Muscarinic Cholinergic Excitation of Smooth Muscle: Signal Transduction and Single Cationic Channel Properties

Molecular mechanisms of cholinergic neurotransmission in visceral smooth muscles with a focus on receptor-operated TRPC4 channel and impairment of gastrointestinal motility by general anaesthetics and anxiolytics

Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, wherein it binds to and activates two muscarinic receptors subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. The first part of this review focuses on the types of cells involved in cholinergic neurotransmission and on the molecular mechanisms underlying acetylcholine-induced membrane depolarisation, which is the central event of excitation-contraction coupling causing Ca2+ entry via L-type Ca2+ channels and smooth muscle contraction. Studies of the muscarinic cation current in intestinal myocytes (mICAT) revealed its main molecular counterpart, receptor-operated TRPC4 channel, which is activated in synergy by both M2 and M3 receptors. M3 receptors activation is of permissive nature, while activation of M2 receptors via Gi/o proteins that are coupled to them plays a direct role in TRPC4 opening. Our understanding of signalling pathways underlying mICAT generation has vastly expanded in recent years through studies of TRPC4 gating in native cells and its regulation in heterologous cells. Recent studies using muscarinic receptor knockout have established that at low agonist concentration activation of both M2 receptor and the M2/M3 receptor complex elicits smooth muscle contraction, while at high agonist concentration M3 receptor function becomes dominant. Based on this knowledge, in the second part of this review we discuss the cellular and molecular mechanisms underlying the numerous anticholinergic effects on neuroactive drugs, in particular general anaesthetics and anxiolytics, which can significantly impair gastrointestinal motility. This article is part of the Special Issue on "Ukrainian Neuroscience".

Mechanisms of regulation of motility of the gastrointestinal tract and the hepatobiliary system under the chronic action of nanocolloids

Modern cutting edge technologies of chemical synthesis enable the production of unique nanostructures with excess energy and high reactivity. Uncontrolled use of such materials in the food industry and pharmacology entail a risk for the development of a nanotoxicity crisis. Using the methods of tensometry, mechanokinetic analysis, biochemical methods, and bioinformatics, the current study showed that chronic (for six months) intragastrical burdening of rats with aqueous nanocolloids (AN) ZnO and TiO₂ caused violations of the pacemaker-dependent mechanisms of regulation of spontaneous and neurotransmitter-induced contractions of the gastrointestinal tract (GIT) smooth muscles (SMs), and transformed the contraction efficiency indices (AU, in Alexandria units). Under the same conditions, the fundamental principle of distribution of physiologically relevant differences in the numeric values of the mechanokinetic parameters of spontaneous SM contractions between different parts of GIT is violated, which can potentially cause its pathological changes. Using molecular docking, typical bonds in the interfaces of the interaction of these nanomaterials with myosin II, a component of the contractile apparatus of smooth muscle cells (SMC) were investigated. In this connection, the study addressed the question of possible competitive relations between ZnO and TiO₂ nanoparticles and actin molecules for binding sites on the myosin II actin-interaction interface. In addition, using biochemical methods, it was shown that chronic long-term exposure to nanocolloids causes changes in the primary active ion transport systems of cell plasma membranes, the activity of marker liver enzymes and disrupts the blood plasma lipid profile, which indicates the hepatotoxic effect of these nanocolloids.

ZnO and TiO2 Nanocolloids: State of Mechanisms that Regulating the Motility of the Gastrointestinal Tract and the Hepatobiliary System

Using the transmission electron microscopy (TEM)/high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) methods, it was shown that the nanocolloids of ZnO contain hydrolyzed ZnO nanoparticles (NPs). Typically, the nanocrystalline ZnO/Zn(OH)2 core is covered by an amorphous shell of zinc hydroxides, preventing the encapsulated crystal core from dissolving. Similar studies were carried out with TiO2 nanocolloids. It was found that burdening of rats for 30 days with a ZnO aqueous nanocolloid (AN) was accompanied by a narrowing of the amplitude range, a decrease (increase) in the frequency of spontaneous contractions (SCs), and an inhibition of the efficiency indices for smooth muscles (SMs) of the antrum and cecum. Under longer (100 days) burdening of rats with AN of ZnO, there was a tendency toward restoring the above parameters. In terms of the value and the direction of changes in most parameters for SCs of SMs, the effects (30 days) of TiO2 AN differed from those for ZnO AN and were almost the same in the case of their long-term impact. It was found that mostly M2-cholinoreceptor-dependent mechanisms of regulating the intracellular concentration of Ca2+ were sensitive to the effect of ZnO and TiO2 ANs. The molecular docking demonstrated that ZnO and TiO2 NPs did not compete with acetylcholine for the site of binding to M3 and M2 cholinoreceptors but may impact the affinity of orthosteric ligands to M2 cholinoreceptors. The studies showed that burdening rats with ZnO and TiO2 ANs was also accompanied by changes in the activity state of both intracellular enzymes and the ion transport systems for Na+, K+, and Ca2+, related to the processes of bile secretion, via the plasma membrane of hepatocytes.

Overview of muscarinic receptor subtypes

The physiological role of muscarinic receptors is highly complex and, although not completely understood, has become clearer over the last decade. Recent pharmacological evidence with novel compounds, together with data from transgenic mice, suggests that all five subtypes have defined functions in the nervous system as well as mediating the non neuronal, hormonal actions of acetylcholine. Numerous novel agonists, allosteric regulators, and antagonists have now been identified with authentic subtype specificity in vitro and in vivo. These compounds provide additional pharmacological opportunities for selective subtype modulation as well as a new generation of muscarinic receptor-based therapeutics.

