Biomedical subjects

C A Berry

Publications and source records attributed to C A Berry.

At least 37 records · Page 2Linked to original sources

PubMed1989

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)

Absorption↗

PubMed1989

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.

Animals↗

PubMed1988

Osmotic gradient dependence of osmotic water permeability in rabbit proximal convoluted tubule.

To assess steady-state transepithelial osmotic water permeability (Pf), rabbit proximal convoluted tubules were perfused in vitro with the impermeant salt, sodium isethionate at 26 degrees C. Osmotic gradients (delta pi) were established by varying the bath concentration of the impermeant solute, raffinose. When lumen osmolality was 300 mOsm and bath osmolality was 320, 360 and 400 mOsm, apparent Pf decreased from 0.5 to 0.10 to 0.08 cm/sec, respectively. Similar data were obtained when lumen osmolality was 400 mOsm. Five possible causes of the delta pi dependence of apparent Pf were considered experimentally and/or theoretically: (1) external unstirred layer (USL); (2) cytoplasmic USL; (3) change in surface area; (4) saturation of water transport; (5) down-regulation of Pf. Apparent Pf was inhibited 83% by p-chloromercuribenzene sulfonate (pCMBS) at 20 mOsm, but not at 60 mOsm delta pi, suggesting presence of a serial barrier resistance to water transport. Increases in perfusate or bath solution flow rate and viscosity did not alter apparent Pf, ruling out an external USL. A simple cytoplasmic USL, described by a constant USL thickness and solute diffusion coefficient, could not account for the delta pi dependence of apparent Pf according to a mathematical model. The activation energy (Ea) for apparent Pf increased from 7.0 to 12.5 kcal/mol when delta pi was increased from 20 to 60 mOsm, not consistent with a simple USL or a change in membrane surface area with transepithelial water flow. These findings are most consistent with a complex cytoplasmic USL, where the average solute diffusion coefficient and/or the area available for osmosis decrease with increasing delta pi. These results (1) indicate that true Pf (at physiologically low delta pi) is very high (greater than 0.5 cm/sec) in the rabbit proximal tubule; (2) provide an explanation for the wide variation in Pf values reported in the literature using different delta pi, and (3) suggest the presence of a flow-dependent cytoplasmic barrier to water flow.

4-Chloromercuribenzenesulfonate↗

PubMed1988

Rapid development of vasopressin-induced hydroosmosis in kidney collecting tubules measured by a new fluorescence technique.

The pre-steady-state kinetics of the vasopressin-induced increase in collecting tubule osmotic water permeability (Pf) has been measured by a new fluorescence technique. Isolated cortical collecting tubules (CCT) from rabbit kidney were perfused with physiological buffers containing the impermeant fluorophores fluorescein sulfonate (FS) and pyrenetetrasulfonic acid (PTSA). Tubules were subject to a 120 mOsm bath-to-lumen osmotic gradient in the presence and absence of 250 microU/ml vasopressin. The magnitude of transepithelial volume flow was determined from the self-quenching of FS, or from the ratio of PTSA/FS fluorescence, measured at 380 nm excitation and 420 +/- 10 nm (PTSA) and greater than 530 nm (FS) emission wavelengths. Pf was calculated from the magnitude of transepithelial volume flow, lumen and bath osmolarities, lumen perfusion rate, and tubule geometry. The instrument response time for a change in bath osmolality was less than 3 s. At 37 degrees C, CCT Pf was (in units of cm/s x 10(4] 13 +/- 2 (mean +/- SE, 16 tubules) before, and 227 +/- 10 after addition of vasopressin to the bath. CCT Pf began to increase in 23 +/- 3 s after vasopressin addition and was half-maximal after 186 +/- 20 s. At 23 degrees C, Pf was 9 +/- 1 (seven tubules) before, and 189 +/- 12 after vasopressin addition. Pf began to increase in 40 +/- 4 s and was half-maximal after 195 +/- 35 s. After vasopressin removal from the bath, Pf decreased to its baseline value with a half-time of 14 min. These results establish a direct fluorescence method to monitor instantaneous transepithelial Pf in perfused tubules and show a very fast stimulation of CCT Pf in response to vasopressin.

Animals↗

PubMed1988

Estimation of intracellular chloride activity in isolated perfused rabbit proximal convoluted tubules using a fluorescent indicator.

