Natriuretic peptide-dependent cGMP signal pathway potentiated the relaxation of gastric smooth muscle in streptozotocin-induced diabetic rats

Mechanism of CNP-mediated DG-PKC and IP4 signaling pathway in diabetic rats with gastric motility disorder

In the precedent research conducted by the same team, it concluded that the activities in C-type natriuretic peptide (CNP)/cyclic guanosine monophosphate (cGMP)/cyclic adenosine monophosphate (cAMP)/β-type phospholipase C (PLCβ) pathways of rat antral smooth muscle were changed due to diabetes, which was the key pathogenetic mechanism for diabetic gastric dysmotility. As the follow-on step, this study was designed to probe into the downstream signaling pathway of CNP/PLCβ. The results showed that level of α-type protein kinase C (PKCα),cell membrane to cytoplasm ratio of PKCα, cell membrane to cytoplasmic ratio of βI-type protein kinase C (PKCβI) and level of Phosphor-PKCα (P-PKCα) were significantly reduced in diabetes rat antral smooth muscle samples. The content of tetraphosphate inositol (IP4) in gastric antral smooth muscle of diabetic rats reduced, and the content of diacyl-glycerol (DG) was unchanged. CNP significantly decreased the content of IP4 and DG, this effect was more obvious in diabetic rats. Subsequent to the addition of protein kinase A (PKA) blocker N-[2- (p-Bromocin-namylamino)ethyl]-5 -isoquinolinesulfonamide dihydrochloride (H-89) before CNP treatment, the inhibitory effect of CNP was reduced; subsequent to the addition of protein kinase G (PKG) blocker KT5823 before CNP treatment, the inhibitory effect of CNP was also reduced. With the addition of the combination of H-89 and KT5823 before CNP treatment, the inhibition by CNP could be offset. These results were concluded that CNP inhibited the activity of PKC family in rat smooth muscle and reduced the levels of IP4 and DG through the PKG/PKA-PLCβ pathways, causing inhibited muscular contractions, which may be a key pathogenetic factor for diabetic gastroparesis.

Electroacupuncture promotes the gastrointestinal motility of diabetic mice by CNP/NPR-B-cGMP and PDE3A-cGMP signaling

BACKGROUND

Electroacupuncture (EA) can promote gastrointestinal (GI) motility of diabetic mice, but the mechanism is not clearly elucidated. Natriuretic peptides (NPs) were related to the diabetes-induced gut dysfunction of mice, which may be associated with ICC (interstitial cells of cajal). Besides, EA could increase the ICC of diabetic mice. Our aim was to explore whether EA can promote the gut motility by CNP/NPR-B-cGMP and PDE3A-cGMP signaling in diabetic mice, and the relationship between NPs and ICC.

METHODS

Wild C57BL/6 male mice were divided into five groups: control group, diabetic mellitus (DM group), diabetic mellitus plus sham EA group (SEA), diabetic mellitus plus low-frequency EA group (LEA), and diabetic mellitus plus high-frequency group (HEA). Gastrointestinal motility was assessed by gastric emptying and GI transit test. Immunofluorescence staining was applied to assess the expression level of CNP, NPR-B, and c-Kit. Western blot, PCR, and ELISA were used to detect the level of CNP, NPR-B, PDE2A, PDE3A, c-Kit, mSCF, and cGMP content. The correlativity between NPR-B and mSCF was evaluated by Pearson's correlation and linear regression analyses.

KEY RESULTS

(a) EA could improve the GI dysfunction of diabetic mice. (b) CNP, NPR-B, and cGMP contents were decreased, but the level of PDE3A, c-Kit, and mSCF was increased in the EA groups. (c) There was a negative correlation between NPR-B and mSCF among the groups.

CONCLUSIONS AND INFERENCES

Electroacupuncture promotes the GI function by CNP/NPR-B-cGMP and PDE3A-cGMP signaling in diabetic mice; up-regulated mSCF/c-Kit signaling by EA may be mediated partially via down-regulation of CNP/NPR-B signaling.

