Voltage-dependent inhibition of the muscarinic cationic current in guinea-pig ileal cells by SK&F 96365

Single-Walled Carbon Nanotubes Inhibit TRPC4-Mediated Muscarinic Cation Current in Mouse Ileal Myocytes

Single-walled carbon nanotubes (SWCNTs) are characterized by a combination of rather unique physical and chemical properties, which makes them interesting biocompatible nanostructured materials for various applications, including in the biomedical field. SWCNTs are not inert carriers of drug molecules, as they may interact with various biological macromolecules, including ion channels. To investigate the mechanisms of the inhibitory effects of SWCNTs on the muscarinic receptor cation current (mI_CAT), induced by intracellular GTPγs (200 μM), in isolated mouse ileal myocytes, we have used the patch-clamp method in the whole-cell configuration. Here, we use molecular docking/molecular dynamics simulations and direct patch-clamp recordings of whole-cell currents to show that SWCNTs, purified and functionalized by carboxylation in water suspension containing single SWCNTs with a diameter of 0.5–1.5 nm, can inhibit mI_CAT, which is mainly carried by TRPC4 cation channels in ileal smooth muscle cells, and is the main regulator of cholinergic excitation–contraction coupling in the small intestinal tract. This inhibition was voltage-independent and associated with a shortening of the mean open time of the channel. These results suggest that SWCNTs cause a direct blockage of the TRPC4 channel and may represent a novel class of TRPC4 modulators.

Functions of Muscarinic Receptor Subtypes in Gastrointestinal Smooth Muscle: A Review of Studies with Receptor-Knockout Mice

Parasympathetic signalling via muscarinic acetylcholine receptors (mAChRs) regulates gastrointestinal smooth muscle function. In most instances, the mAChR population in smooth muscle consists mainly of M2 and M3 subtypes in a roughly 80% to 20% mixture. Stimulation of these mAChRs triggers a complex array of biochemical and electrical events in the cell via associated G proteins, leading to smooth muscle contraction and facilitating gastrointestinal motility. Major signalling events induced by mAChRs include adenylyl cyclase inhibition, phosphoinositide hydrolysis, intracellular Ca2+ mobilisation, myofilament Ca2+ sensitisation, generation of non-selective cationic and chloride currents, K+ current modulation, inhibition or potentiation of voltage-dependent Ca2+ currents and membrane depolarisation. A lack of ligands with a high degree of receptor subtype selectivity and the frequent contribution of multiple receptor subtypes to responses in the same cell type have hampered studies on the signal transduction mechanisms and functions of individual mAChR subtypes. Therefore, novel strategies such as genetic manipulation are required to elucidate both the contributions of specific AChR subtypes to smooth muscle function and the underlying molecular mechanisms. In this article, we review recent studies on muscarinic function in gastrointestinal smooth muscle using mAChR subtype-knockout mice.

Inhibitory effects of SKF96365 on the activities of K(+) channels in mouse small intestinal smooth muscle cells

In order to investigate the effects of SKF96365 (SKF), which is a non-selective cationic channel blocker, on K⁺ channel currents, we recorded currents through ATP sensitive K⁺ (I_KATP), voltage-gated K⁺ (I_Kv) and Ca²⁺ activated K⁺ channels (I_BK) in the absence and presence of SKF in single small intestinal myocytes of mice with patch-clamp techniques. SKF (10 µM) reversibly abolished I_KATP that was induced by cromakalim (10 µM), which is a selective ATP sensitive K⁺ channel opener. These inhibitory effects were induced in a concentration-dependent and voltage-independent manner. The 50% inhibitory concentration (IC₅₀) was 0.85 µM, which was obviously lower than that reported for the muscarinic cationic current. In addition, SKF (1 µM ≈ the IC₅₀ value in I_KATP suppression) reversibly inhibited the I_Kv that was induced by repetitive depolarizing pulses from −80 to 20 mV. However, the extent of the inhibitory effects was only ~30%. In contrast, SKF (1 µM) had no significant effects on spontaneous transient I_BK and caffeine-induced I_BK. These results indicated that SKF inhibited ATP sensitive K⁺ channels and voltage-gated K⁺ channels, with the ATP sensitive K⁺ channels being more sensitive than the voltage-gated K⁺ channels. These inhibitory effects on K⁺ channels should be considered when SKF is used as a cationic channel blocker.

Evidence for M2 and M3 muscarinic receptor involvement in cholinergic excitatory junction potentials through synergistic activation of cation channels in the longitudinal muscle of mouse ileum

Cholinergic nerve-mediated excitatory junction potentials (EJPs) in the longitudinal muscle of mouse ileum were characterized by using M2 or M3 muscarinic receptor-knockout (KO) mice and 1-[β-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SK&F 96365) and pertussis toxin (PTX). EJPs evoked by electrical field stimulation (EFS) in wild-type preparations, initially determined to be cholinergic in origin using tetrodotoxin, atropine, and eserine, were profoundly depressed after SK&F 96365 treatment known to block muscarinic receptor-operated cation channels. A similar depression of the EJPs was also observed by PTX treatment, which is predicted to disrupt M2-mediated pathways linked to cation channel activation. In M2-KO mouse preparations, cholinergic EJPs were evoked by EFS with their relative amplitude of 20%-30% to the wild-type EJP and strongly inhibited by SK&F 96365. No cholinergic EJP was seen in M3-KO as well as M2/M3 double-KO preparations. The results suggest that the wild-type cholinergic EJP is not a simple mixture of M2 and M3 responses, but due to synergistic activation of cation channels by both M2 and M3 receptors in the murine ileal longitudinal muscle.

Store-operated calcium entry in vascular smooth muscle

In non-excitable cells, activation of G-protein-coupled phospholipase C (PLC)-linked receptors causes the release of Ca(2+) from intracellular stores, which is followed by transmembrane Ca(2+) entry. This Ca(2+) entry underlies a small and sustained phase of the cellular [Ca(2+)](i) increases and is important for several cellular functions including gene expression, secretion and cell proliferation. This form of transmembrane Ca(2+) entry is supported by agonist-activated Ca(2+)-permeable ion channels that are activated by store depletion and is referred to as store-operated Ca(2+) entry (SOCE) and represents a major pathway for agonist-induced Ca(2+) entry. In excitable cells such as smooth muscle cells, Ca(2+) entry mechanisms responsible for sustained cellular activation are normally considered to be mediated via either voltage-operated or receptor-operated Ca(2+) channels. Although SOCE occurs following agonist activation of smooth muscle, this was thought to be more important in replenishing Ca(2+) stores rather than acting as a source of activator Ca(2+) for the contractile process. This review summarizes our current knowledge of SOCE as a regulator of vascular smooth muscle tone and discusses its possible role in the cardiovascular function and disease. We propose a possible hypothesis for its activation and suggest that SOCE may represent a novel target for pharmacological therapeutic intervention.

Species dependent expression of intestinal smooth muscle mechanosensitive sodium channels

A mechanosensitive Na(+) current carried by Na(v)1.5 is present in human intestinal circular smooth muscle and contributes to regulation of intestinal motor function. Expression of this channel in different species is unknown. Our aim was to determine if Na(+) currents and message for the alpha subunit of the Na(+) channel (SCN5A) are found in circular smooth muscle cells of human, dog, pig, mouse and guinea pig jejunum. Currents were recorded using patch clamp techniques. Message for SCN5A was investigated using laser capture microdissection and reverse transcription polymerase chain reaction (RT-PCR). Na(+) currents were identified consistently in human and dog smooth muscle cells; however, Na(+) current was not found in pig (0/20) or guinea pig smooth muscle cells (0/21) and found only one mouse cell (1/21). SCN5A mRNA was found in circular muscle of human, dog, and mouse, but not in pig or guinea pig, and not in mouse longitudinal or mucosal layers. In summary, SCN5A message is expressed in, and Na(+) current recorded from, circular muscle layer of human and dog but not from pig and guinea pig. These data show that there are species differences in expression of the SCN5A-encoded Na(v)1.5 channel, suggesting species-specific differences in the electrophysiological response to mechanical and depolarizing stimuli.

Muscarinic receptor-activated cationic channels in murine ileal myocytes

BACKGROUND AND PURPOSE

There is little information about the excitatory cholinergic mechanisms of mouse small intestine although this model is important for gene knock-out studies.

EXPERIMENTAL APPROACH

Using patch-clamp techniques, voltage-dependent and pharmacological properties of carbachol- or intracellular GTPgammaS-activated cationic channels in mouse ileal myocytes were investigated.