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.

G-protein-gated TRP-like cationic channel activated by muscarinic receptors: effect of potential on single-channel gating

There is little information about the mechanisms by which G-protein–coupled receptors gate ion channels although many ionotropic receptors are well studied. We have investigated gating of the muscarinic cationic channel, which mediates the excitatory effect of acetylcholine in smooth muscles, and proposed a scheme consisting of four pairs of closed and open states. Channel kinetics appeared to be the same in cell-attached or outside-out patches whether the channel was activated by carbachol application or by intracellular dialysis with GTPγS. Since in the latter case G-proteins are permanently active, it is concluded that the cationic channel is the major determinant of its own gating, similarly to the K_ACh channel (Ivanova-Nikolova, T.T., and G.E. Breitwieser. 1997. J. Gen. Physiol. 109:245–253). Analysis of adjacent-state dwell times revealed connections between the states that showed features conserved among many other ligand-gated ion channels (e.g., nAChR, BK_Ca channel). Open probability (P_O) of the cationic channel was increased by membrane depolarization consistent with the prominent U-shaped I-V relationship of the muscarinic whole-cell current at negative potentials. Membrane potential affected transitions within each closed-open state pair but had little effect on transitions between pairs; thus, the latter are likely to be caused by interactions of the channel with its ligands, e.g., Ca²⁺ and Gαo-GTP. Channel activity was highly heterogeneous, as was evident from the prominent cycling behavior when P_O was measured over 5-s intervals. This was related to the variable frequency of openings (as in the K_ACh channel) and, especially, to the number of long openings between consecutive long shuttings. Analysis of the underlying Markov chain in terms of probabilities allowed us to evaluate the contribution of each open state to the integral current (from shortest to longest open state: 0.1, 3, 24, and 73%) as P_O increased 525-fold in three stages.

Effects of polyamines on the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle myocytes

The effects of extracellular and intracellular polyamines (PAs), spermine and putrescine, on the cation current (mI(CAT)) evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal myocytes. Extracellular spermine and putrescine rapidly and reversibly inhibited mI(CAT) in a concentration- and voltage-dependent manner with the IC(50) values at -40 mV of about 1 and 5 mM, respectively. Membrane depolarization relieved the blocking action of PAs although cation conductance activation curve remained N-shaped. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents, suggesting that the cation channel rather than the muscarinic receptor was the primary site of the PA action. In outside-out membrane patches, both cation channel unitary conductance and open probability were reduced. In perforated-patch experiments used to retain cytoplasmic PAs sustained 100 microM carbachol-induced mI(CAT) was significantly smaller (478 +/- 76 pA, n = 7) compared to that recorded using conventional whole-cell configuration with nominally PA-free pipette solution (1314 +/- 76 pA, n = 12), but comparable in size to mI(CAT) with 0.3 mM spermine in the pipette solution (509 +/- 41 pA, n = 19). Intracellular putrescine inhibited mI(CAT) less potently compared to spermine. In conclusion, these results show a novel role of intestinal PAs in mI(CAT) inhibition, which can contribute to their well-known suppressing effect on the gastrointestinal smooth muscle excitability and contractility.

Multiple regulation by calcium of murine homologues of transient receptor potential proteins TRPC6 and TRPC7 expressed in HEK293 cells

We investigated, by using the patch clamp technique, Ca2+-mediated regulation of heterologously expressed TRPC6 and TRPC7 proteins in HEK293 cells, two closely related homologues of the transient receptor potential (TRP) family and molecular candidates for native receptor-operated Ca2+ entry channels. With nystatin-perforated recording, the magnitude and time courses of activation and inactivation of carbachol (CCh; 100 microM)-activated TRPC6 currents (I(TRPC6)) were enhanced and accelerated, respectively, by extracellular Ca2+ (Ca2+(o)) whether it was continuously present or applied after receptor stimulation. In contrast, Ca2+(o) solely inhibited TRPC7 currents (I(TRPC7)). Vigorous buffering of intracellular Ca2+ (Ca2+(i)) under conventional whole-cell clamp abolished the slow potentiating (i.e. accelerated activation) and inactivating effects of Ca2+(o), disclosing fast potentiation (EC50: approximately 0.4 mM) and inhibition (IC50: approximately 4 mM) of I(TRPC6) and fast inhibition (IC50: approximately 0.4 mM) of I(TRPC7). This inhibition of I(TRPC6) and I(TRPC7) seems to be associated with voltage-dependent reductions of unitary conductance and open probability at the single channel level, whereas the potentiation of I(TRPC6) showed little voltage dependence and was mimicked by Sr2+ but not Ba2+. The activation process of I(TRPC6) or its acceleration by Ca2+(o) probably involves phosphorylation by calmodulin (CaM)-dependent kinase II (CaMKII), as pretreatment with calmidazolium (3 microM), coexpression of Ca2+-insensitive mutant CaM, and intracellular perfusion of the non-hydrolysable ATP analogue AMP-PNP and a CaMKII-specific inhibitory peptide all effectively prevented channel activation. However, this was not observed for TRPC7. Instead, single CCh-activated TRPC7 channel activity was concentration-dependently suppressed by nanomolar Ca2+(i) via CaM and conversely enhanced by IP3. In addition, the inactivation time course of I(TRPC6) was significantly retarded by pharmacological inhibition of protein kinase C (PKC). These results collectively suggest that TRPC6 and 7 channels are multiply regulated by Ca2+ from both sides of the membrane through differential Ca2+-CaM-dependent and -independent mechanisms.