The role of bicarbonate in proximal tubular sodium chloride transport

112

A B S T R A C T Brush border membrane vesicles were isolated from rabbit renal cortex by Mg++-precipitation and differential centrifugation. NCI-and [3H]glucose uptakes were simultaneously determined by a rapid filtration technique. Lysis of the vesicles with distilled water abolished 90-95% of the radioactivity on the filters, suggesting that nearly all of the 36Cl-and [3H]-glucose counts represented uptake into an osmotically reactive intravesicular space. Inwardly directed K+ gradients plus valinomycin stimulated 36C1-uptake, demonstrating a conductive pathway for chloride uptake into brush-border membrane vesicles. 36C-uptake could also be stimulated by inwardly directed proton gradients (pHoutside < pHinside). This effect was seen in the absence of sodium, as well as in the presence of valinomycin when the vesicles had equal K+ concentrations inside and out. An "overshoot" phenomenon was observed when external 36C-was 2 mM and the external pH was lowered from 7.5 to 6.0 or to 4.5. The effect of the proton gradient was presumed to be different from the conductive mechanism because (a ) the stimulation of3Cl-uptake by inwardly directed K+ diffusion potentials was additive to the proton gradient effect, and (b) competition studies revealed statistically significant effects of thiocyanate on the conductive pathway, but not on the proton-driven pathway.HC1 cotransport or anion exchange are electrically neutral mechanisms which could couple 3Cl-uptake to inwardly-directed proton gradients in a brush border Portions of these results were presented at the 64th Annual

Section: Methodscontrasting

confidence: 46%

Section: Introductionmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Chloride Uptake by Brush Border Membrane Vesicles Isolated from Rabbit Renal Cortex

Warnock¹,

Yee²

1981

112

“…As metabolic acidosis develops during fasting and plasma bicarbonate falls, the filtered load of bicarbonate falls. In absolute terms, less bicarbonate is reabsorbed which in turn could result in less proximal sodium chloride reabsorption as would be predicted from Maude's work (31). In addition, the nonreabsorbable fraction of the partially reabsorbable keto acids, as suggested by Schwab (26), are in the range of 500 ,umol/min which is associated with osmotic diuresis (32).…”

Section: Discussionmentioning

confidence: 69%

The mechanism of the natriuresis of fasting.

Sigler¹

1975

A B S T R A C T This study tests the hypothesis that obligatory cation coverage of metabolically generated anions is the mechanism for the sodium diuresis of fasting. Nine obese female subjects were equilibrated on a constant sodium and caloric intake and then fasted while sodium intake was maintained. Particular attention was paid to maintaining the same upright activity schedule during fasting as during control. Consecutive 3-h increases in urinary sodium, ammonium, and potassium excretion during fasting were matched against simultaneously determined increases in organic acid anions (OAS) and H2PO4-, which would exist in combination with the cations. The changes were significantly correlated (r = 0.891, P < 0.001) in the relationship y = 0.73x + 19 where y equals increases in organic acid salts + H2PO4-and x equals increases in cations. As ammonium excretion rose, sodium conservation occurred with ammonium replacing sodium at the major urinary cation. Corollaries to the hypothesis were also found to be true. They were: (a) Increases in ammonium excretion lagged considerably behind increases in OAS + H2PO4c during the diuretic phase making sodium coverage necessary. (b) Sodium loss was much greater than chloride although chloride balance was minimally negative. (c) After refeeding with glucose, sodium excretion promptly decreased and appeared best correlated with simultaneous decreases in OAS. Ammonium excretion also fell but much less than sodium. The data support the hypothesis that obligatory cation coverage of metabolically generated anions is a major mechanism responsible for the sodium diuresis of fasting. INTRODUCTIONLarge quantities of electrolyte and water are lost in the urine during the 1st wk of fasting. The principal

“…In addition, if the reflection coefficient for NaCl is less than those for glucose, amino acids, and NaHCO,, an effective osmotic pressure gradient existed across tubules perfused with B solution. As will be discussed later, both the Cl gradient and effective osmotic gradient have been proposed as passive driving forces for volume reabsorption (12)(13)(14)(15)(16)(17).2 Solution C was designed to omit those factors that are cturrently Osmolality of all solutions 298-302.…”

Section: Methodsmentioning

confidence: 99%

Characteristics of Volume Reabsorption in Rabbit Superficial and Juxtamedullary Proximal Convoluted Tubules

Jacobson¹

1979

A B S T R A C T Segments of superficial and juxtamedullary proximal convoluted tubules of the rabbit were perfused in vitro to examine the mechanisms responsible for net volume reabsorption. The very early postglomerular segments were not studied. Fluid reabsorptive rates and transepithelial potential differences were compared under various conditions: (a) with perfusate that simulated glomerular filtrate; (b) with perfusate that lacked glucose, amino acids, and acetate and that had HCO3 and Cl concentrations of 5 and 140 mM, respectively; (c) with perfusate that lacked glucose, amino acids, and acetate but with 20 meq ofNaHCO3 replaced with 20 meq ofNa cyclamate; (d) with the same perfusate as in b but in the presence of ouabain in the bath; (e) with ultrafiltrate of rabbit serum titrated with HC1 to final HCO3 and Cl concentrations of 2 and 134 mM, respectively. Tubules were perfused with this titrated ultrafiltrate at 37°C, 21°C, and in the presence of 0.1 mM ouabain in the bath. Bath fluid in all experiments was regular rabbit serum. Under conditions a and b superficial proximal convoluted tubule (SFPCT) and juxtamedullary proximal convoluted tubule (JMPCT) behaved similarly with the exception that SFPCT exhibited a lumen-positive and JMPCT a lumen-negative electrical potential under condition b. However, undeT condition c SFPCT failed to exhibit net volume reabsorption, whereas reabsorption in JMPCT continued unchanged. Ouabain did not affect volume reabsorption in SFPCT under condition