Intracellular signals coupled to different rat ileal muscarinic receptor subtypes

The role of intracellular pathways in the proliferation of human K562 cells mediated by muscarinic receptors

Muscarinic acetylcholine receptors (mAChRs) are members of the superfamily of G protein coupled receptors (GPCRs). Muscarinic receptors are relatively abundant in the central nervous system and in the peripheral parasympathetic nervous system. Several studies have suggested that muscarinic receptors also mediate some cellular events in hematopoietic cells. K562 erythroleukemia cells contain muscarinic receptors M2, M3 and M4, and activation of muscarinic receptors changes cell proliferation. We examined the effects of several compounds on cell proliferation in K562 erythroleukemia cells. These included a muscarinic receptor agonist carbachol (CCh), a protein kinase inhibitor staurosporine; the phospholipase C inhibitor U73122, the MEK 1-2 inhibitor UO126, the PI3-kinase inhibitor wortmannin, the Ca(2+) chelators BAPTA/AM and 2-aminoethoxy-diphenylborate (2APB). In addition, we also investigated muscarinic receptor mediated protein kinase C (PKC) expression in K562 cells. CCh caused a decrease in DNA synthesis in K562 cells supplemented with 1% fetal bovine serum after starvation. Pre-treatment of K562 cells with U73122 and BAPTA/AM antagonized the inhibitory effect of CCh, suggesting that phospholipase C and intracellular calcium are involved in CCh-mediated inhibition of proliferation in K562 cells. Our data also suggest that the regulatory roles of protein kinase C and the MAPK/ERK pathways in K562 cell proliferation are independent of cholinergic activation.

Proliferative actions of muscarinic receptors expressed in macrophages derived from normal and tumor bearing mice

Macrophages (Mps) are essential cellular components of the innate immune system. They are released from the bone marrow as immature monocytes and after circulating in the blood stream, migrate into tissues to undergo final differentiation into resident Mps. In general terms Mps behavior in breast tumors, was described as being either for or against tumor growth. Under certain well defined circumstances Mps are able to kill cells in two ways: direct tumor cytotoxicity or antibody dependent cytotoxicity. We had previously demonstrated that peritoneal Mps from LMM3 mammary tumor bearing mice (TMps) enhanced in vivo the LMM3 induced angiogenesis, promoting tumor growth while Mps from normal BALB/c mice (NMps) did not. In this work, we demonstrate that Mps, expressing functional muscarinic acetylcholine receptors, are able to proliferate in vitro in response to the muscarinic agonist carbachol. These peritoneal cells use two distinct metabolic pathways: TMps are primed by tumor presence and they proliferate mainly by activating arginase pathway and by producing high levels of prostaglandin E(2) via M(1)-M(3) receptors activation. In NMps, carbachol stimulates M(2) receptors function, triggering protein kinase C activity and induces moderate prostaglandin E(2) liberation via M(1) receptor.

Roles of muscarinic receptor subtypes in small intestinal motor dysfunction in acute radiation enteritis

Administration of abdominal radiotherapy results in small intestinal motor dysfunction. We have developed a rat radiation enteritis model that, after exposure in vivo, shows high-amplitude, long-duration (HALD) pressure waves in ex vivo ileal segments. These resemble in vivo dysmotility where giant contractions migrate both antegradely and retrogradely. Mediation of these motor patterns is unclear, although enteric neural components are implicated. After the induction of acute radiation enteritis in vivo, ileal segments were isolated and arterially perfused. TTX, hexamethonium, atropine, or the selective muscarinic antagonists pirenzepine (M(1)), methoctramine (M(2)), and 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP; M(3)) were added to the perfusate. The baseline mean rate per minute per channel of HALD pressure waves was 0.35 +/- 0.047. This was significantly reduced by TTX (83.3%, P < 0.01), hexamethonium (90.3%, P < 0.03), and atropine (98.4%, P < 0.01). The HALD pressure wave mean rate per minute per channel was significantly reduced by pirenzepine (81.1%, P < 0.03), methoctramine (96.8%, P < 0.001), and 4-DAMP (93.1%, P < 0.03) compared with predrug baseline data. As an indicator of normal motility patterns, the frequency of low-amplitude, short-duration pressure waves was also assessed. The mean rate per minute per channel of 5.15 +/- 0.98 was significantly increased by TTX (19%, P < 0.05) but significantly reduced by pirenzepine (35.1%, P < 0.02) and methoctramine (75%, P < 0.0003). However, the rate of small-amplitude pressure waves was not affected by hexamethonium, atropine, or the M(3) antagonist 4-DAMP. The data indicate a role for neuronal mechanisms and the specific involvement of cholinergic receptors in generating dysmotility in acute radiation enteritis. The effect of selective M(3) receptor antagonism suggests that M(3) receptors may provide specific therapeutic targets in acute radiation enteritis.

Interactive effects of dietary resistant starch and fish oil on short-chain fatty acid production and agonist-induced contractility in ileum of young rats

We have shown independently that dietary fiber and n-3 fatty acids can affect gut function. This study investigated the interactive effects of resistant starch (RS) (as high amylose maize starch [HAMS]) and tuna fish oil on ileal contractility. Four-week-old male Sprague Dawley rats were fed 4 diets that contained 100 g/kg fat as sunflower oil or tuna fish oil, with 10% fiber supplied as alpha -cellulose or HAMS for 6 weeks. Fish oil feeding led to higher ileal n-3 fatty acid levels (mainly as DHA) and higher agonist-induced maximal contractility with an RS effect noted for carbachol. HAMS-containing diets resulted in lower colonic pH and higher total short-chain fatty acids (SCFA), but not for butyrate with fish oil. Low prostanoid responses in young rats were enhanced by fish oil independent of RS. The order of muscarinic receptor subtype responses were different compared to older rats; fish oil feeding altered the sensitivity of the M(1) receptor subtype. Although little interactive effects were demonstrated, these data suggest developmental changes in ileal receptor systems with independent effects of RS and fish oil on some bowel properties in juvenile rats.

