Calcium efflux across the plasma membrane of rat parotid acinar cells is unaffected by receptor activation or by the microsomal calcium ATPase inhibitor, thapsigargin.
The rate of Ca2+ extrusion across the plasma membrane of rat parotid acinar cells was determined by measuring the decay of the intracellular calcium concentration, [Ca2+]i, following the addition of EGTA to agonist stimulated cells. In the presence of extracellular Ca2+, the muscarinic cholinergic receptor agonist, methacholine, rapidly increased [Ca2+]i (peaking within 5 s), which then decreased to a higher steady state level. This elevated steady state level was dependent on extracellular Ca2+ concentration. Likewise, thapsigargin, a non-phorbol ester tumor promoter that does not increase inositol phosphates, gradually increased [Ca2+]i, peaking within 1 min and then declining to a new elevated plateau level which was also dependent on extracellular Ca2+. [Ca2+]i, elevated by methacholine or thapsigargin, was rapidly decreased by the addition of EGTA by a process the kinetics of which depended on the value of [Ca2+]i before the addition of EGTA. That is, [Ca2+]i increased as a function of the extracellular Ca2+ concentration and also the apparent half-time for Ca2+ extrusion following the addition of EGTA to cells was increased as the [Ca2+]i increased. This presumably reflects the saturable nature of the Ca2+ extrusion mechanism. The steady state [Ca2+]i in cells stimulated with methacholine or thapsigargin in nominally Ca2+ free medium was similar to the steady state [Ca2+]i in unstimulated cells in normal, Ca2(+)-containing medium. Under these similar [Ca2+]i conditions, stimulated and unstimulated cells showed a similar time course of decay upon addition of EGTA. In addition, neither methacholine nor phorbol myristate acetate decreased the sustained elevation of [Ca2+]i induced by ionomycin.(ABSTRACT TRUNCATED AT 250 WORDS)
Takemura
H
H
Calcium Regulation Section, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina.
Thastrup
O
O
Putney
J W
JW
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002273:Carcinogens / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D016210:Methacholine Chloride; D008688:Methacholine Compounds / Q000494:pharmacology; D008861:Microsomes / Q000201:enzymology; D010306:Parotid Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D002273:Carcinogens; D008688:Methacholine Compounds; D011976:Receptors, Muscarinic; D013729:Terpenes; D016210:Methacholine Chloride; D004533:Egtazic Acid; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium