Opioid-like adverse effects of tianeptine in male rats and mice

RATIONALE

Tianeptine is a mu-opioid receptor (MOR) agonist with increasing reports of abuse in human populations. Preclinical data regarding the abuse potential and other opioid-like adverse effects of tianeptine at supratherapeutic doses are sparse.

OBJECTIVES

The present study evaluated tianeptine in a rat model of abuse potential assessment and in mouse models of motor, gastrointestinal, and respiratory adverse effects.

METHODS

Abuse potential was assessed in adult male Sprague-Dawley rats using an intracranial self-stimulation (ICSS) procedure to determine effects of acute and repeated tianeptine on responding for electrical brain stimulation. Male ICR mice were used to determine the effects of tianeptine in assays of locomotor behavior and gastrointestinal motility. Male Swiss-Webster mice were monitored for respiratory changes using whole-body plethysmography.

RESULTS

In rats, acute tianeptine produced weak and delayed evidence for abuse-related ICSS facilitation at an intermediate dose (10 mg/kg, IP) and pronounced, naltrexone-preventable ICSS depression at a higher dose (32 mg/kg, IP). Repeated 7-day tianeptine (10 and 32 mg/kg/day, IP) produced no increase in abuse-related ICSS facilitation, only modest tolerance to ICSS depression, and no evidence of physical dependence. In mice, tianeptine produced dose-dependent, naltrexone-preventable locomotor activation. Tianeptine (100 mg/kg, SC) also significantly inhibited gastrointestinal motility and produced naloxone-reversible respiratory depression.

CONCLUSIONS

Tianeptine presents as a MOR agonist with resistance to tolerance and dependence in our ICSS assay in rats, and it has lower abuse potential by this metric than many commonly abused opioids. Nonetheless, tianeptine produces MOR agonist-like acute adverse effects that include motor impairment, constipation, and respiratory depression.

Site and mechanism of morphine tolerance in the gastrointestinal tract

Opioid-induced constipation is a major clinical problem. The effects of morphine, and other narcotics, on the gastrointestinal tract persist over long-term use thus limiting the clinical benefit of these excellent pain relievers. The effects of opioids in the gut, including morphine, are largely mediated by the μ-opioid receptors at the soma and nerve terminals of enteric neurons. Recent studies demonstrate that regional differences exist in both acute and chronic morphine along the gastrointestinal tract. While tolerance develops to the analgesic effects and upper gastrointestinal motility upon repeated morphine administration, tolerance does not develop in the colon with chronic opioids resulting in persistent constipation. Here, we review the mechanisms by which tolerance develops in the small but not the large intestine. The regional differences lie in the signaling and regulation of the μ-opioid receptor in the various segments of the gastrointestinal tract. The differential role of β-arrestin2 in tolerance development between central and enteric neurons defines the potential for therapeutic approaches in developing ligands with analgesic properties and minimal constipating effects.

Brain-derived neurotrophic factor enhances cholinergic contraction of longitudinal muscle of rabbit intestine via activation of phospholipase C

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family of proteins best known for its role in neuronal survival, differentiation, migration, and synaptic plasticity in central and peripheral neurons. BDNF is also widely expressed in nonneuronal tissues including the gastrointestinal tract. The role of BDNF in intestinal smooth muscle contractility is not well defined. The aim of this study was to identify the role of BDNF in carbachol (CCh)- and substance P (SP)-induced contraction of intestinal longitudinal smooth muscle. BDNF, selective tropomyosin-related kinase B (TrkB) receptor agonists, and pharmacological inhibitors of signaling pathways were examined for their effects on contraction of rabbit intestinal longitudinal muscle strips induced by CCh and SP. BDNF activation of intracellular signaling pathways was examined by Western blot in homogenates of muscle strips and isolated muscle cells. One-hour preincubation with BDNF enhanced intestinal muscle contraction induced by CCh but not by SP. The selective synthetic TrkB agonists LM 22A4 and 7,8-dihydroxyflavone produced similar effects to BDNF. The Trk antagonist K-252a, a TrkB antibody but not p75NTR antibody, blocked the effect of BDNF. The enhancement of CCh-induced contraction by BDNF was blocked by the phospholipase C (PLC) antagonist U73122, but not by ERK1/2 or Akt antagonists. Direct measurement in muscle strips and isolated muscle cells showed that BDNF caused phosphorylation of TrkB receptors and PLC-γ, but not ERK1/2 or Akt. We conclude that exogenous BDNF augments the CCh-induced contraction of longitudinal muscle from rabbit intestine by activating TrkB receptors and subsequent PLC activation.

