamin (1 microm) reduced both IJP and relaxation to EFS but was without effect on the response to SNP indicating a role for purines, which are also blocked by apamin. 5. Adenosine, AMP, ADP and ATP (all from 1 microm to 1 mm) application caused transient hyperpolarization and muscular relaxation with the following order of potency: adenosine > AMP > ADP > ATP. 6. Inhibitory responses evoked by purines are TTX (1 microm) insensitive but they were inhibited by apamin. This indicates that a purine component for the non-adrenergic non-cholinergic (NANC) response exists but the purine receptor site is not located on the neurone. 7. Overall these results suggest that NANC inhibitory response elicited by EFS presents two different components apamin-sensitive, probably purines-mediated and apamin-insensitive probably NO-mediated as apamin only partially block the response to EFS.

Effect of 18beta-glycyrrhetinic acid on electromechanical coupling in the guinea-pig renal pelvis and ureter

We have tested the effect of the gap junction inhibitor, 18beta-glycyrrhetinic acid (18betaGA) on electromechanical coupling in the guinea-pig renal pelvis and ureter by the sucrose gap technique. In the ureter 18betaGA (3 - 30 microM) produced a concentration-dependent inhibition of the spike component of the action potential (AP) and reduced contraction evoked by electrical stimulation. Neurokinin A (NKA) produced a slow depolarization with superimposed APs and phasic contractions of the ureter. 18betaGA (30 microM) markedly inhibited the depolarization and APs evoked by NKA. However the contractile response was more sustained in the presence than in the absence of 18betaGA. At 100 microM, 18betaGA inhibited the mechanical responses to NKA. KCl (80 mM) produced APs and phasic contractions followed by sustained depolarization and tonic contraction. At 30 microM 18betaGA markedly inhibited the KCl-evoked APs and phasic contractions without affecting the sustained responses. At 100 microM 18betaGA inhibited the tonic contraction to KCl. In the renal pelvis 18betaGA (30 microM) inhibited the amplitude of pacemaker potentials and accompanying contractions and induced the appearance of low-amplitude APs not associated with contraction. We conclude that, up to 30 microM, the action of 18betaGA is consistent with an inhibition of cell-to-cell electrical coupling via gap junctions. The single-unit character of smooth muscles in the guinea-pig upper urinary tract is partly converted to a multi-unit pattern. At high concentrations 18betaGA possesses non specific effects which limit its usefulness as a tool for studying the role of gap junctions in smooth muscles. British Journal of Pharmacology (2000) 129, 163 - 169

Excitatory motor and electrical effects produced by tachykinins in the human and guinea-pig isolated ureter and guinea-pig renal pelvis

1. In isolated tissue experiments, neurokinin A (NKA) produced concentration-dependent contraction of human and guinea-pig ureter (pD2 = 6.7 and 7.2, respectively); an effect greatly reduced (>80% inhibition) by the tachykinin NK2 receptor-selective antagonist MEN 11420 (0.1 microM). The tachykinin NK1 and NK3 receptor agonists septide and senktide, respectively, were ineffective. 2. Electrical field stimulation (EFS) of the guinea-pig isolated renal pelvis produced an inotropic response blocked by MEN 11420 (0.01-1 microM). In the same preparation MEN 11420 (0.1 microM) blocked (apparent pK(B) = 8.2) the potentiation of spontaneous motor activity produced by the NK2 receptor-selective agonist [betaAla8]NKA(4-10). 3. In sucrose-gap experiments, EFS evoked action potentials (APs) accompanied by phasic contractions of human and guinea-pig ureter, which were unaffected by tetrodotoxin or MEN 11420 (3 microM), but were blocked by nifedipine (1-10 microM). NKA (1-3 microM) produced a slow membrane depolarization with superimposed APs and a tonic contraction with superimposed phasic contractions. NKA prolonged the duration of EFS-evoked APs and potentiated the accompanying contractions. MEN 11420 completely prevented the responses to NKA in both the human and guinea-pig ureter. 4. Nifedipine (1-10 microM) suppressed the NKA-evoked APs and phasic contractions in both human and guinea-pig ureter, and slightly reduced the membrane depolarization induced by NKA. A tonic-type contraction of the human ureter in response to NKA persisted in the presence of nifedipine. 5. In conclusion, tachykinins produce smooth muscle excitation in both human and guinea-pig ureter by stimulating receptors of the NK2 type only. NK2 receptor activation depolarizes the membrane to trigger the firing of APs from latent pacemakers.

Sodium nitroprusside enhances contractions of the guinea-pig isolated vas deferens

The effects of sodium nitroprusside on the electrical and mechanical properties of the smooth muscle of the guinea-pig vas deferens, and its responses to transmitter substances, have been investigated by use of the sucrose-gap technique. Isolated longitudinal segments of guinea-pig vas deferens contracted in response to electrical field stimulation (100 V, 0.04-0.1 ms, 1-5 Hz, 10 s train every 60 s) and application of ATP (1 mM) or noradrenaline (10 microM). Sodium nitroprusside (0.1 mM) did not affect resting tension but did enhance contractions evoked by electric-field stimulation but not by ATP or noradrenaline. The sodium nitroprusside-induced enhancement was unaffected by the nitric oxide synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) (0.1 mM). Conversely, electrically evoked contractions were unaffected by the nitric oxide precursor L-arginine (1 mM) or the nitric oxide donor S-nitroso-N-acetyl-DL-penicillamine (SNAP) (0.1 mM). The amplitudes of electrically evoked excitatory junction potentials (EJPs) were not affected by application of sodium nitroprusside, although it caused a small depolarization of 0.7+/-0.3 mV. Similarly, the depolarization caused by exogenous application of ATP or noradrenaline was unaffected by the presence of sodium nitroprusside. L-NAME, L-arginine and SNAP did not affect EJP amplitude or baseline membrane potential. It is concluded that sodium nitroprusside enhances electrically evoked contractions of the guinea-pig vas deferens by reducing the threshold voltage for action potential firing in smooth-muscle cells.

Pharmacological evidence for the existence of multiple P2 receptors in the circular muscle of guinea-pig colon

1. By using the sucrose gap technique, we have investigated the effect of the metabolically stable P2Y receptor agonist, adenosine 5'-O-2-thiodiphosphate (ADPbetaS), on the membrane potential and tension in the circular muscle of the guinea-pig proximal colon. All experiments were performed in the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM), nifedipine (1 microM), L-nitroarginine (L-NOARG, 100 microM) and of the tachykinin NK1 and NK2 receptor antagonists, SR 140333 (0.1 microM) and GR 94800 (0.1 microM), respectively. 2. ADPbetaS (100 microM for 15 s) evoked a tetrodotoxin- (1 microM) resistant hyperpolarization and contraction of the smooth muscle. In the presence of apamin (0.1 microM), the ADPbetaS-induced hyperpolarization was converted to depolarization and the contraction was potentiated while tetraethylammonium (TEA, 10 mM) did not affect significantly the response to ADPbetaS. The combined application of apamin and TEA reproduced the effect observed with apamin alone. 3. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acids (PPADS, 30 microM) slightly but significantly increased the ADPbetaS-induced hyperpolarization, while the contraction evoked by ADPbetaS was reduced by about 80%. Suramin (100 microM) did not affect the ADPbetaS-induced hyperpolarization but totally blocked the ADPbetaS-induced contraction. In the presence of suramin (100 microM), a small relaxation of the circular muscle was observed upon application of ADPbetaS. 4. The contraction and hyperpolarization evoked by ADPbetaS were abolished in Ca2+-free Krebs solution. The blocker of sarcoplasmic reticulum Ca2+ pump, cyclopiazonic acid (10 microM) reduced contraction and hyperpolarization induced by ADPbetaS by about 60 and 50%, respectively. 5. A comparison of our present and previous findings enables to conclude that at least 3 types of P2 receptors are present on the smooth muscle of the guinea-pig colon, as follows: (1) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by alpha,beta-methylene ATP (alpha,beta-meATP) and by endogenously released purines, sensitive to suramin and PPADS; (2) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by ADPbetaS and are resistant to suramin and PPADS; (3) excitatory P2 receptors, producing contraction, which are activated by ADPbetaS and are sensitive to suramin and PPADS. The data also support the idea of the existence of a restricted pool of specialized junctional P2 receptors producing the apamin-sensitive NANC inhibitory junction potential in response to endogenous ligand(s).

