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G Giebisch

Publications and source records attributed to G Giebisch.

At least 145 records · Page 8Linked to original sources
PubMed1988

Morphological heterogeneity of the rabbit collecting duct.

The localization of carbonic anhydrase by histochemistry, of Na-K-ATPase by immunocytochemistry and of rod-shaped intramembranous particles by freeze-fracture electron microscopy, was determined in the collecting duct of rabbits. In the cortical collecting duct (CCD), rod-shaped particles, which are abundant in intercalated cells were observed in both the apical and basolateral membrane of all intercalated cells examined. In the outer stripe of the outer medullary collecting duct (OMCDo) a high density of rod-shaped particles was found only in the apical membrane of intercalated cells. All cells of the inner stripe of the outer medullary collecting duct (OMCDi) had rod-shaped particles in the apical membrane but not in the basolateral membrane. As the collecting duct entered the inner medulla the density of rod-shaped particles decreased until they were virtually absent in the terminal segment. Na-K-ATPase, localized to the basolateral membrane, was more abundant in principal cells than in intercalated cells in the CCD. In the OMCDo, staining was equal in principal and intercalated cells. All cells of the OMCDi and the inner medullary collecting duct (IMCD) stained for Na-K-ATPase. Carbonic anhydrase in the CCD was localized to the cell membranes and cytoplasm of intercalated cells. Principal cells did not stain for carbonic anhydrase. A similar pattern was seen in the OMCDo. In the outer region of the OMCDi most cells did not stain for carbonic anhydrase, whereas in the inner region the apical and lateral membranes of all cells stained for carbonic anhydrase. Weak cytoplasmic staining was occasionally seen. A similar pattern was seen in the initial half of the IMCD, while the terminal half of the IMCD did not stain. In this study, the localization of enzymes and rod-shaped intramembranous particles associated with Na+, K+, and H+ transport shows both segmental and cellular heterogeneity, and correlates with the known transport properties of tubule segments. The distribution of these enzymes and rod-shaped intramembranous particles is different in rabbits and rats, and may explain some of the functional differences between homologous segments in these species.

Animals↗
PubMed1988

Cell rubidium uptake: a method for studying functional heterogeneity in the nephron.

Rubidium uptake into individual tubule cells of rat renal cortex as measured by energy-dispersive X-ray microanalysis on freeze dried cryosections was used as an index of potassium transport. Over a 30 second period following intravenous infusion of rubidium (0.5 mmol/kg body wt) rubidium content increased in all cells. After 30 seconds, rubidium contents were (in mmol/kg dry wt): 225 +/- 8 in distal convoluted tubule cells, 156 +/- 7 in connecting tubule cells, 110 +/- 7 in principal cells, 86 +/- 4 in proximal tubule cells and 24 +/- 2 in intercalated cells (mean +/- SEM). When distal sodium and potassium transport were stimulated by hypertonic saline loading, rubidium uptake was selectively increased into distal convoluted tubule cells by 38%, into connecting tubule cells by 36%, and into principal cells by 52%. However, rubidium uptake into proximal tubule and into intercalated cells remained unchanged. The preferential uptake of rubidium into distal convoluted tubule cells, connecting tubule cells, and principal cells correlates well with the known transport functions of sodium and potassium, whereas intercalated cells are distinguished by low sodium and potassium transport activity.

Animals↗
PubMed1988

Intracellular Na+ and K+ activities and membrane conductances in the collecting tubule of Amphiuma.

Membrane potentials and conductances, and intracellular ionic activities were studied in isolated perfused collecting tubules of K+-adapted Amphiuma. Intracellular Na+ (aNai) and K+ (aKi) activities were measured, using liquid ion-exchanger double-barreled microelectrodes. Apical and basolateral membrane conductances were estimated by cable analysis. The effects of inhibition of the apical conductance by amiloride (10(-5) M) and of inhibition of the basolateral Na-K pump by either a low K+ (0.1 mM) bath or by ouabain (10(-4) M) were studied. Under control conditions, aNai was 8.4 +/- 1.9 mM and aKi 56 +/- 3 mM. With luminal amiloride, aNai decreased to 2.2 +/- 0.4 mM and aKi increased to 66 +/- 3 mM. Ouabain produced an increase of aNai to 44 +/- 4 mM, and a decrease of aKi to 22 +/- 6, and similar changes were observed when the tubule was exposed to a low K+ bath solution. During pump inhibition, there was a progressive decrease of the K+-selective basolateral membrane conductance and of the Na+ permeability of the apical membrane. A similar inhibition of both membrane conductances was observed after pump inhibition by low K+ solution. Upon reintroduction of K+, a basolateral membrane hyperpolarization of -23 +/- 4 mV was observed, indicating an immediate reactivation of the electrogenic Na-K pump. However, the recovery of the membrane conductances occurred over a slower time course. These data imply that both membrane conductances are regulated according to the intracellular ionic composition, but that the basolateral K+ conductance is not directly linked to the pump activity.