Ion channel remodeling in gastrointestinal inflammation

BACKGROUND

 Gastrointestinal inflammation significantly affects the electrical excitability of smooth muscle cells. Considerable progress over the last few years have been made to establish the mechanisms by which ion channel function is altered in the setting of gastrointestinal inflammation. Details have begun to emerge on the molecular basis by which ion channel function may be regulated in smooth muscle following inflammation. These include changes in protein and gene expression of the smooth muscle isoform of L-type Ca(2+) channels and ATP-sensitive K(+) channels. Recent attention has also focused on post-translational modifications as a primary means of altering ion channel function in the absence of changes in protein/gene expression. Protein phosphorylation of serine/theronine or tyrosine residues, cysteine thiol modifications, and tyrosine nitration are potential mechanisms affected by oxidative/nitrosative stress that alter the gating kinetics of ion channels. Collectively, these findings suggest that inflammation results in electrical remodeling of smooth muscle cells in addition to structural remodeling.

PURPOSE

The purpose of this review is to synthesize our current understanding regarding molecular mechanisms that result in altered ion channel function during gastrointestinal inflammation and to address potential areas that can lead to targeted new therapies.

The effect of tyrosine nitration of L-type Ca2+ channels on excitation-transcription coupling in colonic inflammation

BACKGROUND AND PURPOSE

Excitation-transcriptional coupling involves communication between plasma membrane ion channels and gene expression in the nucleus. Calcium influx through L-type Ca(2+) channels induces phosphorylation of the transcription factor, cyclic-AMP response element binding protein (CREB) and downstream activation of the cyclic-AMP response element (CRE) promoter regions. Tyrosine nitration of Ca(2+) channels attenuates interactions with c-Src kinase, decreasing Ca(2+) channel currents and smooth muscle contraction during colonic inflammation. In this study we examined the effect of tyrosine nitration and colonic inflammation on Ca(2+) channel mediated phosphorylation of CREB and CRE activation.

EXPERIMENTAL APPROACH

CREB and phospho-CREB were detected by Western blots and CRE activation measured by dual luciferase assay. Chinese hamster ovary (CHO) cells were transfected with hCa(v)1.2b and hCa(v)1.2b c-terminal mutants. Colonic inflammation was induced by intracolonic instillation of 2,4,6 trinitrobenzene sulphonic acid in mouse colon.

KEY RESULTS

In hCa(v)1.2b transfected CHO cells and in native colonic smooth muscle, depolarization with 80 mM KCl induced CREB phosphorylation (pCREB). Treatment with peroxynitrite inhibited KCl-induced pCREB. Following experimental colitis, KCl-induced CREB phosphorylation was abolished in smooth muscle, concomitant with tyrosine nitration of Ca(2+) channels. Depolarization increased CRE activation in hCa(v)1.2b CHO cells by 2.35 fold which was blocked by nifedipine and by protein nitration of Ca(2+) channels with peroxynitrite. The Src-kinase inhibitor, PP2, blocked depolarization-induced CRE activation. Mutation of the C-terminus tyrosine residue, Y2134F, but not Y1861F, blocked CRE activation.

CONCLUSIONS AND IMPLICATIONS

Post-translational modification of Ca(2+) channels due to tyrosine nitration modified excitation-transcriptional coupling in colonic inflammation.

Acute colitis enhances responsiveness of lumbosacral spinal neurons to colorectal distension in rats

Aim of this study was to examine excitability and responsiveness of lumbosacral spinal neurons to colorectal distension (CRD) in rats with colitis induced by dextran sulphate sodium (DSS). Extracellular potentials of single L6-S2 spinal neurons were recorded in pentobarbital anesthetized and paralyzed rats. Results showed that 40/154 (26%) and 53/156 (34%) neurons responded to noxious CRD (80 mmHg, 20 s) in DSS-treated and control animals, respectively. Neurons with long-lasting and low-threshold excitatory responses to CRD were more frequently encountered in DSS-treated than in control groups (P < 0.05). The mean maximal excitatory responses of neurons to noxious CRD in DSS-treated animals were significantly greater and the duration of responses was longer than those in control animals (P < 0.05). It was suggested that lumbosacral spinal neurons with colorectal input had increased excitability and responsiveness following colitis, which might play an important role in development of colonic hypersensitivity and viscerosomatic referred pain.

Hyperexcitability of convergent colon and bladder dorsal root ganglion neurons after colonic inflammation: mechanism for pelvic organ cross-talk

Clinical studies reveal concomitant occurrence of several gastrointestinal and urologic disorders, including irritable bowel syndrome and interstitial cystitis. The purpose of this study was to determine the mechanisms underlying cross-organ sensitization at the level of dorsal root ganglion (DRG) after acute and subsided gastrointestinal inflammation. DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and Fast Blue were injected into the distal colon and urinary bladder of male rats, respectively. Convergent DRG neurons were found in L1-L3 and L6-S2 ganglia with an average distribution of 14% +/- 2%. The resting membrane potential (RMP) of cells isolated from upper lumbar (UL) ganglia was -59.8 +/- 2.7 mV, whereas lumbosacral (LS) neurons were more depolarized (RMP = -49.4 +/- 2.1 mV, P < or = 0.05) under control conditions. Acute trinitrobenzene sulfonic acid (TNBS) colitis (3 days) decreased voltage and current thresholds for action potential firing in LS but not UL convergent capsaicin-sensitive neurons. This effect persisted for 30 days in the absence of overt colonic inflammation. The current threshold for action potential (AP) firing in UL cells was also decreased from 165.0 +/- 24.5 pA (control) to 85.0 +/- 19.1 pA at 30 days (P < or = 0.05), indicating increased excitability. The presence of a subpopulation of colon-bladder convergent DRG neurons and their persistent hyperexcitability after colonic inflammation provides a basis for pelvic organ cross-sensitization.

Altered ion channel activity in murine colonic smooth muscle myocytes in an experimental colitis model

We have investigated the activity of calcium and potassium channels in a murine model of experimental colitis. Colonic myocytes from dextran sulphate sodium (DSS)-treated mice were examined by whole cell patch clamp techniques. Myeloperoxidase activity was enhanced 3. 5-fold in DSS-treated mouse colon. In whole cell voltage clamp, depolarization predominantly evoked net transient outward currents in DSS-treated mice and inward Ca(2+) currents in control myocytes. Voltage-dependent L-type Ca(2+) currents were studied using intracellular Cs(+) in the patch pipette. Inward Ca(2+) currents were markedly suppressed in inflamed colon. The peak currents at +10 mV depolarization were -3.93 +/- 0.88 pA/pF in control (n = 12) and -1.14 +/- 0.19 (n = 10) in DSS mice. In contrast there was no change in the amplitude, kinetics, or steady-state inactivation properties of the transient outward currents in control or DSS-treated colonic myocytes. Inflammation significantly enhanced activation of the ATP-sensitive K(+) channel. At a holding potential of -50 mV, the K(ATP) channel opener lemakalim induced an inward current of 2.02 +/- 0.5 pA/pF in control (n = 20) and 4.19 +/- 1.17 pA/pF in DSS-treated colon. These currents were abolished by glibenclamide. The present results suggest that inflammation of the colon results in selective changes in ion channel activity of smooth muscle cells.

Role of HERG-like K(+) currents in opossum esophageal circular smooth muscle

An inwardly rectifying K(+) conductance closely resembling the human ether-a-go-go-related gene (HERG) current was identified in single smooth muscle cells of opossum esophageal circular muscle. When cells were voltage clamped at 0 mV, in isotonic K(+) solution (140 mM), step hyperpolarizations to -120 mV in 10-mV increments resulted in large inward currents that activated rapidly and then declined slowly (inactivated) during the test pulse in a time- and voltage- dependent fashion. The HERG K(+) channel blockers E-4031 (1 microM), cisapride (1 microM), and La(3+) (100 microM) strongly inhibited these currents as did millimolar concentrations of Ba(2+). Immunoflourescence staining with anti-HERG antibody in single cells resulted in punctate staining at the sarcolemma. At membrane potentials near the resting membrane potential (-50 to -70 mV), this K(+) conductance did not inactivate completely. In conventional microelectrode recordings, both E-4031 and cisapride depolarized tissue strips by 10 mV and also induced phasic contractions. In combination, these results provide direct experimental evidence for expression of HERG-like K(+) currents in gastrointestinal smooth muscle cells and suggest that HERG plays an important role in modulating the resting membrane potential.

Monochloramine directly modulates Ca(2+)-activated K(+) channels in rabbit colonic muscularis mucosae

BACKGROUND & AIMS

Mesenteric ischemia, infection, and inflammatory bowel disease may eventuate in severe colitis, complicated by toxic megacolon with impending intestinal perforation. Monochloramine (NH(2)Cl) is a membrane-permeant oxidant generated during colitis by the large amount of ambient luminal NH(3) in the colon. Reactive oxygen metabolites can modulate smooth muscle ion channels and thereby affect colonic motility, which is markedly impaired in colitis.

METHODS

Effects of NH(2)Cl on ionic currents in the innermost smooth muscle layer of the colon, the tunica muscularis mucosae, were examined using the patch clamp technique. Membrane potential in whole tissue strips was measured using high-resistance microelectrodes.

