Intracellular calcium mobilization by muscarinic receptors is regulated by micromolar concentrations of external Ca2+
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2001-06-01
Autores
Smaili, Soraya Soubhi 
Carvalho, Solange Maria Torchia
Cavalcanti, Paulo MS
Jurkiewicz, Neide H.
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Carbachol-induced contractions of rat stomach fundus strips, obtained in a nutrient solution containing 1.8 mM Ca2+, were resistant to Ca2+ withdrawal, even after 1 h of bathing the tissues in a nominal 0 Ca2+ solution. This was not observed when K+ was used to evoke contractions, which were rapidly inhibited after Ca2+ removal (t(1/)2=2 min). the effect of carbachol in 0 Ca2+ solution was reduced by using drugs that reduce intracellular pools of Ca2+, such as caffeine (1-3 mM), ryanodine (30 muM) or thapsigargin (1 muM), corroborating the involvement of intracellular Ca2+ stores. On the other hand, when the 0 Ca2+ solution contained EGTA, a complete decline of carbachol effects was observed within about 8 min, indicating the involvement of extracellular Ca2+. Atomic absorption spectrometry showed that our 0 Ca2+ solution still contained 45 muM Ca2+, which was drastically reduced to 5.9 nM in the presence of EGTA. Taken together, our results indicate that the effects of carbachol are due to the mobilization of caffeine-, ryanodine- and thapsigargin-sensitive intracellular Ca2+ stores, and that these stores are not inactivated or depleted if micromolar concentrations (45 muM), but not nanomolar concentrations (5.9 nM) of Ca2+ are maintained in the extracellular milieu.
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Pflugers Archiv-european Journal of Physiology. New York: Springer-verlag, v. 442, n. 3, p. 376-382, 2001.