CHANGES IN INTRACELLULAR SODIUM DURING THE HYDROOSMOTIC RESPONSE TO VASOPRESSIN

During vasopressin (VP)-induced water movement, toad urinary bladder epithelial cells undergo unique morphological changes. The osmolality within these responding cells remains relatively stable despite the large transcellular transport of water. We hypothesized that the hydroosmotic response to VP may be associated with a net increase in sodium either as an aid in maintaining the intracellular osmolality or as part of a Na-Ca exchange process. Changes in intracellular sodium (Na(i)) were monitored over time in individual hemibladders using Na-23 NMR. Hemibladders were mounted as bags on glass pipets and filled with deionized water. During NMR studies, the serosal bath consisted of aerated 2.4 mM HCO3 amphibian Ringer's (pH 8.1) made up with 15% D2O containing the shift reagent, dysprosium tripolyphosphate (1 mM). This reagent allowed for visualization of Na(i) by shifting the extracellular Na signal; it did not affect basal or VP stimulated water flow, short-circuit current, or high energy phosphate metabolism as seen by P-31 NMR. Changes in Na(i) were determined by integrating the area under the unshifted Na peak at each measurement and expressing differences as a ratio relative to baseline. The initial Na(i) signal from unstimulated hemibladders remained stable in these tissues over at least 180 minutes. Within 30 minutes of VP (20 mU/ml) exposure, however, the Na(i) peak increased 2.47 times above pretreatment baseline (N = 16, P < 0.001). The Na(i) signal returned toward baseline values with removal of VP from the serosal bath but only after approximately 90 minutes. When change in cell shape and water movement were prevented by having isotonic sorbitol in the mucosal bath, VP produced no change in the Na(i) signal (N = 10). Thus, Na(i) rose during peak VP-stimulated water flow in parallel with the changes in cell shape. The Na most likely originated from the serosal bath since no Na was present in the mucosal bath.