Activation of the plasma membrane Ca2+ pump during agonist stimulation of pancreatic acini.
The role of internal stores and plasma membrane Ca2+ pumps in controlling [Ca2+]i during agonist stimulation and their regulation by agonists are not well understood. We report here measurements of intracellular ([Ca2+]i) and extracellular ([Ca2+]o) Ca2+ concentrations in agonist-stimulated pancreatic acini in an effort to directly address these questions. Stimulation of acini suspended in Ca(2+)-free or Ca(2+)-containing medium with Ca2+ mobilizing agonists resulted in a typical transient increase in [Ca2+]i. Thapsigargin, a specific inhibitor of internal Ca2+ pumps, inhibited the rate of [Ca2+]i reduction after agonist stimulation by approximately 40%. Under the same conditions, thapsigargin had no effect on the rate of the unidirectional Ca2+ efflux across the plasma membrane as revealed by measurements of [Ca2+]o. These findings suggest that internal Ca2+ pumps actively remove Ca2+ from the cytosol during continued agonist stimulation. The correlation between the reduction in [Ca2+]i and the increase in [Ca2+]o showed that Ca2+ efflux from cells stimulated with agonist and thapsigargin represent Ca2+ efflux across the plasma membrane. Inhibition of cells exposed to agonist and thapsigargin with a specific antagonist sharply reduced the rates of the [Ca2+]i decrease and the accompanied [Ca2+]o increase. Hence, at comparable [Ca2+]i, Ca2+ efflux from stimulated cells was about 3-fold faster than that from resting cells, indicating that agonists directly activate the plasma membrane Ca2+ pump. To study the role of [Ca2+]i increase in plasma membrane Ca2+ pump activation the acini were loaded with 1,2-bis-(2-aminophenoxyethane-N,N,N',N')-tetraacetic acid (BAPTA), and [Ca2+]o was measured during agonist stimulation. Surprisingly, although BAPTA completely prevented the increase in [Ca2+]i, Ca2+ efflux rate was reduced by only 34%. These findings provide the first evidence for Ca(2+)-independent activation of the plasma membrane Ca2+ pump by Ca2+ mobilizing agonists.
Zhang
B X
BX
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235-9040.
Zhao
H
H
Loessberg
P
P
Muallem
S
S
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000502:physiology*; D002217:Carbachol / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives; D016257:Fura-2; D066298:In Vitro Techniques; D007700:Kinetics; D010179:Pancreas / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D013729:Terpenes; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D004533:Egtazic Acid; D019284:Thapsigargin; D002217:Carbachol; D000252:Calcium-Transporting ATPases; D002118:Calcium; D016257:Fura-2