The effect of feeding on ion transport in the rectal gland of the European dogfish (Scyliorhinus canicula)

Abstract

Plasma sodium, chloride and urea concentrations and plasma osmolalities were determined after dogfish were dietary adapted to both acute and chronic feeding regimes. Following single feeding events (20g squid / kg body wt) plasma sodium and chloride concentrations dropped transiently by a maximum of 27% and 18% respectively, two days after feeding. There was however no significant change in the plasma urea concentration or plasma osmolality. Determination of blood vessel calibre by histological examination of rectal glands before, and 12 hours after, a single feeding event suggested an increase in the blood supply to the gland. Repeated feeding events (2.5g pellets / kg body wt) over a period of 1 month were conducted with pelleted diets containing either 1% or 6% w / w NaCl and blood samples were taken weekly for analysis. Plasma sodium concentrations were significantly reduced by approximately 10% after 1 and 3 weeks of feeding and plasma osmolalities fell again by approximately 10% at 1, 2 and 3 weeks when fish were fed on the 1% w / w NaCl pellet diet. However both parameters returned to normal values by the end of the study period. Likewise when fish were fed the high salt (6% w / w NaCl) pellet diet, plasma sodium concentrations and plasma osmolalities again fell by 10-15% after 1, 2 and 3 weeks of repeated feeding. Plasma sodium recovered to normal values within 28 days however the plasma osmolality remained significantly lower than controls. Total RNA was extracted from the rectal gland and homologous and heterologous cDNA probes for the alpha1, beta1 subunits of the Na, K-ATPase and the sCFTR and Na-K-Cl cotransporter were used in Northern and dot blot analyses to identify and quantify the levels of mRNA expression of these major ion transporter proteins during dietary adaptation. Maximal Na, K-ATPase activities in rectal gland homogenates increased transiently by over 40-fold, 9 hours after a single feeding episode (20g squid / kg body wt). There was no concomitant increase in either alpha1 or beta1 subunit of the Na, K-ATPase mRNAs over the same time period however the abundance of both al and (31 Na, K-ATPase subunit mRNAs slowly increased by 75% and 39% respectively two days after feeding and were maintained at that level for either 10 days or 5 days respectively. In chronic dietary adaptations to a squid diet, a 40% increase in beta1 Na, K-ATPase subunit mRNA was found after 4 weeks of repeated feeding however there was no significant increase in al subunit mRNA expression or Na, K-ATPase activity. In contrast chronic dietary adaptation to the 6% w / w NaCl pellet diet resulted in a 3-fold increase in Na, K-ATPase activity however there was no concomitant increase in either the alpha1 or beta1 subunit of the Na, K-ATPase mRNAs. After a single feeding event with the squid diet, mRNAs for both the sCFTR and Na-K-Cl cotransporter proteins were significantly increased, by 55% and 65% respectively, five days after the feeding episode compared to controls.