After brief treatment with ATP, CHME-5 cell monolayers were detached and sedimented by centrifugation (1,000 rpm; 5 min); the pellets were then fixed with 4% paraformaldheyde-2% glutaraldheyde in PBS (for 30 min), post-fixed with 1% OsO4 (1 h), then washed and embedded in Epon. Conventional thin sections were collected on uncoated grids, stained with uranyl acetate and lead citrate, and examined in a Leo 912 electron microscope (Zeiss).