Effect of Reduced Renal Mass on Ammonium Handling and Net Acid Formation by the Superficial and Juxtamedullary Nephron of the Rat

Abstract

A B S T R A C T Papillary and surface micropuncture were used to study the handling of ammonium and the formation of net acid by surface nephrons, deep nephrons, and the terminal segment of collecting duct (CD) after renal mass was reduced by two-thirds. Net acid excretion by the remnant kidney (RK) was significantly reduced, averaging 794±81 neq/min (SE) compared with 1,220±105 neq/min after sham operation (P < 0.001), due to a decrease in ammonium excretion (494±54 vs. 871±79 nmol/min in controls, P < 0.001). Urinary pH and titratable acid excretion were not different in the two groups of animals. After RK formation, ammonium delivery to the end of the proximal tubule increased nearly threefold and averaged 66.2±5.6 compared with 18.4±2.9 pmol/min in controls, (P < 0.001). This greater delivery of ammonium was primarily due to renal tubule entry rather than to changes in the filtered load and was only partially related to the differences in flow rate. Ammonium processing by deep nephrons was profoundly affected by a reduction in renal mass. Although absolute delivery of ammonium was greater to the bend of Henle's loop (BHL), the difference could be accounted for on the basis of an increase in nephron size. Received for publication I November 1982 and in revised form 23 February 1983. with the RK. Hydrogen secretion in the proximal segments of deep and surface nephrons did not increase in proportion to the decrease in renal mass and as a consequence bicarbonate delivery to the end of the proximal tubule of surface nephrons and to the BHL of deep nephrons was increased.When renal mass was reduced FDNH. to the base of the terminal CD doubled but did not change by the tip. In both groups FDNI4+ to the base of the CD was greater than to the end of the distal tubule. However, the increase was the same. On the other hand, the increase in the net acid index between the end of the distal tubule and the base of the CD was profoundly greater in rats with an RK. This difference was primarily due to bicarbonate reabsorption rather than enhanced ammonium reentry. Indeed, >400% of the fractional ammonium delivered to the end of the proximal tubule was lost from the tubule fluid. The data suggest that the decrease in acid excretion by the RK is due to two factors. First, hydrogen secretion in the proximal segments of both nephron populations fails to increase in the proportion to the reduction in renal mass. Second, a reduced reentrapment of ammonia, rather than its impaired production, causes ammonium excretion to decrease.

Load dependence of proximal tubular fluid and bicarbonate reabsorption in the remnant kidney of the Munich-Wistar rat.

“…In the subsequent three studies, absolute rates of proximal bicarbonate reabsorption were measured and uniformly found to be increased (6, 8, 9). The increase above control was variable, however, and fractional reabsorption fell in two (8,9) of the three studies. Although a reduction in fractional reabsorption might, at first glance, support the notion that proximal bicarbonate reabsorptive capacity was diminished in the remnant kidney, the observations might also be explained by the experimental setting.…”

“…Although a reduction in fractional reabsorption might, at first glance, support the notion that proximal bicarbonate reabsorptive capacity was diminished in the remnant kidney, the observations might also be explained by the experimental setting. In two of the three studies, acute ECF volume expansion was induced with hypertonic alkali-containing solutions during the period of study (6,8). Despite ECF volume expansion, fractional reabsorption did not fall in one of these studies (6).…”

“…When kidney tissue is removed surgically in the rat, the single nephron glomerular filtration rate (SNGFR)' in the remaining nephrons increases dramatically (1)(2)(3)(4)(5)(6)(7)(8)(9). This increase in filtration rate imposes an increased reabsorptive burden on the residual nephrons, ifelectrolyte and acid-base balance is to be preserved.…”

“…Part of the discrepancy among studies load; FPR, fractional proximal reabsorption; J, reabsorption rate per millimeter per minute; SNGFR, single nephron glomerular filtration rate. relates to differences in experimental conditions, i.e., extracellular fluid (ECF) volume, age, or species studied. In most studies, fractional fluid and bicarbonate reabsorption in the proximal tubule have been reduced, as compared to control (3,5,8,9,12 (5,6,8,9). An increase in proximal fluid reabsorption in the isolated perfused nephron of the rabbit remnant kidney has also been observed (13)(14)(15).…”

“…Despite ECF volume expansion, fractional reabsorption did not fall in one of these studies (6). In the third study, proximal bicarbonate reabsorption was studied in immature rats under conditions of hydropenia (8 The load dependence of bicarbonate reabsorption in the remnant nephron occurs largely in the early proximal tubule. As shown in Fig.…”