Intracellular Na+ activity as a function of Na+ transport rate across a tight epithelium

Liquid Na+ resin microelectrodes were used to measure intracellular Na+ activities (alpha iNa+) in the mammalian tight epithelium, rabbit urinary bladder. alpha iNa+ averaged 7 +/- 1 mM and was independent of Na+ transport rate over the range of 2 to 8 muA/muF. (1 mF congruent to 1 cm2 apical membrane area). After Na+ loading the cells the Na+ pump in the basolateral membrane was measurably electrogenic. A method for shielding the Na+-sensitive microelectrodes is described which increases the response time and eliminates an electrical shunting artifact.