Tachykinin NK1 and NK2 receptors mediate non-adrenergic non-cholinergic excitatory neuromuscular transmission in the guinea-pig stomach

By using selective tachykinin NK1 and NK2 receptor antagonists and agonists, we studied the excitatory non-adrenergic non-cholinergic transmission to the circular muscle of the corpus of guinea-pig stomach by the sucrose-gap method. After elimination of inhibitory junction potentials by apamin (0.1 microM), L-nitroarginine (30 microM) and tetraethylammonium (10 mM), electrical field stimulation (10 Hz) in the presence of atropine (1 microM) and nifedipine (1 microM) evoked a pure excitatory junction potential and contraction. The selective tachykinin NK2 receptor antagonist, MEN 11420, concentration-dependently inhibited the non-adrenergic non-cholinergic excitatory junction potential (EC50=0.09 microM) and contraction (EC50=0.04 microM) evoked by electrical field stimulation. On the other hand, the selective NK1 receptor antagonist GR 82334 (3 microM) only slightly (by about 30%) inhibited the excitatory junction potential while leaving the contraction unaffected. The combined administration of GR 82334 (1 microM) and MEN 11420 (0.3 microM) produced an additive inhibition of the excitatoryjunction potential, significantly larger than that produced by each antagonist alone. In the presence of both GR 82334 (1 microM) and MEN 11420 (0.3 microM), the P2 purinoreceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (30 microM) remarkably inhibited the fast component of the excitatory junction potential. In the presence of atropine (1 microM), indomethacin (3 microM) and guanethidine (3 microM) either the selective NK2 receptor agonist, [betaAla8]neurokinin A (4-10) (0.01 microM), or the selective NK1 receptor agonist, [Sar9]substance P sulfone (0.3 microM), produced tetrodotoxin-(1 microM) and nifedipine-(1 microM) resistant depolarization and contraction. The [Sar9]substance P sulfone (0.3 microM)-induced contraction, but not that induced by [betaAla8]neurokinin A (4 10) (0.01 microM), was potentiated by apamin (0.1 microM) plus L-nitroarginine (30 microM). In the presence of atropine (1 microM), indomethacin (3 microM), guanethidine (3 microM), apamin (0.1 microM) and L-nitroarginine (30 microM), the selective tachykinin NK2 and NK1 receptor agonists, [betaAla8]neurokinin A (4-10) and [Sar9]substance P sulfone, both produced a concentration-dependent depolarization and contraction of the circular muscle. MEN 11420 inhibited the responses to [[Ala8]neurokinin A (4-10) without affecting the responses to [Sar9]substance P sulfone, while GR 82334 inhibited the responses to [Sar9]substance P sulfone but not that to [betaAla8]neurokinin A (4-10). These data provide evidence that tachykinin NK2 receptors predominantly mediate the non-adrenergic non-cholinergic excitatory transmission to the circular muscle of the corpus of guinea-pig stomach. In addition, after blocking the non-adrenergic non-cholinergic inhibitory junction potential by apamin, L-nitroarginine and tetraethylammonium, the fast component of the non-adrenergic non-cholinergic excitatory junction potential could be mediated by adenosine triphosphate.

Sequential activation of the triple excitatory transmission to the circular muscle of guinea-pig colon

The aim of this study was to resolve the temporal relationships of the triple excitation of the circular muscle of guinea-pig colon that occurs in response to activation of the intrinsic excitatory nerves by using atropine and tachykinin NK1 and NK2 receptor selective antagonists to define the relative contribution of the transmitters involved. In organ bath experiments, performed in the presence of blockers of inhibitory innervation, a train of electrical pulses at 5 Hz for 300 s produced a sustained contraction of the circular muscle of guinea-pig colon: the sequential addition of atropine (1 microM), of the tachykinin NK1 receptor antagonist, SR 140333 (0.3 microM) and of the tachykinin NK2 receptor antagonist, MEN 11420 (1 microM) produced a cumulative inhibitory effect and progressively delayed the onset of the contractile response to nerve stimulation. In the presence of peptidase inhibitors, atropine was less effective in inhibiting the contractile response for prolonged periods of stimulation: however, the pattern of inhibition of the evoked response produced by the sequential addition of blocker drugs was not substantially affected. The selective tachykinin NK3 receptor agonist, senktide, produced a concentration-dependent contraction of guinea-pig colon. The sequential addition of atropine (1 microM), SR 140333 (0.3 microM) and MEN 11420 (1 microM) reproduced the effect of the same drugs on the response to electrical nerve stimulation. The peptide blocker of N-type voltage-dependent calcium channels, omega-conotoxin (0.1 microM) produced a partial inhibitory effect of the response to senktide. The omega-conotoxin-resistant response to 1 microM senktide was inhibited and delayed by the progressive application of atropine, SR 140333 and MEN 11420, similar to the effect observed in the absence of omega-conotoxin. In sucrose gap, single-pulse electrical field stimulation produced a fast excitatory junction potential which was largely (90%) inhibited by atropine; application of a low concentration of the potassium channel blocker, 4-aminopyridine (30 microM), markedly enhanced the atropine-resistant excitatory junction potential which was abolished by the NK1 receptor antagonist, GR 82334. We conclude that, during prolonged electrical or chemical stimulation of excitatory motorneurons, there is a sequential, time-dependent activation of the three excitatory mechanisms in the circular muscle of guinea-pig colon: the pattern of activation is relatively independent of the intensity of stimulation and/or the mechanisms of secretion of released transmitters. Postjunctional factors predominate in determining the relative contribution of the three transmitters, acetylcholine, substance P and neurokinin A, in producing excitation of the circular muscle.

Tachykinin NK1 and NK2 receptors mediate non-adrenergic non-cholinergic excitatory neuromuscular transmission in the human ileum

Tachykinin NK1 and NK2 receptor selective antagonists and agonists were used to study excitatory non-adrenergic non-cholinergic (NANC) transmission in circular muscle strips from human ileum by the sucrose-gap method. In the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM), apamin (0.1 microM), and N omega-nitro-L-arginine (L-NOARG, 30 microM), electrical field simulation (EFS) produced a NANC inhibitory junction potential (i.j.p.) followed by NANC excitatory junction potential (e.j.p.) with superimposed action potentials and contraction of the circular muscle of human ileum. The selective tachykinin NK1 receptor antagonist, GR 82334 (0.1-3 microM) produced a concentration-dependent inhibition of the EFS-evoked NANC e.j.p. (IC50 = 0.21 microM) and contraction (IC50 = 0.21 microM). The selective tachykinin NK2 receptor antagonist, MEN 10627 (0.01-1 microM), likewise produced a concentration-dependent inhibition of the EFS-evoked NANC e.j.p. (IC50 = 0.07 microM) and contraction (IC50 = 0.03 microM). Either antagonist was more effective in inhibiting the mechanical than the electrical response to EFS. Neither GR 82334 nor MEN 10627 had any effect on the apamin- and L-NOARG-resistant NANC i.j.p. Activation of the NK1 or NK2 receptors by the selective receptor agonists, [Sar9]substance P (SP) sulfone and [beta Ala8]neurokinin A (NKA) (4-10), respectively (0.3 microM for 20 s each), produced depolarization with superimposed action potentials and contractions. GR 82334 selectively inhibited the responses to [Sar9]]SP sulfone, without affecting the responses to [beta Ala8]NKA (4-10). MEN 10627 inhibited the responses to [beta Ala8]NKA (4-10), without affecting the responses to [Sar9]SP sulfone. We conclude that both tachykinin NK1 and NK2 receptors co-operate in producing NANC excitation and contraction of the circular muscle in human ileum.

CGRP potentiates excitatory transmission to the circular muscle of guinea-pig colon

We aimed to assess whether calcitonin gene-related peptide (CGRP) can modulate the release of tachykinins which are the main nonadrenergic noncholinergic (NANC) excitatory transmitters to the circular muscle of the guinea-pig proximal colon. In organ bath experiments, electrical field stimulation (EFS) in the presence of atropine (1 microM) and guanethidine (3 microM) evoked twitch phasic NANC contractions which were abolished by the combined administration of tachykinin NK1 and NK2 receptor antagonists. Human alphaCGRP (CGRP, 1-100 nM) produced a concentration-dependent potentiation of the amplitude of the NANC contractions induced by EFS while salmon calcitonin (up to 1 microM) had no effect. The potentiating effect of CGRP was unaffected by in vitro capsaicin pretreatment (10 microM for 15 min), peptidase inhibitors (captopril, bestatin and thiorphan, 1 microM each), apamin (0.3 microM) plus L-nitroarginine (L-NOARG, 100 microM) and by the CGRP1 receptor antagonist, the C-terminal fragment CGRP(8-37) (1 microM). The NK2 receptor antagonist MEN 10627 which, when administered alone, had only a partial inhibitory effect on the amplitude of NANC twitches, concentration-dependently (10 nM-1 microM) inhibited the potentiating effect of CGRP. CGRP (1-100 nM) produced a concentration-dependent potentiation of the atropine-sensitive cholinergic contractions evoked by EFS in the presence of guanethidine and of tachykinin NK1 and NK2 receptor antagonists. Similar to the effect of CGRP, application of capsaicin (0.1-1 microM) potentiated the amplitude of the NANC contraction to EFS, an effect undergoing complete desensitization upon a second application of the drug. CGRP (0.1 microM) did not affect the contractile action of a submaximally effective concentration of neurokinin A (2 nM) while it inhibited that induced by substance P (2 nM). In sucrose gap, single pulse EFS in the presence of atropine (1 microM) and guanethidine (3 microM) induced an inhibitory junction potential (i.j.p.) and a small excitatory junction potential (e.j.p.). CGRP (0.1 microM) produced membrane hyperpolarization and relaxation without affecting i.j.p. amplitude but concomitantly increased the e.j.p. amplitude to induce a contraction in correspondence to each electrical pulse. In the presence of the NK1 receptor antagonist, GR 82334 (3 microM), the membrane hyperpolarization and relaxation produced by CGRP and the EFS-evoked i.j.p. were unaffected, while the potentiating effect of CGRP on the EFS-evoked NANC e.j.p. and the corresponding contraction were abolished. We conclude that, in addition to the previously characterized direct smooth muscle relaxant action via CGRP1 receptors (Maggi et al. Regulatory Peptides 61, 27-36, 1996), CGRP also induces a remarkable potentiation of excitatory neurotransmission to the circular muscle of the guinea-pig colon via CGRP2 receptors. The latter effect, documented in this study, is evidenced on both the atropine-sensitive and the atropine-resistant (tachykinin-mediated) components of excitatory transmission: this effect does not involve mediator(s) release from capsaicin-sensitive primary afferent nerves, nor inhibition of peptide degradation or modulation of NANC inhibitory transmission.

