Isosmotic modulation of Ca2+-regulated exocytosis in guinea-pig antral mucous cells: role of cell volume

1. Exocytotic events and changes of cell volume in mucous cells from guinea -pig antrum were examined by video-enhanced optical microscopy. 2. Acetylcholine (ACh) evoked exocytotic events following cell shrinkage, t he frequency and extent of which depended on the ACh concentration. ACh act ions were mimicked by ionomycin and thapsigargin, and inhibited by Ca2+-fre e solution and Ca2+ channel blockers (Ni2+, Cd2+ and nifedipine). Applicati on of 100 mu M W-7, a calmodulin inhibitor, also inhibited the ACh-induced exocytotic events. These results indicate that ACh actions are mediated by intracellular Ca2+ concentration ([Ca2+](i)) in antral mucous cells. 3. The effects of ion channel blockers on exocytotic events and cell shrink age evoked by ACh were examined. Inhibition of KCl release (quinine, Ba2+ N PPB or KCl solution) suppressed both the exocytotic events and cell shrinka ge evoked by ACh. 4. Bumetanide (inhibition of NaCl entry) or Cl--free solution (increasing C l- release and inhibition of NaCl entry) evoked exocytotic events following cell shrinkage in unstimulated antral mucous cells and caused further cell shrinkage and increases in the frequency of exocytotic events in ACh-stimu lated cells. However, Cl--free solution did not evoke exocytotic events in unstimulated cells in the absence of extracellular Ca2+, although cell shri nkage occurred. 5. To examine the effects of cell volume on ACh-evoked exocytosis, the cell volume was altered by increasing the extracellular K+ concentration. The r esults showed that cell shrinkage increases the frequency of ACh-evoked exo cytotic events and cell swelling decreases them. 6. Osmotic shrinkage or swelling caused the frequency of ACh-evoked exocyto tic events to increase. This suggests that the effects of cell volume on AC h-evoked exocytosis under anisosmotic conditions may not be the same as tho se under isosmotic conditions. 7. In antral mucous cells, Ca2+-regulated exocytosis is modulated by cell s hrinkage under isosmotic conditions.