Inhibitory effect of muscarinic receptor activation on Ca2+ channel current in smooth muscle cells of guinea-pig ileum

M2 Muscarinic Receptor-Dependent Contractions of Airway Smooth Muscle are Inhibited by Activation of β-Adrenoceptors

Beta-adrenoceptor (β-AR) agonists inhibit cholinergic contractions of airway smooth muscle (ASM), but the underlying mechanisms are unclear. ASM cells express M3 and M2 muscarinic receptors, but the bronchoconstrictor effects of acetylcholine are believed to result from activation of M3Rs, while the role of the M2Rs is confined to offsetting β-AR-dependent relaxations. However, a profound M2R-mediated hypersensitization of M3R-dependent contractions of ASM was recently reported, indicating an important role for M2Rs in cholinergic contractions of ASM. Here, we investigated if M2R-dependent contractions of murine bronchial rings were inhibited by activation of β-ARs. M2R-dependent contractions were apparent at low frequency (2Hz) electric field stimulation (EFS) and short (10s) stimulus  intervals. The β1-AR agonist, denopamine inhibited EFS-evoked contractions of ASM induced by reduction in stimulus interval from 100 to 10 s and was more effective at inhibiting contractions evoked by EFS at 2 than 20 Hz. Denopamine also abolished carbachol-evoked contractions that were resistant to the M3R antagonist 4-DAMP, similar to the effects of the M2R antagonists, methoctramine and AFDX-116. The inhibitory effects of denopamine on EFS-evoked contractions of ASM were smaller in preparations taken from M2R ^-/- mice, compared to wild-type (WT) controls. In contrast, inhibitory effects of the β3-AR agonist, BRL37344, on EFS-evoked contractions of detrusor strips taken from M2R ^-/- mice were greater than WT controls. These data suggest that M2R-dependent contractions of ASM were inhibited by activation of β1-ARs and that genetic ablation of M2Rs decreased the efficacy of β-AR agonists on cholinergic contractions.

Contribution of Postjunctional M2 Muscarinic Receptors to Cholinergic Nerve-Mediated Contractions of Murine Airway Smooth Muscle

Postjunctional M2Rs on airway smooth muscle (ASM) outnumber M3Rs by a ratio of 4:1 in most species, however, it is the M3Rs that are thought to mediate the bronchoconstrictor effects of acetylcholine. In this study, we describe a novel and profound M2R-mediated hypersensitization of M3R-dependent contractions of ASM at low stimulus frequencies.. Contractions induced by 2Hz EFS were augmented by > 2.5-fold when the stimulus interval was reduced from 100 to 10 s. This effect was reversed by the M2R antagonists, methoctramine, and AFDX116, and was absent in M2R null mice. The M3R antagonist 4-DAMP abolished the entire response in both WT and M2R KO mice. The M2R-mediated potentiation of EFS-induced contractions was not observed when the stimulus frequency was increased to 20 Hz. A subthreshold concentration of carbachol enhanced the amplitude of EFS-evoked contractions in WT, but not M2R null mice. These data highlight a significant M2R-mediated potentiation of M3R-dependent contractions of ASM at low frequency stimulation that could be relevant in diseases such as asthma and COPD.

Gut bacteria-derived 5-hydroxyindole is a potent stimulant of intestinal motility via its action on L-type calcium channels

Microbial conversion of dietary or drug substrates into small bioactive molecules represents a regulatory mechanism by which the gut microbiota alters intestinal physiology. Here, we show that a wide variety of gut bacteria can metabolize the dietary supplement and antidepressant 5-hydroxytryptophan (5-HTP) to 5-hydroxyindole (5-HI) via the tryptophanase (TnaA) enzyme. Oral administration of 5-HTP results in detection of 5-HI in fecal samples of healthy volunteers with interindividual variation. The production of 5-HI is inhibited upon pH reduction in in vitro studies. When administered orally in rats, 5-HI significantly accelerates the total gut transit time (TGTT). Deciphering the underlying mechanisms of action reveals that 5-HI accelerates gut contractility via activation of L-type calcium channels located on the colonic smooth muscle cells. Moreover, 5-HI stimulation of a cell line model of intestinal enterochromaffin cells results in significant increase in serotonin production. Together, our findings support a role for bacterial metabolism in altering gut motility and lay the foundation for microbiota-targeted interventions.

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.

Mechanisms underlying activation of transient BK current in rabbit urethral smooth muscle cells and its modulation by IP3-generating agonists

We used the perforated patch-clamp technique at 37°C to investigate the mechanisms underlying the activation of a transient large-conductance K(+) (tBK) current in rabbit urethral smooth muscle cells. The tBK current required an elevation of intracellular Ca(2+), resulting from ryanodine receptor (RyR) activation via Ca(2+)-induced Ca(2+) release, triggered by Ca(2+) influx through L-type Ca(2+) (CaV) channels. Carbachol inhibited tBK current by reducing Ca(2+) influx and Ca(2+) release and altered the shape of spike complexes recorded under current-clamp conditions. The tBK currents were blocked by iberiotoxin and penitrem A (300 and 100 nM, respectively) and were also inhibited when external Ca(2+) was removed or the CaV channel inhibitors nifedipine (10 μM) and Cd(2+) (100 μM) were applied. The tBK current was inhibited by caffeine (10 mM), ryanodine (30 μM), and tetracaine (100 μM), suggesting that RyR-mediated Ca(2+) release contributed to the activation of the tBK current. When IP3 receptors (IP3Rs) were blocked with 2-aminoethoxydiphenyl borate (2-APB, 100 μM), the amplitude of the tBK current was not reduced. However, when Ca(2+) release via IP3Rs was evoked with phenylephrine (1 μM) or carbachol (1 μM), the tBK current was inhibited. The effect of carbachol was abolished when IP3Rs were blocked with 2-APB or by inhibition of muscarinic receptors with the M3 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (1 μM). Under current-clamp conditions, bursts of action potentials could be evoked with depolarizing current injection. Carbachol reduced the number and amplitude of spikes in each burst, and these effects were reduced in the presence of 2-APB. In the presence of ryanodine, the number and amplitude of spikes were also reduced, and carbachol was without further effect. These data suggest that IP3-generating agonists can modulate the electrical activity of rabbit urethral smooth muscle cells and may contribute to the effects of neurotransmitters on urethral tone.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

Multiple muscarinic pathways mediate the suppression of voltage-gated Ca2+ channels in mouse intestinal smooth muscle cells

BACKGROUND AND PURPOSE

Stimulation of muscarinic receptors in intestinal smooth muscle cells results in suppression of voltage-gated Ca2+ channel currents (I(Ca)). However, little is known about which receptor subtype(s) mediate this effect.

EXPERIMENTAL APPROACH

The effect of carbachol on I(Ca) was studied in single intestinal myocytes from M2 or M3 muscarinic receptor knockout (KO) and wild-type (WT) mice.

KEY RESULTS

In M2KO cells, carbachol (100 microM) induced a sustained I(Ca) suppression as seen in WT cells. However, this suppression was significantly smaller than that seen in WT cells. Carbachol also suppressed I(Ca) in M3KO cells, but with a phasic time course. In M2/M3-double KO cells, carbachol had no effect on I(Ca). The extent of the suppression in WT cells was greater than the sum of the I(Ca) suppressions in M2KO and M3KO cells, indicating that it is not a simple mixture of M2 and M3 receptor responses. The G(i/o) inhibitor, Pertussis toxin, abolished the I(Ca) suppression in M3KO cells, but not in M2KO cells. In contrast, the G(q/11) inhibitor YM-254890 strongly inhibited only the I(Ca) suppression in M2KO cells. Suppression of I(Ca) in WT cells was markedly reduced by either Pertussis toxin or YM-254890.

CONCLUSION AND IMPLICATIONS

In intestinal myocytes, M2 receptors mediate a phasic I(Ca) suppression via G(i/o) proteins, while M3 receptors mediate a sustained I(Ca) suppression via G(q/11) proteins. In addition, another pathway that requires both M2/G(i/o) and M3/G(q/11) systems may be operative in inducing a sustained I(Ca) suppression.

M2 and M3 muscarinic receptors are involved in enteric nerve-mediated contraction of the mouse ileum: Findings obtained with muscarinic-receptor knockout mouse

The involvement of muscarinic receptors in neurogenic responses of the ileum was studied in wild-type and muscarinic-receptor (M-receptor) knockout (KO) mice. Electrical field stimulation to the wild-type mouse ileum induced a biphasic response, a phasic and sustained contraction that was abolished by tetrodotoxin. The sustained contraction was prolonged for an extended period after the termination of electrical field stimulation. The phasic contraction was completely inhibited by atropine. In contrast, the sustained contraction was enhanced by atropine. Ileal strips prepared from M2-receptor KO mice exhibited a phasic contraction similar to that seen in wild-type mice and a sustained contraction that was larger than that in wild-type mice. In M3-receptor KO mice, the phasic contraction was smaller than that observed in wild-type mice. Acetylcholine exogenously administrated induced concentration-dependent contractions in strips isolated from wild-type, M2- and M3-receptor KO mice. However, contractions in M3-receptor KO mice shifted to the right. The sustained contraction was inhibited by capsaicin and neurokinin NK2 receptor antagonist, suggesting that it is mediated by substance P (SP). SP-induced contraction of M2-receptor KO mice did not differ from that of wild-type mice. SP immunoreactivity was located in enteric neurons, colocalized with M2 receptor immunoreactivity. These results suggest that atropine-sensitive phasic contraction is mainly mediated via the M3 receptor, and SP-mediated sustained contraction is negatively regulated by the M2 receptor at a presynaptic level.

Roles of M2 and M3 muscarinic receptors in cholinergic nerve-induced contractions in mouse ileum studied with receptor knockout mice

BACKGROUND AND PURPOSE

The functional roles of M(2) and M(3) muscarinic receptors in neurogenic cholinergic contractions in gastrointestinal tracts remain to be elucidated. To address this issue, we studied cholinergic nerve-induced contractions in the ileum using mutant mice lacking M(2) or M(3) receptor subtypes.

EXPERIMENTAL APPROACH

Contractile responses to transmural electrical (TE) stimulation were isometrically recorded in ileal segments from M(2)-knockout (KO), M(3)-KO, M(2)/M(3)-double KO, and wild-type mice.

KEY RESULTS

TE stimulation at 2-50 Hz frequency-dependently evoked a fast, brief contraction followed by a slower, longer one in wild-type, M(2)-KO or M(3)-KO mouse preparations. Tetrodotoxin blocked both the initial and later contractions, while atropine only inhibited the initial contractions. The initial cholinergic contractions were significantly greater in wild-type than M(2)-KO or M(3)-KO mice; the respective mean amplitudes at 50 Hz were 91, 74 and 68 % of 70mM K(+)-induced contraction. Pretreatment with pertussis toxin blocked the cholinergic contractions in M(3)-KO but not in M(2)-KO mice. Cholinergic contractions also remained in wild-type preparations, but their sizes were reduced by 20-30 % at 10-50 Hz. In M(2)/M(3)-double KO mice, TE stimulation evoked only slow, noncholinergic contractions, which were significantly greater in sizes than in any of the other three mouse strains.

CONCLUSION AND IMPLICATIONS

These results demonstrate that M(2) and M(3) receptors participate in mediating cholinergic contractions in mouse ileum with the latter receptors assuming a greater role. Our data also suggest that the lack of both M(2) and M(3) receptors causes upregulation of noncholinergic excitatory innervation of the gut smooth muscle.

Muscarinic cationic current in gastrointestinal smooth muscles: signal transduction and role in contraction

1 The muscarinic receptor plays a key role in the parasympathetic nervous control of various peripheral tissues including gastrointestinal tract. The neurotransmitter acetylcholine, via activating muscarinic receptors that exist in smooth muscle, produces its contraction. 2 There is the opening of cationic channels as an underlying mechanism. The opening of cationic channels results in influxes of Ca2+ via the channels into the cell and also via voltage-dependent Ca2+ channels which secondarily opened in response to the depolarization, providing an amount of Ca2+ for activation of the contractile proteins. 3 Electrophysiological and pharmacological studies have shown that the cationic channels as well as muscarinic receptors exist in many visceral smooth muscle cells. However, the activation mechanisms of the cationic channels are still unclear. 4 In this article, we summarize the current knowledge of the muscarinic receptor-operated cationic channels, focusing on the receptor subtype, G protein and other signalling molecules that are involved in activation of these channels and on the molecular characteristics of the channel. This will improve strategies aimed at developing new selective pharmacological agents and understanding the activation mechanism and functions of these channels in physiological systems.

M(2) and M(3) muscarinic receptor-mediated contractions in longitudinal smooth muscle of the ileum studied with receptor knockout mice

Isometric contractile responses to carbachol were studied in ileal longitudinal smooth muscle strips from wild-type mice and mice genetically lacking M(2) or M(3) muscarinic receptors, in order to characterize the mechanisms involved in M(2) and M(3) receptor-mediated contractile responses. Single applications of carbachol (0.1-100 microM) produced concentration-dependent contractions in preparations from M(2)-knockout (KO) and M(3)-KO mice, mediated via M(3) and M(2) receptors, respectively, as judged by the sensitivity of contractile responses to blockade by the M(2)-preferring antagonist methoctramine (300 nM) or the M(3)-preferring antagonist 4-DAMP (30 nM). The M(2)-mediated contractions were mimicked in shape by submaximal stimulation with high K(+) concentrations (up to 35 mM), almost abolished by voltage-dependent Ca(2+) channel (VDCC) antagonists or depolarization with 140 mM K(+) medium, and greatly reduced by pertussis toxin (PTX) treatment. The M(3)-mediated contractions were only partially inhibited by VDCC antagonists or 140 mM K(+)-depolarization medium, and remained unaffected by PTX treatment. The contractions observed during high K(+) depolarization consisted of different components, either sensitive or insensitive to extracellular Ca(2+). The carbachol contractions observed with wild-type preparations consisted of PTX-sensitive and -insensitive components. The PTX-sensitive component was functionally significant only at low carbachol concentrations. The results suggest that the M(2) receptor, through PTX-sensitive mechanisms, induces ileal contractions that depend on voltage-dependent Ca(2+) entry, especially associated with action potential discharge, and that the M(3) receptor, through PTX-insensitive mechanisms, induces contractions that depend on voltage-dependent and -independent Ca(2+) entry and intracellular Ca(2+) release. In intact tissues coexpressing M(2) and M(3) receptors, M(2) receptor activity appears functionally relevant only when fractional receptor occupation is relatively small.

Functional and molecular analysis of L-type calcium channels in human esophagus and lower esophageal sphincter smooth muscle

Excitation of human esophageal smooth muscle involves the release of Ca(2+) from intracellular stores and influx. The lower esophageal sphincter (LES) shows the distinctive property of tonic contraction; however, the mechanisms by which this is maintained are incompletely understood. We examined Ca(2+) channels in human esophageal muscle and investigated their contribution to LES tone. Functional effects were examined with tension recordings, currents were recorded with patch-clamp electrophysiology, channel expression was explored by RT-PCR, and intracellular Ca(2+) concentration was monitored by fura-2 fluorescence. LES muscle strips developed tone that was abolished by the removal of extracellular Ca(2+) and reduced by the application of the L-type Ca(2+) channel blocker nifedipine (to 13 +/- 6% of control) but was unaffected by the inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPase by cyclopiazonic acid (CPA). Carbachol increased tension above basal tone, and this effect was attenuated by treatment with CPA and nifedipine. Voltage-dependent inward currents were studied using patch-clamp techniques and dissociated cells. Similar inward currents were observed in esophageal body (EB) and LES smooth muscle cells. The inward currents in both tissues were blocked by nifedipine, enhanced by Bay K8644, and transiently suppressed by acetylcholine. The molecular form of the Ca(2+) channel was explored using RT-PCR, and similar splice variant combinations of the pore-forming alpha(1C)-subunit were identified in EB and LES. This is the first characterization of Ca(2+) channels in human esophageal smooth muscle, and we establish that L-type Ca(2+) channels play a critical role in maintaining LES tone.

Phospholipase C involvement in activation of the muscarinic receptor-operated cationic current in Guinea pig ileal smooth muscle cells

In guinea pig single ileal smooth muscle cells held under voltage-clamp, the role of phospholipase C (PLC) in activation of the muscarinic receptor-operated cationic current (I(cat)) was studied. U73122, a PLC inhibitor, prevented the generation of I(cat) by the muscarinic agonist carbachol. The effect did not involve muscarinic receptor block since it also blocked I(cat) which was evoked by GTPgammaS applied intracellularly to activate G proteins bypassing muscarinic receptors. Also, neither cationic channel block nor other possible nonspecific actions seemed to be involved since its analogue (U73343), structurally close but deficient of the PLC-inhibiting activity, did not significantly affect carbachol- or GTPgammaS-evoked I(cat). Antibodies against the alpha subunits of G(q)/G(11) proteins (Galpha(q)/Galpha(11)-antibody) blocked only the small component of carbachol-evoked I(cat), which was associated with an increase in [Ca(2+)](i) linked to an increase in G(q/11) protein-regulated PLC activity. 1-Oleoyl-2-acetyl-sn-glycerol (OAG), an analogue of diacylglycerol (DAG) produced via PLC-catalyzed metabolism, produced no or only a small current by itself, with the carbachol-evoked I(cat) remaining unchanged. These results provide evidence for the importance of PLC in I(cat) generation, and they also strongly suggest that the activity of PLC involved in the primary activation of I(cat) is neither under regulation by G(q/11) proteins nor dependent on the action of DAG.

Contractile role of M2 and M3 muscarinic receptors in gastrointestinal, airway and urinary bladder smooth muscle

Both M(2) and M(3) muscarinic receptors are expressed in smooth muscle and influence contraction through distinct signaling pathways. M(3) receptors interact with G(q) to trigger phosphoinositide hydrolysis, Ca(2+) mobilization and a direct contractile response. In contrast, M(2) receptors interact with G(i) and G(o) to inhibit adenylyl cyclase and Ca(2+)-activated K(+) channels and to potentiate a Ca(2+)-dependent, nonselective cation conductance. Ultimately, these mechanisms lead to the prediction that the influence of the M(2) receptor on contraction should be conditional upon mobilization of Ca(2+) by another receptor such as the M(3). Mathematical modeling studies of these mechanisms show that the competitive antagonism of a muscarinic response mediated through activation of both M(2) and M(3) receptors should resemble the profile of the directly acting receptor (i.e., the M(3)) and not that of the conditionally acting receptor (i.e., the M(2)). Using a combination of pharmacological and genetic approaches, we have identified two mechanisms for the M(2) receptor in contraction: 1) a high potency inhibition of the relaxation elicited by agents that increase cytosolic cAMP and 2) a low potency potentiation of contractions elicited by the M(3) receptor. The latter mechanism may be involved in muscarinic agonist-mediated heterologous desensitization of smooth muscle, which requires activation of both M(2) and M(3) receptors.

Muscarinic Agonist-Mediated Heterologous Desensitization in Isolated Ileum Requires Activation of Both Muscarinic M2 and M3 Receptors

We investigated the subtypes of the muscarinic receptor mediating short-term heterologous desensitization in the isolated ileum. Treatment of the ileum from C57BL/6 mice with acetylcholine (30 microM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F2alpha (PGF2alpha) and the muscarinic agonist, oxotremorine-M. This subsensitivity was characterized by 7- and 3-fold increases in the EC50 values of the agonists, respectively, with no significant effect on the maximal response. The subsensitivity to PGF2alpha was prevented in both M2 and M3 muscarinic receptor knockout mice. Similarly, the subsensitivity to oxotremorine-M was prevented in M2 knockout mice. Acetylcholine-mediated desensitization of histamine-induced contractions in the guinea pig ileum was inhibited by both M2- and M3-selective muscarinic antagonists with high potency, although careful analysis of the data suggested behavior more consistent with an M2 antagonistic profile. Modeling studies showed that the competitive antagonism of response contingent upon activation of two receptor subtypes should exhibit a pharmacological profile similar to that of the least sensitive signaling pathway. Our results demonstrate that muscarinic agonist-mediated short-term heterologous desensitization of intestinal smooth muscle is contingent upon activation of both M2 and M3 muscarinic receptors and that activation of either receptor by itself is insufficient to cause desensitization.

