Muscarinic Ca2+ responses resistant to muscarinic antagonists at perisynaptic Schwann cells of the frog neuromuscular junction.
1. Acetylcholine causes a rise of intracellular Ca2+ in perisynaptic Schwann cells (PSCs) of the frog neuromuscular junction. The signalling pathway was characterized using the fluorescent Ca2+ indicator fluo-3 and fluorescence microscopy. 2. Nicotinic antagonists had no effect on Ca2+ responses evoked by ACh and no Ca2+ responses were evoked with the nicotinic agonist nicotine. The muscarinic agonists muscarine and oxotremorine-M induced Ca2+ signals in PSCs. 3. Ca2+ responses remained unchanged when extracellular Ca2+ was removed, indicating that they are due to the release of Ca2+ from internal stores. Incubation with pertussis toxin did not alter the Ca2+ signals induced by muscarine, but did block depression of transmitter release induced by adenosine and prevented Ca2+ responses in PSCs induced by adenosine. 4. The general muscarinic antagonists atropine, quinuclidinyl benzilate and N-methyl-scopolamine failed to block Ca2+ responses to muscarinic agonists. Atropine (at 20,000-fold excess concentration) also failed to reduce the proportion of cells responding to a threshold muscarine concentration sufficient to cause responses in less than 50% of cells. Only the allosteric, non-specific blocker, gallamine (1-10 microM) was effective in blocking muscarine-induced Ca2+ responses. 5. In preparations denervated 7 days prior to experiments, low concentrations of atropine reversibly and completely blocked Ca2+ responses to muscarine. 6. The lack of blockade by general muscarinic antagonists in innervated, in situ preparations suggests that muscarinic Ca2+ responses at PSCs are not mediated by any of the five known muscarinic receptors or that post-translational modification prevented antagonist binding.
Robitaille
R
R
Department of Physiology, University of Toronto, Ontario, Canada. robitair@ere.umontreal.ca
Jahromi
B S
BS
Charlton
M P
MP
D000109:Acetylcholine / Q000494:pharmacology*; D000241:Adenosine / Q000494:pharmacology; D000814:Aniline Compounds / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005456:Fluorescent Dyes / Q000378:metabolism; D005703:Gallamine Triethiodide / Q000494:pharmacology; D008856:Microscopy, Fluorescence; D009116:Muscarine / Q000494:pharmacology; D018721:Muscarinic Agonists / Q000494:pharmacology; D018727:Muscarinic Antagonists / Q000494:pharmacology*; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism*; D009538:Nicotine / Q000494:pharmacology; D018722:Nicotinic Agonists / Q000494:pharmacology; D018733:Nicotinic Antagonists / Q000494:pharmacology; D037342:Pertussis Toxin; D011894:Rana pipiens; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012583:Schwann Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D014966:Xanthenes / Q000378:metabolism
D000814:Aniline Compounds; D005456:Fluorescent Dyes; D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D018722:Nicotinic Agonists; D018733:Nicotinic Antagonists; D011976:Receptors, Muscarinic; D010566:Virulence Factors, Bordetella; D014966:Xanthenes; C059715:Fluo-3; D009538:Nicotine; D009116:Muscarine; D037342:Pertussis Toxin; D000241:Adenosine; D000109:Acetylcholine; D005703:Gallamine Triethiodide; D002118:Calcium