In vitro fluid secretion by epithelium from polycystic kidneys.

The size of the kidneys in patients with autosomal dominant polycystic kidney disease (ADPKD) is due in large measure to the accumulation of secreted fluid within thin-walled epithelial sacs. We measured the net transepithelial movement of liquid in response to forskolin in isolated, intact cysts excised from the surface of human ADPKD kidneys and in cultured, polarized monolayers of epithelial cells derived from ADPKD cysts. 10 excised cysts bathed symmetrically in control culture medium secreted fluid at a rate of 0.19 +/- 0.03 microliter/cm2 per hour after stimulation with forskolin (10 microM). Ouabain (100 microM) addition to the cavity fluid did not change the rate of fluid secretion of 10 forskolin-treated cysts, but addition of the glycoside to the external bathing medium fluid of nine cysts decreased secretion to -0.004 +/- 0.05 microliter/cm2 per hour. 24 monolayers absorbed fluid (range -0.029 to -0.412 microliter/cm2 per hour); by contrast, fluid was secreted (range 0.074 to 1.242 microliters/cm2 per hour) after stimulation with forskolin (10 microM). Ouabain (0.1 microM) in the basolateral but not in the apical medium inhibited fluid secretion. Forskolin increased the intracellular cyclic AMP content of ADPKD and MDCK monolayers by 236 and 196%, respectively. Six ADPKD monolayers had stable lumen negative transepithelial electrical potential differences (PDte) of -1.4 +/- 0.3 mV, positive short circuit currents (SCC) of 11.9 +/- 2.1 microAmp/cm2 and a tissue resistance (Rte) of 116 +/- 14 ohm.cm2. Forskolin increased SCC to 15.5 +/- 1.9 microAmp/cm2 (P < 0.005) and decreased Rte to 95 +/- 13 ohm.cm2 (P < 0.05); PDte remained stable at -1.4 +/- 0.3 mV. Ouabain (10 microM) had no effect when added to the apical medium, but in the basolateral medium decreased SCC to 1.7 +/- 0.3 microAmp/cm2 and PDte to -0.2 +/- 0.1 mV. We conclude that ADPKD cells in surface cysts have the potential to absorb or to secrete solutes and fluid. cAMP-mediated fluid secretion from the basolateral medium into the lumen of surface ADPKD cysts may be driven by anion transport.

A synthetic channel-forming peptide induces Cl(-) secretion: modulation by Ca(2+)-dependent K(+) channels.

A synthetic Cl(-) channel-forming peptide, C-K4-M2GlyR, applied to the apical membrane of human epithelial cell monolayers induces transepithelial Cl(-) and fluid secretion. The sequence of the core peptide, M2GlyR, corresponds to the second membrane-spanning region of the glycine receptor, a domain thought to line the pore of the ligand-gated Cl(-) channel. Using a pharmacological approach, we show that the flux of Cl(-) through the artificial Cl(-) channel can be regulated by modulating basolateral K(+) efflux through Ca(2+)-dependent K(+) channels. Application of C-K4-M2GlyR to the apical surface of monolayers composed of human colonic cells of the T84 cell line generated a sustained increase in short-circuit current (I(SC)) and caused net fluid secretion. The current was inhibited by the application of clotrimazole, a non-specific inhibitor of K(+) channels, and charybdotoxin, a potent inhibitor of Ca(2+)-dependent K(+) channels. Direct activation of these channels with 1-ethyl-2-benzimidazolinone (1-EBIO) greatly amplified the Cl(-) secretory current induced by C-K4-M2GlyR. The effect of the combination of C-K4-M2GlyR and 1-EBIO on I(SC) was significantly greater than the sum of the individual effects of the two compounds and was independent of cAMP. Treatment with 1-EBIO also increased the magnitude of fluid secretion induced by the peptide. The cooperative action of C-K4-M2GlyR and 1-EBIO on I(SC) was attenuated by Cl(-) transport inhibitors, by removing Cl(-) from the bathing solution and by basolateral treatment with K(+) channel blockers. These results indicate that apical membrane insertion of Cl(-) channel-forming peptides such as C-K4-M2GlyR and direct activation of basolateral K(+) channels with benzimidazolones may coordinate the apical Cl(-) conductance and the basolateral K(+) conductance, thereby providing a pharmacological approach to modulating Cl(-) and fluid secretion by human epithelia deficient in cystic fibrosis transmembrane conductance regulator Cl(-) channels.

Intracranial lesions shown by CT scans in 259 cases of first alcohol-related seizures.

We obtained CTs in 259 patients with a first alcohol-related convulsion. Each subject had generalized convulsions, recent abstinence from alcohol abuse, and no obvious etiology for seizures other than alcohol withdrawal. Patients with only focal seizures, major head injury, coma, or a severe toxic-metabolic disorder were excluded. We recorded history and signs of minor head injury, presence of headache, level of consciousness, neurologic signs, routine medical examination findings, and subsequent clinical course. Sixteen patients (6.2%) had intracranial lesions on CT. Eight had subdural hematomas or hygromas, two had vascular malformations, two had neurocysticercosis, and one each showed a Berry aneurysm, possible tumor, skull fracture with subarachnoid hemorrhage, and probable cerebral infarction. In ten cases (3.9%), clinical management was altered because of the CT result. History or signs of minor head trauma, headache, level of consciousness, or focal neurologic signs did not significantly correlate with CT abnormality.

Small-bowel obstruction caused by a long-term indwelling urinary catheter.

The first known case report of a small-bowel obstruction caused by a long-term indwelling Foley catheter is presented. The balloon of the catheter passed into and obstructed the lumen of the distal ileum through a vesicoenteric fistula created by chronic irritation. With the exception of recurrent urinary-tract infections, complications of urinary catheters are rare. The patient presented a diagnostic dilemma that was solved with a preoperative computed tomographic scan.

Compensation of respiratory alkalosis induced after acclimation to simulated altitude.

Conscious intact rats previously acclimated for 3 wk to barometric pressure of 370-380 Torr (3WHx) were made alkalotic for 3 h by a decrease in inspired O2 fraction from 0.10 to 0.075 at ambient barometric pressure (730-740 Torr). Controls were normoxic littermates (Nx) in which inspired O2 fraction was lowered from approximately 0.21 to 0.10 for 3 h. Arterial PCO2 decreased progressively and similarly in both groups (65-70% of control at 15 min). Initially, arterial pH increased less in 3WHx (0.09 +/- 0.004 vs. 0.15 +/- 0.008). As hypocapnia continued, delta[HCO3-]/delta pH (mmol.l-1.pH) became more negative in Nx, from -15.2 +/- 2.5 at 15 min to -37.0 +/- 2.9 at 3 h, indicating nonrespiratory compensation of alkalosis. In 3WHx, delta[HCO3-]/delta pH did not change during alkalosis. Cumulative renal excretion of base (mueq/100 g) during alkalosis increased by 73.2 +/- 11.1 in Nx and 25.4 +/- 7.3 in 3WHx. This difference was mainly due to a larger increase in HCO3- excretion in Nx. The data suggest that the smaller compensation of hypocapnic alkalosis in 3WHx is partly due to the smaller increase in renal base excretion. Because base availability limits renal base excretion, the smaller renal response of 3WHx may be secondary to the low plasma HCO3- concentration that accompanies altitude acclimation.

Resolution of syringomyelia and Chiari I malformation by ventriculoatrial shunting in a patient with pseudotumor cerebri and a lumboperitoneal shunt.

A sneeze caused acute left arm pain in a 36-year-old woman with a lumboperitoneal (LP) shunt that had been placed 3 years earlier for relief of headaches caused by pseudotumor cerebri. Numbness progressed up the left arm, neck, and back of the head and finally into the left face along with weakness of the hand and arm. Magnetic resonance imaging (MRI) and computed tomography revealed new tonsillar herniation and a large eccentric syrinx extending from C2 to T6. The functioning LP shunt was clamped, and a ventriculoatrial shunt was placed. Pain lessened and motor function improved slightly. MRI revealed complete resolution of the syrinx and resolution of the tonsillar herniation. Theories of syringomyelia formation, the relationship to Chiari I malformation, and the implications of this case are discussed.

Specificity of basolateral organic anion exchanger in proximal tubule for cellular and extracellular solutes.

We used an epifluorimetric technique to study the mechanism of organic anion transport across the basolateral surface of isolated S2 segments of rabbit proximal tubules. Fluorescein influx and efflux rates across the basolateral surface of lumen-collapsed tubules were determined from serial measurements of cellular fluorescein content after its addition to or removal from the bathing medium. We examined the effect on fluorescein transport of monocarboxylic and dicarboxylic metabolic intermediates added to the bathing medium or preloaded into the cells. The presence of the monocarboxylates (octanoate, valerate, butyrate, propionate, and acetate) in the bathing medium inhibited fluorescein influx. The eight-carbon-chain fatty acid, octanoate, was nearly as potent an inhibitor as probenecid and was more potent than p-aminohippurate (PAH) (IC50 = 10, 6, and 141 microM, respectively); the shorter chain fatty acids were much less effective (IC50 greater than or equal to 1,000 microM). The dicarboxylates (succinate, adipate, and alpha-ketoglutarate) moderately inhibited fluorescein influx (IC50 = 505, 245, and 257 microM, respectively). To determine the effect of intracellular organic anions on fluorescein influx, tubules were preincubated with organic anions to load the cells; the compounds were then removed from the bath, and fluorescein influx was measured. Preincubation with monocarboxylates, octanoate, butyrate, acetate, and PAH had no effect. In contrast, the dicarboxylates alpha-ketoglutarate, glutarate, and adipate stimulated fluorescein influx by 58, 53, and 43%, respectively, but succinate had no effect. Fluorescein efflux was accelerated by medium octanoate, PAH, alpha-ketoglutarate, and succinate but not by acetate. Probenecid alone had no effect on fluorescein efflux, but it blocked the stimulation caused by medium PAH.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of barium on potassium transport in amphibian kidney.

We have examined the effect of barium on K excretion and transport in the perfused frog kidney. The transepithelial secretory flux of K was calculated from the rate of excretion of 42K from the portal circulation. Barium was applied to the basolateral surfaces of the tubules via the portal circulation. Barium (0.1 mM) increased fractional K excretion from 55 to 178% by stimulating the K secretory flux; 0.5 mM Ba increased fractional K excretion from 39 to 386% primarily by increasing the secretory flux greater than 10-fold. Tissue content increased 10%. When Ba was applied to both the apical and basolateral surfaces by perfusion of the arterial circulation, K excretion and secretion also increased. This same result was obtained when K secretion was first stimulated by acetazolamide. In another series of experiments, tubular cells were loaded with 42K via a pulse injection into the portal circulation, and simultaneous washout of the isotope into the urine and the venous effluent was measured. Analysis of the washout curves revealed that basolateral (portal) application of Ba inhibited efflux of K from the cells into the circulation, reduced the influx from the circulation, and stimulated efflux into the urine. We suggest that blockade of K channels in the basolateral membrane indirectly depolarizes the apical membrane and thereby increases the electrochemical gradient favoring K movement from cells to tubular fluid. When Ba is applied simultaneously to both tubular surfaces, the effect on the basolateral surface apparently overrides any direct effect it may have on K movement across the apical surface. Using 133Ba, we measured a very small transepithelial perfusate-to-tubular fluid flux. However, fractional Ba reabsorption averaged 19% when 0.5 mM Ba was present in the arterial perfusate.

Mechanisms of fluid secretion by polycystic epithelia.

