Impaired G protein function in gallbladder muscle from progesterone-treated guinea pigs

An Update on the Lithogenic Mechanisms of Cholecystokinin a Receptor (CCKAR), an Important Gallstone Gene for Lith13

The cholecystokinin A receptor (CCKAR) is expressed predominantly in the gallbladder and small intestine in the digestive system, where it is responsible for CCK's regulation of gallbladder and small intestinal motility. The effect of CCKAR on small intestinal transit is a physiological response for regulating intestinal cholesterol absorption. The Cckar gene has been identified to be an important gallstone gene, Lith13, in inbred mice by a powerful quantitative trait locus analysis. Knockout of the Cckar gene in mice enhances cholesterol cholelithogenesis by impairing gallbladder contraction and emptying, promoting cholesterol crystallization and crystal growth, and increasing intestinal cholesterol absorption. Clinical and epidemiological studies have demonstrated that several variants in the CCKAR gene are associated with increased prevalence of cholesterol cholelithiasis in humans. Dysfunctional gallbladder emptying in response to exogenously administered CCK-8 is often found in patients with cholesterol gallstones, and patients with pigment gallstones display an intermediate degree of gallbladder motility defect. Gallbladder hypomotility is also revealed in some subjects without gallstones under several conditions: pregnancy, total parenteral nutrition, celiac disease, oral contraceptives and conjugated estrogens, obesity, diabetes, the metabolic syndrome, and administration of CCKAR antagonists. The physical-chemical, genetic, and molecular studies of Lith13 show that dysfunctional CCKAR enhances susceptibility to cholesterol gallstones through two primary mechanisms: impaired gallbladder emptying is a key risk factor for the development of gallbladder hypomotility, biliary sludge (the precursor of gallstones), and microlithiasis, as well as delayed small intestinal transit augments cholesterol absorption as a major source for the hepatic hypersecretion of biliary cholesterol and for the accumulation of excess cholesterol in the gallbladder wall that further worsens impaired gallbladder motor function. If these two defects in the gallbladder and small intestine could be prevented by the potent CCKAR agonists, the risk of developing cholesterol gallstones could be dramatically reduced.

Update on the Molecular Mechanisms Underlying the Effect of Cholecystokinin and Cholecystokinin-1 Receptor on the Formation of Cholesterol Gallstones

Cholecystokinin (CCK) is an important neuro-intestinal peptide hormone produced by the enteroendocrine I-cells in the upper part of small intestine. Protein- and fat-enriched food plays an important role in triggering CCK secretion from the intestine. Carbohydrates stimulate only small amounts of CCK release. The CCK-1 receptor (CCK-1R) is largely localized in the gallbladder, sphincter of Oddi, pancreas, small intestine, gastric mucosa, and pyloric sphincter, where it is responsible for CCK to regulate multiple digestive processes including gallbladder contraction, pancreatic secretion, small intestinal transit, and gastric emptying. Accumulated evidence clearly demonstrates that CCK regulates gallbladder and small intestinal motility through CCK-1R signaling cascade and the effect of CCK-1R on small intestinal transit is a physiological response for regulating intestinal cholesterol absorption. Disruption of the Cck or the Cck-1r gene in mice significantly increases the formation of cholesterol gallstones by disrupting gallbladder emptying and biliary cholesterol metabolism, as well as promoting intestinal absorption of cholesterol. Abnormalities in gallbladder motility function in response to exogenously administered CCK are found primarily in patients with cholesterol gallstones. Patients with pigment gallstones display an intermediate degree of gallbladder motility defect without gallbladder inflammation and enlarged fasting gallbladder. Dysfunctional gallbladder contractility has been found under several conditions such as pregnancy, obesity, diabetes, celiac disease, and total parenteral nutrition although gallstones are not observed. The gallbladder-specific CCK-1R-selective agonist may lead to an efficacious novel way for preventing gallstone formation by promoting gallbladder emptying, particularly for pregnant women and subjects with dysfunctional gallbladder motility function such as celiac patients, as well as patients with total parenteral nutrition.

