Effect of nitric oxide and NO synthase inhibition on nonquantal acetylcholine release in the rat diaphragm

From Frog Muscle to Brain Neurons: Joys and Sorrows in Neuroscience

One element, potassium, can be identified as the connecting link in the research of Czech neurophysiologist Prof. František Vyskočil. It accompanied him from the first student experiments on the frog muscle (Solandt effect) via sodium-potassium pump and quantum and non-quantum release of neurotransmitters (e.g. acetylcholine) to the most appreciated work on the reversible leakage of K+ from brain neurons during the Leao´s spreading cortical depression, often preceding migraine. He used a wide range of methods at the systemic, cellular and genetic levels. The electrophysiology and biochemistry of nerve-muscle contacts and synapses in the muscles and brain led to a range of interesting findings and discoveries on normal, denervated and hibernating laboratory mammals and in tissue cultures. Among others, he co-discovered the facilitating effects of catecholamines (adrenaline in particular) by end-plate synchronization of individual evoked quanta. This helps to understand the general effectiveness of nerve-muscle performance during actual stress. After the transition of the Czech Republic to capitalism, together with Dr. Josef Zicha from our Institute, he was an avid promoter of scientometry as an objective system of estimating a scientist´s success in basic research (journal Vesmír, 69: 644-645, 1990 in Czech).

Glutamate at the Vertebrate Neuromuscular Junction: From Modulation to Neurotransmission

Although acetylcholine is the major neurotransmitter operating at the skeletal neuromuscular junction of many invertebrates and of vertebrates, glutamate participates in modulating cholinergic transmission and plastic changes in the last. Presynaptic terminals of neuromuscular junctions contain and release glutamate that contribute to the regulation of synaptic neurotransmission through its interaction with pre- and post-synaptic receptors activating downstream signaling pathways that tune synaptic efficacy and plasticity. During vertebrate development, the chemical nature of the neurotransmitter at the vertebrate neuromuscular junction can be experimentally shifted from acetylcholine to other mediators (including glutamate) through the modulation of calcium dynamics in motoneurons and, when the neurotransmitter changes, the muscle fiber expresses and assembles new receptors to match the nature of the new mediator. Finally, in adult rodents, by diverting descending spinal glutamatergic axons to a denervated muscle, a functional reinnervation can be achieved with the formation of new neuromuscular junctions that use glutamate as neurotransmitter and express ionotropic glutamate receptors and other markers of central glutamatergic synapses. Here, we summarize the past and recent experimental evidences in support of a role of glutamate as a mediator at the synapse between the motor nerve ending and the skeletal muscle fiber, focusing on the molecules and signaling pathways that are present and activated by glutamate at the vertebrate neuromuscular junction.

Purine P2Y receptors in ATP-mediated regulation of non-quantal acetylcholine release from motor nerve endings of rat diaphragm

We established the effect of ATP, which is released together with acetylcholine (ACh), on the non-quantal ACh release (NQR) in rat diaphragm endplates and checked what kind of purine receptors are involved. NQR was estimated by the amplitude of endplate hyperpolarization (the H-effect) following the blockade of postsynaptic nicotinic receptors and cholinesterase. 100 μM ATP reduced the H-effect to 66% of the control. The action of ATP remained unchanged after the inhibition of ionotropic P2X receptors by Evans blue and PPADS, but disappeared after the application of the broad spectrum P2 receptor antagonist suramin, metabotropic P2Y receptor blocker reactive blue 2 and U73122, an inhibitor of phospholipase C. P2Y-mediated regulation is not coupled to presynaptic voltage-dependent Ca(2+) channels. During the simultaneous application of ATP and glutamate (which is another ACh cotransmitter reducing non-quantal release), the additive depressant effect led to a disappearance of the H-effect. This can be explained by the independence of the action of ATP and glutamate. Unlike the effects of purines on the spontaneous quantal secretion of ACh, its non-quantal release is regulated via P2Y receptors coupled to G(q/11) and PLC. ATP thus regulates the neuromuscular synapse by two different pathways.

