Calcium efflux across the plasma membrane of rat parotid acinar cells is unaffected by receptor activation or by the microsomal calcium ATPase inhibitor, thapsigargin

H Takemura, Ole Thastrup, J W Putney

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Abstract

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)
Original languageEnglish
JournalCell Calcium
Volume11
Issue number1
Pages (from-to)11-7
Number of pages7
ISSN0143-4160
Publication statusPublished - 1990
Externally publishedYes

Keywords

  • Animals
  • Biological Transport, Active
  • Calcium
  • Calcium-Transporting ATPases
  • Carcinogens
  • Cell Membrane
  • Egtazic Acid
  • Kinetics
  • Methacholine Chloride
  • Methacholine Compounds
  • Microsomes
  • Parotid Gland
  • Rats
  • Receptors, Muscarinic
  • Terpenes
  • Thapsigargin

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