Two Cases of Acute Cholecystitis and Symptomatic Choledocholithiasis in Two Women Less than 40-Years-of-Age with Hormonal Intrauterine Devices

Levonorgestrel uterine implants are accepted as a safe and efficacious method of contraception. One of the two major health side effects in a large controlled study of subcutaneous hormonal implants with levonorgestrel was a significant increase in gallbladder disease. Gallbladder hypomotility is recognized as a side effect of the levonorgestrel (progesterone). We recently saw on a Gastroenterology Consult Service, two women under 40-years-of-age who had been transferred from outside hospitals with acute cholecystitis with symptomatic choledocholithiasis. Both required Endoscopic Retrograde Cholangiopancreatography and sphincterotomies in addition to laparoscopic cholecystectomies. Both had hormonal (levonorgestrel-releasing) intrauterine devices in place for contraception. Although one patient had a family history of gallstones, the other did not. Both were nonobese, young women patients. We were struck by the coincidence of seeing two such patients. Few articles in the medical literature detail the clinical risks of gallstone disease in patients with hormonal (levonorgestrel-releasing) intrauterine devices. Our experiences with these two patients led us to believe that patients with risk factors for gallstone disease, such as a positive family history, ethnic predisposition, or obesity, should be warned of possible problems, not only with gallbladder disease, but also of common duct stones.

Effect of progesterone on nitric oxide/cyclic guanosine monophosphate signaling and contraction in gastric smooth muscle cells

Previous studies have shown that progesterone could inhibit muscle contraction in various sites of the gastrointestinal tract. The underlying mechanisms responsible for these inhibitory effects of progesterone are not fully known. The aim of the current study was to investigate the effect of progesterone on the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway and muscle contraction in the stomach. Single gastric smooth muscle cells from female Sprague-Dawley rats were used. The expression of progesterone receptor (PR) mRNA was analyzed by reverse transcription polymerase chain reaction. NO and cGMP levels were measured via specific ELISAs. Acetylcholine (ACh)-induced contraction of single gastric muscle cells preincubated with progesterone was measured via scanning micrometry in the presence or absence of the NO synthase inhibitor, Nω-Nitro-L-arginine (L-NNA), or guanylyl cyclase inhibitor, 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and expressed as percent shortening from resting cell length. PR expression was detected in the stomach muscle cells. Progesterone inhibited ACh-induced gastric muscle cell contraction. Furthermore, progesterone increased NO and cGMP levels in single gastric muscle cells. Most notably, pre-incubation of muscle cells with either L-NNA or ODQ abolished the inhibitory action of progesterone on muscle contraction. These present observations suggest that progesterone promotes muscle cell relaxation in the stomach potentially via the NO/cGMP pathway.

The deletion of the estrogen receptor α gene reduces susceptibility to estrogen-induced cholesterol cholelithiasis in female mice

Compelling evidence has demonstrated that estrogen is a critical risk factor for gallstone formation and enhances cholesterol cholelithogenesis through the hepatic estrogen receptor α (ERα), but not ERβ. To study the lithogenic mechanisms of estrogen through ERα, we investigated whether the deletion of Erα protects against gallstone formation in ovariectomized (OVX) female mice fed a lithogenic diet and treated with 17β-estradiol (E2) at 0 or 6μg/day for 56days. Our results showed that the prevalence of gallstones was reduced from 100% in OVX ERα (+/+) mice to 30% in OVX ERα (-/-) mice in response to high doses of E2 and the lithogenic diet for 56days. Hepatic cholesterol secretion was significantly diminished in OVX ERα (-/-) mice compared to OVX ERα (+/+) mice even fed the lithogenic diet and treated with E2 for 56days. These alterations decreased bile lithogenicity by reducing cholesterol saturation index of gallbladder bile. Immunohistochemical studies revealed that ERα was expressed mainly in the gallbladder smooth muscle cells. High levels of E2 impaired gallbladder emptying function mostly through the ERα and cholecystokinin-1 receptor pathway, leading to gallbladder stasis in OVX ERα (+/+) mice. By contrast, gallbladder emptying function was greatly improved in OVX ERα (-/-) mice. This markedly retarded cholesterol crystallization and the growth and agglomeration of solid cholesterol crystals into microlithiasis and stones. In conclusion, the deletion of Erα reduces susceptibility to the formation of E2-induced gallstones by diminishing hepatic cholesterol secretion, desaturating gallbladder bile, and improving gallbladder contraction function in female mice.