The methodology has been developed to measure cell chloride activity by fluorescence microscopy using the chloride-sensitive dye, 6-methoxy-1-(3-sulfonatopropyl)quinolinium (SPQ). SPQ was loaded into cells of the in vitro microperfused rabbit proximal convoluted tubule by a 10 min luminal perfusion with 20 mM SPQ at 38 degrees C. Fluorescence was excited with a broad band excitation filter (340 and 380 nm) and detected with a 435 nm cut-on filter. The signal to background (autofluorescence) ratio was 4.6 +/- 0.6. The halftime for SPQ leakage from cells at 38 degrees C was 8.6 +/- 1.1 min. In suspended tubules, SPQ did not affect O2 consumption significantly. Intracellular SPQ calibration was performed using the ionophores nigericin and tributyltin, high external potassium concentrations, and varying extracellular chloride concentrations. Cell fluorescence was related to intracellular chloride by a Stern-Volmer relation with a quenching constant of 12 M-1. Apparent chloride concentration in tubules perfused with solutions characteristic for the late proximal convoluted tubule was 27.5 +/- 5 mM (activity 20.6 mM). The halftime of the transient in cell chloride activity upon bath chloride addition was approximately 3 s (38 degrees C). Applications and limitations of this new fluorescence method to study cell chloride transport are discussed.

Animals↗

PubMed1988

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.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗

PubMed1988

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).

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗

PubMed1987

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.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗

PubMed1986

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.

Animals↗

PubMed1986

The beginnings of space medicine.

The beginnings of space medicine are those of aviation medicine and involved, among many pioneers, Dr. Harry G. Armstrong and the Aeromedical Laboratory. Space Medicine has advanced in a short 23 years from arguments about man being capable of survival in space through animal and manned flights to flight durations of 6 months. The public and private arguments about man's capability to survive and work in space and the early decisions about flight duration and safety are chronicled from a personal point of view.

Aerospace Medicine↗

PubMed1985

Peritubular protein modulates neutral active NaCl absorption in rabbit proximal convoluted tubule.

To assess directly the effect of increasing and decreasing peritubular protein concentration on active NaCl absorption, rabbit proximal convoluted tubules were perfused with a high-chloride solution simulating late proximal tubular fluid and bathed in a similar solution containing 0, 6, or 10 g/dl albumin. A direct examination of the active component of NaCl absorption was ensured by 1) the reduction of luminal solutes other than NaCl (no glucose or alanine and only 5 mM HCO-3), and 2) the use of perfusate and bathing solutions in approximate Donnan equilibrium, so that electrochemical driving forces responsible for passive net NaCl absorption are eliminated. When bathing solution albumin concentration was reduced from 6 to 0 g/dl, volume absorption (Jv) was reduced significantly from 0.39 +/- 0.07 to 0.22 +/- 0.08 nl X mm-1 X min-1. When bathing solution albumin concentration was increased from 6 to 10 g/dl, Jv increased significantly from 0.26 +/- 0.04 to 0.37 +/- 0.04 nl X mm-1 X min-1. When proximal convoluted tubules were bathed in 0, 6, or 10 g/dl albumin at 20 degrees C, Jv was not different from zero in all periods, confirming that changes in bathing solution albumin require the presence of active transport to influence NaCl absorption. The transepithelial potential difference attributable to active transport was not different from zero in all periods of all protocols, demonstrating that active NaCl absorption was electroneutral. In conclusion, these data show directly that peritubular protein concentration modulates neutral active NaCl absorption in the rabbit proximal convoluted tubule.

Absorption↗

PubMed1985

Evidence for transcellular osmotic water flow in rat proximal tubules.

To determine the predominant pathway for transepithelial osmotic water flow, the transepithelial osmotic water permeability [Pf(TE)] and the apparent dimensions of paracellular pores and slits were determined in rat proximal convoluted tubules microperfused in vivo. To measure Pf(TE), tubules were perfused with a hyposmotic, cyanide-containing solution. Pf(TE), calculated from the observed volume flux in response to the measured log mean osmotic gradient, was 0.12-0.15 cm/s, assuming sigmaNaCl equal to 1.0-0.7, respectively. The dimensions of the paracellular pathways were determined using measured sucrose and mannitol permeabilities (nonelectrolytes confined to the extracellular space). These were 0.43 and 0.87 X 10(-5) cm/s, respectively. By using the ratio of these permeabilities, their respective free solution diffusion coefficients and molecular radii, and the Renkin equation, the radius of the nonelectrolyte-permeable pores and the total pore area/cm2 surface area/channel length were calculated to be 1.4 nm and 3.56 cm-1, respectively. Similar calculations for slits yielded a slit half-width of 0.8 nm and a total slit area/cm2 surface area/channel length of 3.16 cm-1. The osmotic water permeability of these nonelectrolyte-permeable pathways was calculated by Poiseuille's law to be 0.0018 cm/s (pores) or 0.0014 cm/s (slits), at most 2% of Pf(TE). We conclude that the nonelectrolyte-permeable pathway in the tight junctions is not the major route of transepithelial osmotic water flow in the rat proximal tubule.

