We determined whether aspontaneous luminal disequilibrium pH,pHdq (pHmeasured -pHequilibrium), waspresent inisolated perfused rabbit S2andS3proximal tubules. Luminal pH wasmeasured byperfusing with thefluorescent pHprobe 1,4DHPN,andtheequilibrium pHwascalculated fromthemeasured collected total CO2anddissolved CO2concentrations. S2 tubules failed togenerate aspontaneous pHdq. S3tubules generated aspontaneous acidic pH~lof-0.46±0.15 (P< 0.05), which wasobliterated following theaddition ofcarbonic anhydrase (0.1 mg/ml) totheperfusate. InS3tubules perfused and bathed in4mM total ammonia, luminal total ammonia rose from4.08±0.05 mM (perfusate) to4.95±0.20 mM (collected fluid) (P< 0.02). Carbonic anhydrase addedtotheperfusate prevented therise inthecollected total ammonia concentration. Weconclude that therabbit S3proximal tubule lacks functional luminal carbonic anhydrase. Theacidic pHdintheS3segment enhances thediffusion ofNH3into thelumen.-In contrast, the S2segment hasfunctional luminal carbonic anhydrase.
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