A novel Ca2+-induced Ca2+ release mechanism mediated by neither inositol trisphosphate nor ryanodine receptors.
Members of both major families of intracellular Ca(2+) channels, ryanodine and inositol 1,4,5-trisphosphate (IP3) receptors, are stimulated by substantial increases in cytosolic free Ca(2+) concentration ([Ca(2+)]c). They thereby mediate Ca(2+)-induced Ca(2+) release (CICR), which allows amplification and regenerative propagation of intracellular Ca(2+) signals. In permeabilized hepatocytes, increasing [Ca(2+)]c to 10 microM stimulated release of 30+/-1% of the intracellular stores within 60 s; the EC(50) occurred with a free [Ca(2+)] of 170+/-29 nM. This CICR was abolished at 2 degrees C. The same fraction of the stores was released by CICR before and after depletion of the IP3-sensitive stores, and CICR was not blocked by antagonists of IP3 receptors. Ryanodine, Ruthenium Red and tetracaine affected neither the Ca(2+) content of the stores nor the CICR response. Sr(2+) and Ba(2+) (EC(50)=166 nM and 28 microM respectively) mimicked the effects of increased [Ca(2+)] on the intracellular stores, but Ni(2+) blocked the passive leak of Ca(2+) without blocking CICR. In rapid superfusion experiments, maximal concentrations of IP3 or Ca(2+) stimulated Ca(2+) release within 80 ms. The response to IP3 was complete within 2 s, but CICR continued for tens of seconds despite a slow [half-time (t(1/2))=3.54+/-0.07 s] partial inactivation. CICR reversed rapidly (t(1/2)=529+/-17 ms) and completely when the [Ca(2+)] was reduced. We conclude that hepatocytes express a novel temperature-sensitive, ATP-independent CICR mechanism that is reversibly activated by modest increases in [Ca(2+)], and does not require IP3 or ryanodine receptors or reversal of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase. This mechanism may both regulate the Ca(2+) content of the intracellular stores of unstimulated cells and allow even small intracellular Ca(2+) signals to be amplified by CICR.
Wissing
Frank
F
Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QJ, UK.
Nerou
Edmund P
EP
Taylor
Colin W
CW
D000255:Adenosine Triphosphate / Q000378:metabolism; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D022781:Hepatocytes / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D007700:Kinetics; D009532:Nickel / Q000494:pharmacology; D011485:Protein Binding; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D013324:Strontium / Q000494:pharmacology; D013696:Temperature; D013997:Time Factors
D019837:Ryanodine Receptor Calcium Release Channel; D001464:Barium; D009532:Nickel; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D002118:Calcium; D013324:Strontium