Effects of G-protein-specific antibodies and G beta gamma subunits on the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle cells

(1) The effects on the whole-cell carbachol-induced muscarinic cationic current (mIcat) of antibodies against the alpha-subunits of various G proteins, as well as the effect of a Gbetagamma subunit, were studied in single guinea-pig ileal smooth muscle cells voltage-clamped at -50 mV. Ionized intracellular calcium concentration, [Ca(2+)](i), was clamped at 100 nM using a 1,2-bis(2-aminophenoxyl-ethane-N,N,N',N'-tetraacetic acid)/Ca(2+) mixture. (2) Application of ascending concentrations of carbachol (1-300 micro M) activated mIcat (mean amplitude 0.83 nA at 300 micro M carbachol; EC(50) 8 micro M; Hill slope 1.0). A 20 min or longer intracellular application via the pipette solution of G(i3)/G(o) or G(o) antibodies resulted in about a 70% depression of the maximum response without change in the EC(50) value. In contrast, antibodies against alpha-subunits of G(i1), G(i1)/G(i2), G(i3), G(q)/G(11) or G(s) protein over a similar or longer period did not significantly reduce mIcat. Antibodies to common Gbeta or infusion of the Gbetagamma subunit itself had no effect on mIcat. (3) If cells were exposed briefly to carbachol (50 or 100 micro M) at early times (<3 min) after infusion of antibodies to Galpha(i3)/Galpha(o) or to Galpha(o) had begun, carbachol responses remained unchanged even after 20-60 min; that is, the depression of mIcat by these antibodies was prevented. (4) These data show that Galpha(o) protein couples the muscarinic receptor to the cationic channel in guinea-pig ileal longitudinal smooth muscle and that Gbetagamma is not involved. They also show that prior activation of the muscarinic receptor presumably causes a long-lasting postactivation change of the G protein, which is not reflected in mIcat, but acts to hinder antibody binding.

Glycolytic ATP production regulates muscarinic cation currents in guinea-pig ileum

We investigated the possible sources of intracellular ATP which was previously shown essential for maintaining the muscarinic cationic channel activities (or currents; I(cat)) in guinea-pig ileal myocytes, using two variants of patch clamp techniques. Deprivation of external glucose or its replacement with 2-deoxyglucose significantly reduced the magnitude of I(cat), recorded with nystatin-perforated method, with greater efficacy than for voltage-dependent Ca2+ current Intracellular dialysis of ileal myocytes with key substrates for glycolysis, oxidative metabolism and creatine-phosphocreatine system all resulted in a comparably effective maintenance of I(cat), which was abolished by inhibitors for these ATP-producing systems, 3-bromopyruvate, cyanide and 2,4-dinitrofluorobenzene (DNFB), respectively. However, amongst these inhibitors, only 3-bromopyruvate effectively reduced I(cat) recorded with the nystatin-perforated method. These results strongly suggest the exclusive physiological importance of glycolytic ATP production in maintaining I(cat), activity, and thus this mechanism may play a role in the regulation of gut motility.

Inhibition of oxotremorine-induced desensitization of guinea-pig ileal longitudinal muscle in Ca2+-free conditions

The aim of this study was to investigate the differences between oxotremorine-induced and acetylcholine (ACh)-induced desensitization, particularly under Ca2+-free conditions, in guinea-pig ileal longitudinal muscle, and to elucidate the different mechanisms of desensitization that might exist between these two muscarinic agonists. Pretreatment of the tissue with 10(-7)-10(-5) M oxotremorine (desensitizing treatment) in normal Tyrode solution caused desensitization of the responses to ACh, as did the desensitizing treatment with ACh. However, Ca2+-free conditions significantly reduced oxotremorine-induced desensitization, contrary to the previous findings that Ca2+-free conditions enhanced ACh-induced desensitization. The desensitizing treatment with oxotremorine caused suppression of the responses to high K+ (tonic phase), as did the ACh treatment. Ca2+-free conditions removed this suppression, whereasthis condition enhanced ACh-induced suppression of the K+ response. A protein kinase C inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (10(-4) M) had no effect on oxotremorine-induced desensitization of the ACh response. The results suggest that a voltage-gated Ca2+ channel was involved in oxotremorine-induced desensitization, as in ACh-induced desensitization, but that the process of inactivation of Ca2+ channels was different between oxotremorine and ACh, and that oxotremorine-induced desensitization was due not only to Ca2+ channel, but also to other unknown factors. Protein kinase C did not participate in oxotremorine-induced desensitization.

Muscarinic activation of transient inward current and contraction in canine colon circular smooth muscle cells

Muscarinic receptor mediated membrane currents and contractions were studied in isolated canine colon circular smooth muscle cells. Carbachol (10–5M) evoked a slow transient inward current that was superimposed by a transient outward current at holding potentials greater than –50 mV. Carbachol contracted the cells by 70 ± 2%. The effects of carbachol were blocked by atropine (10–6M), tetraethyl ammonium (20 mM), and BAPTA-AM (25 mM applied for 20 min). The inward current and contraction were not sensitive to diltiazem (10–5M), nitrendipine (3 × 10–7M), niflumic acid (10–5M), or N-phenylanthranilic acid (10–4M), but were gradually inhibited after repetitive stimulations in Ca2+free solution. Ni2+(2 mM) inhibited the inward current by 67 ± 4%. The inward current reversed at +15 mV. The outward component could be selectively inhibited by iberiotoxin (20 nM) or by intracellular Cs+. Repeated stimulation in the presence of cyclopiazonic acid (CPA, 3 µM) inhibited the carbachol-induced outward current and partially inhibited contraction. CPA did not inhibit the inward current. In conclusion, muscarinic receptor stimulation evoked a CPA-sensitive calcium release that caused contraction and a CPA-insensitive transient inward current was activated that is primarily carried by Ca2+ions and is sensitive to Ni2+.Key words: calcium, carbachol, smooth muscle, cyclopiazonic acid, sarcoplasmic reticulum.

