Differential Regulation of Calbindin in the Calcium-Transporting Organs of Birds with High Calcium Requirements

The present review focuses on the differential regulation and differential expression, as well as the possible differential functionality, of avian calbindin-D28k in the three major calcium-transporting tissues: intestine, eggshell gland (ESG) and kidney. Special emphasis is given to their relationships with fast growth and with the laying of long clutches, i.e., unbroken sequences of eggs, separated by one or more days, as affected by the three major regulators, cholecalciferol (vitamin D3), gonadal activity, and intra- or extracellular calcium or its fluxes. The accumulated evidence suggests that intestinal calbindin mostly depends on vitamin D3, ESG calbindin appears to depend on calcium-transport-related factor (s) and to lesser extent also on gonadal activity; whereas renal calbindin partly depends on vitamin D3 and extracellular calcium.