We have sought to determine the mechanisms driving fluid secretion by the cystic epithelium in autosomal dominant polycystic kidney disease (ADPKD). We have performed in vitro experiments on intact cysts dissected from discarded ADPKD kidneys, on monolayers of cells cultured from the cystic epithelium and on microcysts clonally derived from single cultured cells. These preparations absorb fluid in the control state but secrete fluid in response to native cyst fluid, to adenylate cyclase agonists and to permeant analogues of cAMP. Measurements of short-circuit current and transepithelial voltage in the monolayers indicate that anion secretion must drive the fluid secretion. Fluid secretion by the intact cysts was inhibited by basolateral application of ouabain but not by apical application. The effect of ouabain on fluid secretion and short-circuit current in the monolayers followed the same pattern. Thus the functional Na,K-ATPase enzyme complex is located only in the basolateral membrane of the cystic cells and serves to maintain the transmembrane chemical and electrical gradients that drive anion secretion by other transport mechanisms. Fluid secretion and short-circuit current in the cultured monolayers was inhibited by the basolateral application of the Na-K-2Cl cotransporter inhibitors, bumetanide and furosemide, and by apical application of the chloride channel blocker, diphenylamine-2-carboxylate (DPC). These data suggest that chloride is the anion that is actively secreted. Preliminary experiments utilizing the monolayers and the microcysts and measuring cell chloride concentration and chloride efflux across the apical membrane support this conclusion. Other preliminary data indicate that the cystic fibrosis transmembrane conductance regulator is present in the apical membrane. Thus active chloride transport generates fluid secretion by the cystic epithelium.

Ion transport by rabbit descending colon: mechanisms of transepithelial potassium transport.

In vitro preparations of rabbit descending colon were studied under steady-state short-circuit conditions to determine 1) the K concentration dependence of unidirectional K fluxes; 2) the effects of the K channel blocker barium and the diuretic agent furosemide; and 3) the steady-state tissue specific activity of 42K when added to the luminal bathing solution. Results from these studies reveal that 1) labeling of cellular K from the mucosal solution is less than 25% of that from the serosal solution; 2) both unidirectional K fluxes are composed of saturable and nonsaturable components; 3) the serosal-to-mucosal saturable component is abolished by ouabain, and subsequent addition of 2,4-dinitrophenol abolishes the saturable component of the mucosal-to-serosal K flux; 4) luminal or serosal barium alters K transport in a manner consistent with the presence of barium-sensitive K conductances at both membranes; 5) luminal furosemide did not alter K transport; and 6) there is no shunt selectivity for K. We conclude that the majority of both unidirectional K fluxes follow a transcellular pathway and that both the apical and basolateral membranes possess active K uptake mechanisms and barium-sensitive K exit mechanisms.

Effects of sodium and chloride on potassium transport by the bullfrog kidney.

We have studied the effect on K transport of reductions in the Na and Cl concentrations of solutions perfusing the isolated bullfrog kidney. We used recently developed techniques for estimating the unidirectional reabsorptive and secretory K fluxes. Reduction of Na and Cl concentrations in the arterial perfusate from 112 and 100 mM to 22 and 10 mM, respectively, inhibited K secretion 82% and K reabsorption 97%. Reduction of only the Na concentration inhibited K secretion 42% but did not affect K reabsorption. Arterial and portal perfusion with 37 mM Na, 23 mM Cl reduced urine Na concentration to 6 mM and Na reabsorption by 59%. However, K secretion rose 88% and reabsorption fell 76%. Arterial and portal perfusates with 37 mM Na, 100 mM Cl reduced urine Na concentration to 2 mM and Na reabsorption by 46%. Still, K secretion was elevated 57% by an increase in urine flow rate. K reabsorption was not reduced. Arterial and portal perfusates with 112 mM Na, 23 mM Cl, and containing SO4 also stimulated K secretion 26% and inhibited K reabsorption 91%. Thus, reduction of perfusate Na concentration to 22 mM inhibited secretion but 37 mM was sufficient to permit stimulation of secretion by low Cl concentrations and by increased tubular fluid flow rate. Reduction of the perfusate Cl concentration stimulated secretion and inhibited reabsorption. We conclude that a minimum level of Na reabsorption is required to maintain K secretion, but above that minimum level changes in the rate of Na reabsorption do not affect the rate of K secretion. The tubular fluid Cl concentration or the rate of Cl reabsorption affects both reabsorption and secretion of K and, therefore, may be an important regulator of the rate of K excretion.