Animals↗
PubMed1988

Chloride transport in the proximal renal tubule.

Our knowledge of chloride transport along the nephron has greatly expanded. Whereas for a long time it was assumed that chloride ions were reabsorbed entirely passively with sodium--the "mendicant" role of chloride, more recent studies suggest that several distinct reabsorptive transport mechanisms operate in parallel. Accordingly, a new model of proximal chloride transport has evolved that includes both active, transcellular as well as passive, intercellular transport pathways. Transcellular chloride reabsorption involves anion exchange mechanisms in both the luminal and peritubular cell membranes, processes that also depend on sodium, hydrogen, and bicarbonate ions. Chloride transport is thus intimately related to sodium and fluid transport as well as to cell acid-base metabolism. Unresolved problems concern the relative magnitude of transcellular and paracellular chloride transport and the details of luminal and basolateral chloride translocation steps.

Animals↗
PubMed1988

The amphibian diluting segment.

Diluting segments function to reabsorb NaCl and to reduce the osmolality of tubule fluid. These segments in amphibians are important in the conservation of NaCl. The diluting segment of mammals, the thick ascending limb, besides being an important site for the reabsorption of NaCl, supplies the energy that enables the kidney to excrete a concentrated urine. We focus in this review on the mechanisms involved in cellular and paracellular transport of Na+, K+, Cl-, and H+ in the amphibian diluting segment and how the transport of these ions is regulated. Also discussed is the action of loop diuretics and hormones on both transepithelial and cellular function. The large size of amphibian cells and their viability in various artificial conditions have allowed experiments to be performed that are not possible in the mammalian kidney, providing important information on the mechanisms of ion transport common to both mammalian thick ascending limb and amphibian diluting segment.

Aldosterone↗
PubMed1988

An acute increase of peritubular K stimulates K transport through cell pathways of CCT.

In this study, we have investigated net transport and electrical properties of isolated perfused cortical collecting ducts of control and DOCA-treated rabbits to clarify the cell mechanism for secreting K on an acute increase of peritubular K concentration. Raising bath K in control experiments caused an increase in net K secretion (-JK) and net Na absorption (JNa). Barium (5 mM), a known blocker of basolateral K conductance, added to the bath had no effect on this transport stimulation. In the presence of luminal Ba (5 mM) -JK was reduced to zero, but JNa was unaltered. When bath K was increased -JK increased to a value slightly greater than zero, but not different from the predicted passive flux through the paracellular pathway. In tubules of DOCA-treated rabbits, raising K in the bath increased -JK and JNa. The observed stimulation of -JK was blunted in the presence of Ba in the bath, whereas the increase in JNa remained the same. Thus, in DOCA-treated tubules, Ba in the bath significantly attenuated the increase in K secretion without affecting the transport stimulation of JNa. When Ba was added to the perfusate and bath K increased, -JK, which was initially 22.1, increased to 36.4 pmol.mm-1.min-1. Thus a "Ba-insensitive" pathway became more evident in CCTs from DOCA-treated rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗
PubMed1988

Whole-cell potassium currents in single early distal tubule cells.

The apical membrane potassium conductance of amphibian early distal tubules is sensitive to changes in the intracellular pH, with cellular acidification causing a decreased conductance. With the whole-cell patch-clamp technique, we have measured the total potassium conductance of single isolated early distal tubule cells of the frog. With symmetrical potassium gluconate solutions, the whole-cell current was found to be strongly rectifying, with an inward conductance of 12.9 nS (at intracellular pH between 7.6 and 8.0) and an outward conductance of 1.0 nS. The inward current was almost totally inhibited by the addition of 10 mM Ba2+ to the bath solution. The use of pipette solutions with pH between 7.0 and 8.0 showed a positive correlation between intracellular pH and conductance. In contrast, acidification of the extracellular solution caused no significant change in conductance.

Animals↗
PubMed1988

Succinate alters respiration, membrane potential, and intracellular K+ in proximal tubule.

