INTRACELLULAR PH REGULATION IN TROUT URINARY-BLADDER EPITHELIUM - NA+-H+(NH4+) EXCHANGE

We measured intracellular pH (pH(i)) of single epithelial cells in situ in the urinary bladder epithelium using microspectrofluorometry and the cytoplasmically trapped pH-sensitive fluorophore, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The resting pH(i) was 7.21 +/- 0.03 (n = 40 bladders, 489 cells) in pH 7.8 bathing solutions, indicating that H+ is not passively distributed across the plasma membrane and is extruded against its electrochemical gradient. Whereas exposure to hypercapnia (5% CO2 saturation) reversibly decreased pH(i), mucosally added 20 mM NH4+ reversibly increased pH(i). Recovery from the NH4+ effect was slow and lacked an acid-load pH(i) undershoot; this is interpreted as suggesting significant NH4+ permeability. Recovery from hypercapnic acidosis was blocked by mucosally added amiloride, indicating that apical Na+-H+ exchange is involved in pH(i) regulation. Addition of 0.5 mM NH4+ to the basolateral side when the mucosal side was bathed in mock urine (2 mM NaCl) significantly increased undirectional mucosal-to-serosal Na+ flux, and the increase was blocked by mucosally added amiloride. We conclude that an apically located Na+-H+ exchange is important in pH(i) regulation and may also accept NH4+ as the counterion for Na+.