Calcium-ion binding to the chromaffin-granule surface

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Abstract
1. Ca2+ions decreased the surface charge of isolated adrenal-medullary chromaffin granules whether the granules were untreated or previously incubated with neuraminidase. 2. Ca2+binding in both cases followed a Langmuir adsorption isotherm. 3. The chromaffin-granule surface was essentially polyanionic with about 10800 anionic sites per granule, of which 4200 sites are capable of binding Ca2+ions with an electrochemical free energy of adsorption of −1.32kJ·mol−1. 4. The surface region of the chromaffin granule was calculated to bind 1306 Ca2+ions at 2.2mm-Ca2+(ionic strength 0.16mol·litre−1). 5. The importance of Ca2+binding to the chromaffin-granule surface is discussed in relation to the hypothesis of secretion by exocytosis.