RESULTS

Whole cell voltage clamp experiments showed that NH(2)Cl (3-30 micromol/L) enhanced outward currents in a dose-dependent manner, increasing currents more than 8-fold at a test potential of +30 mV. Tail current analysis showed that the currents enhanced by NH(2)Cl were K(+) currents. Inhibition by tetraethylammonium and iberiotoxin suggested that these currents represented activation of large-conductance, Ca(2+)-activated K(+) channels. The membrane-impermeant oxidant taurine monochloramine, however, had no effect on whole cell currents. Single-channel studies in inside-out patches showed that NH(2)Cl increased open probability of a 257-pS channel in symmetrical (140 mmol/L) K(+). In the presence of NH(2)Cl, the steady-state voltage dependence of activation was shifted by -22 mV to the left with no change in the single-channel amplitude. The sulfhydryl alkylating agent N-ethylmaleimide prevented NH(2)Cl-induced channel activation. NH(2)Cl also hyperpolarized intact muscle strips, an effect blocked by iberiotoxin.

CONCLUSIONS

NH(2)Cl, at concentrations expected to be found during colitis, may contribute to smooth muscle dysfunction by a direct oxidant effect on maxi K(+) channels.

Modulation of voltage-dependent Ca2+ channels in rabbit colonic smooth muscle cells by c-Src and focal adhesion kinase

There is emerging evidence indicating that smooth muscle contraction and Ca2+ influx through voltage-dependent L-type Ca2+ channels are regulated by tyrosine kinases; however, the specific kinases involved are largely unknown. In rabbit colonic muscularis mucosae cells, tyrosine-phosphorylated proteins of approximately 60 and 125 kDa were observed in immunoblots using an anti-phosphotyrosine antibody and were identified as c-Src and focal adhesion kinase (FAK) by immunoblotting with specific antibodies. FAK co-immunoprecipitated with c-Src, and the phosphorylation of the c-Src.FAK complex was markedly enhanced by platelet-derived growth factor (PDGF) BB. The presence of activated c-Src in unstimulated cells was identified in cell lysates by immunoblotting with an antibody recognizing the autophosphorylated site (P416Y). In whole-cell patch-clamp studies, intracellular dialysis of a Src substrate peptide and anti-c-Src and anti-FAK antibodies suppressed Ca2+ currents by 60, 62, and 43%, respectively. In contrast, intracellular dialysis of an anti-mouse IgG or anti-Kv1.5 antibody did not inhibit Ca2+ currents. Co-dialysis of anti-c-Src and anti-FAK antibodies inhibited Ca2+ currents (63%) equivalent to dialysis with the anti-c-Src antibody alone. PDGF-BB enhanced Ca2+ currents by 43%, which was abolished by the anti-c-Src and anti-FAK antibodies. Neither the MEK inhibitor PD 098059 nor an anti-Ras antibody inhibited basal Ca2+ currents or PDGF-stimulated Ca2+ currents. The alpha1C subunit of the L-type Ca2+ channel co-immunoprecipitated with anti-c-Src and anti-phosphotyrosine antibodies, indicating direct association of c-Src kinase with the Ca2+ channel. These data suggest that c-Src and FAK, but not the Ras/mitogen-activated protein kinase cascade, modulate basal Ca2+ channel activity and mediate the PDGF-induced enhancement of L-type Ca2+ currents in differentiated smooth muscle cells.

Ammonia effect on calcium-activated chloride secretion in T84 intestinal epithelial monolayers

We recently showed that ammonia profoundly inhibits cyclic nucleotide-regulated Cl- secretion in model human T84 intestinal epithelia but does not impair the secretory response to the Ca2+ agonist carbachol. Using transepithelial transport, fura 2 fluorescence, and radioisotopic efflux techniques, we further explored this dichotomy and arrived at a preliminary explanation for the inhibitory action of ammonia. The secretory response to the Ca(2+)-adenosinetriphosphatase inhibitor thapsigargin is unaffected by ammonia, which suggests that an increase in intracellular Ca2+ stimulates secretory pathways that are insensitive to ammonia. Surprisingly, Cl- secretion elicited by the Ca2+ ionophores ionomycin and A23187 is markedly blunted in monolayers pretreated with ammonia. However, ammonia posttreatment does not inhibit the secretory response to ionophore, which suggests that ammonia may interfere with the ability of these ionophores to increase intracellular [Ca2+]. This hypothesis is directly supported by fura 2 experiments. The inhibitory action of ammonia parallels the behavior of the K+ channel blocker Ba2+, and ammonia reduces the basolateral 86Rb+ efflux rate constant in forskolin- but not in carbachol-treated monolayers. Ammonia, which is present in high concentrations in the normal gastro-intestinal tract, may serve as a novel endogenous regulator of epithelial electrolyte transport by interfering with a Ba(2+)-sensitive basolateral K+ conductance distinct from the Ca(2+)-activated basolateral K+ conductance.