Potassium reabsorption and secretion in the perfused bullfrog kidney.

Techniques have been developed for obtaining comparative estimates of the rates of K secretion and reabsorption in the artificially perfused bullfrog kidney and the use of 42K infused into the portal circulation. In control experiments in which Kexc/Kfilt averaged 0.65, approximately 30% of filtered K escaped reabsorption and excreted K was composed of roughly equal parts of filtered K and secreted K. Fractional reabsorption of K was constant at 70% over a wide range of filtration rates. The rate of K secretion correlated significantly with the rate of Na reabsorption and with the urine flow rate. Acetazolamide stimulated secretion and inhibited reabsorption.

Coupling of cell volume and membrane potential changes to fluid secretion in a model of renal cysts.

Renal tubular epithelia ordinarily absorb NaCl and water, although recent evidence indicates that renal cysts secrete fluid. We have utilized the experimental advantages offered by cultured cysts, formed in a collagen matrix by propagating Madin-Darby canine kidney cells, to investigate the mechanisms involved in fluid secretion by this renal epithelium. The rate of fluid transport (adduced from changes in cavity volume), cell volume and changes in membrane potential were measured simultaneously in isolated cysts. Under basal conditions, cysts absorbed fluid (-0.83 +/- 0.34 x 10(-6) ml/min/cm2 cavity surface area, N = 23). AVP and IBMX changed the direction of net fluid transport to secretion (4.24 +/- 0.49 x 10(-6) ml/min/cm2). Cell volume initially fell 7.4 +/- 0.5% and remained stable thereafter as secretion continued. Membrane electrical potential (bis-oxonol epifluorescence) hyperpolarized in 13 cysts and depolarized in 6, the mean change was 1.9 +/- 3.1%. Fluid secretion was abolished by 0.1 mM ouabain. Secretion was not affected by 0.1 mM DIDS and cell pH (bis-carboxyethyl-carboxyfluorescein epifluorescence) was not altered by the induction of secretion, suggesting that secretion is not dependent on Cl-HCO3 exchange. Barium, in the presence of AVP and IBMX, depolarized the cell membrane potential (bis-oxonol fluorescence increased 22.3 +/- 0.03%), reversed secretion to absorption (from 3.21 +/- 0.93 to -1.52 +/- 0.61 x 10(-6) ml/min/cm2), and increased cell volume 2.7 +/- 0.5%. Bumetanide (100 microM) reduced fluid secretion from 4.49 +/- 1.23 to -0.75 +/- 0.55 x 10(-6) ml/min/cm2, further reduced cell volume 4.4 +/- 1.2% and hyperpolarized the membranes (bis-oxonol fluorescence fell 24.3 +/- 5.0%). In the absence of AVP and IBMX bumetanide had no effect on fluid transport, cell volume or membrane potentials. We conclude that AVP reversed the direction of fluid transport in these cultured renal epithelial cysts from absorption to secretion by stimulating a coordinated interaction of basolateral and apical K, Cl and Na transport mechanisms.

Effect of barium on cell volume regulation in rabbit proximal straight tubules.

Rabbit proximal straight tubules swell abruptly when exposed to hypotonic medium but then shrink in a few minutes as they approach their base-line volume following loss of solute and water. Potassium, the major intracellular cation, as well as sodium, is lost during this process. In the present experiments, we studied hypotonic cell volume regulation in the presence of barium, an agent reported to decrease potassium permeability. Exposure to BaCl2 significantly prolonged hypotonic volume recovery in a dose-dependent manner. Tubules depleted of potassium and loaded with sodium chloride by exposure to 10(-4) M ouabain for 1 h swelled osmometrically and subsequently volume regulated in dilute medium. Volume regulation in such tubules is a consequence of transbasement membrane hydrostatic forces. By contrast, tubules similarly loaded with sodium, but also exposed to 10(-3) M BaCl2, volume regulated only minimally in dilute medium, suggesting BaCl2 might also affect sodium movement. However, hypotonic volume regulation was restored in sodium-loaded BaCl2-treated tubules when cells were more effectively depleted of potassium by incubation in 0-mM potassium medium. We conclude that barium retards hypotonic volume regulation primarily because of its effect on potassium movement.

