The mechanism of bicarbonate reabsorption in the proximal and distal tubules of the kidney. 1965.

The mechanism of HCO3- reabsorption in proximal and distal tubules was examined in rats undergoing NaHCO3 diuresis. The steady-state intratubular pH was measured with pH-sensitive glass microelectrodes and compared with the equilibrium pH calculated from the HCO3- concentration of the tubular fluid (measured with quinhydrone electrodes) and plasma Pco2. In the proximal tubule the intratubular pH and the equilibrium pH were identical, indicating no accumulation of excess H2CO3. After inhibition of carbonic anhydrase, however, intratubular pH was significantly lower (0.85 pH U) than the equilibrium pH. It was concluded that HCO3- reabsorption in the proximal tubule was mediated by H+ secretion, but that carbonic anhydrase located in the luminal membrane of the cell prevented H2CO3 from accumulating in the tubular fluid. In the distal tubule the intratubular pH was 0.85 U lower than the equilibrium pH. This difference could be obliterated by an intravenous injection of carbonic anhydrase. It was concluded that HCO3- reabsorption in this segment was also accomplished by H+ secretion. The accumulation of excess H2CO3 in the tubular fluid indicated that, in contrast to the proximal tubule, carbonic anhydrase was not located in the luminal membrane of distal tubular cells.

Role of Na-dependent Cl/HCO3 exchange in basolateral Cl transport of rabbit proximal tubules.

To analyze the rate of basolateral Cl exit and the magnitude and relative contributions of KCl cotransport and Na-dependent and -independent Cl/HCO3 exchange to Cl exit across the basolateral membrane (BLM) during transcellular Cl absorption, rabbit proximal convoluted tubules (PCT) were perfused with high-Cl, low-HCO3 plus formate solutions and bathed with plasma ultrafiltrate-like plus formate solutions. The initial rates of intracellular Cl activity (AiCl) reduction following bath Cl removal were compared when bath Cl was 0, when bath Na and Cl were 0, and when bath HCO3 and Cl were 0. The initial rate of AiCl reduction following bath Cl removal was 4.4 +/- 0.4 mM/s. After bath Na and Cl removal, this rate was reduced to 25.1 +/- 5.0%. After bath HCO3 and Cl removal, it was reduced to 18.0 +/- 4.8%. The difference between bath Na and Cl removal and bath HCO3 and Cl removal was not significant. Cl efflux following bath HCO3 and Cl removal may be due to a KCl symporter. The contribution of a KCl symporter was examined by raising bath K to 20 mM, thus eliminating the chemical driving force for KCl exit. After bath K increase, the initial rate of AiCl increase was 0.057 +/- 0.005 mM/s. These data suggest that Cl efflux at BLM of rabbit PCT is 1) large enough to explain transcellular Cl transport, 2) predominately due to an Na-dependent Cl/HCO3 exchanger, and 3) negligibly due to an Na-independent Cl/HCO3 exchanger or a KCl symporter.

Mechanism of proximal NaCl reabsorption in the proximal tubule of the mammalian kidney.

