The effects of dietary sodium loading on the activity and expression of Na, K-ATPase in the rectal gland of the European Dogfish ( Scyliorhinus canicula)

Abstract

cDNA fragments of both the α- and β-subunits of the Na, K-ATPase and a cDNA fragment of the secretory form of Na–K–Cl cotransporter from the European dogfish ( Scyliorhinus canicula) were amplified and cloned using degenerate primers in RT-PCR. These clones were used along with a sCFTR cDNA from the related dogfish shark, Squalus acanthias to characterise the expression of mRNAs for these ion transporters in the dogfish rectal gland subsequent to an acute feeding episode. Following a single feeding event where starved dogfish were fed squid portions (20 g squid/kg fish), there was a delayed and transient 40-fold increase in the activity of Na, K-ATPase in crude rectal gland homogenates. Increases in enzyme activity were apparent 3 h after the feeding event and peaked at 9 h before returning to control values within 24 h. These increases in activity were accompanied by small and transient decreases in plasma sodium and chloride concentrations lasting up to 3 days. Significant increases in the expression of mRNAs for α- and β-subunits of the Na, K-ATPase, the Na–K–Cl cotransporter and CFTR chloride channel were detected but not until 1–2 days after the feeding event. It is concluded that the transient increase in Na, K-ATPase activity is not attributable to increases in the abundance of α- and β-subunit mRNAs but must be associated with some, as yet unknown, post-transcriptional activation mechanism.