Intracellular calcium mobilization by muscarinic receptors is regulated bymicromolar concentrations of external Ca2+

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 carba chol effects was observed within about 8 min, indicating the involvement of extracellular Ca2+. Atomic absorption spectrometry showed that our 0 Ca2solution 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 e ffects of carbachol are due to the mobilization of caffeine-, ryanodine- an d thapsigargin-sensitive intracellular Ca2+ stores, and that these stores a re not inactivated or depleted if micromolar concentrations (45 muM), but n ot nanomolar concentrations (5.9 nM) of Ca2+ are maintained in the extracel lular milieu.