Ca2+-induced Ca2+ release from the endoplasmic reticulum amplifies the Ca2+ signal mediated by activation of voltage-gated L-type Ca2+ channels in pancreatic beta-cells.
Stimulus-secretion coupling in pancreatic beta-cells involves membrane depolarization and Ca(2+) entry through voltage-gated L-type Ca(2+) channels, which is one determinant of increases in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). We investigated how the endoplasmic reticulum (ER)-associated Ca(2+) apparatus further modifies this Ca(2+) signal. When fura-2-loaded mouse beta-cells were depolarized by KCl in the presence of 3 mm glucose, [Ca(2+)](i) increased to a peak in two phases. The second phase of the [Ca(2+)](i) increase was abolished when ER Ca(2+) stores were depleted by thapsigargin. The steady-state [Ca(2+)](i) measured at 300 s of depolarization was higher in control cells compared with cells in which the ER Ca(2+) pools were depleted. The amount of Ca(2+) presented to the cytoplasm during depolarization as estimated from the integral of the increment in [Ca(2+)](i) over time (integralDelta[Ca(2+)](i).dt) was approximately 30% higher compared with that in the Ca(2+) pool-depleted cells. neo-thapsigargin, an inactive analog, did not affect [Ca(2+)](i) response. Using Sr(2+) in the extracellular medium and exploiting the differences in the fluorescence properties of Ca(2+)- and Sr(2+)-bound fluo-3, we found that the incoming Sr(2+) triggered Ca(2+) release from the ER. Depolarization-induced [Ca(2+)](i) response was not altered by, an inhibitor of phosphatidylinositol-specific phospholipase C, suggesting that stimulation of the enzyme by Ca(2+) is not essential for amplification of Ca(2+) signaling. [Ca(2+)](i) response was enhanced when cells were depolarized in the presence of 3 mm glucose, forskolin, and caffeine, suggesting involvement of ryanodine receptors in the amplification process. Pretreatment with ryanodine (100 microm) diminished the second phase of the depolarization-induced increase in [Ca(2+)](i). We conclude that Ca(2+) entry through L-type voltage-gated Ca(2+) channels triggers Ca(2+) release from the ER and that such a process amplifies depolarization-induced Ca(2+) signaling in beta-cells.
Lemmens
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Rolf Luft Center for Diabetes Research, Department of Molecular Medicine, Endocrine and Diabetes Unit, Karolinska Institutet, Karolinska Hospital, S-171 76 Stockholm, Sweden.
Larsson
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Berggren
P O
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Islam
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MS
D000818:Animals; D002110:Caffeine / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism* / Q000502:physiology*; D005576:Colforsin / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004594:Electrophysiology; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D004963:Estrenes / Q000494:pharmacology; D016257:Fura-2 / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D008820:Mice, Obese; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011760:Pyrrolidinones / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D015398:Signal Transduction; D013050:Spectrometry, Fluorescence; D013324:Strontium / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology; D013997:Time Factors; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors
D015220:Calcium Channels; D004963:Estrenes; D010726:Phosphodiesterase Inhibitors; D011760:Pyrrolidinones; D019837:Ryanodine Receptor Calcium Release Channel; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; D005576:Colforsin; D002110:Caffeine; D011189:Potassium Chloride; D019284:Thapsigargin; D010738:Type C Phospholipases; D005947:Glucose; D002118:Calcium; D016257:Fura-2; D013324:Strontium