Role of Ca2+ mobilization in muscarinic receptor-mediated membrane depolarization in guinea pig ileal smooth muscle cells

In single smooth muscle cells dispersed from guinea pig ileum, the muscarinic agonist carbachol (CCh) at 2 microM produced an oscillatory or sustained type of depolarization and at 100 microM, the latter type depolarization. Depletion of internal Ca2+ stores blocked the oscillatory response, but not the sustained responses to 2 microM and 100 microM CCh, although their decay after reaching the peak became faster. Blocking voltage-dependent Ca2+ channels (VDCCs) blocked both types of response to 2 microM CCh, but only slowed the initial rising phase of 100 microM CCh responses. Combination of Ca2+ store depletion and VDCC blockade abolished the responses to 2 microM CCh again and decreased those to 100 microM CCh in peak amplitude and persistency. Combination of Ca2+ store depletion with removal of extracellular Ca2+ markedly reduced or abolished the 100 microM CCh responses. The results suggest that muscarinic depolarization of the ileal cells requires Ca2+ mobilization for its generation and persistence; at weak muscarinic stimulation, both Ca2+ entry via VDCCs and Ca2+ release from internal stores may contribute to the Ca2+ mobilization; and under strong muscarinic stimulation, Ca2+ entry pathways resistant to VDCC blockers may also contribute to it.

Two Ca2+ entry pathways mediate InsP3-sensitive store refilling in guinea-pig colonic smooth muscle

Sarcolemma Ca2+ influx, necessary for store refilling, was well maintained, over a wide range (-70 to + 40 mV) of membrane voltages, in guinea-pig single circular colonic smooth muscle cells, as indicated by the magnitude of InsP3-evoked Ca2+ transients. This apparent voltage independence of store refilling was achieved by the activity of sarcolemma Ca2+ channels some of which were voltage gated while others were not. At negative membrane potentials (e.g. -70 mV), Ca2+ influx through channels which lacked voltage gating provided for store refilling while at positive membrane potentials (e.g. +40 mV) voltage-gated Ca2+ channels were largely responsible. Sarcolemma voltage-gated Ca2+ currents were not activated following store depletion. Removal of external Ca2+ or the addition of the Ca2+ channel blocker nimodipine (1 microM) inhibited store refilling, as assessed by the magnitude of InsP3-evoked Ca2+ transients, with little or no change in bulk average cytoplasmic Ca2+ concentration. One hypothesis for these results is that the store may refill from a high subsarcolemma Ca2+ gradient. Influx via channels, some of which are voltage gated and others which lack voltage gating, may permit the establishment of a subsarcolemma Ca2+ gradient. Store access to the gradient allows InsP3-evoked Ca2+ signalling to be maintained over a wide voltage range in colonic smooth muscle.

Microtubule cytoskeleton involvement in muscarinic suppression of voltage-gated calcium channel current in guinea-pig ileal smooth muscle

1. Effects of agents, which affect microtubule polymerization-depolymerization cycle, on Ba2+ current (IBa) flowing through voltage-gated Ca2+ channels and carbachol (CCh)-induced sustained suppression of IBa were examined in whole-cell voltage-clamped smooth muscle cells of guinea-pig ileum. 2. offchicine (100 microM) and vinblastine (100 microM), microtubule depolymerizers, increased the ampitude of IBa. Lumicolchicine (100 microM), an inactive analogue of colchicine, had no effect on IBa. 3. Taxol (1 - 100 microM), a microtubule polymerizer, decreased IBa in a concentration-dependent manner and accelerated the rate of inactivation of IBa. Baccatin III (100 microM), an inactive analogue of taxol, had no effect on IBa. 4. Colchicine (100 microM) and vinblastine (100 microM), but not lumicolchicine (100 microM), decreased or abolished the sustained component of CCh (10 microM)-induced IBa suppression. 5. Pretreatment with taxol (10 - 100 microM) resulted in a concentration-dependent decrease in IBa and the action of CCh on IBa. The inhibitory effects of taxol and CCh on IBa were not additive. 6. Colchicine (100 microM) or taxol (100 microM) had no effect on voltage-gated K+ channel current or CCh-induced non-selective cationic channel current. 7. These results suggest that polymerization of microtubules leads to suppression of Ca2+ channel activity, and that muscarinic sustained suppression of Ca2+ channel current is mediated by a signal transduction element which involves microtubule cytoskeleton.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Ca2+ influx through carbachol-activated non-selective cation channels in guinea-pig gastric myocytes

1. Ca2+ microfluorometry (100 microM K5 fura-2) and the voltage-clamp technique were combined to study the effect of carbachol (CCh, 50 microM) in inducing currents (ICCh) through non-selective cation channels (NSCCCh) and increments in global cytosolic Ca2+ concentration (Delta[Ca2+]c). 2. In Na+-containing bath solution, ICCh fell from an initial phasic to a subsequent small (5 %) tonic component; Delta[Ca2+]c fell to zero. Tonic ICCh and [Ca2+]c became prominent after substitution of extracellular 140 mM Na+ by 140 mM Cs+. Tonic ICCh and Delta[Ca2+]c were insensitive to intracellular heparin (3 mg ml-1) and ryanodine (4 microM), i.e. they did not depend on Ca2+ release from sarcoplasmic reticulum (SR). 3. Single channel currents of NSCCCh could be resolved in whole-cell recordings. Substitution of Na+ by Cs+ increased NSCCCh activity by one order of magnitude and slope conductance from 22 to 30 pS. Extracellular quinidine (3 microM) reversibly blocked the NSCCCh activity. 4. Both tonic ICCh and tonic Delta[Ca2+]c (a) followed a similar time course of activation, desensitization and facilitation, (b) were reversibly blocked by 3 microM quinidine, and (c) persisted upon block of SR Ca2+ release. 5. A Ca2+ fractional current of tonic ICCh (fCa) of 0.009 was calculated by comparing the ratio Delta[Ca2+]c (corrected for simultaneous Ca2+ redistribution) over ICCh with depolarization-induced *Delta[Ca2+]c (Delta[Ca2+]c calculated from ICa induced by a 400 ms depolarization from -60 to 0 mV at 2 mM [Ca2+]o, 145 mM [Cs+]o) over ICa. fCa was 0.023 at [Ca2+]o = 4 mM. 6. With 110 mM extracellular CaCl2 and 145 mM intracellular CsCl, ICCh reversed at +19.5 mV suggesting a permeability ratio PCa/PCs of 2.8. 7. We conclude that Ca2+ influx through NSCCCh under physiological [Ca2+]o could induce Delta[Ca2+]c. The fCa was, however, much smaller than the one calculated from the reversal potential.