The role of CNP-mediated PKG/PKA-PLCβ pathway in diabetes-induced gastric motility disorder

Our previous work demonstrated that the C-type natriuretic peptide (CNP)/cyclic guanosine monophosphate (cGMP)/cyclic adenosine monophosphate (cAMP) pathway in gastric antrum smooth muscle of rats with diabetes was upregulated and played an important role in the development of diabetic gastric dysmotility. Our goal for this study was to explore the downstream signaling pathways of CNP. We found that the expressions of protein kinase G (PKG) and protein kinase A (PKA) in gastric smooth muscle tissue of rats with diabetes were significantly upregulated. The expressions of β-type phospholipase C 3(PLCβ3) and β-type phospholipase C 1(PLCβ1) protein were reduced, whereas Phosphor-PLCβ3Ser1105 (P-PLCβ3Ser1105) was increased. The inhibitory effect of CNP on gastric antral smooth muscle in diabetic rats was significantly greater than in the normal group. The content of trisphosphate inositol (IP3) in the gastric antral smooth muscle of rats with diabetes was significantly lower than that of the normal group. After blocking PKA with N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89, a blockage PKA), the effect of CNP on the production of IP3 was decreased, while blocking PKG with KT5823 (a blockage PKG) simultaneously, and CNP can no longer reduce the IP3 production. CNP promoted the phosphorylation of PLCβ3Ser1105, thereby inhibiting the activity of PLCβ3 in gastric smooth muscle tissue of rats with diabetes; this effect can be abolished by blocking PKA and PKG. These results suggested that CNP can decrease IP3 level in gastric smooth muscle cells and thus inhibit gastric smooth muscle contraction through PKG/PKA-PLCβ pathway, which may play an important role in the development of diabetic gastroparesis.

Effects of insulin-like growth factor-1 on endoplasmic reticulum stress and autophagy in rat gastric smooth muscle cells cultured at different glucose concentrations in vitro

The purpose of the study was to observe changes in endoplasmic reticulum stress (ERS)- and autophagy-related proteins in gastric smooth muscle tissues of diabetic rats with gastroparesis, investigate the effect of insulin-like growth factor 1 (IGF-1) on ERS and autophagy in rat gastric smooth muscle cells cultured under different glucose concentrations, and explore the influence of IGF-1 on development of diabetic gastroparesis (DGP). After establishing a rat model of DGP, rats were divided into normal control (NC) and 6-week diabetic model (DM6W) groups. Expression of ERS-related and autophagy-related proteins was detected by western blot analysis and immunofluorescence assay in rat gastric smooth muscle tissue and in vitro-cultured rat gastric smooth muscle cells exposed to different glucose concentrations and treatment with IGF-1 for 24 or 48 h. Changes in glucose-regulated-protein-78 (GRP78), growth arrest and DNA damage-inducible gene 153 (CHOP), and microtubule-associated protein 1A/1B light chain 3B (LC3) expression levels were detected by western blot analysis, and GRP78 and LC3 expression were examined by confocal laser-scanning microscopy. In vivo expression levels of GRP78, CHOP, and LC3 were significantly higher in the DM6W group compared with the NC group (p < 0.001). Twenty-four hours after cells were cultured at different glucose concentrations in vitro, expression of GRP78, CHOP, and LC3II/I was significantly higher in the high glucose-treated group compared with the normal glucose group (p < 0.05). After IGF-1 intervention, CHOP and GRP78 expression were significantly higher in the normal glucose + IGF-1 group compared with the normal glucose group (p < 0.01), while no significant difference was found between high glucose and high glucose + IGF-1 groups. LC3II/I expression was significantly lower in the normal glucose + IGF-1 group compared with the normal glucose group, and was significantly lower in the high glucose and high glucose + IGF-1 groups (p < 0.05). After 48 h of culture, CHOP expression was significantly higher and LC3II/I expression was significantly lower in the high glucose group compared with the normal glucose group (p < 0.05), but no significant change in GRP78 expression was observed between these two groups. After IGF-1 intervention, there was no difference in CHOP or GRP78 expression between normal glucose + IGF-1 and normal glucose groups. However, CHOP and GRP78 expression were significantly lower in the high glucose + IGF-1 group compared with the high glucose group (p < 0.05). There was no significant difference in LC3II/I expression between normal glucose + IGF-1 and normal glucose groups, or high glucose + IGF-1 and high glucose groups. Results of confocal laser-scanning microscopy showed significantly lower expression of LC3II/I in the high glucose + IGF-1 group compared with the high glucose group (p < 0.05). ERS and autophagy were involved in the occurrence of DGP. IGF-1 exerted an inhibitory effect on ERS in rat gastric smooth muscle cells cultured under high glucose conditions, and this inhibitory effect increased with time. IGF-1 inhibited the level of autophagy in rat gastric smooth muscle cells cultured under high glucose conditions at early stages, which may be achieved through inhibition of ERS.

