Angiotensin (Ang) II directly stimulates epithelial sodium channel activity in the rabbit cortical collecting duct. Because Ang I and converting enzyme analogues might be present in the distal nephron, this raises the possibility of intraluminal generation of Ang II. Conversion of Ang I to Ang II was monitored by Ang II-dependent changes in intracellular sodium concentration as a reflection of sodium transport across the apical membrane. This involved imaging-based fluorescence microscopy with

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... odium-binding benzofuran isophthalate in isolated, perfused, cortical collecting-duct segments from rabbit kidney. Principal and intercalated cells were differentiated by rhodamineconjugated peanut lectin. Control principal cell intracellular sodium concentration, during perfusion with 25 mmol/L NaCl and zero sodium in the bath plus monensin (10 Ϫ5 mol/L) averaged 5.8Ϯ0.14 mmol/L (nϭ156). The increase in intracellular sodium concentration, when luminal NaCl was increased from 25 to 150 mmol/L, was elevated by 3.5-fold in the presence of intraluminal Ang I (10 Ϫ6 mol/L). Also, the effects of Ang I on sodium transport were not significantly different from the effects of Ang II (10 Ϫ9 mol/L). Ang I was used in micromolar concentrations to ensure that there was sufficient substrate available for conversion to Ang II. Inhibition of the angiotensin-converting enzyme with captopril reduced the stimulatory effect of Ang I. These results suggest that intraluminal conversion of Ang I to Ang II can occur in the cortical collecting duct, resulting in enhanced apical sodium entry. (Hypertension. 2003;42:195-199.)