AJP - Renal Physiology, Vol 253, Issue 4 760-F766, Copyright © 1987 by American Physiological Society
Ammonia entry along rat proximal tubule in vivo: effects of luminal pH and flow rate
E. E. Simon and L. L. Hamm
Department of Medicine, Jewish Hospital of St. Louis at Washington University, Missouri 63110.
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.