Depletion of [Ca2+]i inhibits hypoxia-induced vascular permeability factor (vascular endothelial growth factor) gene expression

We have investigated the role of ion channels and intracellular Ca2+ in the regulation of hypoxia-mediated VPF/VEGF activation. Known channel activator and blockers like lemakalim, glibenclamide, tetraethylammonium, 4-aminopyridine and nifedipine do not inhibit VPF/VEGF induction due to hypoxia. Whereas, 5 mM caffeine pretreatment of the 293 cells exhibits a complete inhibition of hypoxia inducted VPF/VEGF expression. Moreover, the cells treated with BAPTA-AM prior to hypoxia also show a dramatic decrease in the VPF/VEGF message level, which suggests an important role of intracellular Ca2+ in this signaling pathway. Caffeine pretreatment also inhibits hypoxia-mediated c-Src kinase activity. These findings demonstrate the importance of intracellular Ca2+ in the event of hypoxia-induced VPF/VEGF expression.

Caffeine- and carbachol-induced Cl- and cation currents in single opossum esophageal circular muscle cells

Cl- and cation currents may play important roles in esophageal smooth muscle membrane potential changes and contraction. We studied Ca2+ release-activated cell-shortening and membrane currents in single cells freshly dispersed from the circular muscle of the opossum esophagus using the standard patch-clamp whole cell recording method. Caffeine (10-20 microM) and carbachol (10-100 microM) shortened the single smooth muscle cells by releasing intracellular Ca2+. At a holding potential of 0 mV, spontaneous transient outward currents STOCs, representing spontaneous Ca(2+)-activated K+ currents) were recorded. Caffeine, carbachol, or ionomycin evoked large outward currents (up to 1,650 pA) and subsequently abolished STOCs. At a holding potential of -50 mV in K(+)-containing solutions, an outward current in response to the agonists was observed; in some cells, the outward current followed an inward current. In K(+)-free solutions, the agonists induced only an inward current whose reversal potential was shifted by alteration of the anion gradient but not by that of the cation. With a low-Cl- pipette solution (Cl- substituted by glucuronate or glutamate), the inward currents were dependent mainly on the external cation gradient. This cation channel was permeable to Ba2+. Inclusion of 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the pipette solution abolished all these currents. These data suggest that in the opossum esophageal circular muscle 1) Ca2+ released from the intracellular stores by caffeine and carbachol is sufficient to induce single smooth muscle cell contraction and 2) the caffeine-, carbachol-, and ionomycin-induced membrane currents consist of Ca(2+)-activated K+, Cl-, and cation conductances.

Tyrosine kinase-dependent modulation of calcium entry in rabbit colonic muscularis mucosae

We studied the role of tyrosine kinase in the regulation of Ca2+ entry in single smooth muscle cells of the rabbit colonic muscularis mucosae using the whole cell patch-clamp technique. Step depolarization to +10 mV from a holding potential of -60 mV produced inward currents that were abolished by 1 microM nifedipine, consistent with the activation of L-type Ca2+ channels. The tyrosine kinase inhibitors, genistein and tyrphostin B42, dose dependently inhibited these Ca2+ currents. The inactive analogue of tyrphostins, tyrphostin A1, did not affect the currents at concentrations of up to 100 microM. Conversely, the tyrosine phosphatase inhibitor, orthovanadate, enhanced peak Ca2+ currents by 30%. Spontaneous transient outward currents (STOCs) (50-600 pA) were elicited with high K+ in the pipette and at 0-mV holding potential. STOCs were activated due to release of Ca2+ from intracellular stores, required the presence of extracellular Ca2+ concentration, and were insensitive to nifedipine. Genistein abolished STOCs; however, in its presence, outward currents activated by caffeine or carbachol were not affected. The refilling of the Ca2+ stores was studied by first depleting intracellular Ca2+ with carbachol in Ca(2+)-free media followed by reperfusing with a Ca(2+)-containing solution for 3-5 min. Under these conditions, a second application of carbachol evoked an outward current due to Ca2+ release. However, this effect was abolished when the refilling of the stores was carried out in the presence of genistein. Carbachol-evoked currents were not attenuated when the refilling was examined in the presence of orthovanadate. Epidermal growth factor (200 ng/ml) enhanced Ca2+ currents by 60% and markedly increased STOCs by over 200%. Western blot analysis, using an anti-phosphotyrosine antibody, showed a tyrosine phosphorylated protein of 60 kDa in control conditions. This was markedly increased after treatment with epidermal growth factor and carbachol. These results suggest that 1) tyrosine kinase modulates the entry of Ca2+ through L-type channels and through nifedipine-resistant pathways involved in refilling of intracellular stores and 2) stimulation of the kinase by agonists enhances Ca2+ entry in the smooth muscle cells of the rabbit colonic muscularis mucosae.