Epithelial transport in polycystic kidney disease.

In autosomal dominant polycystic kidney disease (ADPKD), the genetic defect results in the slow growth of a multitude of epithelial cysts within the renal parenchyma. Cysts originate within the glomeruli and all tubular structures, and their growth is the result of proliferation of incompletely differentiated epithelial cells and the accumulation of fluid within the cysts. The majority of cysts disconnect from tubular structures as they grow but still accumulate fluid within the lumen. The fluid accumulation is the result of secretion of fluid driven by active transepithelial Cl- secretion. Proliferation of the cells and fluid secretion are activated by agonists of the cAMP signaling pathway. The transport mechanisms involved include the cystic fibrosis transmembrane conductance regulator (CFTR) present in the apical membrane of the cystic cells and a bumetanide-sensitive transporter located in the basolateral membrane. A lipid factor, called cyst activating factor, has been found in the cystic fluid. Cyst activating factor stimulates cAMP production, proliferation, and fluid secretion by cultured renal epithelial cells and also is a chemotactic agent. Cysts also appear in the intrahepatic biliary tree in ADPKD. Normal ductal cells secrete Cl- and HCO3-. The cystic ductal cell also secretes Cl-, but HCO3- secretion is diminished, probably as the result of a lower population of Cl-/HCO3- exchangers in the apical membrane as compared with the normal cells. Some segments of the normal renal tubule are also capable of utilizing CFTR to secrete Cl-, particularly the inner medullary collecting duct. The ability of Madin-Darby canine kidney cells and normal human kidney cortex cells to form cysts in culture and to secrete fluid and the functional similarities between these incompletely differentiated, proliferative cells and developing cells in the intestinal crypt and in the fetal lung have led us to suggest that Cl- and fluid secretion may be a common property of at least some renal epithelial cells in an intermediate stage of development. The genetic defect in ADPKD may not directly affect membrane transport mechanisms but rather may arrest the development of certain renal epithelial cells in an incompletely differentiated, proliferative stage.

Chloride and fluid secretion by cultured human polycystic kidney cells.

Epithelial cells cultured from the renal cysts of patients with autosomal dominant polycystic kidney disease (ADPKD) secrete fluid via a process stimulated by adenosine 3',5'-cyclic monophosphate (cAMP). We have investigated the hypothesis that fluid secretion by these cells is dependent on cAMP-mediated chloride secretion. Individual cultured ADPKD cells were suspended within a polymerized collagen matrix and stimulated to form cysts. Individual cultured cysts were placed in a chamber on the stage of an inverted microscope equipped with epifluorescent and video analysis attachments. The rate of fluid secretion, cell volume and changes in intracellular Cl- were measured. In the absence of secretagogues, fluid was absorbed from the cyst cavity (-2.36 +/- 0.64 nl/min/cm2 inner surface area). 8-Bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) plus 3-isobutyl-1-methlyxanthine (IBMX) induced a rapid reversal in the net movement of fluid to secretion (6.79 +/- 1.28 nl/min/cm2). Bumetanide reversibly reduced fluid secretion to 0.95 +/- 0.60 nl/min/cm2. Cell volume rapidly decreased by 7.5 +/- 0.9% with the initiation of secretion and bumetanide caused an additional loss (4.2 +/- 1.0%). Furosemide had a similar effect on forskolin-induced fluid secretion. Cellular chloride concentration was monitored with the use of the indicator, 6-methoxy-N-ethylquinolinium chloride (MEQ). Removal of Cl- from the bath reduced intracellular [Cl-] (MEQ fluorescence increased by 11.4 +/- 2.3%). In cysts pretreated with furosemide to prevent Cl- entry, the application of forskolin caused a decrease in Cl- concentration (MEQ fluorescence increased by 9.3 +/- 2.6%). Using monolayers of cultured ADPKD cells, grown on permeant supports, we compared the changes in short circuit current (ISC) induced by forskolin in the presence and absence of external Cl-. Forskolin increased ISC (from 8.9 +/- 2.7 to 10.6 +/- 2.7 microA/cm2) in the presence of Cl-, but did not significantly affect ISC in its absence. These data indicate that cultured ADPKD cells can direct fluid transport in either the absorptive or the secretory direction, and that cAMP stimulates secretion and this secretion is accompanied by a net loss of cell solute. Inhibition of secretion by bumetanide or furosemide caused an additional loss of cell solute, including Cl-. The ionic transepithelial current induced by forskolin is dependent on the presence of Cl-. These data support the thesis that chloride secretion drives fluid secretion by cultured ADPKD cells.

