Alteration of the properties of gastric smooth muscle in the genetically hyperglycemic OLETF rat

Effects of acupuncture stimulation on blood glucose concentration in the Otsuka Long-Evans Tokushima Fatty (OLETF) rat, an animal model for type-2 diabetes mellitus

BACKGROUND

Effects of acupuncture stimulation on blood glucose concentration and body weight were investigated in the Otsuka Long-Evans Tokushima Fatty (OLETF) rat, a model for type-2 diabetes.

MATERIAL AND METHODS

Three groups of rats were used: OLETF, acupuncture-treated OLETF (AcOLETF), and Long-Evans Tokushima Otsuka (LETO) rats (as control for the OLETF rats). In AcOLETF rats, acupuncture stimulation was applied twice a week to 6 points (zhongwan, tianshu, qihai, ganshu, pishu, shenshu) and changes in blood glucose concentration and body weight were measured.

RESULTS

Initially, at 6 weeks old, there was no significant difference in blood glucose levels between groups. Blood glucose levels increased with age in each group, reaching a maximum of about 430 mg/dl at 37 weeks in OLETF rats. In AcOLETF rats, blood glucose levels increased at a slower rate than in OLETF rats, reaching a maximum concentration of about 280 mg/dl at 37 weeks of age, significantly lower than that in OLETF rats. The concentration of blood glucose in LETO rats had stabilized at a maximum value of 120~140 mg/dl by 16 weeks, remaining at this level for up to 39 weeks. In each group, body weight increased with age and was not affected by acupuncture treatment.

CONCLUSIONS

In OLETF rats, acupuncture treatment significantly reduced blood glucose levels, but not their body weight, suggesting that acupuncture therapy was effective in preventing the development of type-2 diabetes mellitus.

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

A common gastrointestinal complication of diabetes is gastroparesis, and patients with gastroparesis may present with early satiety, nausea, vomiting, bloating, postprandial fullness, or upper abdominal pain. However, the pathogenesis is not clear yet. A recent study indicated that atrial natriuretic peptide (ANP) was secreted from the gastric mucosa and the ANP family plays an inhibitory role in the regulation of gastrointestinal motility, but the effect of the natriuretic peptide signal pathway on diabetic gastroparesis has not been reported. The study investigated the effect of C-type natriuretic peptide (CNP) particulate guanylyl cyclase (pGC) cyclic guanosine monophosphate (cGMP) signaling on gastroparesis in streptozotocin (STZ)-induced diabetic rats. Male Sprague-Dawley rats were divided into two groups; group I: normal control rats; group II: STZ-induced diabetic rats; 4 weeks after induction, the experiments were performed. The spontaneous contraction of gastric smooth muscle strips was recorded by using physiographs in control and diabetic rats. The pGC activity in response to CNP and cGMP production in gastric smooth muscle were measured by using radioimmunoassay (RIA) in normal and diabetic rats. CNP induced a longer lasting relaxation of gastric antral circular smooth muscle strips in STZ-induced diabetic rats. The inhibitory effect of CNP on spontaneous contraction revealed a dose-dependency, and the inhibitory percentages were 25.5 +/- 1.7%, 43.6 +/- 3.2%, 85.1 +/- 2.5% in diabetic rats and 20.5 +/- 1.5%, 31.1 +/- 1.7%, 58.9 +/- 3.7% in the control group at the concentrations of 0.01, 0.03, and 0.1 mumol/l, respectively. The cGMP production and pGC activity in response to CNP in gastric muscle tissues were significantly potentiated in STZ-induced diabetic rats. Natriuretic peptide receptor type B (NPR-B) gene was expressed in the gastric smooth muscles of normal and diabetic rats, and the expression was increased in diabetic rats. The results suggest that natriuretic peptide-dependent pGC-cGMP signal is upregulated and may contribute to diabetic gastroparesis in STZ-induced diabetic rats.

Acupuncture modulates mechanical responses of smooth muscle produced by transmural nerve stimulation in gastric antrum of genetically hyperglycemic rats