KEY RESULTS

Three types of cation channels were identified in outside-out patches (17, 70 and 140 pS). The voltage-dependent behaviour of the 70 pS channel, which was also the most abundantly expressed channel (approximately 0.35 micro(-2)) was most consistent with the properties of the whole-cell muscarinic current (half-maximal activation at -72.3+/-9.3 mV, slope of -9.1+/-7.4 mV and mean open probability of 0.16+/-0.01 at -40 mV; at near maximal activation by 50 microM carbachol). Both channel conductance and open probability depended on the permeant cation in the order: Cs+ (70 pS) >Rb+ (66pS) >Na+ (47 pS) >Li+ (30 pS). External application of divalent cations, quinine, SK&F 96365 or La3+ strongly inhibited the whole-cell current. At the single channel level the nature of the inhibitory effects appeared to be very different. Either reduction of the open probability (quinine and to some extent SK&F 96365 and La3+) or of unitary current amplitude (Ca2+, Mg2+, SK&F 96365, La3+) was observed implying significant differences in the dissociation rates of the blockers.

CONCLUSIONS AND IMPLICATIONS

The muscarinic cation current of murine small intestine is very similar to that in guinea-pig myocytes and murine genetic manipulation should yield important information about muscarinic receptor transduction mechanisms.

Regulation of TRP-like muscarinic cation current in gastrointestinal smooth muscle with special reference to PLC/InsP3/Ca2+ system

Acetylcholine, the main enteric excitatory neuromuscular transmitter, evokes membrane depolarization and contraction of gastrointestinal smooth muscle cells by activating G protein-coupled muscarinic receptors. Although the cholinergic excitation is generally underlined by the multiplicity of ion channel effects, the primary event appears to be the opening of cation-selective channels; among them the 60 pS channel has been recently identified as the main target for the acetylcholine action in gastrointestinal myocytes. The evoked cation current, termed mI(CAT), causes either an oscillatory or a more sustained membrane depolarization response, which in turn leads to increases of the open probability of voltage-gated Ca2+ channels, thus providing Ca2+ entry in parallel with Ca2+ release for intracellular Ca2+ concentration rise and contraction. In recent years there have been several significant developments in our understanding of the signaling processes underlying mICAT generation. They have revealed important synergistic interactions between M2 and M3 receptor subtypes, single channel mechanisms, and the involvement of TRPC-encoded proteins as essential components of native muscarinic cation channels. This review summarizes these recent findings and in particular discusses the roles of the phospholipase C/InsP3/intracellular Ca2+ release system in the mI(CAT) physiological regulation.

Inhibition of carbachol-evoked oscillatory currents by the NO donor sodium nitroprusside in guinea-pig ileal myocytes

The effect of sodium nitroprusside (SNP) on carbachol (CCh)-evoked inward cationic current (Icat) oscillations in guinea-pig ileal longitudinal myocytes was investigated using the whole-cell patch-clamp technique and permeabilized longitudinal muscle strips. SNP (10 microm) completely inhibited I(cat) oscillations evoked by 1 microm CCh. 1H-(1,2,4) Oxadiazole [4,3-a] quinoxaline-1-one (ODQ; 1 microm) almost completely prevented the inhibitory effect of SNP on Icat oscillations. 8-Bromo-guanosine 3',5'-cyclic monophosphate (8-Br-cGMP; 30 microm) in the pipette solution completely abolished Icat oscillations. However, a pipette solution containing Rp-8-Br-cGMP (30 microm) almost completely abolished the inhibitory effect of SNP on Icat oscillations. When the intracellular calcium concentration ([Ca2+]i) was held at a resting level using BAPTA (10 mm) and Ca2+ (4.6 microm) in the pipette solution, CCh (1 microm) evoked only the sustained component of Icat without any oscillations and SNP did not affect the current. A high concentration of inositol 1,4,5-trisphosphate (IP3; 30 microm) in the patch pipette solutions significantly reduced the inhibitory effect of SNP (10 microm) on Icat oscillations. SNP significantly inhibited the Ca2+ release evoked by either CCh or IP3 but not by caffeine in permeabilized preparations of longitudinal muscle strips. These results suggest that the inhibitory effects of SNP on Icat oscillations are mediated, in part, by functional modulation of the IP3 receptor, and not by the inhibition of cationic channels themselves or by muscarinic receptors in the plasma membrane. This inhibition seems to be mediated by an increased cGMP concentration in a protein kinase G-dependent manner.

