Ca-sensitive and Ca-insensitive exocytosis in maize coleoptile protoplasts.

Ca and osmotic driven extension of the surface area of maize coleoptile protoplasts was investigated using capacitance measurements and photolysis of the caged compound DM-nitrophen. Protoplasts responded to an elevation of cytoplasmic Ca (Ca) with a rapid burst in capacitance reaching a maximal increase of 1.3±1.1 % over the resting cell capacitance. Subsequent lowering of the osmotic potential in the external medium by 210 mosmol caused a further increase in C by 26±6 %. These data indicate two independent pathways for insertion of membrane into the plasma membrane. One is driven by Ca and recruits membrane from a small pool. The osmotic evoked rise in surface area draws membrane from a much larger reservoir and may be driven by membrane tension.