Increased non-quantal release of acetylcholine after inhibition of endocytosis by methyl-β-cyclodextrin: the role of vesicular acetylcholine transporter

We investigated the role of the vesicular acetylcholine transporter in the mechanism of non-quantal (non-vesicular) secretion of neurotransmitter in the neuromuscular synapse of the rat diaphragm muscle. Non-quantal secretion was estimated electrophysiologically by the amplitude of end-plate hyperpolarization after inhibition of cholinesterase and nicotinic receptors (H-effect) or measured by the optical detection of acetylcholine in the bathing solution. It was shown that 1 mM methyl-β-cyclodextrin (MCD) reduced both endocytosis and, to much lesser extent, exocytosis of synaptic vesicles (SV) thereby increasing non-quantal secretion of acetylcholine with a concurrent decrease in axoplasm pH. During high-frequency stimulation of the motor nerve, that substantially increases vesicles exocytosis, the non-quantal secretion was further enhanced if the endocytosis of SV was blocked by MCD. In contrast, non-quantal secretion of acetylcholine did not increase when the MCD-treated neuromuscular preparations were superfused with either vesamicol, an inhibitor of vesicular transporter of acetylcholine, or sodium propionate, which decreases intracellular pH. These results suggest that the proton-dependent, vesamicol-sensitive vesicular transporters of acetylcholine, which become inserted into the presynaptic membrane during SV exocytosis and removed during endocytotic recycling of SV, play the major role in the process of non-quantal secretion of neurotransmitter.

Muscarinic M1 acetylcholine receptors regulate the non-quantal release of acetylcholine in the rat neuromuscular junctionviaNO-dependent mechanism

Nitric oxide (NO), previously demonstrated to participate in the regulation of the resting membrane potential in skeletal muscles via muscarinic receptors, also regulates non-quantal acetylcholine (ACh) secretion from rat motor nerve endings. Non-quantal ACh release was estimated by the amplitude of endplate hyperpolarization (H-effect) following a blockade of skeletal muscle post-synaptic nicotinic receptors by (+)-tubocurarine. The muscarinic agonists oxotremorine and muscarine lowered the H-effect and the M1 antagonist pirenzepine prevented this effect occurring at all. Another muscarinic agonist arecaidine but-2-ynyl ester tosylate (ABET), which is more selective for M2 receptors than for M1 receptors and 1,1-dimethyl-4-diphenylacetoxypiperidinium (DAMP), a specific antagonist of M3 cholinergic receptors had no significant effect on the H-effect. The oxotremorine-induced decrease in the H-effect was calcium and calmodulin-dependent. The decrease was negated when either NO synthase was inhibited by N(G)-nitro-L-arginine methyl ester or soluble guanylyl cyclase was inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. The target of muscle-derived NO is apparently nerve terminal guanylyl cyclase, because exogenous hemoglobin, acting as an NO scavenger, prevented the oxotremorine-induced drop in the H-effect. These results suggest that oxotremorine (and probably also non-quantal ACh) selectively inhibit the non-quantal secretion of ACh from motor nerve terminals acting on post-synaptic M1 receptors coupled to Ca(2+) channels in the sarcolemma to induce sarcoplasmic Ca(2+)-dependent synthesis and the release of NO. It seems that a substantial part of the H-effect can be physiologically regulated by this negative feedback loop, i.e., by NO from muscle fiber; there is apparently also Ca(2+)- and calmodulin-dependent regulation of ACh non-quantal release in the nerve terminal itself, as calmidazolium inhibition of the calmodulin led to a doubling of the resting H-effect.

L-citrulline inhibits [3H]acetylcholine release from rat motor nerve terminals by increasing adenosine outflow and activation of A1 receptors

BACKGROUND AND PURPOSE

Nitric oxide (NO) production and depression of neuromuscular transmission are closely related, but little is known about the role of L-citrulline, a co-product of NO biosynthesis, on neurotransmitter release.

EXPERIMENTAL APPROACH

Muscle tension recordings and outflow experiments were performed on rat phrenic nerve-hemidiaphragm preparations stimulated electrically.