Potassium transport by rabbit descending colon.

Unidirectional mucosal-to-serosal (Jm leads to s) and serosal-to-mucosal (Js leads to m) fluxes of potassium and sodium were determined simultaneously on paired sections of descending colon from the same rabbit under short-circuit conditions. In 13-16 pairs of tissues, net potassium secretion and sodium absorption averaged 0.49 +/- 0.08 and 4.0 +/- 0.8 mueq.cm-2.h-1, respectively. Short-circuit current (Jsc) averaged 3.7 +/- 0.4 mueq.cm-2.h-1 and was approximately equal to the algebraic sum of net potassium and sodium fluxes. Treatment of both sides of the colon with 10(-4) M ouabain reduced the Jsc and transmural potential difference to near zero. Ouabain abolished net potassium secretion by reducing JKs leads to m and abolished net sodium absorption by inhibiting JNam leads to s. In the presence of ouabain, net potassium absorption averaging 0.15 +/- 0.07 mueq.cm-2.h-1 (n = 11) was observed. In the presence of 10(-3) M 2,4-dinitrophenol, both net potassium and net sodium fluxes were abolished, primarily as a result of a reduction in JKs leads to m and JNam leads to s without altering JKm leads to s and JNas leads to m. These results suggest that the rabbit descending colon has the capacity to secrete and possibly to absorb potassium by active mechanisms requiring metabolic energy. Comparison of potassium and sodium fluxes suggest that the paracellular pathway in the rabbit colon is not potassium selective.

Relationship between tubular net sodium reabsorption and peritubular potassium uptake in the perfused Necturus kidney.

1. K influx from peritubular space into renal tubular cells, varphi(i) (K), was measured in doubly perfused Necturus kidneys by studying tissue uptake of (42)K added exclusively to the portal circulation. Concomitantly, net tubular Na reabsorption, varphi(n) (Na), was measured by clearance techniques. varphi(n) (Na) and varphi(i) (K) were varied widely by replacing solutions of physiological composition (controls) with solutions containing high K, low K, low Na, cyclamate instead of Cl, ouabain (10(-7)-10(-4)M) or ethacrynic acid (10(-5)-10(-4)M).2. The ratio of varphi(n) (Na) to varphi(i) (K) was found to vary with the experimental conditions, the control value of about 2 was maintained over a threefold variation in absolute Na reabsorption. This ratio increased with low K or ouabain to values near 4. With high K, ethacrynic acid, low Na or cyclamate the relationship was one or lower. Thus, net Na reabsorption can be uncoupled from peritubular K influx.3. These results can be best explained if there are two Na pumps working in parallel: pump A transporting Na (with Cl) and pump B, a Na-for-K-exchange pump. The ratio of Na efflux to K influx could approach infinity if only pump A works (if B is inhibited) and could approach one if only B works. It should vary between these limits in controls when both pumps are active, or when neither of the two pumps is completely inhibited.4. Alternatively, the experimental findings could be explained by a Na pump with a coupling ratio that varies within two extreme values, from high Na-K ratios (with Na reabsorption at, or near, control values but with very low K influx values) to low ratios (with normal K influx values but with low Na reabsorption values).