The possible role of ATP and PACAP as mediators of apaminsensitive NANC inhibitory junction potentials in circular muscle of guinea-pig colon

1. In the presence of atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM), nifedipine (1 microM), L-nitroarginine (L-NOARG, 100 microM), and the selective tachykinin NK1 and NK2 receptor antagonists, SR 140,333 and GR 94,800, respectively (0.1 microM each), a single pulse of electrical field stimulation (EFS) produced a monophasic non-adrenergic non-cholinergic (NANC) inhibitory junction potential (i.j.p., about 10 mV in amplitude) in the circular muscle of guinea-pig proximal colon, recorded by the modified single sucrose gap technique. 2. The P2 purinoceptor agonist, alpha, beta methylene ATP (alpha, beta mATP, 100 microM) and the pituitary adenylyl cyclase activating peptide (PACAP, 1 microM) both produced hyperpolarization (11 +/- 0.8 mV, n = 14 and 10.2 +/- 0.8 mV, n = 19, respectively) and relaxation (1.1 +/- 0.2 mV, n = 14 and 1.5 +/- 0.2 mN, n = 19, respectively) of the circular muscle. 3. Apamin (0.1 microM) nearly abolished (about 90% inhibition) the NANC i.j.p. and the alpha, beta mATP-induced hyperpolarization, markedly reduced the alpha, beta mATP-induced relaxation (73% inhibition) and the PACAP-induced hyperpolarization (65% inhibition), while the PACAP-induced relaxation was unaffected. 4. Tetraethylammonium (TEA, 10 mM) increased the EFS-evoked i.j.p. and revealed an excitatory junction potential (e.j.p.). In the presence of TEA, alpha, beta mATP induced a biphasic response: transient depolarization and contraction followed by hyperpolarization and relaxation. The hyperpolarization to PACAP was reduced by TEA (45% inhibition) but the relaxation was unaffected. 5. The combined application of apamin (0.1 microM) and TEA (10 mM) abolished the i.j.p. and single pulse EFS evoked a pure e.j.p. with latency three times longer than that of the i.j.p. In the majority of strips tested, alpha, beta mATP and PACAP elicited a biphasic response : depolarization and small contraction followed by hyperpolarization and relaxation. 6. The P2 purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) inhibited the NANC i.j.p. in concentration-dependent manner and inhibited the alpha, beta mATP-induced hyperpolarization and relaxation, without affecting the hyperpolarization and relaxation induced by PACAP. On the other hand, the P2 purinoceptor antagonist, suramin (100 microM) inhibited to a similar extent (60-80%) the NANC i.j.p. and the hyperpolarization and relaxation induced by alpha, beta mATP or PACAP. 7. PPADS and suramin reduced the NANC e.j.p. evoked by a single pulse EFS in the presence of apamin and TEA (100 microM of PPADS and 300 microM of suramin inhibited the e.j.p. by about 40%). 8. We conclude that ATP, but not PACAP, mediates the apamin-sensitive NANC i.j.p. in the circular muscle of the guinea-pig colon. After blockade of the NANC i.j.p., ATP may act as an excitatory transmitter by activating excitatory P2 purinoceptors. The subtypes of P2 purinoceptor involved in the inhibitory and excitatory responses remain to be established. The data suggest that excitatory P2 purinoceptors may be located extrajunctionally.

Role of intracellular Ca2+ in the K channel opener action of CGRP in the guinea-pig ureter

1. The aim of this study was to assess the role of sarcoplasmic reticulum (SR) calcium (Ca2+) in the smooth muscle relaxant and hyperpolarizing actions of calcitonin gene-related peptide (CGRP) in the guinea-pig ureter. 2. CGRP (0.1 microM) rapidly and transiently reduced myogenic phasic contractions (twitches) produced by electrical field stimulation (EFS). Approximately 70% of the response to CGRP was antagonized by glibenclamide (1 microM). 3. Cyclopiazonic acid (CPA, 10 microM), ryanodine (100 microM) and thapsigargin (1 microM) reduced only the glibenclamide-sensitive component of the response to CGRP (0.1 microM) but did not modify the mechano-inhibitory effect of cromakalim (3 microM). A low concentration of CPA (1 microM), assumed to produce a limited impairment of Ca2+ uptake from the stores, prolonged the duration of the inhibitory response to CGRP. Pre-exposure to caffeine (5 mM) inhibited the suppression of twitches by CGRP or cromakalim. 4. When the frequency of EFS was increased, the suppression of twitches by CGRP was reduced. Under these conditions, CPA (1 microM) again prolonged the duration of the inhibitory response to CGRP. 5. CGRP (0.1 microM) and cromakalim (3 microM) markedly depressed the phasic component of contractions to 80 mM KCl. CPA (10 microM) antagonized the inhibitory effect of CGRP but not that of cromakalim. Inhibition of the tonic contraction to 80 mM KCl by CGRP was insensitive to CPA. 6. In sucrose gap experiments, a 5 min exposure to CGRP (0.1 microM) or cromakalim (3 microM) produced a sustained membrane hyperpolarization. Caffeine (5 mM) produced a glibenclamide-sensitive transient hyperpolarization followed by a sustained depolarization. When tested in a Ca(2+)-free medium the hyperpolarization produced by CGRP, cromakalim or caffeine was reduced. In normal Krebs, pre-exposure to CPA (10 microM, 60 min) only abolished the hyperpolarization induced by CGRP. In contrast, 5 min after a caffeine challenge (5 mM) the hyperpolarizations induced by CGRP or cromakalim were reduced. The CGRP-induced hyperpolarization was insensitive to apamin (0.1 microM) or charybdotoxin (0.1 microM). 7. We conclude that the K channel-opening action of CGRP in the guinea-pig ureter requires the mobilization of intracellular Ca2+ from a caffeine- and CPA-sensitive store, leading to transient activation of glibenclamide-sensitive K channels. The K channel-opening action of caffeine appears to involve Ca2+ mobilization from a store which is insensitive to depletion by CPA.

Calcitonin gene-related peptide (CGRP) in the circular muscle of guinea-pig colon: role as inhibitory transmitter and mechanisms of relaxation

In the presence of 1 microM tetrodotoxin (TTX), human alpha calcitonin gene-related peptide (CGRP) produced a concentration-dependent relaxation (EC50 1.1 nM; Emax 86% of the relaxation to 1 microM isoprenaline) of mucosa-free circular muscle strips from the guinea-pig proximal colon. In the presence of TTX, the C-terminal fragment CGRP(8-37) produced a concentration (0.3-3 microM)-dependent rightward shift of the curve to CGRP. The TTX-resistant, receptor-mediated, CGRP-induced relaxation was unaffected by apamin (0.3 microM) and L-nitroarginine (L-NOARG, 100 microM), alone or in combination, as well as by glibenclamide (3 microM) or (S)-ketoprofen (10 microM). Tetraethylammonium (TEA, 1-10 mM) and cyclopiazonic acid (CPA, 3-10 microM) produced a concentration-dependent partial inhibition of the relaxant response to CGRP. The inhibitory effect of TEA on the maximal relaxation produced by CGRP was prevented by nifedipine (1 microM) which did not affect the CGRP-relaxation of its own. In the presence of atropine (1 microM), guanethidine (3 microM), SR 140,333 (0.3 microM), MEN 10,627 (1 microM), apamin (0.3 microM) and L-NOARG (100 microM), the application of 1 microM capsaicin produced a transient relaxation of the strips. This response was not reproduced upon a second application of capsaicin, 60 min later, indicating complete desensitization. CGRP(8-37) (0.3-1.0 microM) produced a partial inhibitory effect (about 50% inhibition) of the relaxant response to capsaicin. In the presence of atropine (1 microM), guanethidine (3 microM), SR 140,333 (0.3 microM), MEN 10,627 (1 microM), apamin (0.3 microM), L-NOARG (100 microM) and after capsaicin in vitro pretreatment (10 microM for 15 min), electrical field stimulation (EFS, 10 Hz for 5 s) produced a transient relaxation which was unchanged by CGRP(8-37) (1 microM) while being abolished by TTX. In sucrose gap, brief superfusion with 0.3 microM CGRP produced a TTX (1 microM)- resistant membrane hyperpolarization and relaxation: the hyperpolarization produced by CGRP was inhibited by about 50% by either TEA (10 mM) or CPA (10 microM), while being unaffected by glibenclamide (3 microM). The combined application of TEA and CPA was not more effective (65% inhibition) in inhibiting the CGRP-induced hyperpolarization than each drug alone. We conclude that CGRP produces a direct relaxation of the circular muscle of the guinea-pig proximal colon by activating receptors sensitive to blockade by CGRP(8-37). Activation of Ca-dependent potassium channels and Ca release/reuptake from internal store(s) appear both to be involved in the action of CGRP. Endogenous CGRP mediates part of the relaxant response evoked by stimulation of capsaicin-sensitive primary afferent nerves in the circular muscle of guinea-pig colon, while it is not involved in the apamin and L-NOARG-resistant nonadrenergic noncholinergic (NANC) relaxation produced by electrical field stimulation of intrinsic inhibitory nerves.