Pharmacological profile of store-operated channels in cerebral arteriolar smooth muscle cells

1. In this study, we determined a pharmacological profile of store-operated channels (SOCs) in smooth muscle cells of rabbit pial arterioles. Ca(2+)-indicator dyes, fura-PE3 or fluo-4, were used to track [Ca(2+)](i) and 10 micro M methoxyverapamil (D600) was present in all experiments on SOCs to prevent voltage-dependent Ca(2+) entry. Store depletion was induced using thapsigargin or cyclopiazonic acid. 2. SOC-mediated Ca(2+) entry was inhibited concentration dependently by Gd(3+) (IC(50) 101 nM). It was also inhibited by 10 micro M La(3+) (70% inhibition, N=5), 100 micro M Ni(2+) (57% inhibition, N=5), 75 micro M 2-aminoethoxydiphenylborate (66% inhibition, N=4), 100 micro M capsaicin (12% inhibition, N=3) or preincubation with 10 micro M wortmannin (76% inhibition, N=4). It was completely resistant to 1 micro M nifedipine (N=5), 10 micro M SKF96365 (N=6), 10 micro M LOE908 (N=14), 10-100 micro M ruthenium red (N=1+2), 100 micro M sulindac (N=4), 0.5 mM streptomycin (N=3) or 1 : 10,000 dilution Grammostolla spatulata venom (N=4). 3. RT-PCR experiments on isolated arteriolar fragments showed expression of mRNA species for TRPC1, 3, 4, 5 and 6. 4. The pharmacological profile of SOC-mediated Ca(2+) entry in arterioles supports the hypothesis that these SOCs are distinct from tonically active background channels and several store-operated and other nonselective cation channels described in other cells. Similarities with the pharmacology of TRPC1 support the hypothesis that TRPC1 is a subunit of the arteriolar smooth muscle SOC.

Relaxant mechanisms of parathyroid hormone in rat mesenteric artery

The effects of parathyroid hormone (PTH) on tension and intracellular Ca level ([Ca ] ) were examined in ring preparations of rat mesenteric artery using isometric tension recording and the fura-2 method, respectively. The PTH (30 n ) elicited relaxation in arterial rings precontracted by phenylephrine regardless of the presence or absence of endothelium. In the endothelium-denuded arterial rings precontracted by 3 micro M of phenylephrine or 60 m of potassium chloride (KCl), PTH-related protein and PTH produced concentration-dependent relaxation to the same extent, but inhibited contraction induced by phenylephrine more effectively than that induced by KCl. Phenylephrine-induced tonic contraction was changed to a phasic one with decreased peak tension in the presence of PTH. Similar changes were observed with extracellular Ca removal or methoxyverapamil plus SK&F96365, respective of voltage-gated and receptor-operated Ca channel inhibitors. Phenylephrine evoked a concentration-dependent contraction concomitant with an increase in [Ca ]. PTH reduced both responses to the same extent. In a Ca -free solution, PTH inhibited a phasic contraction and a transient increase in [Ca ] in response to phenylephrine but not caffeine. Reverse transcriptase-polymerase chain reaction showed that PTH and PTH receptors were expressed in the rat mesenteric artery. In this tissue, PTH increased cyclic adenosine monophosphate (cAMP) levels. These results suggest that the inhibitory effect of PTH on alpha -adrenoceptor-mediated contraction results from the inhibition of Ca influx through receptor-operated and voltage-gated Ca channels, and Ca release from Ca stores, probably via increased cAMP in the rat mesenteric artery.

Muscarinic activation of a cation current and associated current noise in entorhinal-cortex layer-II neurons

The effects of muscarinic stimulation on the membrane potential and current of in situ rat entorhinal-cortex layer-II principal neurons were analyzed using the whole cell, patch-clamp technique. In current-clamp experiments, application of carbachol (CCh) induced a slowly developing, prolonged depolarization initially accompanied by a slight decrease or no significant change in input resistance. By contrast, in a later phase of the depolarization input resistance appeared consistently increased. To elucidate the ionic bases of these effects, voltage-clamp experiments were then carried out. In recordings performed in nearly physiological ionic conditions at the holding potential of -60 mV, CCh application promoted the slow development of an inward current deflection consistently associated with a prominent increase in current noise. Similarly to voltage responses to CCh, this inward-current induction was abolished by the muscarinic antagonist, atropine. Current-voltage relationships derived by applying ramp voltage protocols during the different phases of the CCh-induced inward-current deflection revealed the early induction of an inward current that manifested a linear current/voltage relationship in the subthreshold range and the longer-lasting block of an outward K(+) current. The latter current could be blocked by 1 mM extracellular Ba(2+), which allowed us to study the CCh-induced inward current (I(CCh)) in isolation. The extrapolated reversal potential of the isolated I(CCh) was approximately 0 mV and was not modified by complete substitution of intrapipette K(+) with Cs(+). Moreover, the extrapolated I(CCh) reversal shifted to approximately -20 mV on removal of 50% extracellular Na(+). These results are consistent with I(CCh) being a nonspecific cation current. Finally, noise analysis of I(CCh) returned an estimated conductance of the underlying channels of approximately 13.5 pS. We conclude that the depolarizing effect of muscarinic stimuli on entorhinal-cortex layer-II principal neurons depends on both the block of a K(+) conductance and the activation of a "noisy" nonspecific cation current. We suggest that the membrane current fluctuations brought about by I(CCh) channel noise may facilitate the "theta" oscillatory dynamics of these neurons and enhance firing reliability and synchronization.