Symptomatology of irritable bowel syndrome and inflammatory bowel disease during the menstrual cycle

Gender-related physiological variations in gastrointestinal (GI) symptomatology have been observed in women of reproductive age. Many women experience cyclical changes in GI symptomatology during their menstrual cycle, particularly alteration in their bowel habits. Physiological studies of healthy women during the menstrual cycle showed a prolonged GI transit time during the luteal phase, either in the oro-cecum route or in the colon. Worsened GI symptoms, such as abdominal pain, bloating or diarrhea are observed in patients with irritable bowel syndrome (IBS) during menses. This may be due to elevated prostaglandin levels during menses, with an enhanced perception of viscera-somatic stimuli resulting in nausea, abdominal distension and pain. Also patients with IBS or IBD demonstrate a cyclical pattern more closely related to their bowel habits than healthy controls. Women with inflammatory bowel disease (IBD) also have exacerbated symptoms during menses; however, it is unclear whether this relates to physiological variation or disease exacerbation in IBS or IBD. Studies examining the association of the menstrual cycle and GI symptomatology in patients with IBS or IBD, have not yet clarified the underlying mechanisms. Moreover medications—such as non-steroidal anti-inflammatory drugs and oral contraceptive pills used for dysmenorrhea and menstrual migraine in those patients have not well been controlled for in the previous studies, which can contribute to further bias. Understanding changes in GI symptomatology during the menstrual cycle may help to determine the true extent of disease exacerbation and proper management strategy.

Cholesterol gallstone disease: focusing on the role of gallbladder

Gallstone disease (GSD) is one of the most common biliary tract diseases worldwide in which both genetic and environmental factors have roles in its pathogenesis. Biliary cholesterol supersaturation from metabolic defects in the liver is traditionally seen as the main pathogenic factor. Recently, there have been renewed investigative interests in the downstream events that occur in gallbladder lithogenesis. This article focuses on the role of the gallbladder in the pathogenesis of cholesterol GSD (CGD). Various conditions affecting the crystallization process are discussed, such as gallbladder motility, concentrating function, lipid transport, and an imbalance between pro-nucleating and nucleation inhibiting proteins.

Effects of cholesterol on CCK-1 receptors and caveolin-3 proteins recycling in human gallbladder muscle

The contraction of gallbladders (GBs) with cholesterol stones is impaired due to high cholesterol concentrations in caveolae compared with GBs with pigment stones. The reduced contraction is caused by a lower cholecystokinin (CCK)-8 binding to CCK-1 receptors (CCK-1R) due to caveolar sequestration of receptors. We aimed to examine the mechanism of cholesterol-induced sequestration of receptors. Muscle cells from human and guinea pig GBs were studied. Antibodies were used to examine CCK-1R, antigens of early and recycling endosomes, and total (CAV-3) and phosphorylated caveolar-3 protein (pCAV-3) by Western blots. Contraction was measured in muscle cells transfected with CAV3 mRNA or clathrin heavy-chain small-interfering RNA (siRNA). CCK-1R returned back to the bulk plasma membrane (PM) 30 min after CCK-8 recycled by endosomes, peaking at 5 min in early endosomes and at 20 min in recycling endosomes. Pretreatment with cholesterol-rich liposomes inhibited the transfer of CCK-1R and of CAV-3 in the endosomes by blocking CAV-3 phosphorylation. 4-Amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (inhibitor of tyrosine kinase) reproduced these effects by blocking pCAV-3 formation, increasing CAV-3 and CCK-1R sequestration in the caveolae and impairing CCK-8-induced contraction. CAV-3 siRNA reduced CAV-3 protein expression, decreased CCK-8-induced contraction, and accumulated CCK-1R in the caveolae. Abnormal concentrations of caveolar cholesterol had no effect on met-enkephalin that stimulates a delta-opioid receptor that internalizes through clathrin. We found that impaired muscle contraction in GBs with cholesterol stones is due to high caveolar levels of cholesterol that inhibits pCAV-3 generation. Caveolar cholesterol increases the caveolar sequestration of CAV-3 and CCK-1R caused by their reduced recycling to the PM.

