Feedback inhibition of Ca2+ release by Ca2+ is the underlying mechanism of agonist-evoked intracellular Ca2+ oscillations in pancreatic acinar cells.
Oscillations of free intracellular Ca2+ concentration ([Ca2+]i) are known to occur in many cell types during physiological cell signaling. To identify the basis for the oscillations, we measured both [Ca2+]i and extracellular Ca2+ concentration ([Ca2+]o) to follow the fate of Ca2+ during stimulation of [Ca2+]i oscillations in pancreatic acinar cells. [Ca2+]i oscillations were initiated by either t-butyloxycarbonyl-Tyr(SO3)-Nle-Gly-Tyr-Nle-Asp-2-phenylethyl ester (CCK-J), which mobilized Ca2+ from the inositol 1,4,5-trisphosphate (IP3)-insensitive pool, or low concentration of cholecystokinin octapeptide (CCK-OP), which mobilized Ca2+ from the IP3-sensitive internal pool. Little Ca2+ efflux occurred during the oscillations triggered by CCK-J or CCK-OP in spite of a large average increase in [Ca2+]i. When internal store Ca2+ pumps were inhibited with thapsigargin (Tg) during [Ca2+]i oscillations, a rapid Ca2+ efflux occurred similar to that measured in intensely stimulated, nonoscillatory cells. Tg also stimulated 45Ca efflux from internal pools of cells stimulated with CCK-J or a low concentration of CCK-OP. Hence, a large fraction of the Ca2+ released during each spike is reincorporated by the internal store Ca2+ pumps. Surprisingly, when the increase in [Ca2+]i during stimulation of oscillations was prevented by loading the cells with 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, a persistent activation of Ca2+ release and Ca2+ efflux occurred. This was reflected as a persistent increase in [Ca2+]o in cells suspended at low [Ca2+]o or persistent efflux of 45Ca from internal stores of cells maintained at high [Ca2+]o. Since agonist-stimulated Ca2+ release evidently remains activated when [Ca2+]i is highly buffered, the primary mechanism determining Ca2+ oscillations must include an inhibition of Ca2+ release by [Ca2+]i. Loading the cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid had no apparent effect on the levels or kinetics of IP3 formation in agonist-stimulated cells. This suggests that [Ca2+]i regulated the oscillation by inhibition of Ca2+ release independent of its possible effects on cellular levels of IP3.
Zhang
B X
BX
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235-9040.
Muallem
S
S
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000187:drug effects; D002217:Carbachol / Q000494:pharmacology; D002766:Cholecystokinin / Q000031:analogs & derivatives / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005246:Feedback; D016257:Fura-2; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D007700:Kinetics; D009991:Oscillometry; D010179:Pancreas / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010446:Peptide Fragments / Q000494:pharmacology; D051381:Rats; D012844:Sincalide / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D010446:Peptide Fragments; D013729:Terpenes; C078403:cholecystokinin-J; D004533:Egtazic Acid; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D002766:Cholecystokinin; D000252:Calcium-Transporting ATPases; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D012844:Sincalide; D002118:Calcium; D016257:Fura-2