KEY RESULTS

L-citrulline concentration-dependently inhibited evoked [(3)H]ACh release from motor nerve terminals and depressed nerve-evoked muscle contractions. The NO synthase (NOS) substrate, L-arginine, and the NO donor, 3-morpholinosydnonimine chloride (SIN-1), also inhibited [(3)H]ACh release with a potency order of SIN-1>L-arginine>L-citrulline. Co-application of L-citrulline and SIN-1 caused additive effects. NOS inactivation with N(omega)-nitro-L-arginine prevented L-arginine inhibition, but not that of L-citrulline. The NO scavenger, haemoglobin, abolished inhibition of [(3)H]ACh release caused by SIN-1, but not that caused by L-arginine. Inactivation of guanylyl cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) fully blocked SIN-1 inhibition, but only partially attenuated the effects of L-arginine. Reduction of extracellular adenosine accumulation with adenosine deaminase or with the nucleoside transport inhibitor, S-(p-nitrobenzyl)-6-thioinosine, attenuated the effects of L-arginine and L-citrulline, while not affecting inhibition by SIN-1. Similar results were obtained with the selective adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. L-citrulline increased the resting extracellular concentration of adenosine, without changing that of the adenine nucleotides.

CONCLUSIONS AND IMPLICATIONS

NOS catalyses the formation of two neuronally active products, NO and L-citrulline. While, NO may directly reduce transmitter release through stimulation of soluble guanylyl cyclase, the inhibitory action of L-citrulline may be indirect through increasing adenosine outflow and subsequently activating inhibitory A(1) receptors.

Different effects of ATP on the contractile activity of mice diaphragmatic and skeletal muscles

Apart from acetyl-choline (Ach), adenosine-5'-trisphosphate (ATP) is thought to play a role in neuromuscular function, however little information is available on its cellular physiology. As such, effects of ATP and adenosine on contractility of mice diaphragmatic and skeletal muscles (m. extensor digitorum longa-MEDL) have been investigated in in vitro experiments. Application of carbacholine (CCh) in vitro in different concentrations led to pronounced muscle contractions, varying from 9.15+/-4.76 to 513.13+/-15.4 mg and from 44.65+/-5.01 to 101.46+/-9.11 mg for diaphragm and MEDL, respectively. Two hundred micromolars of CCh in both muscles caused the contraction with the 65% (diaphragm) to 75% (MEDL) of maximal contraction force-this concentration was thus used in further experiments. It was found that application of ATP (100 microM) increased the force of diaphragmatic contraction caused by CCh (200 microM) from 335.2+/-51.4 mg (n=21) in controls to 426.5+/-47.8 mg (n=10; P<0.05), but decreased the contractions of MEDL of CCh from 76.6+/-6.5mg (n=26) in control to 40.2+/-9.0mg (n=8; P<0.05). Application of adenosine (100 microM) had no effect on CCh-induced contractions of these muscles. Resting membrane potential (MP) measurements using sharp electrodes were done at 10, 20 and 30 min after the application of ATP and adenosine. Diaphragm showed depolarization from 75+/-0.6 down to 63.2+/-1.05, 57.2+/-0.96 and 53.6+/-1.1 mV after 10, 20 and 30 min of exposition, respectively (20 fibers from 4 muscles each, P<0.05 in all three cases). Adenosine showed no effect on diaphragmatic MP. Both agents were ineffective in case of MEDL. The effects of ATP in both tissues were abolished by suramin (100 microM), a P2-receptor antagonist, and chelerythrin (50 microM), a specific protein-kinase C (PKC) inhibitor, but were not affected by 1H-[1,2,4]-oxadiazolo-[4,3-alpha]-quinoxalin-1-one (ODQ, 1 microM), a guanylyl-cyclase inhibitor, or by adenosine-3,5-monophosphothioate (Rp-cAMP, 1 microM), a protein-kinase A (PKA) inhibitor. Besides the action on contractile activity, ATP (100 microM) led to a significant (P<0.001) depolarization of diaphragm muscle fibers from 74.5+/-2.3 down to 64+/-2.1, 58.2+/-2.2 and 54.3+/-2.4 mV after 10, 20 and 30 min of incubation, respectively. Incubation of MEDL with the same ATP concentration showed no significant change of MP. Denervation of the muscles for 28 days led to a decrease of CCh-induced contractions of diaphragm down to 171.1+/-34.5mg (n=11, P<0.05), but increased the contractile force of MEDL up to 723.9+/-82.3mg (n=9, P<0.01). Application of ATP elevated the contractility of denervated diaphragm caused by CCh up to normal values (311.1+/-79.7 mg, n=6, P>0.05 versus control), but did not significantly affect of contractility of MEDL, which became 848.1+/-62.7 mg (n=6). These results show that the effects of ATP on both diaphragmatic and skeletal muscles are mediated through P2Y receptors coupled to chelerytrin-sensitive protein-kinase C.