Progesterone receptor A mediates VIP inhibition of contraction

The slow transit time of the colon in females with constipation is due to impairment of agonist-induced contraction. The impairment is associated with downregulation of G proteins that mediate contraction and upregulation of Gs proteins that mediate relaxation. These changes are caused by overexpression of progesterone (P4) receptors in the colon, rendering its muscle cells sensitive to physiological P4 concentrations. Downregulation of Gq/11 is mediated by P4 receptor B (PR-B). We examined whether upregulation of Gs proteins increased the inhibition of contraction and whether the increase is mediated by the P4 receptor A (PR-A). These studies were conducted in colon-isolated colon muscle cells from human control and slow-transit constipation (STC) females and from guinea pigs. Muscle cell contraction was induced by CCK-8. Inhibition of contraction was induced by vasoactive intestinal polypeptide (VIP), and 8'bromo-c'AMP (8B-c'AMP) G protein levels were determined by Western blot. VIP-induced inhibition of contraction was greater in muscle cells from STC and P4-treated muscle cells. There were no differences in the inhibition induced by 8B-c'AMP between muscle cells from STC and P4-treated controls. The increased VIP-induced inhibition of muscle cells treated with P4 was blocked by pretreatment with PR-A antibodies and unaffected by PR-B antibodies. These antibodies had no effect on 8B-c'AMP induced-inhibition. The P4 upregulation of Gs proteins was blocked by PR-A antibodies and unaffected by PR-B antibodies. Similar results were obtained in muscle cells from guinea pig colons. We concluded that P4 upregulation of Gs proteins increases VIP-induced inhibition of contraction mediated by PR-A.

Effects of progesterone on motility and prostaglandin levels in the distal guinea pig colon

Progesterone (P4) inhibits the gastrointestinal muscle contraction by downregulating Galpha(q/11) proteins that mediate contraction, by upregulating Galpha(s) proteins that mediate relaxation, and by altering the pattern of cyclooxygenase (COX) enzymes and prostaglandins. We aimed to examine whether P4 treatment of guinea pigs in vivo affects basal colon motility [basal motility index (MI)] by altering the levels and actions of PGF(2alpha) and PGE(2). Guinea pigs were treated with intramuscular (IM) P4 for 4 days. The BASAL MI, the PGF(2alpha)-induced contraction, and PGE(2)-induced inhibition of contraction were examined in muscle strips and cells. The levels of PGF(2alpha) and PGE(2) were measured by radioimmunoassay. Treatment with P4 reduced the basal MI, the levels of PGF(2alpha), and PGF(2alpha)-induced contraction. P4 increased PGE(2) levels, and PGE(2) induced relaxation. Pretreatment with IM RU-486 (10 mg/kg per day), a P4 receptor antagonist, 1 h before P4 blocked the actions of P4. The PGF(2alpha) antagonist Al-1180 abolished basal MI and PGF(2alpha)-induced contraction. N-ethylmaleimide, which blocks unoccupied membrane receptors, blocked Ach and VIP actions but had no effect on PGF(2alpha) and PGE(2) effects. A COX-1 inhibitor decreased and a COX-2 inhibitor increased PGF(2alpha) levels; GTPgammaS increased and GDPbetaS decreased the levels of PGF(2alpha). Galpha(q/11) protein antibodies (Abs) reduced PGF(2alpha) levels, and Galpha(i3) Abs blocked its motor actions. Galphas Abs increased PGF(2alpha) but decreased PGE(2) levels. We concluded that P4 decreases basal MI by reducing PGF(2alpha) levels caused by downregulation of Galpha(q/11) and that PGF(2alpha)-induced contraction was blocked by downregulating Galpha(i3). P4 also decreased the basal MI by increasing PGE(2) levels, and PGE(2) induced relaxation by upregulating Galpha(s) proteins.

High levels of caveolar cholesterol inhibit progesterone-induced genomic actions in human and guinea pig gallbladder muscle

Gallbladder disease is prevalent during pregnancy. It has been suggested that this complication of pregnancy is attributable to increased bile cholesterol (Ch) induced by estrogens and to gallbladder hypomotility caused by increasing levels of progesterone (P4). Studies on nonpregnant gallbladders have shown that increased levels of bile Ch contribute to both gallstone formation and bile stasis. These studies investigated the effects of high levels of plasma membrane Ch on P4 on gallbladder muscle cells from human and guinea pigs. Contraction was studied in intact and permeabilized muscle cells. G proteins were determined by Western blot, and 3H-P4 incorporation by muscle cells was measured in the beta-scintillation counter. High levels of caveolar Ch blocked the effects induced by P4 treatment for 6 h. They suppressed the expected P4 inhibition of GTP-gammaS (a G protein activator)-induced contraction and changes in G proteins by downregulating Gi3 and upregulating Gs protein levels. Ch inhibited these P4 actions at the caveolar 3 (CAV-3) level, since the P4 effects were antagonized by treatment with CAV-3 antibody, by reducing CAV-3 expression through CAV-3 siRNA. CAV-3 antibody and siRNA reduced caveolar Ch levels. High caveolar levels of Ch and CAV-3 antibody blocked the incorporation of 3H-P4 into caveolae. Treatment with GDP-betaS (a G protein antagonist) had no effect on P4 actions. High caveolar Ch levels blocked the P4 effects on muscle contraction and G protein changes probably because both Ch and P4 require CAV-3 proteins for their transport across the plasma membrane.

Role of caveolae in the pathogenesis of cholesterol-induced gallbladder muscle hypomotility

Muscle cells from human gallbladders (GB) with cholesterol stones (ChS) exhibit a defective contraction, excess cholesterol (Ch) in the plasma membrane, and lower binding of CCK-1 receptors. These abnormalities improved after muscle cells were incubated with Ch-free liposomes that remove the excess Ch from the plasma membrane. The present studies were designed to investigate the role of caveolin-3 proteins (Cav-3) in the pathogenesis of these abnormalities. Muscle cells from GB with ChS exhibit higher Ch levels in the plasma membrane that were mostly localized in caveolae and associated with parallel increases in the expression of Cav-3 in the caveolae compared with that in GB with pigment stones (PS). The overall number of CCK-1 receptors in the plasma membrane was not different between muscle cells from GB with ChS and PS, but they were increased in the caveolae in muscle cells from GB with ChS. Treatment of muscle cells from GB with ChS with a Galpha(i3) protein fragment increased the total binding of CCK-1 receptors (from 8.3 to 11.2%) and muscle contraction induced by CCK-8 (from 11.2 to 17.3% shortening). However, Galpha(q/11) protein fragment had no such effect. Moreover, neither fragment had any effect on muscle cells from GB with PS. We conclude that the defective contraction of muscle cells with excessive Ch levels in the plasma membrane is due to an increased expression of Cav-3 that results in the sequestration of CCK-1 receptors in the caveolae, probably by inhibiting the functions of Galpha(i3) proteins.

Prostaglandins mediate tonic contraction of the guinea pig and human gallbladder

The gallbladder (GB) maintains tonic contraction modulated by neurohormonal inputs but generated by myogenic mechanisms. The aim of these studies was to examine the role of prostaglandins in the genesis of GB myogenic tension. Muscle strips and cells were treated with prostaglandin agonists, antagonists, cyclooxygenase (COX) inhibitors, and small interference RNA (siRNA). The results show that PGE2, thromboxane A2 (TxA2), and PGF(2alpha) cause a dose-dependent contraction of muscle strips and cells. However, only TxA2 and PGE2 (E prostanoid 1 receptor type) antagonists induced a dose-dependent decrease in tonic tension. A COX-1 inhibitor decreased partially the tonic contraction and TxB2 (TxA2 stable metabolite) levels; a COX-2 inhibitor lowered the tonic contraction partially and reduced PGE2 levels. Both inhibitors and the nonselective COX inhibitor indomethacin abolished the tonic contraction. Transfection of human GB muscle strips with COX-1 siRNA partially lowered the tonic contraction and reduced COX-1 protein expression and TxB2 levels; COX-2 siRNA also partially reduced the tonic contraction, the protein expression of COX-2, and PGE2. Stretching muscle strips by 1, 2, 3, and 4 g increased the active tension, TxB2, and PGE2 levels; a COX-1 inhibitor prevented the increase in tension and TxB2; and a COX-2 inhibitor inhibited the expected rise in tonic contraction and PGE2. Indomethacin blocked the rise in tension and TxB2 and PGE2 levels. We conclude that PGE2 generated by COX-2 and TxA2 generated by COX-1 contributes to the maintenance of GB tonic contraction and that variations in tonic contraction are associated with concomitant changes in PGE2 and TxA2 levels.

