In our model for calcium-dependent secretion of hormones and neurotransmitters, an important function of calcium is to open calcium-activated ion channels present in secretary vesicles. The opening of these channels would allow the transport of ions from the intermembrane space between the plasma membrane and the secretory vesicle membrane into the secretory vesicle. The resultant osmotic imbalance would remove water from the intermembrane space, allowing the two membranes to move closer together and fuse. According to the model, every secretory granule for calcium-dependent secretion should contain some type of calcium-activated channel. In order to test this, we are currently reconstituting channels from vesicles of the bovine neurohypophysis into lipid bilayers. Preliminary results indicate that the vesicles contain calcium-activated anion channels. Another approach involves measurement of the secretion properties of bovine parathyroid cells. These cells have a unique calcium dose-response curve for secretion: secretion decreases when calcium concentration increases. We have previously examined the vesicle channel properties and the secretion properties of these cells, and found a good correlation, as predicted by our hypothesis. As a further test, we are now trying to block ionic flow through these channels and to determine whether this will block secretion.
