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

Ca²⁺ 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 Ca²⁺ (Ca_i) 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_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 in surface area draws membrane from a much larger reservoir and may be driven by membrane tension.