Activation of calcium oscillations by thapsigargin in parotid acinar cells.
The tumor promoter thapsigargin releases Ca2+ from intracellular stores by specific inhibition of microsomal Ca-ATPase activity without inositol phosphate formation. Recent studies of the actions of thapsigargin support the concept that the level of Ca2+ within the inositol (1,4,5)-trisphosphate (IP3)-sensitive intracellular pool regulates the Ca2+ permeability of the plasma membrane. We examined the effects of thapsigargin on intracellular Ca2+ concentration ([Ca2+]i) in single rat parotid cells using digital fluorescence microscopy. In the absence of extracellular Ca2+ (Ca2+o), thapsigargin transiently increased [Ca2+]i. Following the thapsigargin-induced [Ca2+]i transient, carbachol in the continued absence of Ca2+o was unable to raise [Ca2+]i, indicating that thapsigargin mobilizes Ca2+ from the IP3-sensitive store. In the converse experiment, carbachol prevented a rise of [Ca2+]i by thapsigargin, suggesting that the IP3- and thapsigargin-sensitive Ca2+ pools are the same. Depletion of Ca2+ from the IP3-sensitive pool by thapsigargin enhanced plasma membrane Ca2+ permeability. Thapsigargin triggered sustained Ca2+ oscillations in Ca2(+)-containing medium which are highly reminiscent of agonist-induced oscillations in these cells. Carbachol addition rapidly raised IP3 levels during oscillations triggered by thapsigargin but did not elevate [Ca2+]i, indicating that the IP3-sensitive pool remains continuously depleted during [Ca2+]i fluctuations. The results from this study rule out the involvement of the IP3-sensitive pool in the mechanisms involved in thapsigargin-induced (and by analogy, agonist-induced) oscillations in parotid cells.
Foskett
J K
JK
Division of Cell Biology, Hospital for Sick Children, Toronto, Ontario, Canada.
Roifman
C M
CM
Wong
D
D
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D002273:Carcinogens; D016257:Fura-2; D007295:Inositol Phosphates / Q000032:analysis; D008297:Male; D010306:Parotid Gland / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D010936:Plant Extracts / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013050:Spectrometry, Fluorescence; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D015220:Calcium Channels; D002273:Carcinogens; D007295:Inositol Phosphates; D010936:Plant Extracts; D013729:Terpenes; D019284:Thapsigargin; D002217:Carbachol; D002118:Calcium; D016257:Fura-2