Nitric oxide modulates neuromuscular transmission during hypoxia in rat diaphragm

Hypoxia impairs neuromuscular transmission in the rat diaphragm. In previous studies, we have shown that nitric oxide (NO) plays a role in force modulation of the diaphragm under hypoxic conditions. The role of NO, a neurotransmitter, on neurotransmission in skeletal muscle under hypoxic conditions is unknown. The effects of the NO synthase (NOS) inhibitor nomega-nitro-L-arginine (L-NNA, 1 mM) and the NO donor spermine NONOate (Sp-NO, 1 mM) were evaluated on neurotransmission failure during nonfatiguing and fatiguing contractions of the rat diaphragm under hypoxic (PO2 approximately 5.8 kPa) and hyperoxic conditions (PO2 approximately 64.0 kPa). Hypoxia impaired force generated by both muscle stimulation at 40 HZ (P40M) and by nerve stimulation at 40 HZ (P40N). The effect of hypoxia in the latter was more pronounced. L-NNA increased P40N whereas Sp-NO decreased P40N during hypoxia. In contrast, neither L-NNA nor Sp-NO affected P40N during hyperoxia. L-NNA only slightly reduced neurotransmission failure during fatiguing contractions under hyperoxic conditions. Consequently, neurotransmission failure assessed by comparing force loss during repetitive nerve simulation and superimposed direct muscle stimulation was more pronounced in hypoxia, which was alleviated by L-NNA and aggravated by Sp-NO. These data provide insight in the underlying mechanisms of hypoxia-induced neurotransmission failure. This is important as respiratory muscle failure may result from hypoxia in vivo.

Exogenous nitric oxide enhances the synthesis of type I collagen and heat shock protein 47 by normal human dermal fibroblasts

BACKGROUND

It is well established that the alterations of dermal matrix contributes to skin aging characterized by wrinkles. On the other hand, physiological NO is useful to maintain skin homeostasis such as a vasodilatation. However, a role of NO on production of dermal matrix has been clarified.

OBJECTIVE

In this study, we have attempted to analyze the role of NO on type I collagen synthesis of normal human dermal fibroblasts including expression of procollagen alphaI S(1) mRNA/protein and heat shock protein 47 (HSP47).

METHODS

The effects of NO which was generated by two types of NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP), on type I collagen and HSP47 and their related mRNA expression were examined with ELISA and RT-PCR.

RESULTS

NO was significantly accelerated the production of type I collagen by fibroblasts corresponding with up-regulation of procollagen alphaI (1) mRNA. Furthermore, NO increased both levels of HSP47 protein and mRNA in fibroblasts in a dose-dependent manner.

CONCLUSIONS

These results suggest that NO has dual effects on collagen synthesis by fibroblasts as follows; one is the direct stimulation of collagen synthesis due to the up-regulation of procollagen alphaI(1) mRNA, and the other is an indirect effect through the increase of HSP47 mRNA expression. This is the first report that exogenous NO stimulates HSP47 production by dermal fibroblasts.

Effect of N-acetylaspartylglutamate (NAAG) on non-quantal and spontaneous quantal release of acetylcholine at the neuromuscular synapse of rat

