Na+-K+-ATPase gene expression in the avian eggshell gland: distinct regulation in different cell types

The avian eggshell gland (ESG) is a tissue specialized in transporting the Ca2+ required for eggshell formation and represents a unique biological sys tem in which the calcification process takes place in a circadian fashion. With the use of RNA fingerprinting, a set of genes differentially induced a t the time of calcification was detected, one of which was identified as th e alpha (1)-subunit of Na+-K+-ATPase. The gene was expressed in a circadian manner in both cell types populating the ESG, but in different temporal pa tterns, suggesting distinct mechanisms of regulation. Ca2+ flux and mechani cal strain were found to regulate gene expression in the inner glandular ep ithelium and the pseudostratified epithelium facing the lumen, respectively . Mechanical strain also affected gene expression in cell layers facing the lumen in other parts of the oviduct. Only the alpha (1)-isoform, not the a lpha (2)- or alpha (3)-isoform, of Na+-K+ ATPase was expressed in the ESG. In summary, we demonstrate that the alpha (1)- subunit Na+-K+-ATPase gene i s expressed in different epithelial cell types in the ESG and is regulated by various mechanisms, which may reflect the disparity in the physiological roles of the cells in the process of eggshell formation.