Ca2+-stimulated exocytosis in maize coleoptile cells

Changes in membrane capacitance (C-m) after photolysis of the caged Ca2+ co mpound dimethoxynitrophenamine were studied in protoplasts from maize coleo ptiles. Changes in C-m values resulting from increased concentrations of fr ee Ca2+ in the cytoplasm ([Ca2+](cyt)) were interpreted as representing cha nges in [Ca2+](cyt)-sensitive exocytosis and endocytosis. A continuous incr ease in [Ca2+](cyt) resulted in a sigmoidal increase in C-m values with a h alf-maximal concentration at similar to 1 mu M. The steep increase in C-m v alues was followed by a variable slow phase in changing C-m values. When [C a2+](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, pro toplasts increased in surface area by similar to 2.5%. The biphasic dynamic s of [Ca2+](cyt)-stimulated increases in C-m values can be described by a k inetic model containing two pools of vesicles with two [Ca2+](cyt)-sensitiv e steps in the exocytotic pathway.