Effect of cAMP elevating agents on carbachol-induced phosphoinositide hydrolysis and calcium mobilization in cultured canine tracheal smooth muscle cells

Theophylline-Induced Relaxation Is Enhanced after Testosterone Treatment via Increased KV1.2 and KV1.5 Protein Expression in Guinea Pig Tracheal Smooth Muscle

Theophylline is a drug commonly used to treat asthma due to its anti-inflammatory and bronchodilatory properties. Testosterone (TES) has been suggested to reduce the severity of asthma symptoms. This condition affects boys more than girls in childhood, and this ratio reverses at puberty. We reported that guinea pig tracheal tissue chronic exposure to TES increases the expression of β₂-adrenoreceptors and enhances salbutamol-induced K⁺ currents (IK⁺). Herein, we investigated whether the upregulation of K⁺ channels can enhance the relaxation response to methylxanthines, including theophylline. Chronic incubation of guinea pig tracheas with TES (40 nM, 48 h) enhanced the relaxation induced by caffeine, isobutylmethylxanthine, and theophylline, an effect that was abolished by tetraethylammonium. In tracheal myocytes, chronic incubation with TES increased theophylline-induced IK⁺; flutamide reversed this effect. The increase in IK⁺ was blocked by 4-aminopyridine by ~82%, whereas iberiotoxin reduced IK⁺ by ~17%. Immunofluorescence studies showed that chronic TES exposure increased the expression of K_V1.2 and K_V1.5 in airway smooth muscle (ASM). In conclusion, chronic exposure to TES in guinea pig ASM promotes upregulation of K_V1.2 and K_V1.5 and enhances theophylline relaxation response. Therefore, gender should be considered when prescribing methylxanthines, as teenage boys and males are likely to respond better than females.

YG-1 Extract Improves Acute Pulmonary Inflammation by Inducing Bronchodilation and Inhibiting Inflammatory Cytokines

YG-1 extract used in this study is a mixture of Lonicera japonica, Arctic Fructus, and Scutellariae Radix. The present study was designed to investigate the effect of YG-1 extract on bronchodilatation (ex vivo) and acute bronchial and pulmonary inflammation relief (in vivo). Ex vivo: The bronchodilation reaction was confirmed by treatment with YG-1 concentration-accumulation (0.01, 0.03, 0.1, 0.3, and 1 mg/mL) in the bronchial tissue ring pre-contracted by acetylcholine (10 μM). As a result, YG-1 extract is considered to affect bronchodilation by increased cyclic adenosine monophosphate, cAMP) levels through the β2-adrenergic receptor. In vivo: experiments were performed in C57BL/6 mice were divided into the following groups: control group; PM2.5 (fine particulate matter)-exposed group (PM2.5, 200 μg/kg/mL saline); and PM2.5-exposed + YG-1 extract (200 mg/kg/day) group. The PM2.5 (200 μg/kg/mL saline) was exposed for 1 h for 5 days using an ultrasonic nebulizer aerosol chamber to instill fine dust in the bronchi and lungs, thereby inducing acute lung and bronchial inflammation. From two days before PM2.5 exposure, YG-1 extract (200 mg/kg/day) was administered orally for 7 days. The PM2.5 exposure was involved in airway remodeling and inflammation, suggesting that YG-1 treatment improves acute bronchial and pulmonary inflammation by inhibiting the inflammatory cytokines (NLRP3/caspase-1 pathway). The application of YG-1 extract with broncho-dilating effect to acute bronchial and pulmonary inflammation animal models has great significance in developing therapeutic agents for respiratory diseases. Therefore, these results can provide essential data for the development of novel respiratory symptom relievers. Our study provides strong evidence that YG-1 extracts reduce the prevalence of respiratory symptoms and the incidence of non-specific lung diseases and improve bronchial and lung function.

Bronchodilators, receptors and cross-talk: Together is better?