Succinate, a dicarboxylic acid, is an intermediate in the Krebs cycle that is transported and metabolized by the renal proximal tubule. It is also known to increase proximal tubule transport of phosphate and glucose but not fluid by unknown mechanisms. In the present study, succinate increased proximal tubule respiration in a dose-dependent manner, and a kinetic evaluation indicated that two separate processes were activated. A lower-affinity (Km = 0.9 mM), higher-capacity stimulation (Vmax increase of 49%) was attributed to a decrease in the mitochondrial coupling efficiency. A higher-affinity process (Km = 0.012 mM) was related to an apparent increase in ATP synthesis. The apparent increase in ATP synthesis was not associated with a change in Na+-K+-ATPase activity, however, but rather indicated a 49% increase in ion transport-independent ATP utilization. Basolateral membrane potential hyperpolarized by -7 mV in the presence of succinate, and this was related to an increase in the K+ transference number. Finally, 1 and 5 mM succinate promoted a net cellular uptake of K+, leading to an 11% increase in intracellular K+, which was not the result of an increase in Na+-K+-ATPase activity. Thus the cellular entry and metabolism of succinate promotes multiple changes in ion transport without altering Na+-K+-ATPase activity.

Animals↗
PubMed1988

Effects of a high potassium diet on electrical properties of cortical collecting ducts from adrenalectomized rabbits.

The cortical collecting tubule is one of the main nephron sites where mineralocorticoids and a high potassium diet modulate sodium (Na) and potassium (K) transport. In this study we explored the steroid-independent effects of a high K diet on the electrical transport properties of the isolated rabbit cortical collecting tubule principal cells. The electrophysiological analysis included transepithelial and single-cell potential measurements and equivalent circuit analysis. Rabbits were adrenalectomized (ADX) and received either a control diet (300 meq K/kg diet) or a high K diet (600 meq/kg diet) for 10 d before the experiment. The mean plasma K of ADX control animals was 6.9 mM, that of ADX animals on the high K diet 8.3 mM. The transepithelial potential difference was significantly elevated in the high K group (-3.5 mV, lumen negative), compared with ADX controls (-1.4 mV). The basolateral membrane potential in high K animals was also significantly elevated (-73 mV, cell negative, compared with -63 mV in controls). Estimates of the apical membrane partial Na and K conductances (GaNa and GaK) and of ion currents (IaNa and IaK) also demonstrated stimulation by the high K diet. In the high K group, both GaNa and GaK (0.56 and 2.67 mS.cm-2) were higher than control values (0.27 and 1.17 mS.cm-2). IaNa and IaK were also higher in high K animals (47.8 and -26.2 microA.cm-2) compared with control animals (22.4 and -11.6 microA.cm-2). Thus, a high K intake per se can induce electrophysiological changes consistent with stimulation of Na reabsorption and K secretion.

Adrenalectomy↗
PubMed1988

Calcium-activated epithelial potassium channels.

Calcium-activated K channels have now been directly demonstrated with patch clamping in many epithelia and are present in the apical cell membrane of the proximal tubule, the diluting segment and the cortical collecting tubule of the kidney. These channels with a typical conductance of 50-100 ps (3-6.10(6) ions/s), are highly selective for potassium but also allow passage of Rb, NH4 and Na. Conductance is modified by the concentration of K bathing the channel and depolarization increases the channel open probability. Increases in cell Ca, in intracellular pH or in cyclic AMP increase K channel activity and could thus stimulate secretory K flux from the cell into the lumen. In addition, increased K recycling may also affect reabsorption of other solutes, either by regulating the supply of K to the NaCl cotransporter or, alternatively, by stabilizing the apical membrane potential and thus maintaining an effective electrochemical driving force for electrogenic apical Na entry.

Animals↗
PubMed1987

The effect of sodium taurocholate on proximal tubular reabsorption in the rat kidney.

Microperfusion of tubules in situ was used to study the direct effect of sodium taurocholate on reabsorption of fluid by the proximal tubule of the rat. Sodium taurocholate (0.1 mmol/l) in the tubular perfusate reduced proximal tubular fluid reabsorption by approximately 30%. Thus, the proximal tubule appears to be a major site at which bile salts cause a natriuresis in the rat, and possibly in obstructive jaundice in man.

Absorption↗
PubMed1987

Measurement of Na+-K+ coupling ratio of Na+-K+-ATPase in rabbit proximal tubules.

