Effects of organic solutes on transmural PD and Na transport in freshwater prawn intestine

Abstract

Addition of 1 mM L-alanine or 0.5 mM D-glucose to the mucosal surface of freshwater prawn (Macrobrachium rosenbergii) intestine rapidly increased transmural potential difference (PD) (serosa became more positive). Luminal perfusion with 1 mM D-alanine did not affect the spontaneous PD. Addition of 10 mM serosal L-alanine induced a slow elevation in PD with the same polarity as shown with luminal alanine. The alanine-evoked PD was a hyperbolic function of luminal alanine concentration and was reduced to zero with 0.5 mM serosal ouabain. Luminal 1 mM L-alanine stimulated JNanet from mucosa to serosa from 1.29 +/- 0.45 to 2.83 +/- 0.66 mu mol . cm-2 . h-1. Extensive metabolic breakdown of L-alanine and D-glucose occurred during intestinal transit resulting in a small, but significant, net flux of unaltered sugar from lumen to blood (103.9 +/- 23.1 nmol . cm-2 . h-1) and an absence of net amino acid flow. While 1 mM alanine influx into the epithelium was largely Na dependent, quantitatively insigificant amounts of Na were cotransported into the cell with amino acid (approx 52 nmol . cm-2 . h-1) to account for increased JNanet observed in the presence of this organic solute. Electrogenic effects of luminal nutrients result from their catabolism to fuels for the basolateral Na pump, Increased Na-pump activity stimulates Na entry into the cell by allosteric apical carrier proteins and enhances transmural PD by accelerating net movement of cation across the tissue.