Significance of dynamic changes in gastric smooth muscle cell apoptosis, PI3K-AKT-mTOR and AMPK-mTOR signaling in a rat model of diabetic gastroparesis

The aim of the present study was to investigate the significance of cell apoptosis, the phosphoinositide-3-kinase (PI3K)-protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway, and the 5' adenosine monophosphate-activated protein kinase (AMPK)‑mTOR pathways in the process of diabetic gastroparesis. Changes in gastric smooth muscle cells of diabetic rats with induced gastroparesis were examined. The diabetic rat model was established by dividing animals into a normal control group and diabetic model groups examined at 2, 4 and 6 weeks. Diabetic gastroparesis was evaluated by examining the rates of gastric residual pigment, whereas flow cytometry was used to detect the apoptosis of gastric smooth muscle cells. The expression levels of PI3K and phosphorylated (p‑) AKT, AMPK, mTOR, tuberous sclerosis complex 2, p70 ribosomal S6 kinase, and eukaryotic translation initiation factor 4‑binding protein 1 were determined in gastric muscles using western blot analysis. Diabetic gastroparesis was confirmed in models at 6 weeks. The apoptosis of gastric smooth muscle cells gradually increased in all diabetic groups, and significant changes were observed in key proteins involved in PI3K‑AKT‑mTOR and AMPK‑mTOR signaling. The results indicated that apoptosis was important in the occurrence of diabetic gastroparesis, and the PI3K‑AKT‑mTOR and AMPK‑mTOR pathways were activated during the apoptotic processes, but were incapable of regulating apoptosis.

CNP-pGC-cGMP-PDE3-cAMP Signal Pathway Upregulated in Gastric Smooth Muscle of Diabetic Rats

Our previous studies have shown that CNP-NPR-B/pGC-cGMP is upregulated in the diabetic rats. The present study was designed to determine whether the upregulation of CNP-NPR-B/pGC-cGMP signal pathway affects cGMP-PDE3-cAMP signal pathway in diabetic gastric smooth muscle. The gastric smooth muscle motility was observed by using isometric measurement. PDEs expressions in diabetic gastric smooth muscle tissue were observed by using immunohistochemistry, Western blotting, and RT-PCR methods. The results demonstrated that the inhibitory effect of CNP on the spontaneous contraction of gastric antral circular smooth muscle was potentiated in STZ-induced diabetic rat. CNP-induced increase of cGMP and cAMP was much higher in diabetic gastric smooth muscle tissue than in controls. The expression of PDE3 is downregulated while the levels of gene expression of PDE1, PDE2, PDE4, and PDE5 were not altered in the diabetic gastric smooth muscle tissue. The results suggest that the sensitivity of gastric smooth muscle to CNP is potentiated via activation of CNP-pGC-cGMP-PDE3-cAMP signal pathway in STZ-induced diabetic rats, which may be associated with diabetes-induced gastric motility disorder.

CNP signal pathway up-regulated in rectum of depressed rats and the interventional effect of Xiaoyaosan

AIM: To investigate the distribution and expression of C-type natriuretic peptide (CNP)/natriuretic peptide receptor B (NPR-B) in the rectum of a rodent depression model and the interventional effect of Xiaoyaosan (XYS).

METHODS: Male rats (n = 45) of clean grade (200 ± 20 g) were divided into five groups after one week of adaptive feeding: primary control, depression model, low dose XYS, middle dose XYS, and high dose XYS. The animal experiment continued for 3 wk. Primary controls were fed normally ad libitum. The rats of all other groups were raised in solitary and exposed to classic chronic mild unpredictable stimulation each day. XYS groups were perfused intragastrically with low dose, middle dose, and high dose XYS one hour before stimulation. Primary control and depression model groups were perfused intragastrically with normal saline under similar conditions as the XYS groups. Three weeks later, all rats were sacrificed, and the expression levels of CNP and NPR-B in rectum tissues were analyzed by immunohistochemistry, real-time polymerase chain reaction, and Western blotting.

