Inositol 1,4,5-trisphosphate and the endoplasmic reticulum Ca2+ cycle of a rat insulinoma cell line.
Regulation of endoplasmic reticulum (ER) Ca2+ cycling by inositol 1,4,5-trisphosphate (IP3) was studied in saponin-permeabilized RINm5F insulinoma cells. Cells were incubated with mitochondrial inhibitors, and medium Ca2+ concentration established by nonmitochondrial pool(s) (presumably the ER) was monitored with a Ca2+ electrode. IP3 degradation accounted for the transience of the Ca2+ response induced by pulse additions of the molecule. To compensate for degradation, IP3 was infused into the medium. This resulted in elevation of [Ca2+] from about 0.2 microM to a new steady state between 0.3 and 1.0 microM, depending on both the rate of IP3 infusion and the ER Ca2+ content. The elevated steady state represented a bidirectional buffering of [Ca2+] by the ER, as slight displacements in [Ca2+], by small aliquots of Ca2+ or the Ca2+ chelator quin 2, resulted in net uptake or efflux of Ca2+ to restore the previous steady state. When IP3 infusion was stopped, [Ca2+] returned to its original low level. Ninety per cent of the Ca2+ accumulated by the ER was released by IP3 when the total Ca2+ content did not exceed 15 nmol/mg of cell protein. Above this high Ca2+ content, Ca2+ was accumulated in an IP3-insensitive, A23187-releasable pool. The maximal amount of Ca2+ that could be released from the ER by IP3 was 13 nmol/mg of cell protein. The data support the concept that in the physiological range of Ca2+ contents, almost all the ER is an IP3-sensitive Ca2+ store that is capable of finely regulating [Ca2+] through independent influx (Ca2+-ATPase) and efflux (IP3-modulated component) pathways of Ca2+ transport. IP3 may continuously modulate Ca2+ cycling across the ER and play an important role in determining the ER Ca2+ content and in regulating cytosolic Ca2+ under both stimulated and possibly basal conditions.
Prentki
M
M
Corkey
B E
BE
Matschinsky
F M
FM
D007516:Adenoma, Islet Cell / Q000378:metabolism*; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D002463:Cell Membrane Permeability; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology*; D007340:Insulinoma / Q000378:metabolism*; D007700:Kinetics; D008928:Mitochondria / Q000378:metabolism; D010190:Pancreatic Neoplasms / Q000378:metabolism*; D051381:Rats; D013347:Subcellular Fractions / Q000378:metabolism; D013403:Sugar Phosphates / Q000494:pharmacology*
D007295:Inositol Phosphates; D013403:Sugar Phosphates; D000001:Calcimycin; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium