Effects of MeCh, thapsigargin, and La3+ on plasmalemmal and intracellular Ca2+ transport in lacrimal acinar cells.
The Ca2(+)-mobilizing actions of the muscarinic receptor agonist, methacholine (MeCh), and the microsomal Ca2+ pump inhibitor, thapsigargin, were investigated in lacrimal acinar cells. As previously shown for parotid cells (J. Biol. Chem. 264: 12266-12271, 1989), thapsigargin activates both internal Ca2+ release and Ca2+ entry from the extracellular space without increasing cellular inositol phosphates. The inorganic Ca2+ antagonist La3+ inhibited MeCh- or thapsigargin-activated Ca2+ entry. However, when added before MeCh or thapsigargin, La3+ inhibited the extrusion of Ca2+ at the plasma membrane. This phenomenon was exploited in protocols designed to investigate the pathways for filling agonist-sensitive Ca2+ stores in lacrimal cells. The results show that, in contrast to previous suggestions that external Ca2+ is required to replenish agonist-regulated Ca2+ stores, the inhibition of Ca2+ extrusion permits recycling of Ca2+ released by MeCh back into an MeCh- and thapsigargin-sensitive pool. Thus, although extracellular Ca2+ is the major source for refilling the intracellular Ca2+ stores under physiological conditions, the pathway by which this Ca2+ enters the pool need not be a direct one. These results are consistent with the recently revised capacitative model for the refilling of intracellular Ca2+ stores through Ca2+ influx subsequent to Ca2+ depletion, according to which refilling of intracellular Ca2+ stores occurs via a cytoplasmic route rather than a direct channel between intracellular Ca2+ stores and the extracellular space.
Kwan
C Y
CY
Calcium Regulation Section, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709.
Takemura
H
H
Obie
J F
JF
Thastrup
O
O
Putney
J W
JW
D000818:Animals; D001572:Benzofurans; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002273:Carcinogens / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism*; D005456:Fluorescent Dyes; D016257:Fura-2; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism; D007700:Kinetics; D007765:Lacrimal Apparatus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D007811:Lanthanum / Q000494:pharmacology*; D008688:Methacholine Compounds / Q000494:pharmacology*; D051379:Mice; D008954:Models, Biological; D051381:Rats; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology; D015398:Signal Transduction; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D001572:Benzofurans; D002273:Carcinogens; D005456:Fluorescent Dyes; D007295:Inositol Phosphates; D008688:Methacholine Compounds; D011976:Receptors, Muscarinic; D013729:Terpenes; D019284:Thapsigargin; D007811:Lanthanum; D002118:Calcium; D016257:Fura-2