Relationships between calbindin (Mr 28,000) and calcium transport by the eggshell gland.

1. Eggshell density (mg/cm2) and eggshell gland calbindin decreased in the aged hens. 2. Aged hens which laid eggs with a low shell weight and shell density had significantly lower intestinal and eggshell gland calbindin as compared with those which laid eggs with a high shell weight and shell density. 3. Significant correlations were found in aged hens between duodenal or eggshell calbindin and shell weight or shell density. 4. The results suggest that: (a) aged hens forming light shells absorbed calcium with a lower efficiency than those forming heavy shells or than young hens; (b) the decline in shell density in the aged hens is caused by a physiological calcium deficiency or by a defect in the hens' ability to alter calbindin synthesis in response to calcium needs; (c) in the aged hens, the deposition of calcium into the eggshell is dependent on, or at least associated with, eggshell gland calbindin.