Renal hydrogen ion excretion increases with chronic acid loads and decreases with alkali loads. We examined the mechanism of adaptation by analyzing vacuolar proton-translocating adenosine triphosphatase (H' ATPase) 31-kD subunit protein and mRNA levels, and immunocytochemical distribution in kidneys from rats subjected to acid or alkali loads for 1,3,5,7, and 14 d. Acid-and alkali-loaded rats exhibited adaptive responses in acid excretion, but showed no significant changes in H' ATPase protein or mRNA levels in either cortex or medulla. In contrast, there were profound adaptive changes in the immunocytochemical distribution of H' ATPase in collecting duct intercalated cells. In the medulla, H' ATPase staining in acidloaded rats shifted from cytoplasmic vesicles to plasma membrane, whereas in alkali-loaded rats, cytoplasmic vesicle staining was enhanced, and staining of plasma membrane disappeared. In the cortical collecting tubule, acid loading increased the number of intercalated cells showing enhanced apical H' ATPase staining and decreased the number of cells with basolateral or poorly polarized apical staining. The results indicate that both medulla and cortex participate in the adaptive response to acid and alkali loading by changing the steady-state distribution of H' ATPase, employing mechanisms that do not necessitate postulating interconversion of intercalated cells with opposing polarities. (J. Clin. Invest. 1991. 88:126-136.)

Papillary and surface micropuncture in Munich-Wistar rats was used to assess the role of proximal segments of superficial and juxtamedullary (JM) nephrons, the distal tubule of superficial nephrons, and the terminal collecting duct in acid excretion. The relative role of these segments in ammonium production, bicarbonate reclamation, and net acid formation was assessed under hydropenic conditions and after a chronic acid load. In these two settings the proximal segment of both kinds of nephrons is the major site of ammonium production and bicarbonate reclamation. However, this segment's contribution to net acid formation was only significant during acidosis. On the other hand, segments beyond the distal tubule appear to be the major site of acid formation. In situ pH measurements were lower in these nephron segments and fell even more after the induction of an acidosis. Ammonia appears to enter fluid between the end of the distal tubule and the base of the collecting duct. In vivo pH measurements made near the bend of Henle's loop of JM nephrons were more alkaline than near the end of the proximal tubule of superficial nephrons. It is postulated that this difference in pH allows ammonium to dissociate, permitting the movement of ammonia out of the tubule lumen and into collecting duct fluid where it is protonated and, therefore, reentrapped. This process is enhanced by the ingestion of a chronic acid load.

Pages 1401-1408: Daniel R. Martin, Joe B. Pevahouse, David J. Trigg, David L. Vesely, and John E. Buerkert. “Three peptides from the ANF prohormone NH2- terminus are natriuretic and/or kaliuretic.” Pages 1404 and 1406, Tables 2 and 3, respectively: in the boxhead, the units for UNaV should be μeq·min-1·g kidney wt-1.

The present investigation was designed to determine whether peptides derived from the NH(2)-terminal portion of the 126-amino acid prohormone (pro) of atrial natriuretic factor (ANF) have natriuretic and diuretic properties similar to ANF. Three peptides consisting of amino acids 1-30 [(proANF-(1-30)], 31-67 [proANF-(31-67)], and 79-98 (proANF-(79-98)] of the ANF prohormone were tested and compared with the COOH-terminus peptide (ANF) with respect to their ability to increase urine volume, urine sodium and potassium excretion, and glomerular filtration rate (GFR) in anesthetized Munich-Wistar rats. Each of these peptides except proANF-(79-98) caused a significant diuresis (P less than 0.05) when infused at their respective 100 ng.kg body wt-1.min-1 concentrations for 120 min. ProANFs-(1-30), (31-67), (79-98), and (99-126) (ANF) increased sodium excretion by 231, 973, 167, and 1,405%, respectively. The fractional excretion of sodium compared with control was significant at P less than 0.05, P less than 0.01, and P less than 0.05 for proANFs (1-30), (31-67), and (99-126), respectively. ProANF-(79-98) did not significantly increase the fractional excretion of sodium, but it was the only peptide from the NH(2)-terminus of the prohormone that significantly increased the fractional excretion of potassium's ProANF-(31-67) did not increase urinary potassium excretion. ProANF-(1-30), (79-98), and ANF significantly (P less than 0.05) increased urinary potassium excretion. None of these peptides significantly enhanced GFR. In conclusion, three peptides from the NH(2)-terminus of the ANF prohormone as well as ANF (the COOH-terminus) have either natriuretic, kaliuretic, and/or diuretic properties, but the respective ability of each of these peptides to produce these effects varies considerably.

Pages 1401–1408: Daniel R. Martin, Joe B. Pevahouse, David J. Trigg, David L. Vesely, and John E. Buerkert. “Three peptides from the ANF prohormone NH2- terminus are natriuretic and/or kaliuretic.” Pages 1404 and 1406, Tables 2 and 3, respectively: in the boxhead, the units for UNaV should be μeg·min-1·g kidney wt-1.

Recollection micropuncture in Munich-Wistar rats was used to study the effects of intravenous hypertonic mannitol infusions on fluid reabsorption by surface nephrons, prior to the bend of Henle's loop of deep nephrons, and along the papillary collecting duct. During mannitol diuresis, single nephron glomerular filtration rate rose significantly in surface nephrons but fell in deep nephrons. Although mannitol increased the delivery of sodium and water to the end of the proximal tubule and to the first portion of the distal tubule of surface nephrons, water and sodium were reabsorbed between these two sites. In deep nephrons, water reabsorption prior to the bend of the loop of Henle was significantly decreased. Absolute sodium delivery to this site was reduced despite a marked decrease in fractional sodium reabsorption prior to the bend. Papillary osmolality was decreased. Renal plasma flow and inner medullary plasma flow (IMPF) increased proportionally. The reduced water extraction prior to the bend of deep nephrons and the decrease in papillary osmolality could have been partly due to a concomitant increase in IMPF and a decrease in sodium delivery to the medulla. The reabsorption of delivered sodium and water by the papillary collecting duct was reduced to a greater extent than could be expected from the increase in sodium delivery.

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.

Nonionic diffusion and diffusion equilibrium of ammonia have been generally accepted as the mechanism of urinary ammonium excretion. However, these characteristics have not been examined directly in vitro. In the present studies, nonionic diffusion and diffusion equilibrium of ammonia were examined in rabbit cortical collecting tubules perfused in vitro. Collected fluid ammonium and pH were measured in tubules exposed to chemical gradients of NH3/Nrn. In tubules perfused with an acid perfusate free of ammonia and bathed with solutions containing NH4CI, collected fluid ammonia failed to equilibrate across the epithelium except at slow flow rates. The estimated apparent permeability coefficient to NH3 was -5 X 10-3 cm/s. Predominant nonionic diffusion of NH3, rather than transport of NW, was indicated by alkalinization of luminal fluid in tubules exposed to peritubular NH4C1 and by the relative influence of peritubular NHZ and NH3 on ammonia entry. In tubules perfused with an acid solution containing NH4CI, little loss of ammonium was detectable, indicating a low permeability to NHZ.In contrast to the restricted diffusion of NH3 in cortical collecting tubules, proximal convoluted tubules exhibited a much higher apparent permeability to NH3. In conclusion, nonionic diffusion of NH3 accounted for most ammonium transport in the proximal convoluted tubule and in the cortical collecting tubule. However, there was relatively restricted diffusion in the collecting tubules; this may account for the failure of whole kidney ammonium excretion to obey quantitatively the predictions of nonionic diffusion and diffusion equilibrium of ammonia.