RESULTS: CNP and NPR-B were both expressed in the rectum tissues of all rats. However, the expression levels of CNP and NPR-B at both gene and protein levels in the depression model group were significantly higher when compared to the primary control group (n = 9; P < 0.01). XYS intervention markedly inhibited the expression levels of CNP and NPR-B in depressed rats. The expression levels of CNP and NPR-B in the high dose XYS group did not significantly differ from the expression levels in the primary control group. Additionally, the high and middle dose XYS groups (but not the low dose group) significantly exhibited lower CNP and NPR-B expression levels in the rectum tissues of the respectively treated rats compared to the untreated depression model cohort (n = 9; P < 0.01).

CONCLUSION: The CNP/NPR-B pathway is upregulated in the rectum of depressed rats and may be one mechanism for depression-associated digestive disorders. XYS antagonizes this pathway at least partially.

Aortocaval fistula delays gastric emptying of liquid test meal in awake rats

Arteriovenous anastomoses disrupt cardiovascular and renal homeostasis, eliciting hemodynamic adjustments, resetting the humoral pattern, and inducing cardiac hypertrophy. Because acute circulatory imbalance alters gut motor behavior, we studied the effects of arteriovenous fistula placement on the gastric emptying (GE) of a liquid meal in awake rats. After laparotomy, we created an aortocaval fistula (ACF) by aorta and cava wall puncture with a 21-, 23-, or 26-gauge needle. The ACF was not created in the control group, which underwent sham operation. After 12, 24, or 48 h, mean arterial pressure, heart rate, and central venous pressure were continuously recorded, and cardiac output was estimated by thermal dilution. The rats were then gavage fed a test meal (i.e., phenol red in glucose solution), and fractional dye retention was determined 10, 20, or 30 min later. The effect of prior bleeding on ACF-induced GE delay, the role of neuroautonomic pathways, and changes in plasma hormone levels (i.e., angiotensin II, arginine vasopressin, atrial natriuretic peptide, corticosterone, and oxytocin) were evaluated. When compared with the sham-operated group, ACF rats exhibited arterial hypotension, higher ( P < 0.05) heart rate, central venous pressure, and cardiac output values and increased ( P < 0.05) gastric dye retention, a phenomenon prevented by bilateral subdiaphragmatic vagotomy and hexamethonium treatment. Pirenzepine also impaired the occurrence of gastric delay in subjects with ACF. In addition to causing hyperkinetic circulation, ACF placement delayed the GE of liquid in awake rats, an effect that likely involves a parasympathetic pathway.

Diabetes-induced loss of gastric ICC accompanied by up-regulation of natriuretic peptide signaling pathways in STZ-induced diabetic mice

Our previous study demonstrated that natriuretic peptides (NPs) play an inhibitory role in regulation of gastric smooth muscle motility. However, it is not clear whether NPs are involved in diabetics-induced loss of gastric interstitial cell of Cajal (ICC). The present study was designed to investigate the relationship between diabetics-induced loss of gastric ICC and natriuretic peptide signaling pathway in streptozotocin (STZ)-induced diabetic mice. The results showed that the protein expression levels of c-Kit and membrane-bound stem cell factor (mSCF) in gastric smooth muscle layers were decreased in STZ-induced diabetic mice. However, both mRNA and protein expression levels of natriuretic peptide receptor (NPR)-A, B and C were increased in the same place of the diabetic mice. The amplitude of spontaneous contraction in gastric antral smooth muscles was inhibited by C-type natriuretic peptide (CNP) dose-dependently and the inhibitory effect was potentiated in diabetic mice. Pretreatment of the cultured gastric smooth muscle cells (GSMCs) with different concentration of CNP can significantly decrease the mSCF expression level. 8-Bromoguanosine-3',5'-cyclomo-nophosphate (8-Br-cGMP), a membrane permeable cGMP analog, mimicked the effect of CNP but not cANF (a specific NPR-C agonist). Methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay showed that high concentration of cANF (10(-6) mol/L) inhibited cell proliferation in cultured GSMCs. These findings suggest that up-regulation of NPs/NPR-A, B/cGMP and NPs/NPR-C signaling pathways may be involved in diabetes-induced loss of gastric ICC.