The most widely used maintenance therapies in chronic obstructive pulmonary disease (COPD) are long-acting muscarinic antagonists (LAMAs), and a number of these drugs are now available in combination with long-acting β2-agonists (LABAs). LAMAs inhibit the parasympathetic muscarinic pathway, while LABAs, as sympathomimetics, reduce airway smooth muscle (ASM) tone. As well as directly controlling the constriction and relaxation of ASM, muscarinic and adrenergic receptors are found on inflammatory cells, and drugs that target these receptors may also reduce inflammation in COPD. Evidence suggests that the muscarinic and adrenergic pathways cross-talk at the level of neuronal input to the ASM via second-messenger pathways within ASM cells. Although the cross-talk is not completely understood, pharmacologically targeting both pathways in COPD can maximize bronchodilation. Combining LAMAs and LABAs demonstrated improved efficacy compared with the individual therapies and so, for greater convenience, several fixed-dose combinations for once-daily use have been developed. These fixed-dose combinations demonstrate improvements in both lung-function and patient-reported outcomes compared with well-established monotherapies, with similar tolerability profiles to the individual agents.

The pharmacological rationale for combining muscarinic receptor antagonists and β-adrenoceptor agonists in the treatment of airway and bladder disease

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Muscarinic receptors increase smooth muscle tone in airways and urinary bladder.

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β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.

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Opposition involves transmitter release, signal transduction and receptor expression.

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This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.

Muscarinic receptor antagonists and β-adrenoceptor agonists are used in the treatment of obstructive airway disease and overactive bladder syndrome. Here we review the pharmacological rationale for their combination. Muscarinic receptors and β-adrenoceptors are physiological antagonists for smooth muscle tone in airways and bladder. Muscarinic agonism may attenuate β-adrenoceptor-mediated relaxation more than other contractile stimuli. Chronic treatment with one drug class may regulate expression of the target receptor but also that of the opposing receptor. Prejunctional β₂-adrenoceptors can enhance neuronal acetylcholine release. Moreover, at least in the airways, muscarinic receptors and β-adrenoceptors are expressed in different locations, indicating that only a combined modulation of both systems may cause dilatation along the entire bronchial tree. While all of these factors contribute to a rationale for a combination of muscarinic receptor antagonists and β-adrenoceptor agonists, the full value of such combination as compared to monotherapy can only be determined in clinical studies.

cAMP-dependent protein kinase enhances inositol 1,4,5-trisphosphate-induced Ca2+ release in AR4-2J cells

In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3)R), a ligand-gated Ca(2+) channel, plays an important role in the control of intracellular Ca(2+). There are three subtypes of IP(3)R that are differentially distributed among cell types. AR4-2J cells express almost exclusively the IP(3)R-2 subtype. The purpose of this study was to investigate the effect of cAMP-dependent protein kinase (PKA) on the activity of IP(3)R-2 in AR4-2J cells. We showed that immunoprecipitated IP(3)R-2 is a good substrate for PKA. Using a back-phosphorylation approach, we showed that endogenous PKA phosphorylates IP(3)R-2 in intact AR4-2J cells. Pretreatment with PKA enhanced IP(3)-induced Ca(2+) release in permeabilized AR4-2J cells. Pretreatment with the cAMP generating agent's forskolin and vasoactive intestinal peptide (VIP) enhanced carbachol (Cch)-induced and epidermal growth factor (EGF)-induced Ca(2+) responses in intact AR4-2J cells. Our results are consistent with an enhancing effect of PKA on IP(3)R-2 activity. This conclusion supports the emerging concept of crosstalk between Ca(2+) signaling and cAMP pathways and thus provides another way by which Ca(2+) signals are finely encoded within non-excitable cells.

Airway smooth muscle relaxation results from a reduction in the frequency of Ca2+ oscillations induced by a cAMP-mediated inhibition of the IP3 receptor

Background

It has been shown that the contractile state of airway smooth muscle cells (SMCs) in response to agonists is determined by the frequency of Ca²⁺ oscillations occurring within the SMCs. Therefore, we hypothesized that the relaxation of airway SMCs induced by agents that increase cAMP results from the down-regulation or slowing of the frequency of the Ca²⁺ oscillations.

Methods

The effects of isoproterenol (ISO), forskolin (FSK) and 8-bromo-cAMP on the relaxation and Ca²⁺ signaling of airway SMCs contracted with methacholine (MCh) was investigated in murine lung slices with phase-contrast and laser scanning microscopy.