Neuronal tachykinin NK2 receptors mediate release of non-adrenergic non-cholinergic inhibitory transmitters in the circular muscle of guinea-pig colon

The aims of this study were: (i) verify the usefulness of the recently described non-peptide antagonist, SR 142801, for blocking tachykinin NK3 receptors in the circular muscle of the guinea-pig colon and (ii) after occlusion of NK3 receptors by SR 142801, test the hypothesis that tachykinins may activate non-adrenergic non-cholinergic inhibitory neurons via non-NK3 receptors. In sucrose gap, we found that SR 142801 (0.1 microM) time-dependently inhibited the senktide-induced atropine (1 microM)-sensitive depolarization, action potentials and contractions of circular muscle of guinea-pig colon without affecting the cholinergic excitatory junction potential and contraction produced by single pulse electrical field stimulation. Likewise, SR 142801 (0.1 microM) time-dependently inhibited the senktide-induced non-adrenergic non-cholinergic hyperpolarization and relaxation of the circular muscle, without affecting the non-adrenergic non-cholinergic inhibitory junction potentials and relaxation produced by single pulse electrical field stimulation. Therefore, SR 142801 is a suitable tool to occlude neuronal NK3 receptors in guinea-pig colon. In the presence of SR 142801 (0.1 microM), atropine (1 microM), guanethidine (3 microM), indomethacin (3 microM) and nifedipine (1 microM) superfusion with neurokinin A (0.3 microM) produced depolarization on which a series of inhibitory junction potentials were superimposed. The incidence, number and amplitude of the inhibitory junction potentials evoked by neurokinin A was partly reduced by pretreatment with either apamin (0.1 microM) or L-nitroarginine (30 microM) and was totally blocked by pretreatment with apamin plus L-nitroarginine or by tetrodotoxin (1 microM). None of these treatments affected the depolarization and contraction produced by neurokinin A. The NK1 receptor selective antagonist, GR 82,334 (3 microM), did not affect the responses to neurokinin A, which were abolished by the NK2 receptor-selective antagonist GR 94,800 (0.1 microM). Substance P (0.3 microM) produced a large depolarization of the membrane but was poorly effective in producing superimposed inhibitory junction potentials. The NK1 receptor-selective agonist [Sar9]substance P sulfone (0.3 microM) produced large depolarization without inducing superimposed inhibitory junction potentials, while the NK2 receptor-selective synthetic agonist [beta-Ala8]neurokinin A(4-10) (0.3 microM) produced depolarization and superimposed inhibitory junction potentials. We conclude that neurokinin A, in addition to direct excitation and contraction of circular muscle activates, via neuronal NK2 receptors, inhibitory non-adrenergic non-cholinergic motorneurons. Thus, neuronal NK2 receptors should be considered as targets for endogenous tachykinins in enteric circuitries leading to descending relaxation in guinea-pig colon.

Pre- and postjunctional effects of diadenosine polyphosphates in the guinea-pig vas deferens

The pre- and postjunctional activities of a number of diadenosine polyphosphates were examined in the guinea-pig isolated vas deferens at the level of the membrane potential, using a modified sucrose-gap technique. P1,P3-Di(adenosine 5')triphosphate (Ap3A), P1,P4-di(adenosine 5')tetraphosphate (Ap4A) and P1,P5-di(adenosine 5')pentaphosphate (Ap5A) all caused concentration-dependent depolarization of the smooth muscle membrane. The potency order was: Ap5A > Ap4A > or = Ap3A. P1,P2-Di(adenosine 5')pyrophosphate (Ap2A) did not evoke depolarization even at the highest concentration tested (1 mM). All the dinucleotides caused a reduction in the amplitude of evoked excitatory junction potentials (e.j.ps). The potency order was: Ap5A = Ap4A > Ap3A > Ap2A. The depolarizations evoked by the dinucleotides were markedly reduced by the selective P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM), as was the amplitude of the fully facilitated e.j.p. The inhibition of the e.j.p. evoked by Ap3A and Ap2A was reduced by the P1-purinoceptor antagonist, 8-p-sulphophenyltheophylline (8-pSPT, 50 microM), but that evoked by Ap5A and Ap4A was not. Thus, Ap3A, Ap4A and Ap5A evoke depolarization of the guinea-pig vas deferens via P2X-purinoceptors, and additionally Ap2A and Ap3A exert a prejunctional effect via P1-purinoceptors. The prejunctional activity of Ap4A and Ap5A is mediated via an undefined purinoceptor, which is neither P1 nor P2X.

CGRP inhibition of electromechanical coupling in the guinea-pig isolated renal pelvis

We aimed at studying the mechanism(s) of the inhibitory effect exerted by calcitonin gene-related peptide (CGRP) on the spontaneous activity of the guinea-pig isolated renal pelvis. In organ bath experiments, CGRP (1-100 nM) produced a concentration-dependent (EC50 8 nM) partial inhibition (Emax about 35% inhibition of motility index) of spontaneous contractions. The potassium (K) channel opener, cromakalim (3-10 microM) promptly suppressed the spontaneous contractions in a glibenclamide-(10 microM) sensitive manner. Glibenclamide (10 microM) did not affect the inhibitory action of CGRP. The calcium (Ca) channel agonist, Bay K 8644 (1 microM), markedly enhanced the spontaneous activity of the renal pelvis and reduced the inhibitory effect of CGRP. The protein kinase A inhibitors Rp-cAMPS (300 microM), H8 (100 microM) and H89 (10 microM), and the blockers of intracellular Ca handling by sarcoplasmic reticulum, ryanodine (100 microM) and thapsigargin (1 microM) did not affect the response to CGRP. The response to CGRP was likewise unaffected by the nitric oxide synthase inhibitor, L-nitroarginine (30 microM) and by the protein kinase G inhibitor, KT5823 (3 microM). Furthermore, the inhibitory action of CGRP was not modified by lowering the extracellular concentration of K (from 5.9 to 1.2 mM) nor by increasing (from 2.5 to 3.75 mM) or decreasing (from 2.5 to 0.25 mM) the extracellular Ca concentration. Replacement of 80% glucose with 2-deoxyglucose (2-DOG) reduced the amplitude of spontaneous contractions, both in the absence and presence of 10 microM glibenclamide. In the presence of 2-DOG, the inhibitory action of CGRP was enhanced at a similar extent, either in the absence or presence of glibenclamide. In sucrose gap, the effect of CGRP (0.1 microM for 5 min) was separately analyzed in the proximal (close to the kidney) and distal (close to the ureter) regions of the renal pelvis. Both preparations discharged spontaneous (pacemaker) action potentials having different shape, duration and frequently. CGRP had no effect on pacemaker potentials in the proximal renal pelvis while producing about 30% reduction of the frequency of pacemaker potentials and motility index in the distal renal pelvis. Cromakalim (3 microM) abolished pacemaker potentials in both regions of the renal pelvis. In conjunction with the results of previous studies in the guinea-pig ureter, the present findings document the existence of remarkable regional differences in the effector mechanisms initiated by CGRP receptor occupancy in the guinea-pig pyeloureteral tract. CGRP appears to be inherently unable to activate glibenclamide-sensitive K channels in the guinea-pig renal pelvis, a mechanism which is central for its ability to suppress latent pacemakers in the ureter. Within the renal pelvis, the sensitivity to the inhibitory effect of CGRP appears in the more distal region, from which an 'ureter-like' action potential is recorded.

