Ca2+-stimulated exocytosis in maize coleoptile cells

Changes in membrane capacitance (C-m) after photolysis of the caged Ca2+ compound dimethoxynitrophenamine were studied in protoplasts from maize coleoptiles. Changes in C-m values resulting from increased concentrations of free Ca2+ in the cytoplasm ([Ca2+](cyt)) were interpreted as representing changes in [Ca2+](cyt)-sensitive exocytosis and endocytosis. A continuous increase in [Ca2+](cyt) resulted in a sigmoidal increase in C-m values with a half-maximal concentration at similar to 1 mu M. The steep increase in C-m values was followed by a variable slow phase in changing C-m values. When [Ca2+](cyt) increased at a rate of 0.6 mu mol L-1 sec(-1), the initial steep increase in C-m values lasted similar to 5 to 10 sec. During this time, protoplasts increased in surface area by similar to 2.5%. The biphasic dynamics of [Ca2+](cyt)-stimulated increases in C-m values can be described by a kinetic model containing two pools of vesicles with two [Ca2+](cyt)-sensitive steps in the exocytotic pathway.