Transient outward current in opossum esophageal circular muscle

The whole cell patch-clamp technique was used to record a transient outward K+ current (ITO) from single smooth muscle cells isolated from opossum esophageal circular muscle. The threshold for its activation was -50 mV from holding potentials negative to -70 mV. The current peaked within 10 ms and decayed completely in 200 ms between test depolarization of -40 and -10 mV. ITO was recorded at room temperature in the presence of 5 mM internal ethylene glycol-bis(beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid. Both activation and inactivation kinetics of ITO were markedly changed when recordings were made at higher temperatures (32 degrees C). 4-Amino-pyridine (4-AP, 3 mM) abolished the fast component of the outward current. Tetraethylammonium ion (TEA, 1-30 mM) reduced the sustained component but did not affect ITO. In the presence of TEA and nifedipine, the voltage dependence of the steady-state inactivation data was well fitted by a Boltzmann distribution with a half-inactivation potential of -57 mV. The half-inactivation potential was shifted to a more positive potential in the presence of Cd2+ (-35 mV). The steady-state inactivation and activation data overlap between -50 and -30 mV, suggesting the presence of a "window" current in this potential range. In current-clamp mode, 4-AP depolarized single esophageal cells by approximately 8 mV and shifted the upstroke of the action potential to the left. These results indicate that, in the esophageal circular muscle, ITO is involved in the resting membrane potential and modulation of the onset of action potential.

Muscarinic suppression of ATP-sensitive K+ channel in rabbit esophageal smooth muscle

Smooth muscle cells from the rabbit esophageal muscularis mucosae were studied for the presence of ATP-sensitive K+ channel (KATP) and its inhibition by carbachol. Lemakalim (10 microM), a synthetic K+ channel opener, increased whole cell currents by -174 +/- 15 pA with 0.1 mM intracellular ATP concentration ([ATP]i) and -70 +/- 11 pA with 5 mM [ATP]i. Glibenclamide (10 microM) completely abolished the lemakalim-induced currents. These currents were therefore denoted as KATP. Carbachol (10 microM) suppressed KATP by 74 +/- 4% with 10 mM intracellular ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) concentration and 100% when EGTA was omitted from the pipette solution. Carbachol suppression was attenuated to 23 +/- 16% by the M3 receptor antagonist, p-flurohexahydrosiladifenidol (0.1 microM). KATP was also suppressed by phorbol 12-myristate 13-acetate (PMA; 100 nM) by 63 +/- 9%. The effects of both PMA and carbachol were significantly reduced by inhibitors of protein kinase C and tyrosine kinase. These results suggest that carbachol suppression of KATP is via M3 receptor subtype and the signaling pathway involves Ca2+, protein kinase C, and tyrosine kinase.

Effect of sodium nitroprusside on Ca2+ currents in opossum esophageal circular muscle cells

The effects of sodium nitroprusside (SNP) on ionic currents in single opossum esophageal circular muscle cells were examined. In voltage clamp, Ca2+ currents were studied after K+ currents were blocked with Cs+ in the patch pipette. The threshold for inward Ca2+ currents was -30 mV with peak current between 0 and +10 mV from holding potentials of -90 mV. The Ca2+ currents had both transient and sustained phases. The transient phase was partially resistant to nifedipine (1 microM). SNP (100 microM) reversibly decreased both the transient and sustained phases of the Ca2+ currents by approximately 20%. In cells dialyzed with high-K+ solutions, voltage-clamp recordings demonstrated the presence of an inward current followed by an outward current at potentials positive to -30 mV. SNP under these conditions resulted in a decrease in the Ca2+ current and decreased the outward current during test depolarizations. Action potentials were evoked during current-clamp recordings that consisted of multiple spikes, depending on the stimulus strength. The threshold for spike generation was close to -30 mV and was blocked by Cd2+, suggesting that the upstroke of the action potential was dependent on Ca2+ influx. SNP significantly attenuated action potentials and produced a small hyperpolarization (5-7 mV). These results suggest that SNP has a direct inhibitory effect on Ca2+ currents and thereby decreases evoked action potentials, and that SNP hyperpolarization is not due to Ca(2+)-activated K+ channels.