Effects of vitamin E deficiency on autonomic neuroeffector mechanisms in the rat caecum, vas deferens and urinary bladder

1. Modified sucrose-gap, standard organ-bath techniques and transmitter release studies were used to examine neuromuscular transmission in the caecum, vas deferens and urinary bladder in normal rats and in rats maintained for 12 months on a diet free of vitamin E. 2. In the caecum circular muscle, non-adrenergic, non-cholinergic inhibitory junction potentials were absent from 48 and 15% of preparations from vitamin E-deficient and control animals, respectively. Cholinergic excitatory junction potentials were absent from 83 and 8% of vitamin E-deficient and control preparations, respectively. Responses to applied noradrenaline (0.1-30 microM), alpha,beta-methylene ATP (3-100 microM) and acetylcholine (0.1-30 microM) were attenuated or absent in vitamin E-deficient tissues. Responses to applied KCl were similar in both groups. Release of [3H]noradrenaline or endogenous acetylcholine could not be evoked from vitamin E-deficient tissues. 3. In contrast, in isolated preparations of the vas deferens and urinary bladder, neuromuscular transmission by adrenergic, cholinergic and purinergic components were unaffected by long-term vitamin E deficiency. 4. In conclusion, vitamin E deficiency causes dysfunction of autonomic neuroeffector mechanisms in the smooth muscle of the rat caecum, at both a pre- and postjunctional level. The lesions in autonomic transmission mechanisms brought about by long-term vitamin E deficiency were found only in the caecum; no changes in sympathetic neuromuscular transmission were observed in the vas deferens, or in parasympathetic neuromuscular transmission in the urinary bladder.

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.

Calcitonin gene-related peptide selectively increases cAMP levels in the guinea-pig ureter

Calcitonin gene-related peptide (CGRP, 0.1 microM) and forskolin (10 microM) both produced a time-dependent accumulation of cAMP in homogenates of the guinea-pig ureter, while cromakalim (3 microM) was ineffective. Neither agent did increase the cGMP levels. cAMP accumulation induced by CGRP or forskolin was unchanged by glibenclamide (1 microM). In sucrose gap, the application of forskolin (1-10 microM for 15 s) hyperpolarized the smooth muscle membrane and its effect was greatly enhanced when tested in a low-K+ medium (extracellular K+ reduced from 5.9 to 1.2 mM). The hyperpolarization produced by 10 microM forskolin was reduced and abolished by 1 and 10 microM glibenclamide, respectively, in both normal and low-K+ medium. The present findings demonstrate that CGRP determines a selective cAMP accumulation in the guinea-pig ureter and suggest that elevation of cAMP may be involved in the opening of glibenclamide-sensitive K+ channels in the ureter smooth muscle.

Modulation by stereoselective inhibition of cyclo-oxygenase of electromechanical coupling in the guinea-pig isolated renal pelvis

1. The effects of the (S)- and (R)-enantiomers of the cyclo-oxygenase (COX) inhibitor, ketoprofen, have been investigated on the spontaneous activity of the guinea-pig isolated renal pelvis and on electrical field stimulation-(EFS) induced contractions of the guinea-pig ureter in comparison with the effects of the achiral COX inhibitor, indomethacin. 2. (S)-ketoprofen (0.1-100 microM) produced a concentration- and time-dependent inhibition of the spontaneous myogenic activity of the renal pelvis. The maximal inhibitory effect (% inhibition of motility index) averaged 29, 42, 47 and 56% inhibition of control values at 0.1, 1, 10 and 100 microM. The (R)-enantiomer was ineffective up to 10 microM. 3. Indomethacin (0.1-100 microM) likewise produced a concentration- and time-dependent inhibition of spontaneous motility of the isolated renal pelvis: its maximal inhibitory effect was larger than that produced by (S)-ketoprofen and averaged 21, 40, 69 and 95% inhibition of motility index at 0.1, 1, 10 and 100 microM respectively. In the presence of a maximally effective (100 microM) concentration of (S)-ketoprofen, 100 microM indomethacin produced > 90% inhibition of residual motility. 4. In the guinea-pig isolated ureter, phasic contractions were induced by EFS (5 ms pulse width, 60 V): (S)-ketoprofen (100-500 microM) had no effect on the EFS-evoked contractions. Indomethacin (100-500 microM) produced a concentration-dependent inhibition and/or suppression of the EFS-evoked contractions. When contraction of the ureter was evoked by 80 mM KCl, indomethacin produced about 30 and 80% inhibition at 100 and 300 microM, respectively, while (S)-ketoprofen (300 microM) was ineffective. 5. The effect of (S)-ketoprofen or indomethacin (10 microM each) on the propagation of myogenic impulses along the ureter was determined by use of a three chamber organ bath. The renal end of the ureter was electrically stimulated while recording the mechanical activity of the renal and bladder ends of the ureter: addition of either (S)-ketoprofen or indomethacin (10 microM) did not effect propagation of impulses from the renal to the bladder end of the ureter, while nifedipine (10 microM) promptly blocked the propagated contractions. 6. In sucrose gap experiments, (S)-ketoprofen (10-100 microM) produced a time-dependent shortening of spontaneous action potentials of the guinea-pig renal pelvis and reduced the amplitude and duration of the accompanying phasic contractions. Indomethacin (10 microM) produced comparable effects on the same parameters and significantly reduced the maximal amplitude of depolarization of the pacemaker potential. In the presence of 100 microM (S)-ketoprofen, 100 microM indomethacin promptly suppressed the residual pacemaker potential and contraction.7. Neither (S)-ketoprofen nor indomethacin (10 microM each for 60 min) affected the parameters of action potential and contraction of the guinea-pig ureter evoked by EFS. Both drugs produced a sustained membrane depolarization.8. The present findings demonstrate that stereoselective COX inhibition affects pacemaker potentials and contractility (electromechanical coupling) in the guinea-pig renal pelvis. The modulatory role of endogenous prostanoids involves an amplification of electromechanical coupling in the renal pelvis while excitability, contractility or propagation of impulses along the ureter appear almost independent of prostanoid generation. Previous reports of a total suppression of pyeloureteral motility by indomethacin may reflect a combination of COX inhibition and nonspecific effect on electromechanical coupling.

Effect of the Ca(2+)-ATPase inhibitor, cyclopiazonic acid, on electromechanical coupling in the guinea-pig ureter

1. We have investigated the effect of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase inhibitor, cyclopiazonic acid (CPA), on electromechanical coupling in the guinea-pig ureter. All experiments were performed in capsaicin-pretreated (10 microM for 15 min) ureters to prevent the release of sensory neuropeptides from afferent nerves. 2. In organ bath experiments, electrical field stimulation (EFS, 10 Hz for 1 s, 5 ms pulse width, 60 V) produced tetrodotoxin- (1 microM) resistant phasic contractions which were enhanced by Bay K 8644 (1 microM) and abolished by nifedipine (10-30 microM). 3. CPA (10 microM) enhanced the EFS-evoked contractions both in the absence and presence of Bay K 8644. The effect of CPA was concentration-dependent between 1 and 30 microM. The response to 10 microM CPA was biphasic: the maximal enhancement (58 +/- 3% increase) was observed within 10-20 min from CPA administration, followed by a decline to a new steady state (25 +/- 5% increase over baseline) at 50-60 min. The effect of CPA was reversed by washout. 4. Ryanodine (100 microM) produced a prompt enhancement of the EFS-evoked contractions of the guinea-pig ureter, which peaked at 42 +/- 3% increase over baseline; the co-administration of CPA (10 microM) and ryanodine (100 microM) produced a peak effect (60 +/- 8% enhancement) which was not different from that produced by CPA alone. With either ryanodine alone or ryanodine plus CPA, the enhancement of the EFS-induced contractions was biphasic, showing a time-course similar to that observed with CPA alone. Tetraethylammonium (10 mM) produced a significantly larger effect (93 +/- 13% increase over baseline) and its effect was sustained throughout the 60 min observation period. 5. In the presence of Bay K 8644, superfusion for 30 min with a low Na+ medium (60% of extracellular Na+ replaced by Li+ or choline) reduced the amplitude of EFS-evoked contractions by 20-35%. In both Li(+)- and choline-substituted media, spontaneous activity developed during superfusion with low Na+ Krebs solution which was suppressed by 10 microM nifedipine. CPA (10 microM) produced a marked enhancement of the EFS-evoked contractions in low-Na+ medium (both Li(+)- and choline-substituted) and this effect was sustained throughout the 60 min observation period. 6. In the absence of Bay K 8644, the response of the ureter smooth muscle to EFS is characterized by a refractory period: an interval of about 30 s was required between two applied stimuli to produce a second response comparable in size to that elicited by the first stimulus. CPA (10 micro M, 10-20 min before)markedly reduced the refractory period of the guinea-pig ureter to EFS.7. CPA (10 micro M, 30-60 min before) increased the phasic component of contraction produced by 80 m MKCl. The tonic component of the response to KCl was slightly but not significantly reduced by CPA,and a 'hump' in the tonic contraction was observed at 1-2 min from addition of KCl.8. In sucrose gap experiments, 10 micro M CPA produced a sustained depolarization of the membrane and reduced the latency between application of electrical stimuli and onset of the action potential; these effects were maintained throughout the 60 min superfusion with CPA. CPA also transiently prolonged the plateau phase of the action potential and increased the peak amplitude of contraction: these effects peaked at about 10-20 min from start of superfusion with CPA and then declined. At the peak of its enhancing effect on contraction amplitude, CPA prolonged the contractile phase of the contraction relaxation cycle.9.Superfusion with a low-Na, choline-substituted Krebs solution produced a reversible membrane depolarization. In the presence of Bay K 8644 (1 micro M), action potentials and phasic contractions were superimposed on this depolarization which were abolished by nifedipine (1O micro M).10. These findings indicate that CPA augments the excitability and affects the contraction-relaxation cycle of the smooth muscle of the guinea-pig ureter, implying a role for sarcoplasmic reticulum Ca2+-ATPase in the regulation of electromechanical coupling. The effects of CPA resemble those produced by ryanodine and the effect of the two agents on the amplitude of contractions is non-additive.It appears that following blockade of the CPA-sensitive SR Ca2+ pump, other mechanism(s) may come into action to reduce intracellular Ca2+. The Na+/Ca2+ exchanger could be involved in the compensatory changes responsible for the fading of the response to CPA.

