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Abstract

The ammonia pressure (P_{NH 3}) within the proximal and distal convoluted tubules was investigated in Charles River rats by micropuncture and microperfusion. P_{NH 3} was estimated on free-flow luminal fluid from total ammonia concentration (Berthelot reaction) and pH (quinhydrone electrode), development of disequilibrium pH being prevented in some experiments by simultaneous intravenous infusion of carbonic anhydrase. In free-flow the ammonia concentration rises and the pH decreases along the proximal convoluted tubule. Within the limits of error of the methods, no significant difference of P_{NH 3} was demonstrable between the distal (121±29·10⁻⁶ mm Hg) and proximal convolution (118±27·10⁻⁶ mm Hg) nor between successive segments of the proximal convolution, despite decline of pH of luminal fluid in the segments punctured. The mean value for P_{NH 3} of the cortical convolutions did not significantly differ from the P_{NH 3} of renal venous blood (109±20·10⁻⁶ mm Hg) estimated in the same experiments. Segments of proximal cortical convolutions and distal cortical convolutions were also perfused with an ammonia-free, carbonic-anhydrase-containing phosphate buffer solution at pH 6.30, compounded in such a way as to minimize net gain or loss of luminal water and sodium. Perfusate P_{NH 3} attained a constant value within 0.1–0.2 second, and the value was similar in proximal and distal convolutions (115±32 and 116±30·10⁻⁶ mm Hg respectively). The microperfusion data show that the permeability of the cortical nephrons to ammonia is very high (P = at least 1·10⁻² cm/sec). All the data support the hypothesis that the several compartments of cortical water of rat kidneys are equilibrated to a near approximation with respect to P_{NH 3}.