Upregulation of Ncx Protein in Hepatopancreas and Antennal Gland of Freshwater Crayfish Associated with Elevated Ca Flux

The molting cycle of the freshwater crayfish Procambarus clarkii has emerged as an ideal model to study cellular/molecular mechanisms of Ca homeostasis because of its ability to maximally absorb Ca from a Ca deficient environment. Freshwater contains only 1 mM Ca as opposed to the 10 mM Ca afforded by marine environments. The model is based on a negligible net transepithelial Ca flux during intermolt that changes to vectorial Ca flux around ecdysis. During premolt, 20% of cuticular Ca is reabsorbed for deposition into CaCO3 disks and the remainder is excreted. Postmolt is characterized by net environmental Ca uptake rates of 2-10 mmol/kg/h and remobilization of stored Ca. Both sources are subsequently used for the calcification of a new exoskeleton. The Ca transporting epithelia are the gills, antennal gland (kidney), hepatopancreas (liver) and cuticular hypodermis. Arthropod molting is coordinated by the steroid ecdysone and unknown environmental cues. Previous studies have shown a marked increase in basolateral Ca pump expression during postmolt, suggesting that Ca transporting proteins are upregulated during periods of elevated transepithelial Ca uptake. Kinetic studies suggest that the NCX is the primary extrusion mechanism for basolateral Ca efflux. Therefore, we propose that the NCX may be upregulated during premolt and postmolt compared with intermolt in hepatopancreas and antennal gland.