In the mammalian proximal tubule NaCl reabsorption occurs by both passive and active transport processes. Passive NaCl reabsorption occurs in the presence of a high luminal chloride and a low luminal bicarbonate concentration. These anion gradients provide the driving forces for diffusive Na and Cl movement. Na is driven by the lumen positive PD effected by the greater permeability of the tubular wall to Cl than to HCO3. Cl is driven by its high tubular concentration. Passive NaCl reabsorption accounts for only about 10% to 15% of total proximal NaCl transport. The remaining proximal NaCl is reabsorbed by active transport processes and occurs both in the presence or absence of anion gradients reabsorption. Two mechanisms of active NaCl reabsorption participate in active NaCl reabsorption along the proximal tubule. Firstly, active NaCl reabsorption is electrogenic. In the early proximal tubule Na enters to cell coupled to organic solute transport. This Na reabsorption generates a lumen negative PD and effects "coupled" electrogenic NaCl reabsorption. This mechanism is limited by the supply of organic solutes and is blunted by the greater Na than Cl permeability in the proximal tubule; it probably can account for no more than 10% of proximal NaCl reabsorption. In the terminal proximal tubule, the proximal straight tubule, the apical membrane appears to possess a channel for Na entry. This Na reabsorption also generates a lumen negative PD and effects "simple" electrogenic NaCl reabsorption. This mechanism is limited by the low transport capacity of this segment and probably accounts for no more than 5% to 10% of total proximal NaCl reabsorption. The great bulk of proximal NaCl reabsorption occurs along the entire proximal tubule by active, transcellular electroneutral NaCl reabsorption. The precise cellular transport mechanisms responsible for this process are only recently being defined. At the apical membrane parallel ion exchangers are responsible for NaCl entry into the cell. Na enters via the apical membrane Na-H antiporter. Cl most likely crosses the apical membrane by some combination of Cl-OH and Cl-HCO2 exchangers but not via a Cl-HCO3 exchanger. The relative contributions of Cl-OH and Cl-HCO2 exchange have not been defined. There are two important considerations in this question. First is the availbility of OH versus HCO2. Although there is an infinite supply of OH and a small equilibrium supply of HCO2, it is possible that the luminal concentration of HCO2 could be increased by an USL that raises the concentration of HCO2 to a degree sufficient to supply H2CO2 recycling for physiological transcellular Cl transport rates.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of rat renal Na/H antiporter mRNA levels in response to respiratory and metabolic acidosis.

The mammalian proximal tubule is an important mediator of the renal adaptive response to systemic acidosis. In chronic metabolic and respiratory acidosis the bicarbonate reabsorptive (or proton secretory) capacity is increased. This increase is mediated, at least in part, by an increase in Vmax of the luminal Na/H antiporter. To determine whether this adaptation involves increased mRNA expression, Na/H antiporter mRNA levels were measured by Northern analysis in renal cortex of rats with metabolic (6 mmol/kg body wt NH4Cl for 2 or 5 d) and respiratory (10% CO2/air balanced for 2 or 5 d) acidosis and of normal, pair-fed rats. Na/H antiporter mRNA levels were unchanged after 2 d of both metabolic and respiratory acidosis. After 5 d, however, Na/H antiporter mRNA expression was increased 1.76 +/- 0.12-fold in response to metabolic acidosis (P less than 0.005, n = 8), but was not different from normal in response to respiratory acidosis: 1.1 +/- 0.2 (NS, n = 8). Thus, the renal adaptive response to metabolic acidosis involves increased cortical Na/H antiporter mRNA levels. In contrast, the enhanced proximal tubule Na/H antiporter activity and bicarbonate reabsorption in respiratory acidosis seem to involve mechanisms other than increased Na/H antiporter gene expression.

Chloride transport across the basolateral membrane of rabbit proximal convoluted tubules.

To examine the basolateral Cl transport mechanisms of proximal convoluted tubules (PCT), intracellular Cl activity (AiCl) was measured with double-barreled Cl-selective microelectrodes. When rabbit PCT were perfused in vitro with high Cl, low HCO3, and bathed with ultrafiltrate-like solutions, AiCl was 29.9 +/- 0.4 mM and basolateral membrane voltage (Vbl) was -47.7 +/- 0.4 mV (n = 247). Possible basolateral Cl transport mechanisms that we examined were as follows: Cl conductance, KCl cotransport, and Na-dependent Cl-HCO3 exchange. Cl conductance was negligible, since the voltage clamp of Vbl to 30 mV above and below the spontaneous Vbl did not change AiCl even in the absence of luminal Cl. KCl cotransport was suggested by 1) increasing bath K, increased AiCl, and 2) decreasing bath K decreased AiCl. KCl cotransport was Na independent and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), barium, and furosemide insensitive. Na-dependent Cl-HCO3 exchange was suggested by 1) bath HCO3 reduction increased AiCl, which was greatly inhibited by bath Na removal or bath SITS, and 2) bath Na removal increased AiCl, which was completely blocked by bath SITS. We conclude that 1) Cl conductance is negligibly small at the basolateral membrane and 2) SITS-insensitive KCl cotransport and SITS-sensitive Na-dependent Cl-HCO3 exchange are present at the basolateral membrane.

