title: Threshold calcium levels for lamellar body exocytosis in type II pneumocytes.
abstract: Pulmonary surfactant is secreted via exocytosis of lamellar bodies (LBs) by alveolar type II cells. Here we analyzed the dependence of LB exocytosis on intracellular Ca(2+) concentration ([Ca(2+)](i)). In fura 2-loaded cells, [Ca(2+)](i) was selectively elevated by flash photolysis of a cell-permeant caged Ca(2+) compound (o-nitrophenyl EGTA-AM) or by gradually enhancing cellular Ca(2+) influx. Simultaneously, surfactant secretion by single cells was analyzed with the fluorescent dye FM 1-43, enabling detection of exocytotic events with a high temporal resolution (T. Haller, J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. Proc. Natl. Acad. Sci. USA 95: 1579-1584, 1998). Exocytosis was initiated at a threshold concentration near 320 nmol/l with both instantaneous or gradual [Ca(2+)](i) elevations. The exocytotic response to flash photolysis was highest during the first minute after the rise in [Ca(2+)](i) and thus almost identical to purinoceptor stimulation by ATP. Correspondingly, the effects of ATP on initial secretion could be sufficiently explained by its ability to mobilize Ca(2+). This was further demonstrated by the fact that exocytosis is significantly blocked by suppression of the ATP-induced Ca(2+) signal below approximately 300 nmol/l. Our results suggest a highly Ca(2+)-sensitive step in LB exocytosis.
authors.affiliation: Department of Physiology, University of Innsbruck, A-6020 Innsbruck, Austria. thomas.haller@uibk.ac.at
authors.forename: T
authors.initials: T
authors.lastname: Haller
authors.forename: K
authors.initials: K
authors.lastname: Auktor
authors.forename: M
authors.initials: M
authors.lastname: Frick
authors.forename: N
authors.initials: N
authors.lastname: Mair
authors.forename: P
authors.initials: P
authors.lastname: Dietl
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005089:Exocytosis / Q000502:physiology*; D010777:Photochemistry; D011650:Pulmonary Alveoli / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D011663:Pulmonary Surfactants / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D002614:Chelating Agents; D011663:Pulmonary Surfactants; C085112:2-nitrophenyl-EGTA; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D002118:Calcium