The distribution of calcium,magnesium, copper and iron in the bovine adrenal medulla

The subcellular distributions of calcium, magnesium, catecholamine and protein have been measured in four batches of bovine adrenal medullae; in addition, copper and iron have been measured in the chromaffin granules. Calcium is mainly found in chromaffin granules; magnesium, by contrast, is largely cytoplasmic. The stoichiometry of calcium, magnesium and catecholamines in the chromaffin granule matrix is given and their intragranular concentrations are calculated using measurements of the internal water space of the granules. The chromaffin granules are found to behave as osmometers when the sucrose concentration of the supporting medium is varied between 0.3 and 0.7m; about 30% of the intragranular water is osmotically inactive. Copper is a component of the enzyme dopamine β-hydroxylase; approximately 40% of granule copper is present within the chromaffin granule matrix, the rest being membrane-bound. Iron, a component of cytochrome b₅₆₁, is retained in the membrane; there is twice as much iron as copper in the granule membrane.It is not known how high concentrations of catecholamines and ATP are maintained inside chromaffin granules (or synaptic vesicles). It is possible that metal ions are required to stabilise complexes of high molecular weight. The chromaffin granules are a major reservoir of calcium in adrenal medulla cells.