Ca2+-sensitive and Ca2+-insensitive exocytosis in maize coleoptile protoplasts

Ca2+ and osmotic driven extension of the surface area of maize coleoptile p rotoplasts was investigated using capacitance measurements and photolysis o f the caged compound DM-nitrophen. Protoplasts responded to an elevation of cytoplasmic Ca2+ (Ca-i) with a rapid burst in capacitance reaching a maxim al increase of 1.3 +/- 1.1% over the resting cell capacitance. Subsequent l owering of the osmotic potential in the external medium by 210 mosmol cause d a further increase in C-m by 26+/-6%. These data indicate two independent pathways for insertion of membrane into the plasma membrane. One is driven by Ca-i and recruits membrane from a small pool. The osmotic evoked rise i n surface area draws membrane from a much larger reservoir and may be drive n by membrane tension.