Chromaffin granule membrane-F-actin interactions are calcium sensitive

Exocytosis of neurotransmitters, hormones and other secretory products from many cell types is initiated by a rise in the intracellular free calcium ion concentration¹. By analogy with excitation-contraction coupling in muscle², calcium-triggered acto-myosin interactions have been postulated to push or pull the secretory granule to the plasma membrane ^1,3-6. Indeed, there have been several reports of actin interacting with membranes of secretory vesicles from the adrenal medulla (chromaffin granules)^7-12. Using sedimentation techniques and a novel application^13,14 of falling ball viscometry^15,16 to measure actin binding to membranes, we show here that purified chromaffin granule membranes bind and cause large increases in the viscosity of F-actin. The actin-binding activity of the membranes is trypsin sensitive and thermolabile and interactions between membranes and actin are reversibly inhibited by raising the free calcium ion concentration, [Ca²⁺] _free, above 10^-7 M. We propose that regulation of actin-chromaffin granule membrane interactions may be critical for organelle movement to the plasma membrane during calcium-mediated exocytosis in the chromaffin cell.