Inhibitors of spasmogen-induced Ca2+ channel suppression in smooth muscle cells from small intestine

1. Whole-cell patch-clamp recordings were made from smooth muscle cells isolated from the longitudinal muscle layer of guinea-pig ileum. Carbachol (acting at muscarinic receptors) or histamine (acting at H1 histamine receptors) suppressed Ca2+ channel current. The effect of either agonist had an initial transient component followed by a sustained component. 2. Wortmannin inhibited transient and sustained components of carbachol-induced Ca2+ channel current suppression: half-effective inhibitory concentrations (IC50) were 1.1 microM and 0.6 microM for the two components respectively. Wortmannin also inhibited the transient phase of carbachol-induced cationic current (IC50 1.6 microM) and Ca2+-dependent K+-current (IC50 1.7 microM). Wortmannin did not appear to produce any direct block of cationic channels or Ca2+ channels. 3. Intracellular application of the phospholipase inhibitor D609 (tricyclodecan-9-ylxanthogenate) inhibited transient and sustained components of histamine action on the Ca2+ channel current: the IC50 was about 130 microM for both components. Carbachol action on Ca2+ channels was also inhibited by D609. D609 had no significant direct blocking effect on Ca2+ channels, cationic channels activated by carbachol, or Ca2+-activated K+-current in response to flash-photolysis of caged-inositol 1,4,5-trisphosphate. 4. Micromolar concentrations of wortmannin and D609 are inhibitors of both components of spasmogen-induced Ca2+ channel suppression. The data suggest that both components are mediated by a common, or similar, signal transduction element which is a phospholipase C (PLC) or phospholipase D (PLD) isoform.

Carbachol-induced oscillations in membrane potential and [Ca2+]i in guinea-pig ileal smooth muscle cells

1. Cytosolic free Ca2+ concentration ([Ca2+]i) and membrane potential were simultaneously recorded from single smooth muscle cells of guinea-pig ileum, using a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques. 2. Carbachol (CCh, 2 microM) produced oscillatory changes in [Ca2+]i and membrane potential which coincided well in time with each other, and peaks of membrane potential oscillations reached a saturated level of around -7 mV. Thapsigargin (1 microM) abolished these effects of 2 microM CCh. La3+ (3 microM) immediately prevented the discharge of spike potentials, but allowed both on-going oscillatory responses to persist for a while. 3. CCh (0.25-0.75 microM) caused membrane potential and [Ca2+]i to oscillate in some 20 % of cells studied. Every membrane potential oscillation was preceded by the discharge of single or multiple spike potentials. The effects of CCh were readily abolished by La3+ (3 microM). 4. In cells exhibiting no oscillatory response to 0.25-0.75 microM CCh, an electrically evoked action potential usually generated changes in [Ca2+]i and membrane potential similar to those following spontaneously evoked action potentials, and sometimes it did so only after [Ca2+]i or InsP3 had been slightly elevated by repeatedly evoking action potentials or by increasing CCh concentration in the bath medium. 5. The results suggest that in ileal smooth muscle cells, the oscillations of [Ca2+]i and membrane potential arising from muscarinic stimulation result from release of Ca2+ from internal stores and that there is a Ca2+-induced potentiation of coincidently elicited cation channel openings. Under weak muscarinic stimulation, Ca2+ entry upon action potential discharge can trigger such a release of stored Ca2+, resulting in synchronous generation of a large rise in [Ca2+]i and a slow, large membrane depolarization.

Muscarinic cation current and suppression of Ca2+ current in guinea pig ileal smooth muscle cells

Cationic current (Icat) and inhibition of the voltage-dependent Ca2+ current (ICa) evoked by muscarinic receptor activation with carbachol were studied using whole-cell patch clamp technique in smooth muscle cells isolated from longitudinal muscle of guinea pig small intestine. With low buffering of [Ca2+]i (0.1 mM BAPTA [1,2-bis-(2-aminophenoxy)-ethane-N,N, N', N'-tetraacetic acid] in pipette solution) Icat and ICa inhibitory responses had a rapid onset to an initial peak followed by a sustained phase. The sustained phase of ICa suppression was bigger than in the case when [Ca2+]i was clamped to 100 nM, but decreased with repeated stimulation. Upon repeated stimulation with 50 microM carbachol in cells where [Ca2+]i was clamped to 100 nM and when GTP was absent, Icat amplitude decreased strongly and more substantially compared to ICa inhibition, but both responses declined only slightly when 1 mM GTP was present in the pipette solution. GDP-betaS (1 or 5 mM) in pipette solution or pre-treatment of cells with pertussis toxin (6 microg/ml, for 4 h or longer) blocked Icat more than ICa suppression by carbachol, whereas L-NAME (N-omega-nitro-L-arginine methyl ester hydrochloride) (100 microM in pipette solution) affected neither of them significantly. We conclude that the cationic current and the suppression of the voltage-dependent Ca2+ current evoked by muscarinic receptor activation are mediated by pertussis toxin-sensitive G-protein(s) but the latter response was less sensitive to blockade by GDP-betaS and to GTP deficiency in the cell.

M2 and M3 muscarinic receptors couple, respectively, with activation of nonselective cationic channels and potassium channels in intestinal smooth muscle cells

Smooth muscle cells of guinea pig ileum express both M2 and M3 subtypes of muscarinic receptors. Under voltage clamp, activation of the muscarinic receptors with carbachol (CCh) induces Ca2+-activated K+ current (I[K-Ca]) and nonselective cationic current (Icat). Receptor subtypes mediating the current responses were characterized by using pirenzepine, AF-DX116, 4-DAMP and atropine, which have different profiles of the affinity constants for muscarinic receptor subtypes. The muscarinic antagonists inhibited either CCh-evoked I(K-Ca) or Icat with different potencies. Their relative potencies for I(K-Ca) and Icat inhibition resembled the relative affinity constants for M3 and M2 subtypes, respectively. Thus, the I(K-Ca) is mediated via the M3 subtype and the Icat via the M2 subtype.

Bethanechol activates a post-receptor negative feedback mechanism in rabbit urinary bladder smooth muscle

PURPOSE

Recent studies using vascular and gut smooth muscles indicate that contractile receptor agonists may activate post-receptor down-regulatory mechanisms causing a temporary reduction in the strength of subsequent contractions. Our data indicate a similar mechanism exists in detrusor smooth muscle of the urinary bladder.

MATERIALS AND METHODS

Each isolated strip of female rabbit detrusor was placed in a tissue bath, secured to an isometric force transducer, and length-adjusted until depolarization with 110 mM KCl produced a maximum contraction (S0). Subsequent contractions were normalized to S0 (S/S0) or to a first stimulus with 30 mM KCl or caffeine (S/S1). Tissues were pretreated with the muscarinic receptor agonist, bethanechol (BE), then stimulated with KCl, caffeine, or Bay k 8644 to identify potential post-receptor down-regulation.