Restoration of depressed prostanoid-induced ileal contraction in spontaneously hypertensive rats by dietary fish oil

We have reported that dietary fish oil (FO) rich in n-3 PUFA modulates gut contractility. It was further demonstrated that the gut of spontaneously hypertensive rats (SHR) has a depressed contractility response to prostaglandins (PG) compared with normotensive Wistar-Kyoto (WKY) rats. We investigated whether feeding diets supplemented with n-3 PUFA increased gut contractility and restored the depressed prostanoid response in SHR gut. Thirteen-week-old SHR were fed diets containing fat at 5 g/100 g as coconut oil (CO), lard, canola oil containing 10% (w/w) n-3 FA as alpha-linolenic acid (1 8:3n-3), or FO (as HiDHA, 22:6n-3) for 12 wk. A control WKY group was fed 5 g/100 g CO in the diet. As confirmed, the SHR CO group had a significantly lower gut response to PGE2 and PGF2alpha compared with the WKY CO group. Feeding FO increased the maximal contraction response to acetylcholine in the ileum compared with all diets and in the colon compared with lard, and restored the depressed response to PGE2 and PGF2alpha in the ileum but not the colon of SHR. FO feeding also led to a significant increase in gut total phospholipid n-3 PUFA as DHA (22:6n-3) with lower n-6 PUFA as arachidonic acid (20:4n-6). Canola feeding led to a small increase in ileal EPA (20:5n-3) and DHA and in colonic DHA without affecting contractility. However, there was no change in ileal membrane muscarinic binding properties due to FO feeding. This report confirms that dietary FO increases muscarinic- and eicosanoid receptor-induced contractility in ileum and that the depressed prostanoid response in SHR ileum, but not colon, is restored by tissue incorporation of DHA as the active nutrient.

Upregulation of muscarinic receptors by long-term nitric oxide inhibition in the rat ileum

1. The aim of the present study was to examine the effects of long-term nitric oxide (NO) blockade on contractions of the rat ileum induced by muscarinic agonists. 2. Male Wistar rats received the NO synthesis inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 20 mg/rat per day) in drinking water for 7, 15, 30 and 60 days. Concentration-responses curves to methacholine and carbachol were obtained and pEC50 values were calculated. Saturation binding assays were performed in membranes prepared from rat ileum after 60 days of l-NAME treatment and the dissociation constant (KD) and maximal number of binding sites (Bmax) were determined by Scatchard analysis. 3. The NO synthase activity of the ileum was markedly reduced in all l-NAME-treated groups. At 60 days after l-NAME treatment, a significant increase in the potency of methacholine (fourfold) and carbachol (threefold) was observed. In binding studies, we found a significant increase in Bmax for [3H]-quinuclidinyl benzilate of approximately 57% in the l-NAME treated group without any significant change in KD values. The contractile response to methacholine was not modified by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (3 micro mol/L). No morphological alterations in the rat ileum were observed in l-NAME-treated rats. 4. Our findings suggest that treatment with l-NAME for 60 days induces a marked increase in the potency of methacholine and carbachol, as well as an increase in receptor number in the rat ileum.

Participation of nitric oxide synthase and cyclo-oxygenase in the signal transduction pathway of ileal muscarinic acetylcholine receptors

Parasympathetic activation of ileal motility is essential for intestinal physiology. We have previously demonstrated that carbachol activates muscarinic acetylcholine receptors (mAChR) of rat intestine and stimulates ileal motility via phospholipase C. This activation induces phosphoinositide turnover and intracellular calcium mobilization. We show here that carbachol stimulation of rat ileal motility is potentiated by the nitric oxide synthase (NOS) inhibitor N(G)-monomethyl arginine. Thus, we confirm that carbachol increases, in a dose-dependent manner, the activity of a NOS isoform that depends on calcium-calmodulin binding. Its product, nitric oxide (NO), activates not only guanylyl cyclase, inducing cGMP synthesis, but also cyclo-oxygenase, producing prostaglandin E(2). The prostanoid probably cooperates with NO to induce ileal smooth muscle relaxation.

Tyrosine kinase regulatory action on ileal muscarinic effects of IFN-gamma

Interferon-gamma (IFN-gamma) is a pleiotropic cytokine that has a large number of immunologic and nonimmunologic functions. We have described that IFN-gamma could activate muscarinic cholinergic receptors (mAchR) of rat intestine, stimulating ileal motility. We also observed that mAchR activation induced inhibition of cAMP levels and stimulation of cGMP formation. The objectives of our work were to clarify the signal transduction pathways involved in regulation of ileal motility through mAchR activation by IFN-gamma. Our results demonstrate that this cytokine produces an ileal cholinergic response through tyrosine kinase activity. The activation of tyrosine kinase mediates ileal contractility, phosphoinositide hydrolysis by phospholipase C, nitric oxide synthase via protein kinase C, and cGMP synthesis. The increment in ileal motility is probably due to hyperproduction of prostaglandin E2 (PGE2) by ileal tissue. This prostanoid is an important mediator because it stimulates ileal motility. We conclude that IFN-gamma not only immunomodulates the gut microenvironment but also exerts a local nonimmunologic regulation on intestinal motility.