Tachykinin NK3 receptor mediates NANC hyperpolarization and relaxation via nitric oxide release in the circular muscle of the guinea-pig colon

In the presence of atropine (1 microM), guanethidine (3 microM) and of the tachykinin NK1 (SR 140,333 0.1 microM) and NK2 (GR 94,800 3 microM) receptor antagonists, the application of the tachykinin NK3 receptor selective agonist senktide, or that of neurokinin B, produced concentration-dependent sustained nonadrenergic noncholinergic (NANC) relaxation of mucosa-free circular muscle strips from the guinea-pig proximal colon. The maximal relaxant responses to senktide and neurokinin B were similar, approaching about 70% of the relaxation to 1 microM isoprenaline. Senktide (EC50 0.16 nM) was about 64-fold more potent than neurokinin B (EC50 10.3 nM). When tested in the presence of peptidase inhibitors (thiorphan 1 microM, captopril 1 microM and amastatin 10 microM), neurokinin B (EC50 0.24 nM) was equipotent to senktide (EC50 0.19 nM). At 1 nM, substance P and neurokinin A were ineffective in producing a NANC relaxation of the colon. At 1 microM substance P, neurokinin A and neurokinin B produced a NANC relaxation, which averaged 23, 40 and 79% of the maximal response to isoprenaline, respectively. In the presence of peptidase inhibitors, 1 nM substance P and neurokinin A produced threshold relaxant responses and, at 1 microM, the three natural tachykinins were equieffective (66 +/- 8, 72 +/- 5 and 75 +/- 5% of the relaxation to isoprenaline for substance P, neurokinin A and neurokinin B, respectively). The relaxant response to 1 nM senktide (producing about 70-80% of its maximal effect) was totally abolished by 1 microM tetrodotoxin and largely (> 90%) inhibited by 100 microM L-nitroarginine (L-NOARG). The inhibition by L-NOARG was partially reversed by L-arginine (3 mM) but not D-arginine. Apamin (1 microM) produced a slight (about 20%) inhibition of the response to senktide. The peptide blocker of N-type calcium channels, omega-conotoxin (0.1 microM) was ineffective. In sucrose gap electrophysiological experiments, superfusion with senktide (0.1 microM for 10 s) produced a slowly developing and prolonged hyperpolarization of the membrane and relaxation. Both effects were inhibited by L-NOARG while apamin had no effect. These findings indicate that a neuronal NK3 receptor mediates NANC hyperpolarization and relaxation of the circular muscle of the guinea-pig proximal colon, principally through the release of NO. NO generation/release in response to NK3 receptor stimulation does not require calcium influx through N-type calcium channels.

Inhibitory transmitter action of calcitonin gene-related peptide in guinea-pig ureter via activation of glibenclamide-sensitive K channels

1. In single sucrose gap, electrical field stimulation (EFS, 1-5 Hz) produced graded hyperpolarization of the membrane of the guinea-pig ureter smooth muscle, which was blocked by tetrodotoxin (0.3 microM) or in vitro capsaicin desensitization (3 microM for 15 min). Capsaicin itself produced a transient hyperpolarization of the membrane on its first application. 2. Superfusion with human alpha calcitonin gene-related peptide (CGRP, 30-300 mM) likewise produced a transient hyperpolarization of the membrane, mimicking the neurogenic inhibitory junction potential (i.j.p.). The hyperpolarization by CGRP was unaffected by tetrodotoxin, indicating a postjunctional site of action. 3. Both the EFS-evoked i.j.p. and the CGRP-induced hyperpolarization were inhibited by the CGRP receptor antagonist, CGRP(8-37) (0.3-3 microM) which did not affect the hyperpolarization produced by the KATP channel opener, cromakalim (0.3 microM). 4. The KATP channel blocker, glibenclamide (1 microM) blocked both the EFS-evoked i.j.p. and the CGRP-induced hyperpolarization. 5. When evoked in a low K medium (1.2 mM, KCl being replaced by an equimolar amount of NaCl), the EFS-evoked i.j.p. and the CGRP-induced hyperpolarization were both markedly enhanced, consistent with the idea that opening of K channels underlies both responses. 6. The present findings provide direct electrophysiological evidence for a neurotransmitter role of CGRP, released from the peripheral endings of capsaicin-sensitive primary afferent neurones, in the guinea-pig ureter. The action of both exogenous and endogenous CGRP involves the activation of glibenclamide-sensitive (KATP) potassium channels.

Electrophysiological evidence for different release mechanism of ATP and NO as inhibitory NANC transmitters in guinea-pig colon

1. The effect of the P2-purinoceptor antagonist, suramin, the specific N-type voltage-dependent calcium channel blocker, omega-conotoxin GVIA (omega-CgTx) and the delta-opioid receptor agonist [D-Pen2,D-Pen5] enkephalin (DPDPE) on the apamin-sensitive and apamin-resistant inhibitory junction potentials (i.j.ps) produced by electrical field stimulation (EFS) were investigated by means of a sucrose-gap technique in the circular muscle of the guinea-pig colon. 2. After incubation of muscle strips in either atropine (1 microM), guanethidine (3 microM) and NG-nitro-L-arginine (L-NOARG, 30 microM) or atropine, guanethidine and apamin (0.3 microM), the addition of the NK1 receptor antagonist, SR 140,333 (1 microM) abolished the non-adrenergic, non-cholinergic (NANC) excitatory junction potential (e.j.p.) and unmasked a pure apamin-sensitive i.j.p. (in the presence of L-NOARG) or a pure apamin-resistant i.j.p. (in the presence of apamin). Both types of i.j.p. were abolished by tetrodotoxin. 3. Suramin (30-300 microM) concentration-dependently inhibited the apamin-sensitive i.j.p., while the apamin-resistant i.j.p. was not significantly affected by suramin (up to 300 microM). L-NOARG (30 microM) markedly reduced the apamin-resistant i.j.p. 4. The delta-opioid receptor agonist, DPDPE (0.03-3 microM) concentration-dependently reduced the apamin-sensitive i.j.p., while leaving the apamin-resistant i.j.p. unaffected. Naloxone (1 microM) prevented the i.j.p. inhibition evoked by DPDPE (0.3 microM). 5. omega-CgTx (0.3 microM) markedly reduced the apamin-sensitive but not the apamin-resistant i.j.p. The application of DPDPE (3 MicroM), after development of a steady state inhibitory effect by omega-CgTx, evoked further inhibition of the apamin-sensitive ij.p., similar to the effect produced by DPDPE alone. The L-type calcium channel blocker, nifedipine (1 MicroM) did not significantly affect either the apamin-sensitive or the apamin-resistant ij.ps.6. These findings support the purinergic origin of the fast, apamin-sensitive ij.p. produced by EFS in the circular muscle of the guinea-pig colon and strongly suggest that the apamin-sensitive and the apamin-resistant components of the evoked ij.p. utilize different mechanisms for the secretion of theNANC transmitters, ATP and NO, respectively.

Effect of Bay K 8644 and ryanodine on the refractory period, action potential and mechanical response of the guinea-pig ureter to electrical stimulation

We have investigated the effect of the dihydropyridine calcium channel agonist, Bay K 8644, and of the plant alkaloid blocker of calcium-induced calcium release (CICR) from the sarcoplasmic reticulum, ryanodine, on the refractory period, action potential and mechanical response of the guinea-pig isolated ureter to electrical stimulation. All experiments were performed in ureters pre-exposed to 10 microM capsaicin to eliminate the inhibitory influence exerted by local release of sensory neuropeptides on ureteral excitability and contraction. In organ bath experiments, electrical field stimulation with parameters which produce direct excitation of ureteral smooth muscle (train of pulses at 10 Hz, 5 ms pulse width, 60 V for 1 s) produced tetrodotoxin- (1 microM) resistant phasic contractions. The response to EFS was abolished by nifedipine (1 nM-3 microM) and was enhanced by Bay K 8644 (1 nM-3 microM). In the presence of Bay K 8644 (1 microM), nifedipine (30 microM) abolished the evoked contractions. Ryanodine (10-100 microM) had no significant effect on the amplitude of evoked contraction. The response of the guinea-pig ureter to direct electrical stimulation of smooth muscle is characterized by a refractory period: at least 40 s interstimulus interval was required to produce a second response in all preparations tested. Bay K 8644 (1 microM) markedly reduced the refractory period of the ureter and a similar effect was observed with ryanodine (100 microM). To further analyze the effect of Bay K 8644 and ryanodine on the refractory period, the response of the ureter was investigated over a 10 s period of stimulation (other parameters as above). In control ureters, continuous stimulation for 10 s produced only one phasic contraction just after the beginning of the train of stimuli. In the presence of Bay K 8644 or ryanodine, more than one phasic contraction developed during a 10 s stimulation, i.e. the refractory period became shorter than the train duration. When both Bay K 8644 and ryanodine were tested on the same preparations, an additive excitatory effect was observed on the mechanical response to electrical stimulation. A slight elevation of KCl concentration (5-10 mM) reduced the refractory period of the ureter as observed with ryanodine or Bay K 8644. Application of KCl (80 mM) produced a biphasic contractile response of the ureter: a series of phasic contractions occurred first, which were then replaced by a slowly developing tonic response. Bay K 8644 (1 microM) enhanced both components of the response to KCl. Ryanodine (10 and 100 microM) markedly prolonged the duration of phasic contractions evoked by KCl and, at 100 microM, slightly (about 25%) reduced the amplitude of tonic contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Different Ca2+ influx pathways mediate tachykinin receptor-induced contraction in circular muscle of guinea-pig colon