Gender and estradiol as major factors in the expression and dimerization of nNOSα in rats with experimental diabetic gastroparesis

BACKGROUND

The molecular mechanisms of cellular changes responsible for diabetic gastroparesis, primarily seen in middle-aged women, still remain incompletely defined. We hypothesized that a decrease in the expression, dimerization, and post-translational modification of neuronal nitric oxide synthase alpha (nNOSα) is estrogen mediated and responsible for the gender-specific prevalence of this malady.

METHODS

We induced diabetes by injecting male and female rats with streptozotocin. Male diabetic rats without gastroparesis were then injected with estrogen for 3 weeks and evaluated for gastroparesis development. Gastric tissues were analyzed for the elucidation of biochemical events associated with diabetes and gastroparetic dysfunction.

RESULTS

Although male diabetic, gastroparetic (either streptozotocin- or estrogen-induced) rats exhibited similarity in disease pathology to that of females, the molecular mechanisms of development were different. Our results indicate that slow gastric emptying in both male diabetic, gastroparetic rat groups was not associated with the level of expression of nNOSα in gastric tissues. However, nNOSα dimerization, which reflects nNOSα activation, did decline slightly in the antrum of diabetic males with estrogen-induced gastroparesis, suggesting a possible estrogen role. Females with diabetic gastroparesis, in contrast, demonstrated significantly impaired levels and dimerization of nNOSα in the antrum and pylorus. Although the precise mechanism remains unknown, nNOSα dimerization impairment in female antrum is apparently associated with reduced phosphorylation of Ser(1416) in the activation loop of nNOSα.

CONCLUSION

Taken together, these results demonstrate that gender and estrogens may be leading factors, through molecular changes involved in nitric oxide synthesis down-regulation, within the antrum and pylorus of female diabetic, gastroparetic rats.

Atrial natriuretic peptide signal pathway upregulated in stomach of streptozotocin-induced diabetic mice

AIM

To investigate atrial natriuretic peptide (ANP) secretion from gastric mucosa and the relationship between the ANP/natriuretic peptide receptor type A (NPR-A) pathway and diabetic gastroparesis.

METHODS

Male imprinting control region (ICR) mice (4 wk old) were divided into two groups: control mice, and streptozotocin-induced diabetic mice. Eight weeks after injection, spontaneous gastric contraction was recorded by using physiography in control and streptozotocin-induced diabetic mice. The ANP-positive cells in gastric mucosa and among dispersed gastric epithelial cells were detected by using immunohistochemistry and flow cytometry, respectively. ANP and natriuretic peptide receptor type A (NPR-A) gene expression in gastric tissue was observed by using the reverse transcriptase polymerase chain reaction.

RESULTS

The frequency of spontaneous gastric contraction was reduced from 12.9 +/- 0.8 cycles/min in the control group to 8.4 +/- 0.6 cycles/min in the diabetic mice (n = 8, P < 0.05). However, the amplitude of contraction was not significantly affected in the diabetic group. The depletion of interstitial cells of Cajal in the gastric muscle layer was observed in the diabetic mice. ANP-positive cells were distributed in the gastric mucosal layer and the density index of ANP-positive cells was increased from 20.9 +/- 2.2 cells/field in control mice to 51.8 +/- 2.9 cells/field in diabetic mice (n = 8, P < 0.05). The percentage of ANP-positive cells among the dispersed gastric epithelial cells was increased from 10.0% +/- 0.9% in the control mice to 41.2% +/- 1.0% in the diabetic mice (n = 3, P < 0.05). ANP and NPR-A genes were both expressed in mouse stomach, and the expression was significantly increased in the diabetic mice.

CONCLUSION

These results suggest that the ANP/NPR-A signaling pathway is upregulated in streptozotocin-induced diabetic mice, and contributes to the development of diabetic gastroparesis.