A combination of 23Na nuclear magnetic resonance (NMR) spectroscopy and a K+-selective electrode was used to make simultaneous measurements of net Na+ and K+ fluxes across plasma membranes of rabbit renal proximal tubules after an abrupt stimulation of Na+-K+-ATPase. After a step in extracellular K+ concentration ([K+]o) from low to higher concentration (0.1-0.3 mM to 0.5-5.2 mM) at 25 degrees C, net extrusion of Na+ and uptake of K+ were observed. These fluxes were completely inhibited by ouabain (10(-3) M). Because initial rates of K+ uptake in presence or absence of Ba2+ (a known inhibitor of plasma membrane K+ conductance) were indistinguishable, net K+ flux was virtually unidirectional. Because suspension buffers contained neither glucose nor amino acids and the ratio of net Na+ and K+ fluxes (JNa and JK, respectively) was constant over a wide range of transmembrane Na+ gradients and absolute values of the JNa and JK, it is likely that changes in electrogenic or passive net fluxes across plasma membranes were insignificant in the first 30-45 s after the [K+]o step. Thus the ratio of these initial net Na+ and K+ fluxes corresponds closely to the Na+-K+ coupling ratio of the Na+-K+-ATPase. In 12 experiments, the measured Na+-K+-ATPase coupling ratio was 1.54 +/- 0.07 (SE). The coupling ratio was constant over a wide range of intracellular Na+ content, intracellular sodium concentration, [K+]o and transmembrane Na+ gradient. The coupling ratio also remained constant over an eightfold range of Na+-K+-ATPase rates.

Animals↗
PubMed1987

Effect of insulin on renal potassium metabolism.

The effect of insulin on renal potassium excretion was examined by employing the euglycemic insulin clamp technique in combination with renal clearance measurements. While euglycemia was maintained, insulin was infused at rates of 4.8 (n = 7) and 12 (n = 5) mU X kg-1 X min-1. Steady-state plasma insulin levels of 164 +/- 8 and 370 +/- 15 microU/ml were achieved in the low- and high-dose studies, respectively. Base-line plasma potassium concentration declined progressively by a mean of 0.14 +/- 0.09 (P less than 0.05) and 0.40 +/- 0.05 meq/liter (P less than 0.01) during the low- and high-dose insulin infusion protocols. Urinary potassium excretion did not change significantly from base line with either insulin dose. Because the decline in plasma potassium concentration could have masked a stimulatory effect of insulin on UKV, six rats received a 12-mU X kg-1 X min-1 euglycemic insulin clamp in combination with an exogenous potassium infusion to maintain the plasma potassium concentration constant at the basal level (4.03 +/- 0.03 vs. 4.05 +/- 0.05 meq/l). Under these conditions of normokalemia, insulin augmented UKV 2.4-fold, from 0.20 +/- 0.05 to 0.48 +/- 0.04 meq/l (P less than 0.001).

Animals↗
PubMed1987

Electroneutral H+ secretion in distal tubule of Amphiuma.

This study examines the cellular mechanisms of acid secretion by the in vitro perfused late distal tubule of Amphiuma kidney. Acidification of tubule fluid occurred against an electrochemical gradient of 16 mV; thus H+ secretion was active. Amiloride (1 mM) or a reduction of sodium in the perfusion fluid (from 83.7 to 7.7 mM) partially reduced acidification. Amiloride, in the presence of low sodium, completely inhibited acidification. Furthermore, acetazolamide and ouabain in the bath solution (0.1 mM) also inhibited acidification. Conductive properties of the epithelium and of individual cell membranes were determined by means of cable analysis of the tubule and intracellular voltage recordings. The transepithelial voltage and resistance averaged -0.4 +/- 0.4 mV, lumen negative, and 7,147 +/- 845 omega X cm, respectively. Two functionally different cell types were identified by intracellular microelectrodes. Type I cells had a basolateral membrane voltage (Vbl) of -67.7 mV. As determined by ion substitution experiments, the basolateral membrane was conductive to K+ and Cl-. This cell also had a 4-acetamido-4'-isothiocyanostilbene-2-2'-disulfonic acid (SITS)-sensitive Na+-dependent HCO3- exit pathway in the basolateral membrane. Type II cells had a Vbl of -76.1 mV (P less than 0.05 vs. type I) and the basolateral membrane was conductive to K+ and Cl- but not to HCO3-. HCO3- movement across the basolateral membrane in this cell may occur by electroneutral Cl- -HCO3- exchange. The apical cell membrane of both cell types did not contain measurable ionic conductances, as evidenced by a high value of apical membrane fractional resistance (0.98 +/- 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1987

Electrogenic Na/HCO3 cotransport across basolateral membrane of isolated perfused Necturus proximal tubule.