Results

All three cAMP-elevating agents simultaneously induced a reduction in the frequency of Ca²⁺ oscillations within the SMCs and the relaxation of contracted airways. The decrease in the Ca²⁺ oscillation frequency correlated with the extent of airway relaxation and was concentration-dependent. The mechanism by which cAMP reduced the frequency of the Ca²⁺ oscillations was investigated. Elevated cAMP did not affect the re-filling rate of the internal Ca²⁺ stores after emptying by repetitive exposure to 20 mM caffeine. Neither did elevated cAMP limit the Ca²⁺ available to stimulate contraction because an elevation of intracellular Ca²⁺ concentration induced by exposure to a Ca²⁺ ionophore (ionomycin) or by photolysis of caged-Ca²⁺ did not reverse the effect of cAMP. Similar results were obtained with iberiotoxin, a blocker of Ca²⁺-activated K⁺ channels, which would be expected to increase Ca²⁺ influx and contraction. By contrast, the photolysis of caged-IP₃ in the presence of agonist, to further elevate the intracellular IP₃ concentration, reversed the slowing of the frequency of the Ca²⁺ oscillations and relaxation of the airway induced by FSK. This result implied that the sensitivity of the IP₃R to IP₃ was reduced by FSK and this was supported by the reduced ability of IP₃ to release Ca²⁺ in SMCs in the presence of FSK.

Conclusion

These results indicate that the relaxant effect of cAMP-elevating agents on airway SMCs is achieved by decreasing the Ca²⁺ oscillation frequency by reducing internal Ca²⁺ release through IP₃ receptors.

M2 and M3 muscarinic receptor activation of urinary bladder contractile signal transduction. I. Normal rat bladder

The muscarinic receptor subtype-activated signal transduction mechanisms mediating rat urinary bladder contraction are incompletely understood. M(3) mediates normal rat bladder contractions; however, the M(2) receptor subtype has a more dominant role in contractions of the hypertrophied bladder. Normal bladder muscle strips were exposed to inhibitors of enzymes thought to be involved in signal transduction in vitro followed by a single cumulative concentration-response curve to the muscarinic receptor agonist carbachol. The outcome measures were the maximal contraction, the potency of carbachol, and the affinity of the M(3) -selective antimuscarinic agent darifenacin for inhibition of contraction. Inhibition of phosphoinositide-specific phospholipase C (PI-PLC) with 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH(3)) reduces carbachol potency and reduces darifenacin affinity, whereas inhibition of phosphatidyl choline-specific phospholipase C (PC-PLC) with O-tricyclo[5.2.1.02,6]dec-9-yl dithiocarbonate potassium salt (D609) attenuates the carbachol maximal contraction. Inhibition of rho kinase with (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclohexanecarboxamide dihydrochloride (Y-27632) reduces carbachol potency and increases darifenacin affinity. Inhibition of rho kinase, protein kinase A (PKA), and protein kinase G (PKG) with 1-(5-isoquinolinesulfonyl)-homopiperazine.HCl (HA-1077) reduces the carbachol maximal contraction, carbachol potency, and darifenacin affinity. Inhibition of protein kinase C (PKC) with chelerythrine increases darifenacin affinity, whereas inhibition of rho kinase, PKA, PKG, and PKC with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine.2HCl (H7) reduces the carbachol maximum and carbachol potency while increasing darifenacin affinity. Inhibition of rho kinase, PKA, and PKG with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl (H89) reduces carbachol maximum and carbachol potency. Both the M(2) and the M(3) receptor subtype are involved in normal rat bladder contractions. The M(3)subtype seems to mediate contraction by activation of PI-PLC, PC-PLC, and PKA, whereas the M(2) signal transduction cascade may include activation of rho kinase, PKC, and an additional contractile signal transduction mechanism independent of rho kinase or PKC.

Selective restoration of calcium coupling to muscarinic M(3) receptors in contractile cultured airway myocytes

