Ammonia entry along rat proximal tubule in vivo: effects of luminal pH and flow rate.
The roles of luminal pH and flow rate in determining ammonia entry along the rat proximal tubule were examined using in vivo microperfusion. With perfusion rate constant at 15 nl/min, perfusate bicarbonate concentration was varied. Collected fluid ammonia concentration correlated with collected fluid bicarbonate concentration, consistent with nonionic diffusion (r = 0.726; P less than 0.001). Hence ammonia entry was dependent on luminal pH. With perfusate bicarbonate constant at 5 or 25 mM, perfusion rate was varied. In all groups, there was little change in collected fluid ammonia concentration with flow rate. Thus ammonia entry was also highly dependent on flow rate. With paired collections using a 25 mM bicarbonate perfusate, collected fluid bicarbonate was higher at a 30 nl/min perfusion rate than at 15 nl/min (16.8 +/- 1.1 vs. 10.3 +/- 1.1 mM), whereas total ammonia concentrations were similar (0.54 +/- 0.1 and 0.55 +/- 0.1). Thus the NH3 concentration was higher at 30 than at 15 nl/min (6.1 +/- 1.2 vs. 3.4 +/- 0.5 microM; P less than 0.025), a result not predicted by simple nonionic diffusion. Thus these studies demonstrate the importance of nonionic diffusion in determining ammonia entry along the proximal tubule. However, the results also demonstrate that flow rate importantly determines ammonia entry in vivo in a manner not predicted by simple nonionic diffusion of NH3. This augmentation of ammonia entry with increasing flow rate may involve flow-dependent alterations in ammonia synthesis or transport of NH+4.
Simon
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Department of Medicine, Jewish Hospital of St. Louis at Washington University, Missouri 63110.
Hamm
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D000641:Ammonia / Q000493:pharmacokinetics*; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001784:Blood Gas Analysis; D006863:Hydrogen-Ion Concentration; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008433:Mathematics; D010477:Perfusion; D051381:Rats
D001639:Bicarbonates; D000641:Ammonia