Electroneutral NaCl absorption in the proximal tubule: mechanisms of apical Na-coupled transport.

The proximal tubule utilizes multiple mechanisms to reabsorb filtered NaCl. In the early PCT electrogenic Na-coupled organic solute transport generates a lumen-negative PD which drives Cl- passively through the paracellular pathway. Preferential reabsorption of HCO3- and organic solutes in the early PCT elevates luminal Cl- concentration, which in the late PCT provides the driving force for passive reabsorption of both Na+ and Cl-. However, most of the NaCl reabsorbed in the PCT is mediated by an electroneutral mechanism in which equivalent amounts of Na+ and Cl- move transcellularly across apical and basolateral membranes. In the mammalian PCT the evidence overwhelmingly supports parallel Na+-H+ and Cl- -base exchangers as the mechanism by which Na+ and Cl- cross the apical membrane during electroneutral, transcellular NaCl reabsorption. OH-, HCO3-, formate and Ox- have all been suggested to be the anion exchanged for Cl-. An important physiologic contribution of formate has been shown in in vitro microperfusion studies [29]. Measurements of intracellular pH using fluorescent dyes [59, 60] support a quantitatively important role for formate and argue against a large contribution of OH- and HCO3-. The absence of a role for HCO3- is also supported by in vivo microperfusion studies using methoxazolamide [53]. The potential role of oxalate requires physiologic evaluation. To date, the experimental data suggest that Cl- -formate is probably the predominant anion exchange mechanism. One may ask why, in a process so critical as NaCl reabsorption, the tubule would choose to use a "toxin" rather than one of those ions more familiar to renal physiologists?(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lumen pH on cell pH and cell potential in rabbit proximal tubules.

To determine the effect of luminal pH on cell pH and basolateral cell membrane potential difference (Vbl) of rabbit proximal convoluted tubules, Vbl was measured by conventional microelectrodes and intracellular pH was measured microfluorometrically. Lowering lumen pH acidified the cell and depolarized Vbl. Three factors contributed to depolarization of Vbl. Lowering lumen pH decreased apical cell membrane potassium permeability (PK) as indicated by the following: 1) at lumen pH 7.4 raising lumen [K] depolarized Vbl; 2) lowering lumen pH eliminated the depolarization of Vbl induced by increasing lumen [K]. An additional effect was suggested by the following: lumen Ba2+ blunted, but did not eliminate, the Vbl response to lowering lumen pH. An effect on basolateral K permeability (PK) via its effect on cell pH was suggested by the fact that lowering lumen pH dramatically reduced the depolarization induced by increasing bath [K]. Lowering lumen pH might influence Vbl by inhibiting H+-HCO3- transport. Addition of 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) to the bath solution hyperpolarized Vbl and enhanced the depolarization induced by lowering luminal pH. At luminal pH 6.0 SITS had no effect, suggesting elimination of H+ secretion. Addition of 1 mM luminal amiloride had no effect on Vbl or the response of Vbl to lowering luminal pH, but in the presence of amiloride SITS still hyperpolarized Vbl, suggesting amiloride-insensitive electrogenic H+ secretion. These results suggest that lumen pH-dependent depolarization of Vbl is due to 1) a decrease in apical PK; 2) cell acidification with secondary effects on basolateral PK; and 3) a decrease in apical electrogenic H+ transport.

Rapid, contrast-enhanced, diuretic magnetic resonance imaging of unilateral partial ureteral obstruction. An experimental study in micropigs.

