Cellular activation by Ca2+ release from stores in the endoplasmic reticulum but not by increased free Ca2+ in the cytosol.
Ca(2+) release from intracellular stores and/or transmembrane influx can increase the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). Such changes in [Ca(2+)](i) might transduce signals regulating transcription, motility, secretion, and so on. Surfactant secretagogues such as ATP and ionomycin stimulate the release and transmembrane influx of Ca(2+), both of which increase [Ca(2+)](i). The addition of surfactant protein A (SP-A) or depleting cellular Ca(2+) inhibited both surfactant secretion and Ca(2+) transients. Current results suggest that Ca(2+) signalling stimulates surfactant secretion by type II pneumocytes, but not via increased [Ca(2+)](i). Treatment of cells with a Ca(2+) chelator, bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM), stimulated secretion but decreased [Ca(2+)](i). Adding SP-A or depleting Ca(2+) inhibited BAPTA-AM-induced secretion. When studied directly, Ca(2+) in the endoplasmic reticulum store ([Ca(2+)](l)) decreased in response to BAPTA, ionomycin and thapsigargin, and increased in response to SP-A. Phorbol ester (PMA) induced surfactant secretion without altering [Ca(2+)](i) or [Ca(2+)](l) and was unaffected by Ca(2+) depletion. The addition of PMA to Ca(2+)-releasing secretagogues increased secretion, but combining two Ca(2+)-releasing secretagogues did not. These results suggest that (1) Ca(2+) signalling of type II cell surfactant secretion reflects changes in [Ca(2+)](l), not [Ca(2+)](i), (2) PMA elicits secretion differently from Ca(2+)-releasing secretagogues, and (3) SP-A inhibits secretion by enhancing Ca(2+) sequestration within endoplasmic reticulum stores. Whether other cell types signal via changes in [Ca(2+)](l) is unknown.
Strayer
D S
DS
Department of Pathology, Jefferson Medical College, 1020 Locust Street, Philadelphia, PA 19107, USA. david.strayer@mail.tju.edu
Hoek
J B
JB
Thomas
A P
AP
White
M K
MK
D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002614:Chelating Agents / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism*; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005260:Female; D066298:In Vitro Techniques; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D008168:Lung / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D010743:Phospholipids / Q000378:metabolism; D011510:Proteolipids / Q000494:pharmacology; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D002614:Chelating Agents; D007476:Ionophores; D010743:Phospholipids; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D004533:Egtazic Acid; D015759:Ionomycin; D019284:Thapsigargin; D013755:Tetradecanoylphorbol Acetate