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Inhibition of mouse neuromuscular transmission and contractile function by okadaic acid and cantharidin
Author(s) -
Hong S J
Publication year - 2000
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1038/sj.bjp.0703418
Subject(s) - okadaic acid , cantharidin , cardiotoxin , neuromuscular transmission , neuromuscular junction , tetrodotoxin , chemistry , muscle contraction , neurotransmission , endocrinology , medicine , biochemistry , skeletal muscle , biophysics , biology , phosphatase , phosphorylation , neuroscience , receptor , organic chemistry
Phosphorylations of cellular proteins modulate biological activities. The effects of okadaic acid (0.1–10 μ M ) and cantharidin (1–100 μ M ), inhibitors of protein phosphatases, on the synaptic transmission at the mouse neuromuscular junction were explored. Both inhibitors almost completely depressed twitch forces elicited by electrical stimulation of diaphragm muscles (the IC 50 s for okadaic acid and cantharidin were 1.1±0.2 and 13±1 μ M , n =5, respectively) and suppressed contractures evoked by high K + and ryanodine more than 70%. Contractures caused by cardiotoxin, which destroys the integrity of sarcolemma, were not depressed. Both okadaic acid (10 μ M ) and cantharidin (100 μ M ) depolarized muscle membranes from ∼−80 to ∼−60 mV in a partially reversible and tetrodotoxin‐sensitive manner. The initial short‐term enhancement of twitch responses (up to ∼40%) was correlated with the inhibitors‐induced repetitive firings of muscle action potential. Treatment with either agent resulted in nearly complete inhibitions of endplate potential (epp). The IC 50 s were 0.8±0.2 and 9±2 μ M ( n =5), respectively, for okadaic acid and cantharidin. On high frequency stimulation, the coefficient of epps was increased more than 10 fold and the extent of epp run‐down during stimulations intensified from ∼25 to ∼75%. Analyses of presynaptic quantal releases revealed decreases in epp quantal content and the immediately available vesicle pool. The frequency of miniature epp was initially elevated up to 2 fold then suppressed down to ∼30%. The small reduction in the amplitude was antagonized when the membrane of endplate area was repolarized. The data suggest that okadaic acid and cantharidin inhibit mobilizations of synaptic vesicles and depress Ca 2+ release from sarcoplasmic reticulum and that protein phosphatases participate in the modulation of motor function.British Journal of Pharmacology (2000) 130 , 1211–1218; doi: 10.1038/sj.bjp.0703418