Effects of acupuncture treatment on mechanical responses produced by transmural nerve stimulation (TNS) and acetylcholine (ACh) were investigated in circular smooth muscle preparations isolated from the antrum of the stomach of genetically hyperglycemic rats. While control rats had blood glucose levels of about 140 mg/dl, this was approximately tripled in the genetically hyperglycemic rats, but only doubled in the acupuncture treated genetically hyperglycemic rats. Antrum smooth muscle produced phasic contractions spontaneously, with a similar frequency and amplitude in the three groups of rats. Effects of atropine and Nomega-nitro-L-arginine (L-NA) on TNS-induced responses revealed that in the antrum smooth muscle of the control rats, cholinergic excitatory, non-adrenergic non-cholinergic excitatory (NANCE), nitrergic inhibitory and off-responses produced projections: the last projection was considered to be non-adrenergic non-cholinergic non-nitrergic (NANCNN) in nature. In genetically hyperglycemic rats, nitrergic and NANCNN projections were enhanced and NANCE projections were absent. Acupuncture treated genetically hyperglycemic rats showed a reduction of NANCNN projection and enhancement of cholinergic projection, with no alteration to nitrergic projection, but a recovery of NANCE projection. ACh elicited inhibitory responses at low concentrations (1-30 nM) and excitatory responses at high concentrations (100-300 nM), in the three groups of rats. L-NA converted the ACh-induced inhibitory responses to excitatory responses. Immunohistochemical examination indicated no significant difference in the distribution of c-Kit expressing cells in the antrum smooth muscle from the three groups of rats. The results indicated that in antral smooth muscle, hyperglycemia was associated with enhanced activity in nitrergic and NANCNN projections and attenuation of NANCE projections, and that acupuncture treatment caused both a reduced blood glucose level and attenuated NANCNN projections. In genetically hyperglycemic rats, cholinergic responses were enhanced by acupuncture, possibly due to the enhanced cholinergic projections, with no change in the sensitivity of postjunctional muscarinic receptors to ACh.

C-type natriuretic-peptide-potentiated relaxation response of gastric smooth muscle in streptozotocin-induced diabetic rats

AIM: To study the sensitivity of gastric smooth muscle to C-type natriuretic peptide (CNP) in streptozotocin (STZ)-induced diabetic rats.

METHODS: The spontaneous contraction of a gastric smooth muscle strip was recorded by using physiological methods in rats. The expressions of CNP and natriuretic peptide receptor-B (NPR-B) in gastric tissue were examined by using immunohistochemistry techniques in the diabetic rat.

RESULTS: At 4 wk after injection of STZ and vehicle, the frequency of spontaneous contraction of gastric smooth muscle was significantly reduced in diabetic rats, and the frequency was decreased from 3.10 ± 0.14 cycle/min in controls to 2.23 ± 0.13 cycle/min (n = 8, P < 0.01). However, the amplitude of spontaneous contraction was not significant different from the normal rat. CNP significantly inhibited spontaneous contraction of gastric smooth muscle in normal and diabetic rats, but the inhibitory effect was significantly potentiated in the diabetic rats. The amplitudes of spontaneous contraction were suppressed by 75.15% ± 0.71% and 58.92% ± 1.32% while the frequencies were decreased by 53.33% ± 2.03% and 26.95% ± 2.82% in diabetic and normal rats, respectively (n = 8, P < 0.01). The expression of CNP in gastric tissue was not changed in diabetic rats, however the expression of NPR-B was significantly increased in diabetic rats, and the staining indexes of NPR-B were 30.67 ± 1.59 and 17.63 ± 1.49 in diabetic and normal rat, respectively (n = 8, P < 0.01).

CONCLUSION: The results suggest that CNP induced an inhibitory effect on spontaneous contraction of gastric smooth muscle, potentiated in diabetic rat via up-regulation of the natriuretic peptides-NPR-B-particulate guanylyl cyclase-cyclic GMP signal pathway.

Alteration of the responses of gastric smooth muscle to endothelin in streptozotocin-induced diabetic rats

Diabetic gastropathy is suggested to be the result of not only an autonomic neuropathy but also to disorder of the spontaneous rhythmic motility of the gastric smooth muscle. Attempts were made to investigate the alteration of the effects of endothelin-1 (ET-1), which is known to enhance the spontaneous activity of gastrointestinal smooth muscle, on gastric activity in streptozotocin (STZ)-induced diabetic rats. STZ-induced diabetic rats were prepared by the injection of Sprague-Dawley (SD) rats with STZ (i.p.). Isometric mechanical responses were recorded in isolated circular smooth muscle strips of the stomach antrum, to measure changes in the rhythmicity of the smooth muscle. ET-1 (10 nM) significantly elevated the resting tension and the frequency of spontaneous contraction, but did not alter the amplitude of the spontaneous oscillatory contractions in normal rats. In diabetic rats, ET-1 elevated the resting tension, and spontaneous contractions were increased in frequency, however they were decreased in amplitude. In normal rats, sarafotoxin S6c (S6c, 10 nM), a selective ET(B) receptor agonist, elevated the resting tension slightly and increased both the frequency and amplitude of the spontaneous contractions. However, S6c significantly elevated the resting tension alone in STZ-induced diabetic rats. Selective stimulation of endothelin type A (ET(A)) receptors with ET-1, in the presence of a selective antagonist of ET(B) receptors, produced similar responses in the gastric muscle of both normal and diabetic rats. These results indicate that ET-1 elevates the resting tension and increases the frequency of the spontaneous oscillatory contractions in both normal and STZ-induced diabetic rats, to a similar extent. However, the specific actions on ET(B) receptors were quite different between the two: the elevating actions on the resting tension were much greater in STZ-diabetic rats than in normal rats. The results suggested the facilitation of ET(B) receptor signaling in the antrum during the pathogenesis of diabetic gastropathy.