Animals↗

PubMed1985

Characteristics of water diffusion in the rabbit proximal convoluted tubule.

Water diffusion in the in vitro microperfused rabbit proximal convoluted tubule was measured to assess the transepithelial pathway for water movement and the physical characteristics of the water permeation pathway. The measured diffusive water permeability (PDW(MEASURED] was high, 5 X 10(-3) cm/s, suggesting a transcellular transport route. Simultaneously measured n-butanol permeability (PDNB(MEASURED], an index of the cytoplasmic resistance to water diffusion, was 3.6 X 10(-3) cm/s. PDW(MEASURED], PDNB(MEASURED) and the free solution diffusion coefficients for water and n-butanol were used to derive a minimum membrane PDW (PDW(MEMBRANE] of 12.5 X 10(-3) cm/s. These data suggest that at least 60% of the transepithelial resistance to water diffusion resides in the cell cytoplasm. Measurement of the temperature dependence of PDW(MEASURED) and PDNB(MEASURED) gave an apparent activation energy of PDW(MEMBRANE) that was constant between 20 and 40 degrees C at about 4.3 kcal/mol. The organic mercurial sulfhydryl reagent p-chloromercuribenzene sulfonate, which has been shown to reduce diffusive water permeability in the red cell membrane by 50%, reduced PDW(MEMBRANE) by 54% without affecting PDNB(MEASURED). These last two independent lines of evidence are consistent with water diffusion through small, aqueous, protein-bounded channels.

4-Chloromercuribenzenesulfonate↗

PubMed1985

Girls with attention deficit disorder: a silent minority? A report on behavioral and cognitive characteristics.

A sample of girls and boys with attention deficit disorder (ADD) were examined for within-group sex differences and compared with control boys and girls on historical, behavioral, and cognitive characteristics. Profiles of essential and secondary features of ADD support the Diagnostic and Statistical Manual of Mental Disorders nosology for ADD with and without hyperactivity as clinically distinct subtypes of ADD, with the girls and boys with ADD with hyperactivity exhibiting attentional, behavioral, and cognitive impairments, and the children with ADD without hyperactivity exhibiting deficits along an attentional/cognitive axis. Management problems and antisocial behaviors were correlates of hyperactivity, and increased impulsivity was not associated with attentional deficits in the absence of hyperactivity. Within the ADD with hyperactivity group, the girls demonstrated more severe cognitive impairments, particularly in the area of language function, were younger at the time referral for medical attention, and tended to come from families of lower socioeconomic status. Disruptive, uncontrolled behaviors were more frequent among the boys with ADD with hyperactivity. The girls with ADD without hyperactivity demonstrated poorer self-esteem and were significantly older than the boys with ADD without hyperactivity at referral. Girls in both ADD with and without hyperactivity groups were more likely to suffer peer rejection than their male counterparts. The results of this study suggest that girls with ADD may be underidentified and that cognitive deficits have a more prominent role in the identification of girls, whereas behavioral disturbances increase the likelihood of identification for boys.

Adolescent↗

PubMed1984

Mechanism of bicarbonate exit across basolateral membrane of the rabbit proximal convoluted tubule.

To clarify the mechanism(s) of HCO-3 movement across the basolateral membrane, rabbit proximal convoluted tubules were perfused in vitro. Two possible mechanisms were examined: neutral HCO-3 exit coupled to chloride and rheogenic HCO-3 exit. A complete C1- substitution with isethionate in the lumen and bath did not affect HCO-3 reabsorption, suggesting that HCO-3 exit is not coupled to chloride. Addition of 2 mM Ba2+ to the bath, which has been shown to depolarize the basolateral membrane potential difference, caused a 42% inhibition of HCO-3 reabsorption and a 32% inhibition of volume flux, suggesting that HCO-3 exit is rheogenic. Ba2+ did not affect the volume flux when HCO-3 reabsorption was inhibited by acetazolamide, suggesting that the Ba2+ effect is not due to a general inhibition of cell metabolism. From these data we propose that HCO-3 exits the basolateral membrane by a rheogenic, chloride-independent mechanism.

Animals↗

PubMed1984

Evidence for neutral transcellular NaCl transport and neutral basolateral chloride exit in the rabbit proximal convoluted tubule.