K+ currents in rabbit esophageal muscularis mucosae

Single cells were obtained from esophageal muscularis mucosae of the rabbit using enzymatic dispersion. Their electrophysiological properties were studied with both conventional whole cell and nystatin-perforated patch techniques. The latter technique was used to prevent "washout" of intracellular constituents and to maintain endogenous buffering of Ca2+. The average resting potential of these cells was -54 +/- 3.2 mV in the conventional recording and -51 +/- 4.4 mV in perforated patch recordings. In the current-clamp mode, regenerative responses were consistently observed in perforated patch recordings, but not when conventional whole cell gigaseal methods were used. Conventional whole cell voltage-clamp methods revealed outward currents on depolarization from a holding potential of -70 mV. These currents were inhibited by extracellular tetraethylammonium (TEA, 5-10 mM) and CoCl2 (4 mM), indicating that the predominant outward current is a Ca(2+)-activated K+ current. In the presence of TEA, inward Ca2+ currents were unmasked. In contrast, when the nystatin-perforated patch technique was used, depolarizations resulted in a net inward current followed by an outward current. The outward current was inhibited by CoCl2 (2 mM) and TEA (5 mM) to the same extent as in conventional recordings. A second component of K+ current was observed in both types of recordings when extracellular Ca2+ influx was abolished and also in the presence of TEA. This slowly activating persistent K+ current resembled a delayed rectifier K+ current. These studies show that the rabbit tunica muscularis mucosal cells possess voltage-activated Ca2+ and K+ channels as well as the capability to elicit action potentials.

Ca2+ and Ca(2+)-activated Cl- currents in rabbit oesophageal smooth muscle

1. An inward current carried by Ca2+ was recorded from single smooth muscle cells of rabbit oesophageal muscularis mucosae using a whole-cell gigaseal technique with physiological (2 mM) external calcium concentration ([Ca2+]o) in the presence of intracellular Cs+ ([Cs+]i 130 mM). Only one type of Ca2+ current could be identified. The threshold for its activation was approximately -30 mV and maximum inward current (approximately 300 pA) was recorded at 0 mV. 2. This inward current was blocked by Co2+ (4 mM), Cd2+ (0.5 mM) and nifedipine (1 microM) and was enhanced by Bay K 8644 (5 microM). We therefore classify it as a L-type Ca2+ current and denote it ICa. 3. Steady-state inactivation data were well-fitted by a Boltzmann distribution, indicating that inactivation of the Ca2+ current is strongly modulated by membrane potential. However, the inactivation of ICa slowed significantly and became less complete when BaCl2 replaced CaCl2 in the Tyrode solution suggesting that the inactivation of ICa may also be dependent on [Ca2+]i. The steady-state activation and inactivation curves for ICa overlap between -40 and 0 mV indicating that there may be a Ca2+ window current in this range of potentials. 4. When EGTA was omitted from the pipette-filling solution, depolarizations positive to -10 mV resulted in a transient as opposed to a maintained inward Ca2+ current which was followed by a relatively large outward current. Under these conditions, slowly decaying inward tail currents were also recorded upon repolarization to the holding potential, -60 mV. However, when EGTA was omitted from the pipette, marked 'run-down' of the Ca2+ current occurred within 10 min after starting the whole-cell recording. 5. This run-down of ICa was reduced significantly when the nystatin perforated patch technique was used. Under these conditions stable ICa records could be obtained for over 1 h. Outward currents and slow decaying inward tail currents similar to those recorded with no EGTA in the pipette were also obtained consistently using the nystatin recording technique. 6. In nystatin perforated patch recordings, CoCl2 (2 mM) completely abolished the Ca2+ current, the outward currents and the slow inward tails. These findings suggest that the outward currents and slow inward tails are activated by a transmembrane influx of Ca2+. 7. Ion replacement and pharmacological tests provided evidence that both the outward currents and the slow inward tails are due to Ca(2+)-activated Cl- current (ICl(Ca)).(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic currents in single cells from human cystic artery

The patch-clamp technique was used to study the electrophysiological properties of single smooth muscle cells obtained from the human cystic artery. These cells contracted on exposure to high K+ and had a mean resting potential of -36 +/- 7 mV. Under current clamp, regenerative responses could not be elicited when depolarizing pulses were applied. Voltage-clamp measurements demonstrated that a large fraction of the outward current was inhibited by tetraethylammonium (5-10 mM) or Ca2+ channel blockers and that it was enhanced by increasing [Ca2+]o, suggesting that it is a Ca(2+)-activated K+ current. In addition, spontaneous transient outward currents that were sensitive to extracellular Ca2+ were observed in some cells. In cell-attached patch-clamp recordings, Ca(2+)-activated K+ channels that had a conductance of 117 pS were consistently identified. At negative potentials (approximately -60 mV), these single-channel events deactivated completely and very quickly, suggesting that they do not control the resting membrane potential in healthy cystic artery cells. Ca2+ currents that were recorded using Ba2+ (10 mM) as the charge carrier were enhanced by the dihydropyridine agonist, Bay K 8644, and blocked by nifedipine (0.1 microM). Only one type of Ca2+ current, the L-type, could be identified in these cells. These results demonstrate that the major ionic currents in the human cystic artery are similar to other mammalian arteries and indicate that this tissue will be a useful model for studying the metabolic and pharmacological modulation of ionic currents in human vascular smooth muscle.