We previously demonstrated that after several days of serum deprivation about one-sixth of confluent cultured canine tracheal myocytes acquire an elongated, structurally and functionally contractile phenotype. These myocytes demonstrated significant shortening on ACh exposure. To evaluate the mechanism by which these myocytes acquire responsiveness to ACh, we assessed receptor-Ca(2+) coupling using fura 2-AM fluorescence imaging and muscarinic receptor expression using Western analysis. Cells were grown to confluence in 10% fetal bovine serum and then maintained for 7-13 days in serum-free medium. A fraction of serum-deprived cells exhibited reproducible intracellular Ca(2+) mobilization in response to ACh that was uniformly absent from airway myocytes before serum deprivation. The Ca(2+) response to 10(-4) M ACh was ablated by inositol 1,4,5-trisphosphate (IP(3)) receptor blockade using 10(-6) M xestospongin C but not by removal of extracellular Ca(2+). Also, 10(-7) M atropine or 10(-7) M 4-diphenylacetoxy-N-methylpiperidine completely blocked the response to ACh, but intracellular Ca(2+) mobilization was not ablated by 10(-6) M pirenzepine or 10(-6) M methoctramine. In contrast, 10(-5) M bradykinin (BK) was without effect in these ACh-responsive myocytes. Interestingly, myocytes that did not respond to ACh demonstrated robust increases in intracellular Ca(2+) on exposure to 10(-5) M BK that were blocked by removal of extracellular Ca(2+) and were only modestly affected by IP(3) receptor blockade. Serum deprivation increased the abundance of M(3) receptor protein and of BK(2) receptor protein by two- to threefold in whole cell lysates within 2 days of serum deprivation, whereas M(2) receptor protein fell by >75%. An increase in M(3) receptor abundance and restoration of M(3) receptor-mediated Ca(2+) mobilization occur concomitant with reacquisition of a contractile phenotype during prolonged serum deprivation. These data demonstrate plasticity in muscarinic surface receptor expression and function in a subpopulation of airway myocytes that show mutually exclusive physiological and pharmacological diversity with other cells in the same culture.

Effects of forksolin, dibutyryl cAMP and H89 on Ca2+ mobilization in submandibular salivary cells of newborn rats

The effects of substances which affect cAMP or the cAMP-dependent protein kinase (PKA) on the inositol 1,4,5-trisphosphate (IP3) and Ca2+ responses to acetylcholine or thapsigargin were investigated in submandibular gland cells of newborn rats. Exposure to forskolin, dibutyryl cAMP or the PKA inhibitor H89 did not affect the formation of IP3 or the release of Ca2+ from intracellular stores elicited by acetylcholine. However, the thapsigargin-induced Ca2+ release was reduced by dibutyryl cAMP and enhanced by H89 in immature cells. Ca2+ influx activated by acetylcholine and thapsigargin was additive in immature cells but not in mature cells, suggesting the presence of a separate Ca2+ entry pathway in immature cells. Moreover, the acetylcholine-stimulated Ca2+ influx was significantly potentiated by forskolin and dibutyrylcAMP, but not by H89 in immature cells. In contrast, the thapsigargin-activated Ca2+ influx was dramatically enhanced by H89, but not by forskolin and dibutyrylcAMP in these cells. This modulation of Ca2+ mobilization by the test substances is different from that observed in mature submandibular cells in which forskolin, dibutyrylcAMP and H89 affected both IP3 formation and Ca2+ release in response to acetylcholine. Therefore, these results suggest differences in the interaction between the cAMP-PKA and the phosphoinositide-Ca2+ signalling pathways of mature and immature salivary cells. The modulation of Ca2+ influx by the cAMP-PKA pathway in immature cells is likely to play a part in the maturation of salivary cells.

Involvement of protein kinase C and protein kinase A in the muscarinic receptor signalling pathways mediating phospholipase C activation, arachidonic acid release and calcium mobilisation

The involvement of protein kinase C (PKC) and protein kinase A (PKA) in cholinergic signalling in CHO cells expressing the M3 subtype of the muscarinic acetylcholine receptor was examined. Muscarinic signalling was assessed by measuring carbachol-induced activation of phospholipase C (PLC), arachidonic acid release, and calcium mobilisation. Carbachol activation of PLC was not altered by inhibition of PKC with chelerythrine chloride, bisindolylmaleimide or chronic treatment with phorbol myristate acetate (PMA). Activation of PKC by acute treatment with PMA was similarly without effect. In contrast, inhibition of PKC blocked carbachol stimulation of arachidonic acid release. Likewise, PKC inhibition resulted in a decreased ability of carbachol to mobilise calcium, whereas PKC activation potentiated calcium mobilisation. Inhibition of PKA with H89 or Rp-cAMP did not alter the ability of carbachol to activate PLC. Similarly, PKA activation with Sp-cAMP or forskolin had no effect on PLC stimulation by carbachol. Carbachol-mediated release of arachidonic acid was decreased by H89 but only slightly increased by forskolin. Forskolin also increased calcium mobilisation by carbachol. These results suggest a function for PKC and PKA in M3 stimulation of arachidonic acid release and calcium mobilisation but not in PLC activation.

