On the mechanism of ouabain-induced release of acetylcholine from synaptosomes.
Ouabain (5 x 10(-8)-5 x 10(-4) M) was confirmed to cause a dose-dependent increase in [3H]acetylcholine ([3H]ACh) release, cytosolic free Ca2+ concentration ([Ca2+]i), and 22Na+ uptake in cerebrocortical synaptosomes of rats in the presence of extracellular Ca2+. Ouabain also caused a dose-dependent decrease in membrane potential. In a low-Na+ (10 mM) medium, ouabain failed to increase [3H]ACh release and [Ca2+]i. Tetrodotoxin (10(-6) M) had no effect on the ouabain-induced increase in both [3H]ACh release and [Ca2+]i but abolished the increase in 22Na+ uptake and partially inhibited the depolarizing effect. Verapamil (10(-6)-5 x 10(-4) M) inhibited the ouabain-induced increase in both [3H]ACh release and [Ca2+]i in a dose-dependent manner. Removal of extracellular Ca2+ abolished the effect of ouabain on [Ca2+]i but not on [3H]ACh release and 22Na+ uptake, regardless of the presence or absence of EGTA. In the absence of extracellular Ca2+, 10 mM Mg2+ blocked ouabain-induced [3H]ACh release, which was resistant to verapamil. These results suggest that ouabain can increase ACh release from synaptosomes without the preceding increases in intracellular Ca2+ and/or Na+ content. It seems likely that the removal of extracellular Ca2+ unmasks mechanisms of ouabain action different from those operating in the presence of Ca2+.
Satoh
E
E
Department of Veterinary Pharmacology, Obihiro University of Agriculture and Veterinary Medicine, Japan.
Nakazato
Y
Y
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D008274:Magnesium / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009994:Osmolar Concentration; D010042:Ouabain / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000493:pharmacokinetics; D013574:Synaptosomes / Q000378:metabolism* / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D013779:Tetrodotoxin; D010042:Ouabain; D012964:Sodium; D014700:Verapamil; D008274:Magnesium; D000109:Acetylcholine; D002118:Calcium