Inhibition of field stimulation-evoked relaxations in rat oesophageal smooth muscle by the calcium antagonist PN 200-110

1. The inhibitory effects of the 1,4-dihydropyridine calcium channel antagonist, PN 200-110 (isradipine), on field stimulation-evoked tetrodotoxin (TTX)-sensitive and -insensitive relaxations were studied in rat oesophageal smooth muscle of the tunica muscularis mucosae. 2. The TTX-insensitive relaxation was inhibited by PN 200-110 in a stereoselective manner with the (+)-(S)-isomer displaying a 1000 fold greater inhibitory potency than the (--)-(R) isomer. A similar potency was noted for inhibition of high K+ -evoked contractions. 3. TTX-sensitive relaxations evoked by field stimulation and contractions elicited by the muscarinic cholinoceptor agonist, cis-2-methyl-4-dimethylamino-methyl-1,3-dioxolane methiodide (cisdioxolane) were considerably less sensitive to inhibition by PN 200-110, although, again, stereoselectivity for PN 200-110 was apparent. 4. Pretreatment with (+)-(S)-PN 200-110 resulted in a non-competitive displacement of the Ca2+ concentration-response curves obtained in the presence of either isotonic 50 mM KCl or cisdioxolane. The effect of K+ was 10 fold more sensitive than that of cis-dioxolane. 5. The potency rank orders for inhibition of TTX-insensitive field stimulation-evoked relaxations and K+ -mediated contractions in a series of calcium channel antagonists were closely correlated; (+)-(S)-PN 200-110 showing highest potency followed by nifedipine, verapamil, diltiazem, (--)-(R)-PN 200-110. 6. It is concluded that TTX-insensitive relaxations are dependent upon an influx of extracellular Ca2+ through potential-operated calcium channels.

Similarity of relaxations evoked by BRL 34915, pinacidil and field-stimulation in rat oesophageal tunica muscularis mucosae

1. In the rat oesophageal tunica muscularis mucosae (TMM) the potassium channel openers, BRL 34915 and pinacidil, raised the threshold for concentration-dependent K+ contractions, suppressed contractions evoked by field stimulation of the TMM in the presence of tetrodotoxin (TTX) and tetraethylammonium (TEA), and relaxed tonic contractions resulting from muscarinic cholinoceptor stimulation. 2. BRL 34915 and both (+)- and (--)-pinacidil increased 86Rb efflux from tracer-loaded tissues; nifedipine abolished this effect. 3. Relaxations produced by potassium channel openers were inhibited by a temperature drop from 37 degrees C to 26.5 degrees C, an increase in extracellular K+ concentration to 64 mM, and treatment with the calcium channel antagonist, nifedipine. The same treatments also blocked field stimulation-evoked TTX-insensitive relaxations. 4. It is concluded that field stimulation of rat oesophageal smooth muscle in the presence of cholinoceptor-induced tone results in an increase in K+ permeability that is directly or indirectly coupled to Ca2+ influx through potential-operated channels.

Tetrodotoxin-sensitive and -insensitive relaxations in the rat oesophageal tunica muscularis mucosae

1. Relaxation responses were produced by vagal and field stimulation, respectively, of the whole oesophagus preparation from the rat and of the isolated tunica muscularis mucosae (t.m.m.) preparation from the rat. These relaxation responses persisted in the presence of antagonists of histamine, serotonin, noradrenaline and acetylcholine. 2. Unlike vagally evoked relaxation, that evoked by low-frequency field stimulation, i.e. field-stimulated relaxation (f.s.r.) was generally resistant to tetrodotoxin (TTX). 3. Both types of relaxations exhibited remarkable temperature sensitivity and were abolished by lowering the bath temperature from 37 to 28 degrees C. 4. TTX-resistant relaxations were also produced by scorpion (Leiurus quinquestriatus) venom, the calcium ionophore, A23187 (calimycin) and by increasing the extracellular potassium by 2 mM. The failure of these agents to inhibit f.s.r. is inconsistent with a releasing and/or depleting action on any endogenous mediator. 5. Relaxations produced by vasoactive intestinal peptide (VIP) could be blocked by alpha-chymotrypsin which, however, failed to abolish f.s.r., suggesting that VIP is not the mediator of f.s.r. 6. F.s.r. was completely blocked by the calcium channel antagonists, verapamil (10(-6) M), nifedipine (10(-7) M), and by magnesium (20 mM). 7. Our results indicate that TTX-insensitive relaxations in the isolated t.m.m. are dependent upon extracellular calcium, are due to activation of potential-operated calcium channels and are not mediated by VIP.