Mechanism of inhibition of VIP-induced LES relaxation by heme oxygenase inhibitor zinc protoporphyrin IX

The putative heme oxygenase inhibitor zinc protoporphyrin IX (ZnPP IX) is known to exert diverse actions, including inhibitory action on smooth muscle relaxation by vasoactive intestinal polypeptide (VIP). The studies were performed in the opossum lower esophageal sphincter (LES) smooth muscle to determine the site of the inhibitory action of ZnPP IX in the smooth muscle relaxation by VIP. We also examined the effect of a direct Gs protein activator, cholera toxin (CTX), known to stimulate adenylate cyclase (AC). CTX caused relaxation of the LES smooth muscle by its action directly at the smooth muscle cells. The convergence of the common mechanisms of actions of VIP and CTX on AC was determined by the suppression of their effects by the AC inhibitor and CTX desensitization. ZnPP IX caused attenuation of the LES smooth muscle relaxation by VIP but not by CTX. ZnPP IX but not zinc deuteroporphyrin IX caused significant inhibition of VIP binding to the membrane receptor. We conclude that ZnPP IX attenuates VIP-induced LES smooth muscle relaxation by inhibition of VIP binding to G protein-coupled receptors linked to AC at a point proximal to G protein activation.

Inhibition of receptor-mediated calcium responses by corticotrophin-releasing hormone in the CATH.a cell line

A region of the brain believed to be important in the CNS response to stress is the locus coeruleus, the predominant site of noradrenergic cell bodies. Corticotrophin releasing hormone (CRH) is the primary hypothalamic releasing hormone responsible for the activation of the pituitary-adrenal axis in response to stress and, in this study, we employed a locus coeruleus-like cell line, CATH.a, to investigate the modulation of receptor signalling pathways by CRH. Pituitary adenylyl cyclase-activating polypeptide (PACAP) (10 nM), vasoactive intestinal peptide (VIP) (1 microM) and carbachol (1 mM) produced transient increases in intracellular [Ca2+]. The inhibition of the carbachol (1 mM) response by CRH was concentration-dependent (EC50 = 154 +/- 1.8 nM). Calcium responses to sub-maximally effective concentrations of PACAP (5 nM), VIP (400 nM) and carbachol (1 mM) were abolished by prior exposure to CRH (1 microM). At the concentrations employed, CRH and VIP both substantially increased intracellular [3H]-cyclic AMP accumulation. The adenylyl cyclase activator forskolin (10 microM) was also effective at eliminating the agonist-induced calcium responses. Incubation with the cell permeant cyclic AMP analogue dibutyryl cyclic AMP (dbcAMP) (1 mM), an activator of protein kinase A (PKA), for 12 min prior to agonist exposure similarly abolished the intracellular calcium response to carbachol. Carbachol increased [3H]-inositol phosphate ([3H]-IP) accumulation to a maximum of 2.4 +/- 0.11-fold basal (EC50 = 6.75 +/- 0.26 microM). PACAP produced a much greater accumulation (19.9 +/- 2.1 fold basal; EC50 = 24 nM). In the presence of forskolin (10 microM), neither carbachol- nor PACAP-induced [3H]-IP accumulation was significantly different from in its absence. These results demonstrate that CRH inhibits receptor-mediated intracellular calcium responses in a locus coeruleus-like cell line possibly via activation of PKA. This modulation could be important in controlling neuronal function in vivo in stressful situations in which the levels of CRH are increased in the locus coeruleus.