Interstitial cells of Cajal in diabetic gastroenteropathy

Gastroenteropathy causes considerable morbidity in patients with diabetes mellitus and represents a major healthcare burden. Current treatments are largely symptomatic and frequently ineffective. Development of new therapeutic options is hampered by poor understanding of the underlying pathomechanisms. Experimental studies and sparse human data indicate that diabetic gastroenteropathy is multifactorial and involves not only parasympathetic and sympathetic autonomic nerves, but also enteric neurons, smooth muscle cells and interstitial cells of Cajal (ICC). ICC are mesenchymal cells that occur throughout the muscular coat of the gastrointestinal tract and provide functions critical for normal gastrointestinal motility including generation and propagation of electrical slow waves and mediation of bidirectional communication between the autonomic nervous system and smooth muscle cells. Through these functions, and in concert with other cell types of the gastrointestinal muscles, ICC support basic gastrointestinal functions such as digestion, absorption and waste removal. Loss or dysfunction of ICC in various dysmotilities and their animal models has been shown to lead to gastric dysrhythmias, gastroparesis, slow intestinal transit, impaired neuroeffector mechanisms and altered visceral afferent signalling that are considered hallmarks of diabetic gastroenteropathy. These findings and an increasing body of evidence indicating disruptions of ICC networks in diabetes suggest that the loss of ICC in this disorder is probably of functional significance and could even be a major pathogenetic factor. Future research should focus on the identification of the molecular and cellular mechanisms underlying ICC loss in diabetes and the translation of the experimental findings into treatments.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Electrophysiological properties of gastric pacemaker potentials

Electrophysiological properties of pacemaker potentials recorded from myenteric interstitial cells of Cajal (ICC-MY) within the guinea-pig gastric antrum are reviewed briefly. Pacemaker potentials consist of two components, a primary component forming a transient depolarization with a rapidly rising initial phase, followed by a secondary component as a plateau with sustained depolarization. The primary component is inhibited by low [Ca2+]o solutions or depolarization of the membrane with high [K+]o solutions. This inhibition could be mimicked by chelating [Ca2+]i with BAPTA-AM, suggesting that this component is produced by activation of voltage-dependent Ca2+ permeable channels. The plateau component is inhibited by low [Cl-]o solution or DIDS, an inhibitor of Ca2+-activated Cl(-)-channels, suggesting that this component is formed by Ca2+-activated Cl(-)-currents. Reduction of Ca2+ release from internal stores by inhibiting the internal Ca-pump with cyclopiazonic acid results in a shortened duration of the plateau component, with no alteration in the rate of rise of the primary component. 2-APB, an inhibitor of the IP3-receptor mediated Ca2+ release from internal stores, abolishes pacemaker potentials, suggesting that the release of Ca2+ from internal IP3-sensitive Ca2+ stores is required for generation of pacemaker potentials. CCCP, a mitochondrial protonophore, depolarizes the membrane and abolishes pacemaker potentials, suggesting that mitochondrial Ca2+ handling functions may be coupled with generation of pacemaker potentials. These results indicate that the two components of pacemaker potentials are generated by different mechanisms; the primary component may be produced by activation of voltage-dependent Ca2+-permeable channels, while the plateau component may be produced by the opening of Ca2+-activated Cl(-)-channels. It is hypothesized that pacemaker potentials are initiated by depolarization of the membrane due to generation of unitary potentials in response to mitochondrial Ca2+ handling. Activation of voltage-dependent Ca2+ influx, IP3-receptor mediated Ca2+ release from the internal stores and Ca2+-activated Cl(-)-channels may be involved as successive steps downstream to the generation of unitary potentials.