The developing relationship between receptor-operated and store-operated calcium channels in smooth muscle

Contraction of smooth muscle is initiated, and to a lesser extent maintained, by a rise in the concentration of free calcium in the cell cytoplasm ([Ca(2+)](i)). This activator calcium can originate from two intimately linked sources--the extracellular space and intracellular stores, most notably the sarcoplasmic reticulum. Smooth muscle contraction activated by excitatory neurotransmitters or hormones usually involves a combination of calcium release and calcium entry. The latter occurs through a variety of calcium permeable ion channels in the sarcolemma membrane. The best-characterized calcium entry pathway utilizes voltage-operated calcium channels (VOCCs). However, also present are several types of calcium-permeable channels which are non-voltage-gated, including the so-called receptor-operated calcium channels (ROCCs), activated by agonists acting on a range of G-protein-coupled receptors, and store-operated calcium channels (SOCCs), activated by depletion of the calcium stores within the sarcoplasmic reticulum. In this article we will review the electrophysiological, functional and pharmacological properties of ROCCs and SOCCs in smooth muscle and highlight emerging evidence that suggests that the two channel types may be closely related, being formed from proteins of the Transient Receptor Potential Channel (TRPC) family.

The mechanism for the contraction induced by leukotriene C4 in guinea-pig taenia coli

The mechanism underlying the LTC(4)-induced contraction of guinea-pig taenia coli was determined using the simultaneous measurements of [Ca(2+)](i) and force in whole muscle preparations. Additional experiments were performed in receptor coupled permeabilized preparation. For comparison purposes, the contraction which was induced by a typical G-protein mediated agonist, carbachol was also characterized. LTC(4) induced a contraction in the guinea-pig taenia coli in a concentration-dependent manner. The maximal response was obtained at 100 nM and the EC(50) value was 5.4+/-1.9 nM. Both LTC(4) and carbachol induced increases in [Ca(2+)](i) and force. The maximum force induced by 100 nM LTC(4) was significantly smaller than that induced by 10 microM carbachol, although an increase in [Ca(2+)](i) produced by both agonists was similar. In the permeabilized preparations, carbachol, but not LTC(4), induced an additional force development at a fixed Ca(2+) concentration. LTC(4) induced no increase in [Ca(2+)](i) and force in the Ca(2+)-free solution, while carbachol induced transient increases in both [Ca(2+)](i) and force in a Ca(2+)-free solution. Both diltiazem and SK&F 96365 significantly inhibited the LTC(4)- and carbachol-induced increases in [Ca(2+)](i) and force in normal PSS. The inhibitory pattern of [Ca(2+)](i) by these drugs was also similar. We thus conclude that LTC(4) induces the contraction of the guinea-pig taenia coli mainly through Ca(2+) influx via both the diltiazem-sensitive and SK&F 96365-sensitive Ca(2+) channels, without affecting either the Ca(2+)-sensitivity or the intracellular Ca(2+) release. These results indicated that the mechanism underlying the LTC(4)-induced contraction differs greatly from that for conventional G-protein mediated agonists, such as carbachol.

Vasopressin-induced contraction in the rat basilar artery in vitro

Vasopressin ([Arg(8)]vasopressin)-induced contraction was characterized using receptor agonists and antagonists for vasopressin and channel blockers in the rat basilar artery ring preparations. Vasopressin induced rhythmic contractions superimposed on a contraction in endothelium-intact preparations but not in denuded ones. Endothelium removal shifted the concentration-response curve for vasopressin leftward and upward. In endothelium-denuded preparations, vasopressin V(1) receptor antagonist shifted the concentration-response curve for vasopressin downward and rightward. Vasopressin V(1) receptor agonist caused contraction but V(2) receptor agonist did not. The contractile response to vasopressin was partly inhibited by nifedipine, SK&F 96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole) and niflumic acid. In the absence of extracellular Ca(2+), vasopressin produced a transient contraction. Charybdotoxin produced an upward and leftward shift of the concentration-response curve for vasopressin. These results suggest that vasopressin elicits contraction due to Ca(2+) influx through voltage-dependent and receptor-operated Ca(2+) channels and to Ca(2+) release from Ca(2+) stores by activating vasopressin V(1) receptors in the rat basilar artery.