THE SITE OF ACIDIFICATION OF THE URINE WITHIN THE RENAL TUBULE IN AMPHIBIA

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Abstract
It is clear from the quantitative data given that acidification of urine in the tubule of Nec-turus is a function of the distal, not of the proximal segment of the tubule. Qualitative expts. show with equal convincingness that this is true of frogs (Rana pipiens). None of the expts. confirms the belief of Ellinger and Hirt that the proximal tubule is concerned in the acidification process. Nothing was seen which indicates secretion of an acid fluid by the proximal tubule. In the expts. made by Bensley and Steen comparable to the arterial ligation expts. of Ellinger and Hirt the fluid which was seen in the lumina of proximal tubules following the intraven. injection of phenol red was red. The cells of only about 1/5 of the extent of the distal tubule possess the power of acidification and these are situated somewhat nearer to the distal than to the proximal end of the distal convolution. In many instances the site of these cells was recognizable by a widening of the lumen which they inclose. The demonstration that the acidifying cells can change the reaction of 0.33 M Na phosphate from 7.5 to 6.8 in 1 min. furnishes an intimation that the functional capacity of these cells is far greater than is required to effect acidification of the normal tubule fluid. This soln. has more than 100 times the buffer value of glomerular urine. The change descr. is equivalent to that which would follow the addition of 1/4 volume of 0.33 N HC1 to 1 vol. of the buffer soln. Indications were found that the function of the acidifying cells persists after doses of bicarbonate sufficient to produce marked rise in blood pH. There were no indications that cells other than those normally involved assume a power of acidification when the pH of fluid which bathes them is excessively low. None of the results obtained provides proof of the nature of the acidification process. The facts that the concn. of NaHCO3 in urine approaches zero; that Cl is rarely present in more than a trace; that the concn. of urinary phosphates is only 2-3 times that of the plasma, that in turn being very low; all make it easier to believe that acidification is accomplished by reabsorption of bicarbonate than by secretion of acid. In the absence of conclusive evidence to the contrary, however, the authors are unable to deny that the acidifying cells may secrete acid; if they do, the salt resulting from its neutralization must be absorbed.