We used an electrophysiological approach (single sucrose gap) to compare the mechanism of action of selective tachykinin NK1 and NK2 receptor agonists ([Sar9]substance P sulfone and [beta ala8]neurokinin A-(4-10), respectively) in producing contraction of the circular muscle of the guinea-pig proximal colon. [Sar9]Substance P sulfone produced a marked depolarization, action potentials and increase in membrane conductance. On the other hand, [beta Ala8]neurokinin A-(4-10) produced less depolarization of the cell membrane and did not change membrane resistance. Nifedipine (1 microM) greatly reduced (80% inhibition) the contraction due to [Sar9]substance P sulfone while that due to [beta Ala8]neurokinin A-(4-10) was slightly affected (13% inhibition). Action potentials induced by either agonist were suppressed by nifedipine, while depolarization was reduced only to a minor extent. When tested in a Ca(2+)-free medium, the contraction produced by either agonist was greatly reduced (84-89%) as compared to the control. In organ bath experiments [Sar9]substance P sulfone and [beta Ala8]neurokinin A-(4-10) produced concentration-dependent contraction of the circular muscle of the colon (EC50 8 and 12 nM, respectively). Nifedipine (1 microM) markedly suppressed the response to [Sar9]substance P sulfone while that to [beta Ala8]neurokinin A-(4-10) was only slightly depressed. These findings demonstrate that NK1 receptor-mediated contraction is strictly linked to membrane depolarization and action potentials generation through nifedipine-sensitive Ca2+ channels (electromechanical coupling) while the NK2 receptor-mediated contraction is substantially unrelated to depolarization and, while being largely dependent upon extracellular Ca2+, is nifedipine-resistant, possibly linked to the opening of non-selective (Ca(2+)-permeable) receptor-gated cation channels (pharmacomechanical coupling).

Effect of cromakalim and glibenclamide on spontaneous and evoked motility of the guinea-pig isolated renal pelvis and ureter

1. We have investigated the effect of the potassium (K) channel opener, cromakalim, on the spontaneous myogenic activity of the guinea-pig isolated renal pelvis and on myogenic contractions evoked by direct electrical stimulation of the guinea-pig isolated ureter. 2. In the presence of Bay K 8644 (1 microM), electrical stimulation of the guinea-pig ureter (10 Hz for 1 s, pulse width 5 ms, 60 V) produced regular tetrodotoxin-(1 microM) resistant phasic contractions which were suppressed by 3 microM cromakalim. Glibenclamide (0.1-3 microM), 4-aminopyridine (4-AP, 0.1-2 mM) and tetraethylammonium (TEA, 1-10 mM) produced a concentration-dependent inhibition of the effect of cromakalim with the rank order of potency (EC50 in parentheses): glibenclamide (0.64 microM) >> 4-AP (1.11 mM) > TEA (6.6 mM). Apamin (0.1-0.3 microM) was without effect. 3. Cromakalim (0.1-10 microM) produced concentration-dependent inhibition and suppression of spontaneous contractions of the guinea-pig isolated renal pelvis and of evoked contractions of the ureter with EC50 values of 0.71 and 0.47 microM, respectively. 4. Glibenclamide (1 microM) produced a rightward shift of the concentration-response curve to cromakalim in both the renal pelvis and ureter, without producing depression of the maximal inhibitory effect. Glibenclamide did not affect the spontaneous activity of the renal pelvis while it produced a slight enhancement (10-15% increase) of evoked contractions of the ureter. Glibenclamide did not affect the inhibitory action of the adenylate cyclase activator, forskolin, in the renal pelvis or ureter. 5. In electrophysiological experiments (sucrose gap), cromakalim (0.3 and 1 microM) produced hyperpolarization of ureter smooth muscle. Cromakalim also produced a transient suppression of action potentials and accompanying phasic contractions evoked by electrical stimulation. Before suppression of evoked contractions, a shortening of action potential duration was observed concomitant with the developing hyperpolarization produced by cromakalim. A lower concentration (0.1 MicroM) of cromakalim did not affect membrane potential but shortened action potential duration and reduced the evoked contraction.6. Glibenclamide (1 MicroM) inhibited the hyperpolarizing action of cromakalim and prevented its inhibitory action on evoked action potentials and contractions of the ureter. Glibenclamide also produced a slight prolongation of action potential duration and increased the amplitude and duration of the accompanying mechanical response.7. These findings demonstrate that activation of cromakalim- and glibenclamide-sensitive K channels produces a powerful mechanism for regulation of pyeloureteral motility and suppression of latent pacemakers of the ureter in guinea-pig. Glibenclamide-sensitive K channels take part in determining action potential shape and duration in the guinea-pig ureter.

Tachykinin NK1 but not NK2 receptors mediate non-cholinergic excitatory junction potentials in the circular muscle of guinea-pig colon

1. The effect of tachykinin NK1 and NK2 receptor antagonists on noncholinergic excitatory junction potentials (e.j.ps) evoked by electric field stimulation (EFS) in the circular muscle of the guinea-pig proximal colon was investigated by means of a sucrose-gap technique. 2. In the presence of 1 microM atropine, submaximal EFS (10 Hz, 20-30 V, 0.5 ms pulse width, 1 s train duration) evoked an inhibitory junction potential (i.j.p.) followed by e.j.p. with superimposed action potentials (APs) and contraction. Addition of either NG-nitro-L-arginine (L-NOARG, 0.1 mM) or apamin (0.1 microM) inhibited the evoked i.j.p. and the combined administration of the two agents almost abolished it. In the presence of both L-NOARG and apamin, an atropine-resistant e.j.p. was the only electrical response evoked by EFS in 50% of cases and a small i.j.p. (10% of original amplitude) followed by e.j.p. was evident in the remainder. 3. In the presence of L-NOARG and apamin, the tachykinin NK1 receptor antagonists, (+/-)-CP 96,345 and GR 82,334 (10 nM-3 microM) concentration-dependently inhibited the atropine-resistant e.j.p. and accompanying contraction evoked by EFS. EC50 values were: 0.77 microM (e.j.p. inhibition) and 0.22 microM (inhibition of contraction) for (+/-)-CP 96,345; 0.61 microM (e.j.p. inhibition) and 0.20 microM (inhibition of contraction) for GR 82,334. The tachykinin NK2 receptor antagonists, MEN 10,376 (up to 3 microM) and SR 48,968 (up to 1 microM) had no effect on the atropine-resistant e.j.p. MEN 10,376 (3 microM) but not SR 48,968 produced a slight inhibition of the evoked contraction. 4. (+/- )-CP 96,345 (3 microM) and GR 82,334 (3 microM) markedly reduced (81 and 89% inhibition, respectively)the atropine-resistant ej.p. in the absence of L-NOARG and apamin, without affecting the ij.p. MEN 10,376 (3 microM) and SR 48,968 (1 microM) had no significant effect on noncholinergic ij.p. and ej.p. evoked in the absence of apamin and L-NOARG.5. The electrical and mechanical responses to the NK, receptor agonist [Sar9]substance P (SP) sulfone were blocked by (+/-)-CP 96,345 (3 1M) or GR 82,334 (3 microM) which, at the same concentration, failed to affect the responses to the NK2 receptor agonist [PAla8] neurokinin A (NKA) (4-10). In contrast, MEN10,376 (3 microM) or SR 48,968 (1 microM) blocked the response to [beta Ala8]NKA(4-10) without affecting the response to [Sar9]SP sulfone.6. In the presence of L-NOARG and apamin, and in the absence of atropine, EFS of low pulse width(0.02-0.03 ms, other parameters as above) produced cholinergic ej.ps and contraction which were unaffected by GR 82,334 (3 microM). (+/-)-CP 96,345 (3 JAM) produced 24% reduction in the area of the atropine-sensitive ej.p. without affecting the peak amplitude of ej.p. or contraction.7. These findings demonstrate that the noncholinergic ej.ps and accompanying contraction of the circular muscle of the guinea-pig colon are produced through activation of intramural tachykininergic nerves and that the resultant smooth muscle response is almost entirely mediated through NK1 receptors.

