Intracellular calcium mobilization by muscarinic receptors is regulated by micromolar 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 (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.
Smaili
S S
SS
Department of Pharmacology, Universidade Federal de São Paulo-Escola Paulista de Medicina, Brazil. ssmaili.farm@infar.epm.br
Carvalho
S M
SM
Cavalcanti
P M
PM
Jurkiewicz
N H
NH
García
A G
AG
Jurkiewicz
A
A
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005748:Gastric Fundus / Q000502:physiology*; D009130:Muscle, Smooth / Q000502:physiology; D009532:Nickel / Q000494:pharmacology; D010528:Peristalsis / Q000187:drug effects; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D051381:Rats; D011920:Rats, Inbred WF; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012433:Ryanodine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D002614:Chelating Agents; D018679:Cholinergic Agonists; D004791:Enzyme Inhibitors; D010726:Phosphodiesterase Inhibitors; D011976:Receptors, Muscarinic; D012433:Ryanodine; D002110:Caffeine; D004533:Egtazic Acid; D019284:Thapsigargin; D009532:Nickel; D002217:Carbachol; D002118:Calcium