N-Acetylaspartylglutamate (NAAG), known to be present in rat motor neurons, may participate in neuronal modulation of non-quantal secretion of acetylcholine (ACh) from motor nerve terminals. Non-quantal release of ACh was estimated by the amplitude of the endplate membrane hyperpolarization (H-effect) caused by inhibition of nicotinic receptors by (+)-tubocurarine and acetylcholinesterase by armin (diethoxy-p-nitrophenyl phosphate). Application of exogenous NAAG decreased the H-effect in a dose-dependent manner. The reduction of the H-effect by NAAG was completely removed when N-acetyl-beta-aspartylglutamate (betaNAAG) or 2-(phosphonomethyl)-pentanedioic acid (2-PMPA) was used to inhibit glutamate carboxypeptidase II (GCP II), a presynaptic Schwann cell membrane-associated ectoenzyme that hydrolyzes NAAG to glutamate and N-acetylaspartate. Bath application of glutamate decreased the H-effect similarly to the action of NAAG but N-acetylaspartate was without effect. Inhibition of NMDA receptors by dl-2-amino-5-phosphopentanoic acid, (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine (MK801), and 7-chlorokynurenic acid or inhibition of muscle nitric oxide synthase (NO synthase) by N(G)-nitro-l-arginine methyl ester and 3-bromo-7-nitroindazole completely prevented the decrease of the H-effect by NAAG. These results suggest that glutamate, produced by enzymatic hydrolysis of bath-applied NAAG, can modulate non-quantal secretion of ACh from the presynaptic terminal of the neuromuscular synapse via activation of postsynaptic NMDA receptors and synthesis of nitric oxide (NO) in muscle fibers. NAAG also increased the frequency of miniature endplate potentials (mEPPs) generated by spontaneous quantal secretion of ACh, whereas the mean amplitude and time constants for rise time and for decay of mEPPs did not change.

Intraganglionic laminar endings and their relationships with neuronal and glial structures of myenteric ganglia in the esophagus of rat and mouse

Intraganglionic laminar endings (IGLEs) represent the only vagal mechanosensory terminals in the tunica muscularis of the esophagus and may be involved in local reflex control. We recently detected extensive though not complete colocalization of the vesicular glutamate transporter 2 (VGLUT2) with markers for IGLEs. To elucidate this colocalization mismatch, this study aimed at identifying markers for nitrergic, cholinergic, peptidergic, and adrenergic neurons and glial cells, which may colocalize with VGLUT2 outside of IGLEs. Confocal imaging revealed, besides substantial colocalization of VGLUT2 and substance P (SP), no other significant colocalizations of VGLUT2 and immunoreactivity for any of these markers within the same varicosities. However, we found close contacts of VGLUT2-positive structures to vesicular acetylcholine transporter, choline acetyltransferase, neuronal nitric oxide synthase, galanin, neuropeptide Y, and vasoactive intestinal peptide immunoreactive cell bodies and varicosities, as well as to glial cells. Neuronal perikarya were never positive for VGLUT2. Thus, VGLUT2 was almost exclusively found in IGLEs and may serve as a specific marker for them. In addition, many IGLEs also contained SP. The close contacts established by IGLEs to myenteric cell bodies, dendrites, and varicose fibers suggest that IGLEs modulate various types of enteric neurons and vice versa.

Vesicular glutamate transporter 1 immunoreactivity in motor endplates of striated esophageal but not skeletal muscles in the mouse

Glutamate, the major excitatory transmitter in the central nervous system, has been speculated for years to influence mammalian motor endplates but trials to identify glutamatergic motor terminals failed because specific markers were not available. Recently, antibodies to vesicular glutamate transporters (VGLUTs) opened new possibilities for further morphological investigations. We detected VGLUT1 immunoreactivity (-ir), but not VGLUT2-ir and VGLUT3-ir, respectively, in many motor nerve terminals in motor endplates of the mouse esophagus as identified by alpha-bungarotoxin or colocalization of VGLUT1 with choline acetyltransferase. These findings suggest that glutamate is co-stored with acetylcholine in esophageal neuromuscular junctions. Surprisingly, we found neither VGLUT1-ir nor VGLUT2-ir or VGLUT3-ir in neuromuscular junctions of somitic and branchiogenic skeletal muscles. This may reflect differences in functional properties and the embryonic origin between skeletal and esophageal striated muscle fibers.

Glutamate regulation of non-quantal release of acetylcholine in the rat neuromuscular junction