The electrical nature of active NaCl transport and the significance of a basolateral membrane chloride conductance were examined in isolated perfused rabbit proximal convoluted tubules (PCT). PCT were perfused with a high chloride solution that simulated late proximal tubular fluid and were bathed in an albumin solution that simulated rabbit serum in the control and recovery periods. The electrical nature of NaCl transport was examined by bathing the tubules in a high chloride albumin solution where there were no anion gradients. Volume reabsorption (Jv) during the control and recovery period was 0.56 and 0.51 nl/mm X min, respectively, and 0.45 nl/mm X min when the tubules were bathed in a high chloride bath. The transepithelial potential difference (PD) during the control and recovery periods averaged 2.3 mV, but decreased to 0.0 mV in the absence of anion gradients, which indicated that NaCl transport is electroneutral. Further evidence that NaCl transport is electroneutral was obtained by examining the effect of addition of 0.01 mM ouabain in PCT perfused and bathed with high chloride solutions. The Jv was 0.54 nl/mm X min in the control period and not statistically different from zero after inhibition of active transport. The PD was not different from zero in both periods. Two groups of studies examined the role of basolateral membrane Cl- conductance in NaCl transport. First, depolarizing the basolateral membrane with 2 mM bath Ba++ did not significantly affect Jv or PD. Second, the effect of the presumptive Cl- conductance inhibitor anthracene-9-CO2H was examined. Anthracene-9-CO2H did not significantly affect Jv or PD. In conclusion, these data show that NaCl transport in the PCT is electroneutral and transcellular and provide evidence against a significant role for basolateral membrane chloride conductance in the rabbit PCT.

Animals↗

PubMed1983

Effect of potassium concentration on bicarbonate reabsorption in the rabbit proximal convoluted tubule.

The direct effects of acute changes in K+ concentration on HCO-3 (JnettCO2) and volume reabsorption (Jv) were examined in isolated perfused rabbit proximal convoluted tubules (PCT). Increasing ambient K+ concentration from 5 to 8 mM did not change JnettCO2 (94.5 +/- 16.1 vs. 98.8 +/- 17.7 pmol X mm-1 X min-1) or Jv (1.27 +/- 0.15 vs. 1.24 +/- 0.16 nl X mm-1 X min-1). In contrast, reducing ambient K+ concentration from 5 to 2 mM inhibited JnettCO2 by 22% and Jv by 29%. Reducing luminal K+ concentration from 5 to 0 mM with constant bath K+ concentration at 5 mM did not affect JnettCO2 or Jv. Further reductions in bath K+ concentration to 0.5 and 0 mM showed a similar dependence of both fluxes on K+ concentration. Half-maximum inhibition of JnettCO2 was obtained at 1.1 mM ambient K+ concentration and of Jv at 0.85 mM. At zero bath K+ concentration JnettCO2 was 6.6 +/- 2.5 pmol X mm-1 X min-1 and Jv was 0.03 +/- 0.04 nl X mm-1 X min-1. To determine whether this rate of acidification was significantly different from zero, we examined the ability of the PCT to generate tCO2 concentration gradients with zero bath K+ concentration at slow perfusion rates. The tCO2 concentration gradient generated (0.94 mM) was not different from that found when the perfusate was inserted directly into the collection pipette in the absence of a tubule (0.71 mM). These data are consistent with the view that HCO-3 reabsorption is totally dependent on the Na+-K+-ATPase pump system.

Animals↗

PubMed1983

Water permeability and pathways in the proximal tubule.

The route of water transport in the proximal tubule could be either transjunctional or transcellular. A transjunctional route is supported by data showing high osmotic-to-diffusive water permeability ratios, the possible correlation of junctional leakiness to ions and nonelectrolytes with water permeability, and solvent drag of nonelectrolytes and ions. These data, however, are not convincing. A transcellular route of water transport is supported by data showing that the osmotic water permeability (Pf) for apical and/or basolateral cell membranes is sufficiently high to account for the transepithelial Pf, making a tentative conclusion for a transcellular route of water transport possible. In addition, measurements of Pf have yielded insights into the mechanism of solute-solvent coupling. Pf has been reported to be mostly between 0.1 and 0.3 cm/s. In the rabbit proximal straight and the Necturus proximal convoluted tubule, in which water transport rates are low, this range of Pf will account for volume absorption with only small osmotic gradients (less than 6 mosmol). Higher osmotic gradients are required in the rat and possibly the rabbit proximal convoluted tubule, where water transport rates are higher. Solute-solvent coupling in all species is probably due to both luminal hypotonicity and lateral intercellular space hypertonicity. These two processes are directly linked. Mass balance requires that generation of luminal hypotonicity also generates a hypertonic absorbate and, thus, some degree of lateral intercellular space hypertonicity. It is likely that, in the rabbit at least, effective osmotic pressure gradients due to differences in solute reflection coefficients play little role in solute-solvent coupling.

Animals↗