RAISING THE CYTOSOLIC CA2- EVIDENCE FOR CA2+-STIMULATED EXOCYTOSIS( CONCENTRATION INCREASES THE MEMBRANE CAPACITANCE OF MAIZE COLEOPTILE PROTOPLASTS )

Enhanced elongation of coleoptile cells has been proposed to be relate d to a rise in secretory activity. Therefore, to obtain a direct measu rement of exocytotic events in maize (Zea mays L.) coleoptile protopla sts we used the patch-clamp method to record changes in membrane capac itance (Cm) as a parameter proportional to fluctuations of the membran e surface area. The secretory activity of protoplasts was correlated w ith the cytosolic free Ca2+ concentration ([Ca2+](cyt)): dialyzing pro toplasts with 1 mu M [Ca2+](cyt) caused a steady rise in Cm of 3.3 +/- pF.s(-1). In contrast, dialysis with a solution containing <20 nM Ca2 + produced a small and persistent decrease in Cm. This demonstrates th at secretory activity in coleoptile cells can be controlled by factors which modulate [Ca2+](cyt).