Transcellular sodium transport and intracellular sodium activities in rabbit gallbladder

This study was designed to explore the relation between the rate of transcellular active Na⁺ transport by rabbit gallbladder epithelium, J(Na), and the intracellular Na⁺ activity, (Na)(c); the latter was determined by use of highly selective Na⁺ microelectrodes. The underlying strategy was based on the well-established observation that J(Na) is stimulated by the presence of bicarbonate in the bathing solution. Our results confirm previous observations that the addition of bicarbonate to the bathing solutions results in a twofold increase in J(Na). In the absence of bicarbonate, (Na)(c) averaged 16 mM. Within 2-4 min after the addition of bicarbonate to both bathing solutions, (Na)(c) increased to an average value of 22 mM and then gradually declined and by 15 min did not differ significantly from the value observed in the absence of bicarbonate. Thus, a twofold increase in J(Na) is not associated with an increase in (Na)(c). These results are in accord with earlier observations on Necturus urinary bladder and small intestine and contradict the notion that an increase in the rate of active Na⁺ extrusion from the cell across the basolateral membrane in response to an increase in the rate of Na⁺ entry across the apical membrane is necessarily the result of a sustained increase in (Na)(c).