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Ca 2+ dependency of ‘Ca 2+ ‐independent’ exocytosis in SPOC1 airway goblet cells
Author(s) -
Rossi Andrea H.,
Sears Patrick R.,
Davis C. William
Publication year - 2004
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2004.070433
Subject(s) - egta , exocytosis , ionomycin , secretion , bapta , mucin , phorbol , chemistry , secretagogue , biophysics , protein kinase c , calcium , biochemistry , biology , medicine , endocrinology , receptor , intracellular , signal transduction , organic chemistry
SPOC1 airway goblet cells secrete mucin in response to P2Y 2 receptor agonists and to secretagogues, phorbol 12‐myristate 13‐acetate (PMA) and ionomycin, which mobilize elements of the phospholipase C pathway, PKC and Ca 2+ , respectively. Previous studies demonstrated that mucin secretion from SLO‐permeabilized, EGTA‐buffered SPOC1 cells was stimulated by PMA at low Ca 2+ levels (< 0.1 μ m ), consistent with the notion that regulated exocytosis may occur by Ca 2+ ‐independent pathways. We tested the alternative hypothesis that PMA‐induced mucin secretion is, in fact, a Ca 2+ ‐dependent process under the conditions of low bulk Ca 2+ , one that is permitted in the typical SLO‐permeabilized cell model by the slow binding kinetics of EGTA. Both IP 3 and elevated bulk Ca 2+ activated mucin secretion in SPOC1 cells buffered by EGTA, suggesting that IP 3 generates a local Ca 2+ gradient in the vicinity of the secretory granules to the degree necessary to trigger exocytosis. BAPTA, which binds Ca 2+ approximately 100‐fold faster than EGTA, diminished IP 3 ‐induced mucin release over a range of concentrations by ≥ 69%, yet maintained an essentially normal mucin secretory response to elevated bulk Ca 2+ in permeabilized SPOC1 cells. BAPTA also diminished the mucin secretory response of permeabilized cells to PMA, relative to the EGTA‐buffered control: at PMA below 30 n m , BAPTA abolished the secretory response, and at higher concentrations it was reduced significantly relative to the EGTA‐buffered controls. PMA‐induced secretion in EGTA was insensitive to heparin. These results suggest that Ca 2+ is released locally during PMA‐induced exocytosis, by an IP 3 ‐independent mechanism.