RESULTS

Contractions induced by 30 mM KCl had three phases labeled fast peak (FP), slow peak (SP) and steady-state (SS). In tissues exposed for 30 min. to a maximum BE concentration then washed for 5 min., the KCl-induced FP and SP, but not SS, responses were reduced by approximately 40%. Smaller reductions in peak KCl-induced contractions occurred in tissues pretreated for a shorter duration or with a 100-fold lower BE concentration. This down-regulation induced by bethanechol pretreatment was reversible, lasting approximately 1-2 h. Not only were KCl-induced contractions reduced by BE pretreatment, but also those produced by the intracellular Ca(2+)-mobilizer, caffeine, and the L-type Ca2+ channel agonist, Bay k 8644.

CONCLUSIONS

Pretreatment of isolated strips of rabbit detrusor with a muscarinic receptor agonist produced short-term down-regulation of KCl-induced peak contractions that may have involved inhibition of both influx of extracellular Ca2+ and release of intracellular Ca2+. Reductions in the degree of this novel modulatory response during disease conditions and aging could enhance contractile activity, possibly causing detrusor instability.

Subtypes of the muscarinic receptor in smooth muscle

Muscarinic receptors are expressed in smooth muscle throughout the body. In most instances, the muscarinic receptor population in smooth muscle is composed of mainly the M2 and M3 subtypes in an 80% to 20% mixture. The M3 subtype mediates phosphoinositide hydrolysis and calcium mobilization, whereas the M2 subtype mediates an inhibition of cAMP accumulation. In addition, a variety of ionic conductances are elicited by muscarinic receptors. Muscarinic agonists stimulate a nonselective cation conductance that is pertussis toxin-sensitive and dependent on calcium. The pertussis toxin-sensitivity of this response suggests that it is mediated by M2 receptors. Following agonist induced depolarization of smooth muscle, voltage dependent calcium channels are activated to enable an influx of calcium. In some instances, muscarinic agonists enhance this conductance through a mechanism involving protein kinase C, whereas in other instances, muscarinic agonists suppress this calcium conductance. Smooth muscle often contains calcium activated potassium channels that tend to repolarize the membrane following calcium influx. Activation of muscarinic receptors suppresses this potassium conductance in some smooth muscles. Under standard conditions, muscarinic agonists elicit pertussis toxin-insensitive contractions through activation of the M3 receptor. When most of the M3 receptors are inactivated, it is possible to measure a pertussis toxin-sensitive contractile response to muscarinic agonists that is most likely mediated through M2 receptors. M2 receptors also cause an indirect contraction by inhibiting the relaxant effects of agents that increase cAMP (e.g., forskolin and isoproterenol).

Protein kinase C requirement of Ca2+ channel stimulation by intracellular ATP in guinea-pig basilar artery smooth muscle cells

1. Smooth muscle cells were isolated from guinea-pig basilar artery and conventional whole-cell recordings of Ca2+ channel activity were made at room temperature within 7 h of the isolation procedure. The purpose of the study was to investigate the mechanism of the stimulatory action of intracellular ATP on Ca2+ channels. 2. High (millimolar) concentrations of ATP were needed to produce stimulation of Ca2+ channels, and neither ADP nor AMP mimicked the action of ATP. 3. The ATP effect was not mimicked by stable ATP derivatives (AMP-PNP or AMP-PCP) and was abolished by incubation of cells in non-specific protein kinase inhibitors (staurosporine or H-7) or specific protein kinase C inhibitors (GF109203x, calphostin C or chelerythrine) but not by tyrosine kinase inhibitors (tyrphostin B42 and genistein). 4. The data suggest that ATP-induced stimulation of L-type Ca2+ channels requires functional activity of a protein kinase C isozyme.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Some evidence against the involvement of arachidonic acid in muscarinic suppression of voltage-gated calcium channel current in guinea-pig ileal smooth muscle cells

1. To see if arachidonic acid (AA) plays a role in the sustained suppression of voltage-gated calcium channel currents produced by muscarinic receptor stimulation by carbachol (CCh), the effects of AA on membrane currents were examined in whole-cell voltage-clamped smooth muscle cells of the guinea-pig ileum. 2. In cells bathed in Ba2+ PSS and dialysed with Cs(+)-based low EGTA (0.05 mM) pipette solution, and in which Ba2+ current (IBa) flowing through voltage-gated calcium channels was evoked repeatedly by stepping to 0 mV from the holding potential of -60 mV, AA (1-30 microM), applied extracellularly, gradually suppressed IBa in a concentration-dependent manner. The IBa suppression was observed even with 20 mM EGTA in the pipette. 3. AA (3 microM) and CCh (10 microM) shifted the voltage-dependent inactivation curve of IBa in the negative potential direction, but the effect of AA differed from that of CCh in that an accompanying appreciable decrease in the slope was observed. 4. The sustained suppression of IBa induced by CCh (10 microM) remained almost unaltered after pretreatment with 4-bromophenacyl bromide (10 microM), an inhibitor of phospholipase A2, or a combination of indomethacin (10 microM), an inhibitor of the cyclo-oxygenase pathway, and nordihydroguaiaretic acid (10 microM), an inhibitor of the lipoxygenase pathway. 5. In cells bathed in Ca2+ PSS and dialysed with K(+)-based pCa 6.5 pipette solution, voltage-dependent Ca2+ current (ICa) and K+ current (IK) were recorded simultaneously. AA (3 microM) suppressed IK as well as ICa, whereas CCh (10 microM) suppressed ICa but not IK. 6. We conclude from these results that AA or its metabolite is unlikely to be involved in the sustained suppression of voltage-gated calcium channel current induced by muscarinic receptor stimulation in guinea-pig ileal smooth muscle cells.

Modulation of carbachol-induced [Ca2+]i oscillations by Ca2+ influx in single intestinal smooth muscle cells

1. Oscillations of cytosolic Ca2+ concentration ([Ca2+]i) evoked by carbachol (CCh; 2 microM), a muscarinic agonist, were detected as oscillatory changes of muscarinic receptor-coupled cationic current (Icat) in guinea-pig ileal smooth muscle cells by the whole cell patch-clamp technique. 2. Reduction of extracellular Ca2+ from 2 mM to 0.2 or 0.05 mM, during CCh-induced Icat oscillations, caused them to disappear or to decrease markedly in frequency. A return to 2 mM Ca2+ concentration restored the initial Icat oscillations. 3. Application of nifedipine (1-3 microM) or D600 (2-5 microM) to block the voltage-gated Ca2+ channel (VGCC) decreased the frequency of the ongoing Icat oscillations in the cells held at -20 mV, but it was without effect in cells held at -60 mV. 4. Displacement of the holding potential of -20 mV to -60 mV to deactivate VGCC produced a decrease, an increase or no noticeable change in the frequency of the Icat oscillations in different cells. Displacement to 20 mV to inactivate VGCC invariably produced a decrease in the frequency. In nifedipine-treated cells, the Icat oscillations varied in frequency voltage-dependently in a reverse and linear way within the range -80 to 40 mV. 5. Application of thapsigargin (1 or 2 microM), an inhibitor of Ca(2+)-ATPase in the membrane of internal Ca2+ stores, caused CCh-induced Icat oscillations to disappear with a progressing phase during which their amplitude, but not frequency, declined. 6. The results suggest that membrane Ca2+ entry has a crucial role to play in regulation of the frequency of CCh-induced [Ca2+]i oscillations in addition to persistence of their generation, and that the effect is brought about by a potential mechanism independent of Ca2+ store replenishment. They also provide evidence that two types of Ca2+ permeant channels, VGCC and an as yet unidentified channel, are involved in the Ca2+ entry responsible for modulation of [Ca2+]i oscillations.

