Ammonia loss from rat proximal tubule in vivo: effects of luminal pH and flow rate.

The roles of luminal pH and flow rate in determining ammonia loss from proximal tubules perfused with solutions containing 10 mM NH4Cl were examined using in vivo microperfusion. Perfusate bicarbonate concentration was varied between 5, 25, and 40 mM in tubules perfused at 50 nl/min. As expected, ammonia loss from the 25 or 40 mM bicarbonate-containing perfusates was greater than from that containing 5 mM bicarbonate. Furthermore, there was a correlation between ammonia loss and the log mean luminal bicarbonate concentration (r = 0.39, P less than 0.01). From the collected fluid ammonia and bicarbonate concentrations, the transtubular gradients for NH+4 and NH3 were estimated, allowing a calculation of the apparent permeability coefficients for NH3 (PNH3) and NH+4 (PNH+4). The calculated PNH3 of 2.2 +/- 0.5 X 10(-2) cm/s was similar to previous estimates in the rabbit; the calculated PNH+4 of 5.5 +/- 0.8 X 10(-4) cm/s was approximately 10 times that previously found in the rabbit proximal straight tubule in vitro. Next, flow rate was varied between 25 and 50 nl/min using the 5 mM bicarbonate perfusate. Ammonia loss was significantly higher from the latter. Thus these studies suggest that NH+4 loss from the proximal tubule may be an important determinant of ammonia movement along this segment. Ammonia loss is flow-rate dependent, similar to ammonia entry in previous studies.