The value of rapid, contrast-enhanced, diuretic magnetic resonance (MR) imaging (using ferrioxamine B and furosemide) in demonstrating partial unilateral ureteral obstruction and the potential of such MR imaging in differentiating obstructive from nonobstructive hydronephrosis was assessed in six micropigs. MR imaging (0.35 Tesla, partial-flip technique with repetition time [TR] of 125 milliseconds, echo-delay time [TE] of 20 milliseconds, and flip angle of 70 degrees) was performed before, and at 5, 12, and 19 days after partial ureteral obstruction. Additionally, MR images were acquired 5, 12, and 19 days after release of obstruction. The diuretic was injected 10 minutes after the contrast medium. MR findings were correlated with results from nuclear scintigraphy (99mTc-DMSA uptake). MR images provided good morphologic detail from which renal size, parenchymal thickness, and degree of hydronephrosis could be determined. Contrast medium allowed assessment of cortical uptake and urinary excretion. The course of cortical signal enhancement best characterized the difference between obstructive and nonobstructive hydronephrosis. Normal kidneys and kidneys with nonobstructive hydronephrosis showed progressive decrease in cortical signal enhancement (-11.7% within 40 minutes) after furosemide injection. The kidneys with obstructive hydronephrosis demonstrated a plateau of signal enhancement without decrease (-0.7% within 40 minutes). These results demonstrate the utility of rapid contrast-enhancing, diuretic MR imaging in differentiating obstructive from nonobstructive hydronephrosis.

Role of Na+-H+ antiport in rat proximal tubule NaCl absorption.

We recently showed that, in the presence of physiological sodium concentrations, 4.3 mM luminal amiloride inhibits 90% of apical membrane Na+-H+ antiporter activity in the in vivo microperfused rat proximal convoluted tubule. In the present studies we examined the effect of 4.3 mM luminal amiloride on transepithelial NaCl absorption from a high-chloride, low-bicarbonate perfusate, simulating the tubular fluid of the late proximal tubule. Both chloride and volume absorption were inhibited approximately 44%, consistent with inhibition of most of transcellular NaCl absorption, and suggestive of parallel Na+-H+ and Cl(-)-base exchange as the mechanism of NaCl uptake across the apical membrane. Methazolamide (10(-4) M), a potent inhibitor of renal carbonic anhydrase, had no significant effect on either volume or chloride absorption, suggesting that a carbonic anhydrase-independent mechanism is at least partially involved in chloride absorption. Hydrochlorothiazide (1 mM), an inhibitor of electroneutral NaCl cotransport in tight epithelia, did not significantly affect either volume or chloride absorption. Thus these studies suggest that, in the rat, the mechanism of apical membrane electroneutral NaCl uptake is Na+-H+ and Cl- -base exchange.

SITS-sensitive basolateral anion current in rabbit proximal convoluted tubules.

To assess the presence and nature of steady-state anion current across the basolateral membrane in in vitro rabbit proximal convoluted tubules bathed and perfused with a high-chloride, low-bicarbonate solution simulating late proximal tubular fluid, steady-state basolateral cell membrane potential difference (Vb1) was measured by conventional microelectrodes. The mean value of Vb1 was -52 mV. Addition of 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) to the bath solution hyperpolarized Vb1 by 30 mV, suggesting the presence of basolateral anion current. Total chloride removal did not change Vb1 significantly, and formate, a presumptive stimulant of electroneutral sodium chloride transport, depolarized Vb1 both in the presence and absence of chloride, suggesting that the formate-stimulated change in Vb1 was chloride independent. In the total absence of chloride and bicarbonate, 1 mM bath SITS and 0.1 mM lumen and bath acetazolamide hyperpolarized Vb1 by 27-35 and 23 mV, respectively. These results suggest that the SITS-sensitive change in Vb1 is independent of chloride and associated with a basolateral anion current that is predominantly due to bicarbonate exit. In the absence of exogenous CO2, cell-to-bath HCO3-dependent anion current can be derived from metabolic CO2.

Regulation of cell pH by ambient bicarbonate, carbon dioxide tension, and pH in the rabbit proximal convoluted tubule.