This study was undertaken to determine whether the proximal tubule of the mud puppy Necturus maculosus possesses a basolateral Na/HCO3 cotransporter. We examined the effects on basolateral membrane potential (Vbl) and intracellular pH (pHi) of 1) lowering basolateral [HCO3-] at constant PCO2, and 2) replacing Na+ with N-methyl-D-glucamine. Vbl and pHi were measured with Ling-Gerard and liquid-membrane pH microelectrodes, respectively, in isolated tubules perfused in vitro. We found that decreasing basolateral [HCO3-] from 10 mM (pH 7.5) to 2 mM (pH 6.8) resulted in an immediate depolarization of 14.9 mV, and a pHi decrease of 0.35. SITS (4-acetamido-4'-isothiocyanostibene-2,2'-disulfonic acid, 0.5 mM) inhibited the HCO3-induced depolarization by 87% and inhibited the initial rate of the pHi decrease by 79%. Replacement of basolateral Na+ with N-methyl-D-glucamine resulted in an immediate depolarization of 11.3 mV, and a pHi decrease of 0.36. SITS inhibited the zero Na-induced depolarization by 86% and the initial rate of the pHi decrease by 81%. Nominal removal of basolateral HCO3- (replaced with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) inhibited the zero Na-induced depolarization by 64%, whereas nominal removal of Na+ inhibited the 2 mM HCO3-induced depolarization by 67%. Replacement of all basolateral Cl- with glucuronate did not inhibit the changes in Vbl induced by changing [HCO3-] or [Na+]. Observations similar to those described above have been made previously on Ambystoma proximal tubules, and attributed to an electrogenic Na/HCO3 cotransport mechanism that carries HCO3-, Na+, and net negative charge in the same direction. We conclude that Necturus proximal tubules possess a similar, if not identical, electrogenic Na/HCO3 cotransport mechanism.

Animals↗
PubMed1987

Potassium channels in Necturus proximal tubule.

Potassium channels from the apical and basolateral membranes of Necturus proximal tubule were studied using the patch-clamp technique. The conductance of the basolateral channel was dependent on the pipette K+ concentration (apparent Km, 65.5 mM K+; maximum channel conductance, 49.8 pS). The permeability ratio (PK+/PNa+) was approximately 10:1. The fractional open time increased with hyperpolarization, whereas mean open times did not change. Ba2+ (0.1 mM pipette concentration) blocked the channel and reduced the mean open time. The apical K+ channel was activated with depolarization and had a slope conductance of 60 pS in the inside-out configuration (100 mM KCl in the pipette and 2.5 mM KCl in the bath). The K+-Na+ selectivity ratio was 32:1. These different channel types will allow independent control of the apical and basolateral membrane K+ conductances.

Animals↗
PubMed1987

Effects of adrenalectomy on CCD: evidence for differential response of two cell types.

Electrophysiological and chemical methods were used to determine the Na and K transport properties of the isolated cortical collecting duct (CCD) of control and adrenalectomized (ADX) rabbits. Net fluxes of Na (JNa) and K (-JK) in controls were 5.7 and 3.2 pmol . mm-1 . min-1 and in ADX were 1.0 and 0.7 pmol . mm-1 . min-1, respectively, similar to electrically determined rates. In separate experiments, blind impalement of cells from adrenal intact (group 1), ADX (group 2), and ADX rabbits treated with deoxycorticosterone (group 3) allowed identification of two distinct cell types, majority cells (MA) and minority cells (MI). In all groups, MA were distinguished from MI by a relatively high basolateral membrane potential (-Vb), low apical membrane fractional resistance (FRa), and presence of apical and basolateral membrane K conductances. Vb of MA (-82.4 mV) was significantly hyperpolarized in groups 1 and 3 combined, when compared with group 2 (-66.4 mV). However, there was no significant difference between Vb of MI in group 2 (-38.9 mV) and Vb of MI in groups 1 and 3 (-36.2 mV). In MA of group 1 equivalent circuit values of apical membrane Na and K conductances (GNaa, GKa) and maximum pump current (Ipmax) were 0.84 and 6.72 mS/cm2 and 46.7 microA/cm2, respectively. These values in group 2 were significantly lower (0.28 and 1.52 mS/cm2 and 8.7 microA/cm2, respectively). It is concluded that two cell types can be distinguished electrically in the CCD. MA have properties consistent with principal cells and MI have properties consistent with intercalated cells. ADX causes a decrease in GNaa, GKa, and Ipmax of PC that results in proportionate decreases in INaa and IKa.

Adrenalectomy↗