Characterization of the inositol 1,4,5-trisphosphate-induced Ca2+ release in pancreatic beta-cells.
Pancreatic beta-cells isolated from obese-hyperglycaemic mice released intracellular Ca2+ in response to carbamoylcholine, an effect dependent on the presence of glucose. The effective Ca2+ concentration reached was sufficient to evoke a transient release of insulin. When the cells were deficient in Ca2+, the Ca2+ pool sensitive to carbamoylcholine stimulation was equivalent to that released by ionomycin. Unlike intact cells, cells permeabilized by high-voltage discharges failed to generate either inositol 1,4,5-triphosphate (InsP3) or to release Ca2+ after exposure to carbamoylcholine. However, the permeabilized cells released insulin sigmoidally in response to increasing concentrations of Ca2+. Also in the absence of functional mitochondria these cells exhibited a large ATP-dependent buffering of Ca2+, enabling the maintenance of an ambient Ca2+ concentration corresponding to about 150 nM even after several additional pulses of Ca2+. InsP3, maximally effective at 6 microM, promoted a rapid and pronounced release of Ca2+. The InsP3-sensitive Ca2+ pool was rapidly filled and lost its Ca2+ late after ATP depletion. The transient nature of the Ca2+ signal was not overcome by repetitive additions of InsP3. It was possible to restore the response to InsP3 after a delay of approx. 20 min, an effect which had less latency after the addition of Ca2+. These latter findings argue against degradation and/or desensitization as factors responsible for the transiency in InsP3 response. It is suggested that Ca2+ released by InsP3 is taken up by a part of the endoplasmic reticulum (ER) not sensitive to InsP3. On metabolism of InsP3, Ca2+ recycles to the InsP3-sensitive pool, implying that this pool indeed has a very high affinity for the ion. The presence of functional mitochondria did not interfere with the recycling process. The ER in pancreatic beta-cells is of major importance in buffering Ca2+, but InsP3 only modulates Ca2+ transport for a restricted period of time following immediately upon its formation. Thereafter the non-sensitive part of the ER takes over the continuous regulation of Ca2+ cycling.
Nilsson
T
T
Department of Medical Cell Biology, Biomedicum, Uppsala, Sweden.
Arkhammar
P
P
Hallberg
A
A
Hellman
B
B
Berggren
P O
PO
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000493:pharmacokinetics; D002463:Cell Membrane Permeability / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004987:Ethers / Q000493:pharmacokinetics; D005947:Glucose / Q000493:pharmacokinetics; D006593:Hexokinase / Q000493:pharmacokinetics; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000493:pharmacokinetics*; D007328:Insulin / Q000378:metabolism; D015759:Ionomycin; D007476:Ionophores / Q000493:pharmacokinetics; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D008820:Mice, Obese; D013403:Sugar Phosphates / Q000493:pharmacokinetics*
D004987:Ethers; D007295:Inositol Phosphates; D007328:Insulin; D007476:Ionophores; D013403:Sugar Phosphates; D015759:Ionomycin; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D006593:Hexokinase; D005947:Glucose; D002118:Calcium