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 mu M) than at high (greater than or equal to 10 mu M) 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 mu M ACh but inhibited by 100 mu M 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 Ca-45(2+) 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.