beta-Adrenergic modulation of Ba2+ currents and K+ contractures in frog slow skeletal muscle fibers

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

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.

The enhanced spontaneous activity of the diabetic colon is not the consequence of impaired inhibitory control mechanisms

1. Approximately 45% of patients with diabetes mellitus have gastrointestinal complications such as diarrhoea and constipation, but the underlying aetiology is unclear. The present study investigates alterations in spontaneous motility of the colon that may be, in part, responsible for these symptoms using an established animal model of diabetes. 2. Rats were rendered diabetic by a single intraperitoneal injection of streptozotocin and age-matched controls were injected with citrate buffer. Rats were sacrificed after 8-weeks and proximal colonic circular muscle tissue mounted in organ baths. 3. Spontaneous activity was observed in both control and diabetic tissues, but this activity was almost doubled in colonic tissue taken from diabetic rats. It was hypothesized that this increase was due to a deficit in inhibitory control of the colon in the diabetic state. 4. Possible alterations in nitrergic and vasoactive intestinal polypeptide (VIP)ergic control were investigated using a range of pharmacological tools. 5. Sodium nitroprusside, VIP and antioxidants (reduced glutathione, ascorbate and alpha-tocopherol) inhibited the spontaneous activity, but the level of inhibition observed was not different in diabetic tissue compared with control. 6. Arginine, [D-p-Cl-Phe6, Leu17]-VIP and alpha-chymotrypsin had no effect on spontaneous activity in either sets of tissue. 7. N omega-nitro-L-arginine produced a small, but significant, increase in the level of spontaneous activity, but the degree of increase was not different between control and diabetic tissues. 8. Western blots demonstrated that there was no inducible-nitric oxide synthase (iNOS) in control or diabetic tissues and that the levels of endothelial-NOS (eNOS) and neuronal-NOS (nNOS) detected were not statistically significantly different. The [3H]-citrulline assay established that the functionality of the NOS isoforms present were unaltered in the diabetic state. 9. This study demonstrates that there is a marked alteration in motility in the colon taken from diabetic animals. However, the change in motility is unlikely to be due to a change in inhibitory control mechanisms and may be due to an increased excitability.

Adrenaline diminishes K+ contractures and Ba2+-current in chicken slow skeletal muscle fibres

The effects of adrenaline and the beta-adrenergic agonist isoprenaline on K+-evoked tension (K+-contracture) and Ba2+ current were investigated in chicken slow (anterior latissimus dorsi (ald)) muscle using isometric-tension measurements and current recording. Addition of adrenaline (10(-7) - 10(-5) M) or isoprenaline (10(-6) - 10(-5) M) to the bath reduced the amplitude of the K+-contractures. These effects were blocked by the beta-antagonist propranolol (5 x 10(-6) M). External application of a cAMP analogue (8-bromo cyclic AMP; 1 x 10(-4) M) also decreased the amplitude of the K+-contractures. To analyze the possible relationship between the induced tension reduction and effects on sarcolemmal Ca2+ channels, a slow action potential and a slow inward membrane current were studied in intact ald chicken muscle fibres. When the ald muscle was immersed in a Na+- and Cl--free solution containing Ba2+ and depolarizing pulses were delivered from a -80 mV holding potential, the muscle fibres exhibited a small, slow Ba2+-dependent potential (observed at about -26 mV, peak amplitude, around -10 mV). The response was blocked by the addition of Co2+ (5 mM) or Cd2+ (2 mM). Using the three-microelectrode voltage-clamp technique, a slow inward membrane current underlying the Ba2+ potential could be discerned. The current had a mean threshold of -60 mV, reached maximum at about -5 mV and ranged from ca. 9 to 19 pA/cm2 (depending on the external Ba2+ concentration). It had a mean reversal potential of +45 mV. The Ba2+ inward current was diminished when adrenaline or isoprenaline was added to the bath (1 x 10(-5) M); however, this decrease did not occur when propranolol was present (5 x 10-6 M). These results suggest that the decreases in the tension of K+-contractures induced by adrenaline and isoprenaline may occur through beta-adrenergic effects on sarcolemmal Ca2+ channels in ald chicken slow muscle fibres.

Measurement of Ca(2+) currents in intact slow skeletal muscle fibers of the frog by the three-microelectrode technique

The recording of currents passing through calcium channels in intact skeletal muscle fibers presents several difficulties. However, use of the three-microelectrode voltage-clamp technique at the end of the fiber provided us with a good approximation of current values in such fibers. Using this technique, we were able to measure the calcium-channel current in slow skeletal muscle fibers of the frog (Rana pipiens) and to quantify the effects of denervation on this current.

Effects of denervation on Ca2+ channels in slow skeletal muscle fibers of the frog

Effects of denervation on calcium channels in slow skeletal muscle fibers in the frog (Rana pipiens) were studied using the three-microelectrode voltage-clamp technique in intact fibers. Ca2+, Ba2+, and Sr2+ currents were all significantly reduced in amplitude during the first 2 weeks after denervation. After nerve section the selectivity sequence Ba congruent with Ca > Sr was changed to Ba > Sr > Ca and the values for relative ratio increased from 1.04 to 2.65 for Ba2+ and from 0.58 to 1.20 for Sr2+ (with respect to Ca2+). Barium current saturation was more obvious in denervated fibers than in non-denervated fibers. The values obtained with the Michaelis-Menten type expression, I = Imax/(1+Kd/[Ba]e) were Kd = 2.7 mM and Imax = 20 microA/cm2 in fibers 2 weeks after nerve section compared with the values Kd = 4.4 mM and Imax = 60 microA/cm2 obtained in non-denervated fibers. Additionally, the effects of two calcium channel blockers (cobalt and nifedipine) were greater by a factor of two in denervated fibers than in non-denervated fibers. Three weeks or so after nerve section, all the biophysical properties studied began to show a tendency to recover toward the values obtained in non-denervated muscles (controls). These results suggest that calcium channels are modified or that there is a change in the types of calcium channels present in frog slow skeletal muscle fibers after denervation.

Effects of pimobendan, a new cardiotonic agent, on contractile responses in single skeletal muscle fibres of the frog

Effects of a new cardiotonic agent, pimobendan, on contraction were investigated in single intact skeletal muscle fibres of the frog. Pimobendan increased twitch tension in a concentration-dependent manner regardless of the presence or absence of Ca2+ without any effect on tetanic tension, the resting membrane potential and the shape of the action potential. Pimobendan caused a further increase in twitch tension potentiated by caffeine (1 mM). Adenine, an inhibitor of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum, inhibited twitch tension potentiated by caffeine but not by pimobendan, suggesting that twitch potentiation by pimobendan is not attributed to increases in Ca2+-induced Ca2+ release. Pimobendan failed to increase cAMP levels in the skeletal muscle, though forskolin significantly increased it without any effect on twitch tension. Contractile responses to high concentrations of caffeine and K+ were also potentiated by pimobendan. These results suggest that the potentiating effect of pimobendan on skeletal muscle contraction is mainly due to the increase in Ca2+ sensitivity to the contractile apparatus.