Ca2+ dependency of 'Ca2+-independent' exocytosis in SPOC1 airway goblet cells.
SPOC1 airway goblet cells secrete mucin in response to P2Y2 receptor agonists and to secretagogues, phorbol 12-myristate 13-acetate (PMA) and ionomycin, which mobilize elements of the phospholipase C pathway, PKC and Ca2+, respectively. Previous studies demonstrated that mucin secretion from SLO-permeabilized, EGTA-buffered SPOC1 cells was stimulated by PMA at low Ca2+ levels (< 0.1 microm), consistent with the notion that regulated exocytosis may occur by Ca2+-independent pathways. We tested the alternative hypothesis that PMA-induced mucin secretion is, in fact, a Ca2+-dependent process under the conditions of low bulk Ca2+, one that is permitted in the typical SLO-permeabilized cell model by the slow binding kinetics of EGTA. Both IP3 and elevated bulk Ca2+ activated mucin secretion in SPOC1 cells buffered by EGTA, suggesting that IP3 generates a local Ca2+ gradient in the vicinity of the secretory granules to the degree necessary to trigger exocytosis. BAPTA, which binds Ca2+ approximately 100-fold faster than EGTA, diminished IP3-induced mucin release over a range of concentrations by > or = 69%, yet maintained an essentially normal mucin secretory response to elevated bulk Ca2+ 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 nm, 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 Ca2+ is released locally during PMA-induced exocytosis, by an IP3-independent mechanism.
Rossi
Andrea H
AH
6009 Thurston-Bowles, University of North Carolina, Chapel Hill, NC 27599-7248, USA.
Sears
Patrick R
PR
Davis
C William
CW
D000818:Animals; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects* / Q000502:physiology; D020397:Goblet Cells / Q000187:drug effects* / Q000502:physiology; D051381:Rats
D004533:Egtazic Acid; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium