The intracellular distribution of calcium has been studied in the mucosa of the avian shell gland, a tissue which transports large quantities of calcium during discrete time intervals. Ca45 was administered to hens either in a single dose followed by sacrifice 5 min later or in repeated doses over an extended period followed by sacrifice 2 hr or 24 hr after the last injection. Subcellular fractions were isolated by differential centrifugation and analyzed for Ca45. The Ca45 was located principally in the particulate fractions; the concentration (CPM Ca45/mg N) was highest in the mitochondrial fraction. Comparisons of (1) the Ca45 distribution in shell gland cells with that of liver cells, (2) the alterations which occur due to the phase of the egg laying cycle, (3) the effects due to the time elapsed since the last injection of Ca45, and (4) the Ca45 distribution of the short term experiments with that of the long term experiments revealed that the mitochondrial fraction of the shell gland appeared to be active in the movement of calcium. The microsomal fraction showed increased values in CPM Ca45/mg N when calcification was occurring, which may indicate that the subcellular components of this fraction have a role in calcium transport. The nuclear and supernatant fractions did not seem to be involved in the transport process. The implications of these results concerning the manner by which calcium may be controlled on a cellular level in this system are discussed.

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