Nongenomic effects of progesterone on the contraction of muscle cells from the guinea pig colon

Progesterone (PG) affects muscle cells by genomic mechanisms through nuclear receptors and by nongenomic mechanisms through unidentified pathways. This study aimed to determine the pathways mediating its nongenomic actions. Experiments were performed in dissociated muscle cells from guinea pig colons. Nongenomic actions were defined as those occurring within 10 min of PG exposure. PG blocked the contraction to CCK-8 and NKA (10(-7) M) but did not impair ACh (10(-7) M) and KCl (2.5 x 10(-2) M)-induced contraction. Both CCK-8 and NKA contract muscle cells by releasing calcium from intracellular stores, whereas ACh and KCl can utilize extracellular calcium. PG also blocked the contraction induced by inositol 1,4,5-trisphosphate, thapsigargin, and caffeine, agents that contract muscle cells by releasing calcium from storage sites. The nongenomic actions of PG were transient because they were absent 1 h after the first PG dose, remaining unresponsive after a second PG dose was administered. Furthermore, PG had no effect on the contraction induced by CCK-8 and thapsigargin in muscle cells from animals pretreated with daily intramuscular PG for 4 days. Cytosolic incorporation experiments of [(3)H]PG showed that pretreatment with unlabeled PG significantly reduced the radiolabeled PG incorporation in the cytosol. We conclude that the nongenomic actions of PG on colonic muscle cells transiently blocked calcium release from storage sites, and this response became rapidly desensitized. This effect does not appear to be specific to PG because other steroid hormones such as aldosterone and testosterone can also induce it.

Impaired cytoprotective function of muscle in human gallbladders with cholesterol stones

Acute cholecystitis develops in gallbladders (GB) with excessive bile cholesterol (Ch). Increased membrane Ch content affects membrane function and may affect PGE(2) receptors involved in the cytoprotection against acute inflammation. This study was aimed at determining whether the cytoprotective response to PGE(2) is affected by lithogenic bile with Ch. Muscle cells from human GB with cholesterol stones (ChS) or pigment stones (PS) were obtained by enzymatic digestion. PGE(2) levels were measured by radioimmunoassay, and activities of superoxide dismutase (SOD) and catalase were assayed by spectrophotometry. The contraction in response to H(2)O(2) in muscle cells from PS was 14 +/- 0.3%, not different from normal controls, and decreased after the cells were incubated with Ch-rich liposomes (P < 0.05), which increase the Ch content in the plasma membranes. In muscle cells from GB with ChS, H(2)O(2)-induced contraction was only 9.2 +/- 1.3% and increased to 14 +/- 0.2% after Ch-free liposome treatment to remove Ch from the plasma membranes (P < 0.01). H(2)O(2) caused a similar increase in the levels of lipid peroxidation and PGE(2) content in muscle cells from GBs with ChS and PS. However, the activities of SOD and catalase were significantly lower in muscle cells from GBs with ChS compared with those with PS. The binding capacity of PGE(2) receptors was also significantly lower in muscle cells from GBs with ChS compared with those with PS. In conclusion, the cytoprotective response to reactive oxygen species is reduced in muscle cells from GBs with ChS despite a normal increase in the cellular levels of PGE(2). This impaired cytoprotective response may be due to a dysfunction of PGE(2) receptors with decreased binding capacity resulting from excessive Ch levels in the plasma membrane.

