Na microelectrode study of pathways of Na entry into Amphiuma intestinal absorptive cells

To define the pathways of Na+ entry into intestinal villus cells, intracellular Na+ activity (aiNa) was measured in Amphiuma duodenum using conventional and Na-sensitive microelectrodes. Replacement of Na+ in the luminal medium reduced aiNa rapidly; replacement of Na+ in the serosal medium caused a slow decline of aiNa. Hence, mucosal and serosal membranes are both permeable to Na+. Ouabain addition to the serosal medium caused aiNa to increase over 4 h. When Na+ was present only in the mucosal medium and Na+ transport was inhibited with ouabain, aiNa increased over 4 h. With galactose or valine (20 mM) in the mucosal medium aiNa was greater at 2 h relative to paired control tissues. The gain in aiNa was unaffected by replacement of luminal medium Cl- with gluconate or exposure to 1 mM furosemide or amiloride. Amiloride, at 1 mM, was detected by the Na-sensitive neutral carrier. Over a wide range of Na+ concentrations in the luminal medium the rate of Na+ entry across the mucosal membrane correlated strongly (r = 0.95) with the electrochemical gradient for Na+ across the luminal membrane. It is concluded that aiNa of urodele intestinal cells is maintained at a low level by the operation of a Na+-K+ pump. Na+ entry across the luminal membrane occurs by diffusion and by the cotransport with sugars and amino acids. Luminal NaCl cotransport and Na+-H+ exchange do not appear to contribute to Na entry to a measurable extent but it is possible that these transport processes operate at a slow rate, but were inhibited secondary to inhibition of the Na-K pump.