Glutamate, previously demonstrated to participate in regulation of the resting membrane potential in skeletal muscles, also regulates non-quantal acetylcholine (ACh) secretion from rat motor nerve endings. Non-quantal ACh secretion was estimated by the amplitude of endplate hyperpolarization (H-effect) following blockade of skeletal muscle post-synaptic nicotinic receptors by (+)-tubocurarine and cholinesterase by armin (diethoxy-p-nitrophenyl phosphate). Glutamate was shown to inhibit non-quantal release but not spontaneous and evoked quantal secretion of ACh. Glutamate-induced decrease of the H-effect was enhanced by glycine. Glycine alone also lowered the H-effect, probably due to potentiation of the effect of endogenous glutamate present in the synaptic cleft. Inhibition of N-methyl-d-aspartate (NMDA) receptors with (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine (MK801), dl-2-amino-5-phosphopentanoic acid (AP5) and 7-chlorokynurenic acid or the elimination of Ca2+ from the bathing solution prevented the glutamate-induced decrease of the H-effect with or without glycine. Inhibition of muscle nitric oxide synthase by NG-nitro-l-arginine methyl ester (l-NAME), soluble guanylyl cyclase by 1H[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and binding and inactivation of extracellular nitric oxide (NO) by haemoglobin removed the action of glutamate and glycine on the H-effect. The results suggest that glutamate, acting on post-synaptic NMDA receptors to induce sarcoplasmic synthesis and release of NO, selectively inhibits non-quantal secretion of ACh from motor nerve terminals. Non-quantal ACh is known to modulate the resting membrane potential of muscle membrane via control of activity of chloride transport and a decrease in secretion of non-quantal transmitter following muscle denervation triggers the early post-denervation depolarization of muscle fibres.

Early postdenervation depolarization is controlled by acetylcholine and glutamate via nitric oxide regulation of the chloride transporter

Resting non-quantal acetylcholine (ACh) and probably glutamate (Glu) release from nerve endings activates M1- and NMDA receptor-mediated Ca2+ entry into the sarcoplasm with following activation of NOS and production of NO. This is a trophic message from motoneurons, which keeps the Cl- transport inactive in the innervated sarcolemma. After denervation, the secretion of ACh and Glu at the neuromuscular junction is eliminated within 3-4 h and the production of NO in the sarcoplasm is lowered. As a result, the Cl- influx is probably activated by dephosphorylation of the Cl- transporter with subsequent elevation of intracellular Cl- concentration. The equilibrium Cl- potential becomes more positive and the muscle membrane becomes depolarized.

Effects of modulation of nitric oxide on rat diaphragm isotonic contractility during hypoxia

Nitric oxide (NO) is essential for optimal myofilament function of the rat diaphragm in vitro during active shortening. Little is known about the role of NO in muscle contraction under hypoxic conditions. Hypoxia might increase the NO synthase (NOS) activity within the rat diaphragm. We hypothesized that NO plays a protective role in isotonic contractile and fatigue properties during hypoxia in vitro. The effects of the NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA), the NO scavenger hemoglobin, and the NO donor spermine NONOate on shortening velocity, power generation, and isotonic fatigability during hypoxia were evaluated (Po(2) approximately 7 kPa). l-NMMA and hemoglobin slowed the shortening velocity, depressed power generation, and increased isotonic fatigability during hypoxia. The effects of l-NMMA were prevented by coadministration with the NOS substrate l-arginine. Spermine NONOate did not alter isotonic contractile and fatigue properties during hypoxia. These results indicate that endogenous NO is needed for optimal muscle contraction of the rat diaphragm in vitro during hypoxia.

An investigation of the effects of zaprinast, a PDE inhibitor, on the nitrergic control of the urethra in anaesthetized female rats

1. The effects of L-NAME and zaprinast were investigated (i.v.) on reflex-evoked changes in bladder and urethral pressures in urethane-anaesthetized female rats. 2. L-NAME attenuated reflex-evoked urethral relaxations (65+/-10%), while zaprinast potentiated these responses (68+/-24%). L-NAME and zaprinast also increased baseline urethral pressure and urethral striated muscle (EUS-EMG) activity. These drugs had little effect on the bladder. 3. Following pre-treatment with alpha-bungarotoxin (i.v.) to block urethral striated muscle, L-NAME and zaprinast failed to increase baseline urethral pressure. Further zaprinast failed to alter the size of reflex-evoked urethral relaxations. 4. Intra-urethral zaprinast caused a significant increase while sodium nitroprusside (SNP) and isoprenaline caused decreases in urethral pressure (+14+/-3%, -25+/-5%, -29+/-7%, respectively). These changes were associated with increases in EUS-EMG activity. After chlorisondamine (i.v.), zaprinast caused a significant fall in urethral pressure, while the decrease in urethral pressure caused by SNP and isoprenaline was potentiated. No changes in EUS-EMG activity occurred. 5. These results indicate that a nitrergic pathway mediates reflex-evoked urethral smooth muscle relaxations. The data also indicates that there is a background release of NO, which reduces sphincter skeletal muscle activity. Further, the ability of zaprinast to potentiate nitrergic evoked urethral relaxations involves an increase in striated muscle tone. This appears to be an indirect result of smooth muscle relaxation and is mediated, at least in part, by a chlorisondamine-sensitive mechanism.

