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

• Published in: Pflügers Archiv 2001-06, Vol.442 (3), p.376-382
• Main Authors: Smaili, S S, Carvalho, S M, Cavalcanti, P M, Jurkiewicz, N H, García, A G, Jurkiewicz, A
• Format: Article
• Language: English
• Subjects: Animals; Biological Transport - drug effects; Caffeine - pharmacology; Calcium - metabolism; Calcium - pharmacology; Carbachol - pharmacology; Chelating Agents - pharmacology; Cholinergic Agonists - pharmacology; Egtazic Acid - pharmacology; Enzyme Inhibitors - pharmacology; Female; Gastric Fundus - physiology; Muscle, Smooth - physiology; Nickel - pharmacology; Peristalsis - drug effects; Phosphodiesterase Inhibitors - pharmacology; Rats; Rats, Inbred WF; Receptors, Muscarinic - metabolism; Ryanodine - pharmacology; Thapsigargin - pharmacology
• ISSN: 0031-6768; 1432-2013
• DOI: 10.1007/s004240100535

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 (t1/2=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 microM) or thapsigargin (1 microM), 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 microM 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 microM), but not nanomolar concentrations (5.9 nM) of Ca2+ are maintained in the extracellular milieu.