Muscarinic stimulation of airway smooth muscle cells

1. Acetylcholine, the principal neurotransmitter of the parasympathetic nervous system, is released at both ganglionic synapses and postganglionic neuroeffector junctions and acts by activation of nicotinic and muscarinic cholinoceptors. This review focuses on the effects of postjunctional muscarinic stimulation of airway smooth muscle. 2. On pharmacological criteria, four distinct subtypes of muscarinic cholinoceptor, denoted M1, M2, M3 and M4 receptors, have been identified by use of selective antagonists. Cloned muscarinic cholinoceptors are members of the family of GTP-binding protein-coupled receptors, which are characterized by seven transmembrane (TM) regions connected by intra- and extracellular loops. Between the fifth and the sixth TM regions, muscarinic receptors possess a large intracytoplasmic loop that is considered to be responsible for G-protein-coupling selectivity and exhibits high divergence between the different subtypes. 3. At the site of the smooth muscle itself, both binding and Northern blot studies have demonstrated, in a variety of species, that muscarinic receptor subtypes present are M2 and M3. M2 receptors are coupled to Gi proteins and adenylyl cyclase inhibition and thus to cAMP signaling. M3 receptors are coupled to Gq/11 protein and phosphoinositide hydrolysis and thus to calcium signaling. 4. Muscarinic-induced contraction of airway smooth muscle is mediated by M3 receptors. M2-mediated inhibition of adenylyl cyclase contributes to the prevention of bronchodilation. Cross-talk between muscarinic and beta2 adrenoceptors is likely to be present in airway smooth muscle. The pathophysiological role of this cross-talk requires further investigation.

Alzheimer's alpha-secretase may be a calcium-dependent protease

Proteolytic processing of beta-amyloid precursor protein (APP) is believed to be fundamental to the understanding of Alzheimer's disease. The identities and the regulatory elements of the proteases involved in the process, known as alpha/beta/gamma secretases, are unclear. In this study, by examining reported data, we found some indications suggesting that the putative alpha-secretase may be a calcium-dependent protease, and that this enzyme may play a primary role in the regulation of APP processing. Based on this, we proposed a model for the membrane orientations of the secretases for further discussions.

Inhibition of muscarinic-stimulated polyphosphoinositide hydrolysis and Ca2+ mobilization in cat iris sphincter smooth muscle cells by cAMP-elevating agents

The effects of carbachol (CCh) on inositol 1,4,5-trisphosphate (IP3) production and intracellular calcium ([Ca2+]i) mobilization, and their regulation by cAMP-elevating agents were investigated in SV-40 transformed cat iris sphincter smooth muscle (SV-CISM-2) cells. CCh produced time- and dose-dependent increases in IP3 production; the t1/2 and EC50 values were 68 s and 0.5 microM, respectively. The muscarinic agonist provoked a transient increase in [Ca2+]i which reached maximum within 77 s, and increased [Ca2+]i mobilization in a concentration-dependent manner with an EC50 of 1.4 microM. Thapsigargin, a Ca(2+)-pump inhibitor, caused a rapid rise in [Ca2+]i and subsequent addition of CCh was without effect. Both CCh-induced IP3 production and CCh-induced [Ca2+]i mobilization were more potently antagonized by 4-DAMP, an M3 muscarinic receptor antagonist, than by pirenzepine, an M1 receptor antagonist, suggesting that both responses are mediated through the M3 receptor subtype. Treatment of the cells with U73122, a phospholipase C (PLC) inhibitor, resulted in a concentration-dependent decrease in both CCh-stimulated IP3 production and [Ca2+]i mobilization. These data indicate close correlation between enhanced IP3 production and [Ca2+]i mobilization in these smooth muscle cells and suggest that the CCh-stimulated increase in [Ca2+]i could be mediated through increased IP3 production. Isoproterenol (ISO) inhibited CCh-induced IP3 production (IC50 = 80 nM) and [Ca2+]i mobilization (IC50 = 0.17 microM) in a concentration-dependent manner. Microsomal fractions isolated from SV-CISM-2 cells contained phospholipase C (PLC) which was stimulated by CCh (10 microM) and GTP gamma S (0.1 microM). Pretreatment of the cells with ISO or forskolin, 5 microM each, produced membrane fractions in which CCh-stimulated PLC activity was significantly attenuated. Furthermore, when microsomal fractions isolated from SV-CISM-2 cells were phosphorylated with Protein kinase A (PKA), the CCh- and GTP gamma S-stimulated IP3 production were significantly inhibited. It can be concluded from these studies that in SV-CISM-2 cells, activation of M3 muscarinic receptors results in stimulation of PLC-mediated PIP2 hydrolysis, generating IP3 which mobilizes [Ca2+]i. Furthermore, elevation of cAMP may inhibit IP3 production and [Ca2+]i mobilization through mechanisms involving PKA-dependent phosphorylation of PLC, G-proteins, IP3 receptor and/or IP3 metabolizing enzymes.