Ca2+ and Secretion
The substantial body of work showing that Ca2+ affected the permeability properties of the cell membrane, while interesting in itself, did not provide a very compelling or complete understanding of the mechanism of action of the ion. Although usually not explicitly stated, there are different studies where you can almost hear the authors struggling with this rather vague concept and where they are attempting to formulate a more specific model. An example is the induction of α-amylase release in barley aleurone cells by gibberellic acid. Studies on this model system had gained attention because it seemed that the molecular basis for gibberellic acid might emerge. An early and influential observation was that of Chrispeels and Varner (1967), who showed that the presence of Ca2+e (mM) greatly facilitated the appearance of gibberellic acid–induced α-amylase in the medium. The increase was not trivial; as shown by Jones (1973), 20 mM CaCl2 stimulated an 18-fold increase over the water control. If an increase in the [Ca2+]e renders the membrane less permeable, why then would there be an increased release of α-amylase? It is my supposition that this conundrum puzzled different researchers leading eventually to the realization that Ca2+ specifically stimulated the process of enzyme secretion (Jones and Jacobsen, 1983), an idea that stands as a paradigm today in both plant and animal systems (Zorec and Tester, 1992).