UNLABELLED: To study the regulation of cell pH by ambient pH, carbon dioxide tension (PCO2), and bicarbonate (HCO3), cell pH was measured in the isolated, in vitro microperfused rabbit proximal convoluted tubule using the fluorescent dye (2',7')-bis-(carboxyethyl)-(5,6)-carboxyfluorescein. For the same changes in external pH, changes in [HCO3] and PCO2 affected cell pH similarly ([HCO3]: pHi/pHe = 0.67, PCO2: pHi/pHe = 0.64, NS). Isohydric changes in extracellular [HCO3] and PCO2 did not change cell pH significantly. Changes in peritubular [HCO3] elicited larger changes in cell pH than changes in luminal [HCO3], which were enhanced by peritubular 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS). The cell pH defense against acute increases and decreases in PCO2 was inhibited by sodium, but not by chloride removal. Peritubular SITS inhibited the cell pH defense against increases and decreases of PCO2, whereas luminal amiloride inhibited cell pH defense against increases in PCO2. CONCLUSIONS: (a) Steady-state cell pH changes in response to changes in extracellular [HCO3] and PCO2 are quantitatively similar for a given change in extracellular pH; (b) the rate of the basolateral Na/(HCO3)3 cotransporter is a more important determinant of cell pH than the rate of the apical membrane mechanism(s); (c) cell pH defense against acute changes in PCO2 depends on the basolateral Na/(HCO3)3 cotransporter (acid and alkaline loads) and the luminal Na/H antiporter (acid loads).

Basolateral membrane Na/base cotransport is dependent on CO2/HCO3 in the proximal convoluted tubule.

The mechanism of basolateral membrane base transport was examined in the in vitro microperfused rabbit proximal convoluted tubule (PCT) in the absence and presence of ambient CO2/HCO3- by means of the microfluorometric measurement of cell pH. The buffer capacity of the cells measured using rapid NH3 washout was 42.8 +/- 5.6 mmol.liter-1.pH unit-1 in the absence and 84.6 +/- 7.3 mmol.liter-1.pH unit-1 in the presence of CO2/HCO3-. In the presence of CO2/HCO3-, lowering peritubular pH from 7.4 to 6.8 acidified the cell by 0.30 pH units and lowering peritubular Na from 147 to 0 mM acidified the cell by 0.25 pH units. Both effects were inhibited by peritubular 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS). In the absence of exogenous CO2/HCO3-, lowering peritubular pH from 7.4 to 6.8 acidified the cell by 0.25 pH units and lowering peritubular Na from 147 to 0 mM decreased cell pH by 0.20 pH units. Lowering bath pH from 7.4 to 6.8 induced a proton flux of 643 +/- 51 pmol.mm-1.min-1 in the presence of exogenous CO2/HCO3- and 223 +/- 27 pmol.mm-1.min-1 in its absence. Lowering bath Na from 147 to 0 mM induced proton fluxes of 596 +/- 77 pmol.mm-1.min-1 in its absence. The cell acidification induced by lowering bath pH or bath Na in the absence of CO2/HCO3- was inhibited by peritubular SITS or by acetazolamide, whereas peritubular amiloride had no effect. In the absence of exogenous CO2/HCO3-, cyanide blocked the cell acidification induced by bath Na removal, but was without effect in the presence of exogenous CO2/HCO3-. We reached the following conclusions. (a) The basolateral Na/base n greater than 1 cotransporter in the rabbit PCT has an absolute requirement for CO2/HCO3-. (b) In spite of this CO2 dependence, in the absence of exogenous CO2/HCO3-, metabolically produced CO2/HCO3- is sufficient to keep the transporter running at 30% of its control rate in the presence of ambient CO2/HCO3-. (c) There is no apparent amiloride-sensitive Na/H antiporter on the basolateral membrane of the rabbit PCT.

Role of the Na+/H+ antiporter in rat proximal tubule bicarbonate absorption.