Streptozotocin-induced diabetes modulates presynaptic and postsynaptic function in the rat ileum

Altered gastrointestinal motility frequently occurs in diabetic patients and also in animal models of diabetes but the underlying causes are not clear. In the present study, contractile responses to agonists and electrical field stimulation (EFS) and the inhibitory actions of an adenosine A(1) receptor agonist were investigated on ilea from 8-week streptozotocin (STZ)-induced diabetic rats. Contractile responses to carbachol, prostaglandin F(2 alpha) (PGF(2 alpha)), the calcium ionophore A23187 and to EFS were increased in diabetic tissues compared to controls. In contrast, the inhibitory effects of a potent and selective adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) on electrical field stimulation-evoked contractions were decreased in diabetic tissues compared to controls but its ability to relax carbachol-contracted tissues was unaltered. These results suggest that diabetes may cause alterations at both pre- and postsynaptic sites and this may lead in turn to the gastrointestinal complications seen in diabetic patients.

Plasticity of electrical pacemaking by interstitial cells of Cajal and gastric dysrhythmias in W/W mutant mice

BACKGROUND & AIMS

Interstitial cells of Cajal (ICC) generate and propagate slow waves in the stomach. Gastric peristalsis depends on a proximal-to-distal gradient in slow wave frequency. We tested whether the gastric frequency gradient was an intrinsic property of ICC and whether dysrhythmias result from disruptions of ICC networks.

METHODS

We studied wild-type (WT) and W/W(V) mice, which have only myenteric (pacemaker) ICC in the stomach. ICC distributions were analyzed by Kit immunofluorescence. Pacemaking in tissues was studied by intracellular electrophysiologic recording and in cultured ICC by monitoring mitochondrial [Ca(2+)] oscillations with rhod-2 fluorescence or membrane potential with DiBAC(4)(3) fluorescence.

RESULTS

Slow wave frequencies were constant throughout WT gastric muscle sheets containing corpus and antrum. Separating the antrum from the corpus caused a significant drop in antral slow wave frequency. ICC from WT antrums also displayed significantly slower pacemaker frequencies than corpus ICC, but the corpus pacemaker frequency dominated in cocultures of corpus and antrum ICC. Myenteric ICC networks were reduced in W/W(V) mice, particularly in the corpus. In W/W(V) mice, separating the antrum from the corpus failed to reduce antral slow wave frequency. Antral pacemaker frequency in ICC from W/W(V) stomachs was the same as in corpus ICC.

CONCLUSIONS

The proximal-to-distal slow wave frequency gradient and entrainment of distal electrical activity by proximal pacemakers are fundamental properties of gastric ICC. Chronic depletion of ICC networks disrupts the proximal-to-distal frequency gradient, and emergence of ectopic pacemakers in the antrum may be caused by "reprogramming" of the ICC pacemaker apparatus.

Altered beta-adrenoceptor-mediated responses in the gastric smooth muscle of hypertensive rats

Effects of isoproterenol on contraction and membrane potential of gastric smooth muscle were studied in stroke prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar Kyoto rats (WKY). Circular muscle preparation from the gastric fundus developed tonic contraction by re administration of Ca2+ to a nominally Ca2+-free solution. The contraction was inhibited by nifedipine or nicardipine. Isoproterenol induced relaxation when it was applied to the Ca2+-induced contraction. The amplitude of isoproterenol induced relaxation was concentration-dependent. Propranolol 10(-6) M abolished the relaxation induced by isoproterenol 10(-7) M. In the preparation from SHRSP, the amplitude of isoproterenol induced relaxation was smaller than that from WKY between 3 x 10(-9) and 10(-7) M. Forskolin, an adenylate cyclase activator, induced concentration-dependent relaxation. There was no difference in the relaxation induced by forskolin between preparations from WKY and SHRSP. Dibutilyl cyclic AMP, a membrane permeable analogue of cyclic AMP, also induced similar relaxation in preparations from WKY and SHRSP. Resting membrane potential of smooth muscle cell was not different between preparations from WKY and SHRSP. Isoproterenol hyperpolarized the membrane concentration-dependently. Isoproterenol-induced hyperpolarization in the preparation from SHRSP was smaller than that from WKY between 10(-8) and 10(-6) M. When the membrane was depolarized by Tyrode's solution containing 40 mM K+, isoproterenol-induced hyperpolarization was almost abolished. In this condition, the isoproterenol-induced relaxation was inhibited partly, however, there was no difference in the amplitude of relaxation between preparations from WKY and SHRSP. Therefore, isoproterenol-induced hyperpolarization contributed at least partly to the relaxation. Forskolin hyperpolarized the membrane by the same amplitude in the preparations from WKY and SHRSP. These results indicate that a decrease in hyperpolarization may contribute to the decreased relaxation by isoproterenol in the preparation from SHRSP.