Activity, abundance, distribution and expression of Na+/K+-ATPase in the salt glands of Crocodylus porosus following chronic saltwater acclimation.

Saltwater crocodiles, Crocodylus porosus, possess lingual salt glands which function to remove excess Na(+) and Cl(-) accumulated as a consequence of living in salt water. Little is known about the nature of ion transport systems in C. porosus salt glands and how these systems respond to an osmotic challenge. In the present study, we examined the distribution and regulation of the Na(+)/K(+)-ATPase (NKA) pump, specifically the alpha-(catalytic) subunit in the salt glands of C. porosus chronically acclimated (6 months) to freshwater (FW) or 70% seawater (SW). We hypothesised that in the SW-acclimated C. porosus there would be an up-regulation of the abundance, activity and gene expression of the NKA transporter. NKA was immunolocalised to the lateral and basal membrane of secretory cells. As predicted, the NKA alpha-subunit was 2-fold more abundant in SW-acclimated C. porosus salt glands. NKA gene expression was also elevated in the salt glands of SW- vs FW-acclimated crocodiles. There was no increase in the specific activity of NKA in SW-acclimated animals and the in vitro rate of oxygen consumption by salt gland slices from SW-acclimated animals was not significantly different from that of FW-acclimated animals. The proportion of tissue oxygen consumption rate attributable to NKA activity was not different between SW- and FW-acclimated animals (approximately 50%). These data suggest that either chronic SW acclimation does not affect NKA in crocodile salt glands in the same manner as seen in other models or crocodiles possess the capacity to moderate NKA activity following prolonged exposure to SW.