Muscarinic stimulation exerts both stimulatory and inhibitory effects on the concentration of cytoplasmic Ca2+ in the electrically excitable pancreatic B-cell.
Mouse pancreatic islets were used to investigate how muscarinic stimulation influences the cytoplasmic Ca2+ concentration ([Ca2+]i) in insulin-secreting B-cells. In the absence of extracellular Ca2+, acetylcholine (ACh) triggered a transient, concentration-dependent and thapsigargin-inhibited increase in [Ca2+]i. In the presence of extracellular Ca2+ and 15 mM glucose, ACh induced a biphasic rise in [Ca2+]i. The initial, transient phase increased with the concentration of ACh, whereas the second, sustained, phase was higher at low (0.1-1 microM) than at high (> or = 10 microM) concentrations of ACh. Thapsigargin attenuated (did not suppress) the first phase of the [Ca2+]i rise and did not affect the sustained response. This sustained rise was inhibited by omission of extracellular Na+ (which prevents the depolarizing action of ACh) and by D600 or diazoxide (which prevent activation of voltage-dependent Ca2+ channels). During steady-state stimulation, the Ca2+ action potentials in B-cells were stimulated by 1 microM ACh but inhibited by 100 microM ACh. When B-cells were depolarized by 45 mM K+, ACh induced a concentration-dependent, biphasic change in [Ca2+]i, consisting of a first peak rapidly followed by a decrease. Thapsigargin suppressed the peak without affecting the drop in [Ca2+]i. Measurements of 45Ca2+ efflux under similar conditions indicated that ACh decreases Ca2+ influx and slightly increases the efflux. All effects of ACh were blocked by atropine. In conclusion, three mechanisms at least are involved in the biphasic change in [Ca2+]i that muscarinic stimulation exerts in excitable pancreatic B-cells. A mobilization of Ca2+ from the endoplasmic reticulum contributes significantly to the first peak, but little to the steady-state rise in [Ca2+]i. This second phase results from an influx of Ca2+ through voltage-dependent Ca2+ channels activated by a Na(+)-dependent depolarization. However, when high concentrations of ACh are used, Ca2+ influx is attenuated.
Gilon
P
P
Unité d'Endocrinologie et Métabolisme, University of Louvain Faculty of Medicine, Brussels, Belgium.
Nenquin
M
M
Henquin
J C
JC
D000109:Acetylcholine / Q000494:pharmacology; D000200:Action Potentials; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism*; D003981:Diazoxide / Q000494:pharmacology; D004558:Electric Stimulation; D005260:Female; D005711:Gallopamil / Q000494:pharmacology; D007515:Islets of Langerhans / Q000502:physiology*; D051379:Mice; D011188:Potassium / Q000494:pharmacology; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D002121:Calcium Channel Blockers; D011976:Receptors, Muscarinic; D013729:Terpenes; D005711:Gallopamil; D019284:Thapsigargin; D001285:Atropine; D012964:Sodium; D000109:Acetylcholine; D003981:Diazoxide; D011188:Potassium; D002118:Calcium