-Stimulated Exocytosis in Maize Coleoptile Cells

in the cytoplasm ([Ca 2 1 ] cyt ) were interpreted as representing changes in [Ca 2 1 ] cyt –sensitive exocytosis and endocytosis. A continuous increase in [Ca 2 1 ] cyt resulted in a sigmoidal increase in C m values with a half-maximal concentration at z 1 m M. The steep increase in C m values was followed by a variable slow phase in changing C m values. When [Ca 2 1 ] cyt increased at a rate of 0.6 m mol L 2 1 sec 2 1 , the initial steep increase in C m values lasted z 5 to 10 sec. During this time, protoplasts increased in surface area by z 2.5%. The biphasic dynamics of [Ca 2 1 ] cyt –stimulated increases in C m values can be described by a kinetic model containing two pools of vesicles with two [Ca 2 1 ] cyt –sensitive steps in the exocytotic pathway.