Nitric oxide: biologic effects on muscle and role in muscle diseases

Nitric oxide is a ubiquitous cell-signaling molecule involved in regulation of numerous homeostatic mechanisms and in mediation of tissue injury. Nitric oxide influences contraction, blood flow, and metabolism, as well as myogenesis. Nitric oxide exerts its influence by activation of guanylate cyclase and nitrosylation of proteins, which include glyceraldehyde-3-phosphate dehydrogenase, the ryanodine receptor and actomyosin ATPase. Skeletal muscle expresses all three isoforms of the nitric oxide synthase, including a muscle-specific splice variant; expression of the isoforms is fiber-type specific and influenced by age and disease. Nitric oxide produced with certain systemic conditions and local inflammation is likely toxic to skeletal muscle, either directly or in reactions with oxygen-derived radicals. Although nitric oxide impacts on many functions in muscle, its effects are subtle, and much work remains to be done to determine its importance in the pathogenesis of muscle diseases.

ATP but not adenosine inhibits nonquantal acetylcholine release at the mouse neuromuscular junction

The postsynaptic membrane of the neuromuscular synapse treated with antiacetylcholinesterase is depolarized due to nonquantal release of acetylcholine (ACh) from the motor nerve ending. This can be demonstrated by the hyperpolarization produced by the application of curare (H-effect). ATP (1 x 10-5 M) decreased the magnitude of the H-effect from 5 to 1.5 mV. The membrane input resistance and the ACh sensitivity were unchanged, and so changes in these cannot explain the ATP effect. Adenosine alone was without effect on the nonquantal release. On the other hand, both ATP and adenosine depressed the frequency of spontaneous miniature endplate potentials, to 56% and 43% respectively. The protein kinase A inhibitor Rp-cAMP or the guanylyl cyclase inhibitor 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one did not affect the inhibitory influence of ATP on the H-effect, whereas staurosporine, an inhibitor of protein kinase C, completely abolished the action of ATP. Suramin, an ATP antagonist, enhanced the H-effect to 8.6 mV and, like staurosporine, prevented the inhibitory effect of ATP. ATP thus suppresses the nonquantal release via a direct action on presynaptic metabotropic P2 receptors coupled to protein kinase C, whilst adenosine exerts its action mainly by affecting the mechanisms underlying quantal release. These data, together with earlier evidence, show that nonquantal release of ACh can be modulated by several distinct regulatory pathways, in particular by endogenous substances which may or may not be present in the synaptic cleft at rest or during activity.

The role of desensitisation in decay time of miniature endplate currents in frogs Rana ridibunda and Rana temporaria

A new comparative characteristic of endplate microphysiology has been introduced. It is the feasibility of receptors to become desensitised as demonstrated on two frog species, Rana temporaria and Rana ridibunda: the decay times (tau(MEPC)) of single quantum miniature endplate currents (MEPCs) in the sartorius muscles of both species were about 1 ms and were not affected by the desensitisation-promoting agent proadifen when AChE was active. However, when the desensitisation was induced by anticholinesterase neostigmine and promoted by proadifen, the prolongation of tau(MEPC) from 1 ms was almost twice as great in Rana temporaria (tau(MEPC) = 4.4 ms) than in Rana ridibunda (tau(MEPC) = 3.1). This indicates that desensitisation reduces the number of available receptors and lowers the number of available ACh molecules for repetitive binding by trapping them by desensitised, high-affinity receptors significantly more in Rana ridibunda than in Rana temporaria. The application of proadifen, a promoter of desensitisation, decreased the prolongation of MEPCs in both species, but this shortening was more rapid in Rana ridibunda than in Rana temporaria. It is concluded that the desensitisation-induced reduction in the density, and the number of postsynaptic receptors is significantly higher at Rana ridibunda than in Rana temporaria endplates.