G protein-mediated dysfunction of excitation-contraction coupling in ileal inflammation

Inflammation impairs the circular muscle contractile response to muscarinic (M) receptor activation. The aim of this study was to investigate whether the expression of muscarinic receptors, their binding affinity, and the expression and activation of receptor-coupled G proteins contribute to the suppression of contractility in inflammation. The studies were performed on freshly dissociated single smooth muscle cells from normal and inflamed canine ileum. Northern blotting indicated the presence of only M(2) and M(3) receptors on canine ileal circular muscle cells. Inflammation did not alter the mRNA or protein expression of M(2) and M(3) receptors. The maximal binding and K(d) values also did not differ between normal and inflamed cells. However, the contractile response to ACh in M(3) receptor-protected cells was suppressed, whereas that in M(2) receptor-protected cells was enhanced. Further experiments indicated that the expression and binding activity of G alpha(q/11) protein, which couples to M(3) receptors, were downregulated, whereas those of G alpha(i3), which couples to M(2) receptors, were upregulated in inflamed cells. We concluded that inflammation depresses M(3) receptor function, but it enhances M(2) receptor function in ileum. These effects are mediated by the differentially altered expression and binding activity of their respective coupled G alpha(q/11) and G alpha(i3) proteins.

Smooth muscle function and dysfunction in gallbladder disease

The gallbladder epithelium and smooth muscle layer are exposed to concentrated biliary solutes, including cholesterol and potentially toxic hydrophobic bile salts, which are able to influence muscle contraction. Physiologically, gallbladder tone is regulated by spontaneous muscle activity, hormones, and neurotransmitters released into the muscle from intrinsic neurons and extrinsic sympathetic nerves. Methods to explore gallbladder smooth muscle function in vitro include cholecystokinin (CCK) receptor-binding studies and contractility studies. In human and animal models, studies have focused on cellular and molecular events in health and disease, and in vitro findings mirror in vivo events. The interplay between contraction and relaxation of the gallbladder muscularis leads in vivo to appropriate gallbladder emptying and refilling during fasting and postprandially. Defective smooth muscle contractility and/or relaxation are found in cholesterol stone-containing gallbladders, featuring a type of gallbladder leiomyopathy; defects of CCKA receptors and signal transduction may coexist with abnormal responses to oxidative stress and inflammatory mediators. Abnormal smooth musculature contractility, impaired gallbladder motility, and increased stasis are key factors in the pathogenesis of cholesterol gallstones.

Effect of cortisol on norepinephrine-mediated contractions in ovine uterine arteries

Cortisol potentiated norepinephrine (NE)-mediated contractions in ovine uterine arteries (UA). We tested the hypothesis that cortisol regulated alpha(1)-adrenoceptor-mediated pharmacomechanical coupling differentially in nonpregnant UA (NUA) and pregnant UA (PUA). Cortisol (10 ng/ml for 24 h) significantly increased contractile coupling efficiency of alpha(1)-adrenoceptors in NUA, but increased alpha(1)-adrenoceptor density in PUA. Cortisol potentiated NE-induced inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] synthesis in both NUA and PUA, but increased coupling efficiency of alpha(1)-adrenoceptors to Ins(1,4,5)P(3) synthesis only in NUA. Carbenoxolone alone did not affect NE-mediated Ins(1,4,5)P(3) production, but significantly enhanced cortisol-mediated potentiation of NE-stimulated Ins(1,4,5)P(3) synthesis in PUA. In addition, cortisol potentiated the NE-induced increase in Ca(2+) concentration in PUA, but increased NE-mediated contraction for a given amount of Ca(2+) concentration in NUA. Collectively, the results indicate that cortisol potentiates NE-mediated contractions differentially in NUA and PUA, i.e., by upregulating alpha(1)-adrenoceptor density leading to increased Ca(2+) mobilization in PUA while increasing alpha(1)-adrenoceptor coupling efficiency and myofilament Ca(2+) sensitivity in NUA. In addition, the results suggest that pregnancy increases type 2 11 beta-hydroxysteroid dehydrogenase activity in the UA.