Amiloride and the more potent amiloride analog, 5-(N-t-butyl) amiloride (t-butylamiloride), were used to examine the role of the Na+/H+ antiporter in bicarbonate absorption in the in vivo microperfused rat proximal convoluted tubule. Bicarbonate absorption was inhibited 29, 46, and 47% by 0.9 mM or 4.3 mM amiloride, or 1 mM t-butylamiloride, respectively. Sensitivity of the Na+/H+ antiporter to these compounds in vivo was examined using fluorescent measurements of intracellular pH with (2', 7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein (BCECF). Amiloride and t-butylamiloride were shown to be as potent against the antiporter in vivo as in brush border membrane vesicles. A model of proximal tubule bicarbonate absorption was used to correct for changes in the luminal profiles for pH and inhibitor concentration, and for changes in luminal flow rate in the various series. We conclude that the majority of apical membrane proton secretion involved in transepithelial bicarbonate absorption is mediated by the Na+-dependent, amiloride-sensitive Na+H+ antiporter. However, a second mechanism of proton secretion contributes significantly to bicarbonate absorption. This mechanism is Na+-independent and amiloride-insensitive.

Evidence for electroneutral sodium chloride transport in rat proximal convoluted tubule.

One- to two-thirds of NaCl absorption in the late proximal convoluted tubule (no luminal organic solutes present) is inhibited by cyanide and thus is dependent on active transport. To examine whether this active transport-dependent NaCl transport is electrogenic or electroneutral, the effect of cyanide on transepithelial potential difference (PD) was measured in the rat proximal convoluted tubule microperfused in vivo. In the presence of an ultrafiltrate-like luminal perfusate containing glucose and alanine, cyanide addition caused the transepithelial PD to change from -0.44 +/- 0.04 to -0.05 +/- 0.03 mV (P less than 0.001). In the presence of a late proximal tubular fluid (high chloride, low bicarbonate, no organics), the transepithelial PD was 1.23 +/- 0.06 mV and was unchanged at 1.19 +/- 0.05 mV after cyanide addition (NS). To eliminate the possibility that an effect of cyanide on a putative acidification-dependent lumen-positive PD was concealing an effect on an electrogenic sodium transport-dependent lumen-negative PD, the above studies were repeated in the presence of acetazolamide. Cyanide did not affect the transepithelial PD (1.17 +/- 0.05 vs. 1.07 +/- 0.06 mV, NS). We conclude that, although cyanide-inhibitable NaCl transport is electrogenic in the presence of luminal organic solutes, it does not generate a transepithelial PD in their absence and therefore is electroneutral.

Amiloride inhibition of proximal tubular acidification.

In brush border membrane vesicles prepared from mammalian kidney cortex, amiloride is a potent inhibitor of the Na+/H+ exchanger. In the present study, in vivo microperfusion was used to examine the effect of luminal amiloride on transport in the rat superficial proximal convoluted tubule. At a perfusion rate of 14 nl/min, addition of 10(-3) M amiloride to artificial early proximal tubular fluid reduced bicarbonate absorption from 103 +/- 7 to 81 +/- 5 pmol mm-1 X min-1 and volume absorption from 2.03 +/- 0.15 to 1.57 +/- 0.06 nl X mm-1 X min-1. Glucose efflux was unchanged, excluding nonspecific inhibition of Na+-K+-ATPase. Luminal amiloride at 10(-4) M did not affect bicarbonate absorption or volume absorption. At a perfusion rate of 41 nl/min, 10(-3) M amiloride reduced bicarbonate absorption from 179 +/- 8 to 114 +/- 9 pmol X mm-1 X min-1, a significantly greater inhibition than that seen in tubules perfused at 14 nl/min. Amiloride at 10(-3) M had no significant effect on sodium chloride absorption as measured by volume flux from an artificial late proximal tubular fluid. The results show that luminal amiloride specifically inhibits proximal acidification and demonstrate involvement of the Na+/H+ antiporter in proximal tubular acidification. However, the inhibition of acidification is less than the inhibition of Na+/H+ exchange predicted by vesicle studies.

A model of proximal tubular bicarbonate absorption.

A model is presented that utilizes determinants of acidification defined from microperfusion studies in the rat to stimulate the effect on absolute bicarbonate absorption along the entire proximal convoluted tubule. Net bicarbonate absorption is considered to consist of active transcellular proton secretion in parallel with passive paracellular bicarbonate diffusion. The rate of proton secretion is calculated as a function of luminal bicarbonate concentration using Michaelis-Menten kinetics. The K1/2 is modified by luminal flow rate and the Vmax by peritubular bicarbonate concentration. Solute-solvent interactions and axial heterogeneity are also included as determinants of proton secretion rate. The model demonstrates that a given percentage stimulation or inhibition of active proton secretion leads to a much smaller effect on absolute proximal bicarbonate absorption along the entire tubular length. This blunting of the stimulation or inhibition is greatest when filtered bicarbonate load is limited by decreases in glomerular filtration rate or plasma bicarbonate concentration. In addition, the model shows that flow dependence is greater at low plasma bicarbonate concentrations, whereas the effect of extracellular fluid volume expansion is greater at high plasma bicarbonate concentrations. Agreement between the model predictions and the results of free-flow micropuncture studies from our laboratory is good with the exception of the effect of raising plasma bicarbonate concentration. This discrepancy is resolvable by allowing the effect of peritubular pH to increase along the length of the tubule, a hypothesis that requires verification.

Active and passive components of NaCl absorption in the proximal convoluted tubule of the rat kidney.

The active and passive components of NaCl absorption were examined in doubly perfused proximal convoluted tubules (PCT) of the rat kidney. When anion concentration gradients were generated by perfusing the lumen with a high chloride, low bicarbonate solution and the peritubular capillaries with a complete solution resembling plasma ultrafiltrate, volume absorption (JV) was 1.79 nl/mm/min and estimated chloride absorption (JCl) was 270 pEq/mm/min. When anion gradients were eliminated by perfusing the peritubular capillaries with a high chloride solution, JV was reduced to 0.91 nl/mm/min and JCl to 140 pEq/mm/min. These residual rates of absorption were reduced to zero by removing potassium from the perfusates. In the presence of anion gradients, removal of potassium reduced JV from 1.79 to 0.60 nl/mm/min and JCl from 270 to 90 pEq/mm/min. It is concluded that: (1) when PCT lumen are perfused with high chloride solution and the peritubular capillaries perfused with an ultrafiltrate-like solution, approximately 50% of NaCl and water absorption is passive, driven by the anion gradients, and 50% is active; (2) when PCT lumen and peritubular capillaries are both perfused with high chloride solution, anion gradients are absent and all NaCl absorption is active.

Gas production after reaction of sodium bicarbonate and hydrochloric acid.

Ingestion of sodium bicarbonate has been implicated as one of the proximate causes of spontaneous gastric rupture. However, the volume and rate of gas released from the reaction of ingested sodium bicarbonate and gastric acid has not been previously studied in detail. We, therefore, developed an in vitro method for measuring gas release after addition of sodium bicarbonate to a solution containing hydrochloric acid. From the results of our studies, we conclude that even though hydrochloric acid and sodium bicarbonate react instantaneously, the resulting gas production is slow, mainly because CO2 produced from the dehydration of carbonic acid dissolves in water and is only slowly released into the gas phase. The major exogenous factors that determine the rate of gas release are the volume of the solution, the quantity of reactants, the air volume over the reaction mixture, the partial pressure of CO2 of the acid solution before the addition of bicarbonate, and the stirring rate. The presence of food, alcohol, and carbonic anhydrase had relatively little if any effect. Based on our results, we believe that ingestion of the recommended dose of sodium bicarbonate (one-half teaspoon) would result in only small amounts of sudden gas release, probably not enough to be an important factor in causing spontaneous gastric rupture. On the other hand, we measured the amount of sodium bicarbonate that people actually select to take for indigestion, and all exceeded the recommended dose. Some people selected doses of bicarbonate that would result in several hundred milliliters of gas release within 3 min; it seems likely that such injudicious ingestion of sodium bicarbonate, if taken when the stomach was distended with air, food, and liquid, could be an important factor in spontaneous gastric rupture.