ABSTRACT
The inflammatory response in acute gouty arthritis is in large part a result of the interaction between neutrophils and monosodium urate (MSU) crystals. The tyrosine kinase Syk, which has been largely associated with the phagocytic response by Fc receptors and with spreading mediated by integrins, has been identified as one of the major proteins tyrosine-phosphorylated in human neutrophils upon stimulation by MSU crystals and is known to be mediated in part by the Fc receptor, CD16. This has led to the present examination of the implication of Syk in the activation pathways used by MSU crystals. The tyrosine-phosphorylation patterns induced by MSU crystals and by the ligation of CD16 were inhibited by piceatannol, which, conversely, only slightly delayed but did not diminish the peak of tyrosine phosphorylation induced by cross-linking CD32 or by the addition of fMet-Leu-Phe. Moreover, piceatannol inhibited the activity of Syk as monitored by in vitro kinase assays, by its in situ tyrosine phosphorylation, and by its activity toward exogenous substrates after stimulation by MSU crystals. We also measured the impact of piceatannol on the mobilization of calcium, the production of superoxide anions, and the activity of PLD stimulated by MSU crystals. We noted a distinct inhibition of all these responses by piceatannol. Finally, the morphological changes observed in neutrophils as characteristic of MSU crystal internalization were diminished significantly by piceatannol. The results obtained show that Syk plays a critical and central role in the signal-transduction pathways called upon by MSU crystals subsequent to their interaction with human neutrophils.
Subject(s)
Enzyme Precursors/physiology , Neutrophil Activation/drug effects , Neutrophils/enzymology , Protein-Tyrosine Kinases/physiology , Uric Acid/pharmacology , Adult , Antibodies/immunology , Cells, Cultured , Crystallization , Enzyme Inhibitors/pharmacology , Enzyme Precursors/antagonists & inhibitors , Gout/enzymology , Humans , Intracellular Signaling Peptides and Proteins , N-Formylmethionine Leucyl-Phenylalanine/pharmacology , Neutrophils/cytology , Neutrophils/drug effects , Phospholipase D/metabolism , Phosphorylation/drug effects , Phosphotyrosine/metabolism , Protein-Tyrosine Kinases/antagonists & inhibitors , Receptors, IgG/immunology , Stilbenes/pharmacology , Superoxides/metabolism , Syk Kinase , Uric Acid/administration & dosageABSTRACT
Rat motor nerve terminals and the endplates they interact with exhibit changes to varying patterns of use, as when exposed to increased activation in the form of endurance exercise training. The extent to which these changes affect neuromuscular transmission efficacy is uncertain. In this study, the effects of habitual exercise on the electrophysiological properties of neuromuscular transmission in rat soleus muscle were investigated using a novel in situ approach. Consistent with previous reports, miniature endplate potential frequency was enhanced by habitual exercise. Other passive properties, such as resting membrane potential, miniature endplate potential amplitude, and "giant" miniature endplate potential characteristics were unaltered by the training program. Full-size endplate potentials were obtained by blocking soleus muscle action potentials with mu-conotoxin GIIIb. Quantal content values were 91.5 and 119.9 for control and active groups, respectively (P < 0.01). We also measured the rate and extent of endplate potential amplitude rundown during 3-s trains of continuous stimulation at 25, 50, and 75 Hz; at 50 and 75 Hz, we found both the rate and extent of rundown to be significantly attenuated (10--20%) in a specific population of cells from active rats (P < 0.05). The results establish the degree of activity-dependent plasticity as it pertains to neuromuscular transmission in a mammalian slow-twitch muscle.
Subject(s)
Muscle, Skeletal/physiology , Neuromuscular Junction/physiology , Physical Conditioning, Animal/physiology , Synaptic Transmission/physiology , Action Potentials/drug effects , Animals , Conotoxins/pharmacology , Electric Stimulation , Female , Hindlimb , Membrane Potentials/physiology , Motor Endplate/physiology , Muscle, Skeletal/innervation , Physical Exertion/physiology , Rats , Rats, Sprague-Dawley , Sodium Channels/drug effectsABSTRACT
The aim of the study was to test the hypothesis that a 16 week endurance training program would alter the abundance of endplate-associated nicotinic acetylcholine receptors (nAChR) in various rat skeletal muscles. We found a 20% increase in endplate-specific [125I]alpha-bungarotoxin binding in several muscles of trained rats, accompanied by equal susceptibility of toxin binding to the inhibitory effect of D-tubocurarine in sedentary and trained muscles. We conclude that the neuromuscular junction adaptations that occur with increased chronic activation include an increase in nAChR number. Results of experiments designed to determine nAChR turnover also suggest that this effect is mediated by an alteration in the receptor's metabolic state. The potential implications and mechanisms of this adaptation are discussed.