Abnormalities of gallbladder muscle associated with acute inflammation in guinea pigs

Muscle strips from experimental acute cholecystitis (AC) exhibit a defective contraction. The mechanisms responsible for this impaired contraction are not known. The present studies investigated the nature of these abnormalities. AC was induced by ligating the common bile duct of guinea pigs for 3 days. Contraction was studied in enzymatic dissociated muscle cells. Cholecystokinin (CCK) and prostaglandin E2 (PGE2) receptor binding studies were performed by radioreceptor assay. The levels of lipid peroxidation, cholesterol, phospholipid, and H2O2 as well as the catalase and superoxide dismutase (SOD) activities were determined. PGE2 content was measured by radioimmunoassay. Muscle contraction induced by CCK, ACh, or KCl was significantly reduced in AC, but PGE2-induced contraction remained normal. GTPgammaS, diacyglycerol (DAG), and 1,4,5-trisphosphate (IP3), which bypass the plasma membrane, caused a normal contraction in AC. The number of functional receptors for CCK was significantly decreased, whereas those for PGE2 remained unchanged in AC. There was a reduction in the phospholipid content and increase in the level of lipid peroxidation as well as H2O2 content in the plasma membrane in AC. The PGE2 content and the activities of catalase and SOD were also elevated. These data suggest that AC cause damage to the constituents of the plasma membrane of muscle cells. The preservation of the PGE2 receptors may be the result of muscle cytoprotection.

Defect of receptor-G protein coupling in human gallbladder with cholesterol stones

Human gallbladders with cholesterol stones (ChS) exhibit an impaired muscle contraction and relaxation and a lower CCK receptor-binding capacity compared with those with pigment stones (PS). This study was designed to determine whether there is an abnormal receptor-G protein coupling in human gallbladders with ChS using (35)S-labeled guanosine 5'-O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding, (125)I-labeled CCK-8 autoradiography, immunoblotting, and G protein quantitation. CCK and vasoactive intestinal peptide caused significant increases in [(35)S]GTPgammaS binding to Galpha(i-3) and G(s)alpha, respectively. The binding was lower in ChS than in PS (P < 0.01). The reduced [(35)S]GTPgammaS binding in ChS was normalized after the muscles were treated with cholesterol-free liposomes (P < 0.01). Autoradiography and immunoblots showed a decreased optical density (OD) for CCK receptors, an even lower OD value for receptor-G protein coupling, and a higher OD for uncoupled receptors or Galpha(i-3) protein in ChS compared with PS (P < 0.001). G protein quantitation also showed that there were no significant differences in the Galpha(i-3) and G(s)alpha content in ChS and PS. We conclude that, in addition to an impaired CCK receptor-binding capacity, there is a defect in receptor-G protein coupling in muscle cells from gallbladder with ChS. These changes may be normalized after removal of excess cholesterol from the plasma membrane.

The Rho-related protein Rnd1 inhibits Ca2+ sensitization of rat smooth muscle

1. The small GTP-binding Rho proteins are involved in the agonist-induced Ca2+ sensitization of smooth muscle. The action and the expression of Rnd1, a new member of the Rho protein family constitutively bound to GTP, has been studied in rat smooth muscle. 2. Recombinant prenylated Rnd1 (0.01-0.1 mg ml-1) dose dependently inhibited carbachol- and GTPgammaS-induced Ca2+ sensitization in beta-escin-permeabilized ileal smooth muscle strips but had no effect on the tension at submaximal [Ca2+] (pCa 6.3). Rnd1 inhibited GTPgammaS-induced tension without shifting the dose-response curves to GTPgammaS. 3. pCa-tension relationships were not modified by Rnd1 and the rise in tension induced through the inhibition of myosin light chain phosphatase by calyculin A was not affected by Rnd1. 4. The Ca2+ sensitization induced by recombinant RhoA was completely abolished when RhoA and Rnd1 were applied together. 5. Rnd1 was expressed at a low level in membrane fractions prepared from intestinal or arterial smooth muscles. The expression of Rnd1 was strongly increased in ileal and aortic smooth muscle from rats treated with progesterone or oestrogen. Progesterone-treated ileal muscle strips showed a decrease in agonist-induced Ca2+ sensitization. 6. The present study shows that (i) Rnd1 inhibits agonist- and GTPgammaS-induced Ca2+ sensitization of smooth muscle by specifically interfering with a RhoA-dependent mechanism and (ii) an increase in Rnd1 expression may account, at least in part, for the steroid-induced decrease in agonist-induced Ca2+ sensitization.