An electrophysiological study of developmental changes in the innervation of the guinea-pig taenia coli

A sucrose-gap technique was used to study the development of neuromuscular transmission in the taenia coli of fetal, 1- to 2-day-old, 3- to 4-week-old and 3-month-old guinea-pigs. In addition, the effects of exogenous, alpha,beta-methylene adenosine 5'-triphosphate (ATP), noradrenaline, vasoactive intestinal polypeptide (VIP) and carbachol were examined. Taking the response to a single pulse of electrical field stimulation as the index of a developed neuromuscular junction, it was apparent that the non-adrenergic inhibitory system arose before, and matured more quickly than, the cholinergic system. The inhibitory system was present by 8 weeks of gestation in some fetuses, but, in contrast, excitatory cholinergic transmission was not seen until birth. As evidenced by responses to carbachol, alpha,beta-methylene ATP and VIP, cholinergic, purinergic and VIP receptors were present on the smooth muscle at the earliest ages studied. No changes in sensitivity to these agents were noted throughout development, although in adults the level of the maximum responses increased.

An in vitro electrophysiological study of the colon from patients with idiopathic chronic constipation

Preparations of the circular muscle layer from the sigmoid colon resected from patients with idiopathic chronic constipation were compared, at an electrophysiological level using the sucrose-gap technique, with preparations of the same region of the intestine resected from patients with rectal carcinoma. Non-adrenergic, non-cholinergic inhibitory neuromuscular transmission, represented by inhibitory junction potentials, was present in preparations from both groups. However, the inhibitory response in preparations from constipated patients had a slower or longer time-course than in those from cancer patients. Also, rebound activity following inhibitory transmission was observed in 34% of preparations from constipated patients but was observed in 67% of preparations from cancer patients. Preparations from both groups displayed the same patterns of spontaneous activity and the same proportion of each group was quiescent. The threshold for generation of action potentials and the passive resistance of the smooth muscle membrane were the same in both groups. However, quiescent preparations from constipated patients were less likely to discharge trains of action potentials when the smooth muscle membrane was depolarized than were preparations from cancer patients. These changes in transmission processes and excitability in tissue from constipated patients are discussed in relation to altered states of colonic motility found in people with idiopathic chronic constipation.

Hereditary internal anal sphincter myopathy causing proctalgia fugax and constipation. A newly identified condition

A newly identified myopathy of the internal anal sphincter is described. In the affected family, at least one member from each of five generations had severe proctalgia fugax; onset was usually in the third to fifth decades of life. Three members of the family have been studied in detail. Each had severe pain intermittently during the day and hourly during the night. Constipation was an associated symptom, in particular difficulty with rectal evacuation. Clinically the internal anal sphincter was thickened and of decreased compliance. The maximum anal canal pressure was usually increased with marked ultraslow wave activity. Anal endosonography confirmed a grossly thickened internal anal sphincter. Two patients were treated by internal anal sphincter strip myectomy; one showed marked improvement and one was relieved of the constipation but had only slight improvement of the pain. The hypertrophied muscle in two of the patients showed unique myopathic changes, consisting of vacuolar changes with periodic acid-Schiff-positive polyglycosan bodies in the smooth muscle fibers and increased endomysial fibrosis. In vitro organ-bath studies showed insensitivity of the muscle to noradrenaline, isoprenaline, carbachol, dimethylpiperazinium, and electrical-field stimulation. Immunohistochemical studies for substance P, calcitonin gene-related peptide, galanin, neuropeptide Y, and vasoactive intestinal peptide showed staining in a similar distribution to that in control tissue. A specific autosomal-dominant inherited myopathy of the internal anal sphincter that causes anal pain and constipation has been identified and characterized.

Enkephalins modulate inhibitory neuromuscular transmission in circular muscle of human colon via delta-opioid receptors

1. A sucrose-gap technique was used to investigate the neuromodulatory actions of enkephalins on non-adrenergic, non-cholinergic inhibitory junction potentials (IJPs) in the circular muscle of the human large intestine. 2. The native enkephalins, [Leu5]enkephalin (LENK) and [Met5]enkephalin (MENK) caused a concentration-dependent reduction in amplitude of IJPs without a significant effect on the smooth muscle membrane. 3. The actions of LENK and MENK were mimicked by the delta-selective opioid receptor agonists [D-Pen2, D-Pen5]enkephalin (DPDPE) and [D-Ala2, D-Leu5]enkephalin (DADLE). 4. The actions of LENK, MENK and DPDPE were antagonized to similar extents by the delta-selective opioid receptor antagonist ICI 174,864. 5. The mu-selective opioid receptor agonist [D-Ala2, Me Phe, Gly-ol5]enkephalin was approximately 100-fold less potent than any of the native or synthetic enkephalins at reducing the amplitude of the IJP. Dynorphin A and beta-endorphin both had very weak activity. 6. Responses to all of the agonists were inhibited by naloxone. The degree of antagonism of DPDPE or DADLE by naloxone (1 microM) was the same as that of LENK or MENK. 7. Neither MENK nor LENK affected hyperpolarization of the smooth muscle membrane induced by ATP or 5-hydroxytryptamine. Vasoactive intestinal polypeptide (1 pM-1 microM) did not produce any observable responses and this lack of reactivity was not affected by the enkephalins. 8. It is concluded that in the circular muscle of the human colon, LENK and MENK can act on prejunctional delta-opioid receptors to produce inhibition of non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. Possible physiological significance of this prejunctional receptor is discussed.

Pre- and postjunctional actions of purine and xanthine compounds in the guinea-pig caecum circular muscle

1. The sucrose-gap technique was used to study pre- and postjunctional actions of P1-purinoceptor and P2-purinoceptor agonists and a range of xanthine derivatives in the guinea-pig caecum circular muscle. 2. Adenosine, 2-chloroadenosine (2-ClAd), ATP and alpha,beta-methylene ATP all caused concentration-dependent hyperpolarization of the smooth muscle membrane with a rank order of potency of 2-ClAd greater than alpha,beta-methylene ATP greater than adenosine. 3. The xanthine derivatives caffeine, theophylline, 8-phenyltheophylline and 1,3-dipropyl-8-(2-amino-4-chlorophenyl) xanthine (PACPX) at submicromolar concentrations evoked depolarization of the smooth muscle membrane. At higher concentrations, all these compounds and enprofylline caused concentration-dependent hyperpolarization. 4. All the purine compounds tested caused a reduction in the amplitude of the non-adrenergic, non-cholinergic inhibitory junction potential (i.j.p.). For the P1-purinoceptor agonists adenosine and 2-ClAd this was almost entirely a prejunctional effect. For the P2-purinoceptor agonists this was mostly a postjunctional effect because both ATP and alpha,beta-methylene ATP caused significantly greater increases in the conductance of the smooth muscle membrane than did adenosine or 2-ClAd. 5. All the xanthine compounds tested (up to 100 microM), except enprofylline, were capable of increasing the amplitude of the i.j.p. At millimolar concentrations both caffeine and theophylline could reduce the i.j.p. amplitude. 6. It is concluded that there are inhibitory prejunctional P1-purinoceptors on the i.j.p.-producing neurones in the guinea-pig caecum circular muscle and that, of the xanthine derivatives tested, none of them would be suitable to use as a P1-purinoceptor antagonist in this preparation because of their own direct effects.

Adrenergic, but not cholinergic or purinergic, responses are potentiated in the cecum of diabetic rats

Electrophysiologic properties of smooth muscle strips from the circular muscle of the cecum were compared in streptozocin-treated diabetic (8 wk) and untreated control rats using the sucrose-gap technique. Changes in membrane potential elicited by field stimulation (0.03-32 Hz) and by exogenously applied adenosine triphosphate (10-1000 microM) and noradrenaline (0.03-10 microM) were measured. Nonadrenergic, noncholinergic inhibitory junction potentials were the predominant response to field stimulation. However, in some preparations from both diabetic and control tissues, the inhibitory junction potential was preceded by a small nonadrenergic, noncholinergic excitatory junction potential. In nonatropinized preparations, a cholinergic excitatory junction potential was occasionally elicited; there was no difference between these cholinergic responses in diabetic and control tissues. The inhibitory junction potentials and the hyperpolarization in response to adenosine triphosphate were similar in diabetic and control tissues, although the rate of hyperpolarization of the single inhibitory junction potential was slower in the diabetic tissues. In contrast, exogenous application of noradrenaline revealed significantly greater hyperpolarizing responses in diabetic compared with control tissues. This increase in potency appeared to be due, in part, to an increased sensitivity of alpha-adrenoceptors on smooth muscle. There was no evidence for beta-adrenoceptor activation by noradrenaline. Prejunctional inhibition of the nonadrenergic, noncholinergic neuromuscular transmission by noradrenaline was not affected by streptozocin-induced diabetes. The induced changes in adrenoceptor activity were selective for the postjunctional alpha-adrenoceptors.