Carbachol but not acetylcholine inhibits contraction by the protein kinase C-dependent and -independent pathways in the smooth muscle of guinea pig taenia caeci

In the intestinal smooth muscle of guinea pig taenia caeci, acetylcholine and carbachol induced a transient contraction followed by a sustained contraction. The magnitudes of the transient and sustained contractions were similar when muscle was stimulated with acetylcholine (0.1 microM-1 mM) or a lower concentration (0.1 microM) of carbachol. However, higher concentrations of carbachol (1 - 100 microM) induced significantly smaller sustained contraction than the transient contraction. In the 45 mM KCI-stimulated strips, addition of 100 microM carbachol induced a transient increase followed by a sustained decrease in the contractile tension. In contrast, acetylcholine (0.1 microM-1 mM) showed only weak inhibitory effect on the high K(+)-induced contraction either in the absence or presence of a cholinesterase inhibitor, 0.5 microM diisopropylfluorophosphate. The same concentration of diisopropylfluorophosphate shifted the concentration-response curve for acetylcholine to lower concentrations. In the muscles pretreated with 3 microM phorbol 12-myristate 13-acetate for 24 hr to desensitize protein kinase C, sustained contractions induced by higher concentrations of carbachol (1-100 microM) were significantly greater than those in the strips without the treatment with phorbol ester. However, the transient contraction and the contraction induced by a lower concentration (0.1 microM) of carbachol were not changed by the treatment with phorbol ester. Pretreatment with phorbol ester attenuated the inhibitory effect of carbachol on the high K(+)-induced contraction. These results suggest that the inhibitory effects of carbachol is composed of two phases: protein kinase C-independent transient inhibition and protein kinase C-dependent sustained inhibition.

Carbachol-induced [Ca2+]i oscillations in single smooth muscle cells of guinea-pig ileum

1. Changes in cytosolic Ca2+ concentration ([Ca2+]i) produced by carbachol (CCh) were measured in single smooth muscle cells of guinea-pig ileum using a Ca(2+)-sensitive fluorescent dye, fura-2, to clarify the underlying mechanisms of muscarinic [Ca2+]i oscillations. 2. Half of the cells, when exposed to 0.2 microM CCh, exhibited repeated changes in [Ca2+]i giving a serrated appearance. The oscillatory changes in [Ca2+]i were very similar to those evoked by increasing extracellular K(+) concentration ([K+]o) to 30 mM, which were abolished by removal of extracellular Ca2+, nifedipine and La3+, but remained unchanged after depletion of internal Ca2+ stores with cyclopiazonic acid, thapsigargin and ryanodine. 3. Every individual [Ca2+]i oscillation was just like a [Ca2+]i increase generated spontaneously in about 8% of cells or triggered by an action potential evoked by a current pulse in current-clamped cells. 4. In the remaining half of the cells exposed to 0.2 microM CCh, slower [Ca2+]i oscillations were elicited and every individual [Ca2+]i oscillation was always preceded by the fast brief increase in [Ca2+]i. 5. [Ca2+]i oscillations elicited by 2 microM CCh were temporally and functionally distinct from those induced by high [K+]o. They were more or less regular in the periodicity and pattern, comprised pacemaker potential-like [Ca2+]i increases or sinusoidal types of [Ca2+]i increases, and could be elicited even in 100 mM K+(o). 6. Removal of extracellular Ca2+ or application of nifedipine, methoxyverapamil (D600), diltiazem or La3+ during CCh (2 micro M)-induced [Ca2+]i oscillations caused them to disappear. In cells i which internal Ca2+ stores were depleted, 2 microM CCh did not evoke [Ca2+]i oscillations but occasionally induced single or repeated generation of the increase in [Ca2+]i with a serrated appearance. 7. The results indicate that CCh can induce two types of [Ca2+]i oscillation in guinea-pig ileal smooth muscle cells; one arises from Ca2+ influx associated with action potential discharges and the other from periodic release of Ca2+ from internal stores. The latter [Ca2+]i oscillation requires extracellular Ca2+ to sustain it.

Cholinergic inhibition of Ca2+ current in guinea-pig gastric and tracheal smooth muscle cells

1. Cholinergic regulation of L-type Ca2+ channels was investigated in freshly dissociated guinea-pig gastric and tracheal smooth muscle cells. Acetylcholine (ACh, 50 microM) decreased Ca2+ channel current (ICa) by 37 +/- 3% (mean +/- S.E.M., 46 cells). 2. ACh reduced ICa at all voltages, with no shift in the current-voltage relationship. Effects of ACh were rapid (within 5 s) and repeatable, with multiple applications reproducibly inhibiting ICa in the continued presence of extracellular Ca2+ and in the presence of protein kinase C inhibitors. 3. The involvement of Ca2+ stores in this inhibition was investigated using Ca(2+)-free solution or cyclopiazonic acid (CPA) to deplete the stores. ACh initially inhibited ICa in the Ca(2+)-free solution (Na+ as charge carrier, 53 +/- 4% decrease, 18 cells) with subsequent responses significantly attenuated (n = 9). CPA (1 microM) reduced, then abolished, the effects of ACh on ICa (n = 5). 4. When studied in cell-attached patches (Ba2+ as charge carrier), ACh reduced Ca2+ channel open probability in twenty-two of thirty-six cells, consistent with the involvement of a diffusible cytosolic messenger. 5. ACh also inhibited ICa in tracheal muscle cells (reduction of 38 +/- 6% in 1 mM Ca2+, 4 cells; 77 +/- 3% in Ca(2+)-free solution, 7 cells). Furthermore, in cells where ACh elicited oscillating Ca(2+)-activated Cl- current, oscillatory inhibition of ICa was also observed (3 cells). 6. In summary, ACh causes rapid and reversible inhibition of ICa in gastric and tracheal muscles. Ca2+ stores were required to initiate this effect, with the rapid onset and oscillatory inhibition consistent with Ca2+ inhibition of the channel. Suppression of ICa would reduce Ca2+ entry during cholinergic excitation.