The electrophysiological analysis of tubular transport

Voltages and resistances of the anterior Malpighian tubule of Drosophila melanogaster

The small size of Malpighian tubules in the fruit fly Drosophila melanogaster has discouraged measurements of the transepithelial electrical resistance. The present study introduces two methods for measuring the transepithelial resistance in isolated D . melanogaster Malpighian tubules using conventional microelectrodes and PClamp hardware and software. The first method uses three microelectrodes to measure the specific transepithelial resistance normalized to tubule length or luminal surface area for comparison with resistances of other epithelia. The second method uses only two microelectrodes to measure the relative resistance for comparing before and after effects in a single Malpighian tubule. Knowledge of the specific transepithelial resistance allows the first electrical model of electrolyte secretion by the main segment of the anterior Malpighian tubule of D . melanogaster The electrical model is remarkably similar to that of the distal Malpighian tubule of Aedes aegypti when tubules of Drosophila and Aedes are studied in vitro under the same experimental conditions. Thus, despite 189 millions of years of evolution separating these two genera, the electrophysiological properties of their Malpighian tubules remains remarkably conserved.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Peritubular membrane potential in kidney proximal tubular cells of spontaneously hypertensive rats

Peritubular membrane potential in kidney proximal tubular cells of spontaneously hypertensive rats (SHR-Okamoto strain adult rats) was measured with conventional 3 mol KCl microelectrodes, in vivo. Peritubular cell membrane potential was not different in SHR (-66.5 ± 0.7 mV) as compared with normotensive control Wistar rats (-67.5 ± 1.2 mV). To test the effects of possible altered sodium membrane transport in SHR on proximal tubule peritubular membrane potential, we allowed SHR and control rats to drink 1% NaCl for two weeks. Again, proximal tubule peritubular membrane potential was not different in SHR on 1% NaCl (-67.0 ± 1.0 mV) as compared with control rats on 1% NaCl (-64.7 ± 1.3 mV). From these results we concluded that peritubular membrane potential in kidney proximal tubular cells of SHR was not different from normotensive Wistar control rats, and if some alteration of sodium transport in kidney proximal tubular cells of SHR could exist, that was not possible to evaluate from the measurements of peritubular membrane potential in kidney proximal tubular cells.

Micropuncturing the nephron

To achieve the role of the kidney in maintaining body homeostasis, the renal vasculature, the glomeruli, and the various segments of the nephron and the collecting duct system have to fulfill very diverse and specific functions. These functions are dependent on a complex renal architecture and are regulated by systemic hemodynamics, hormones, and nerves. As a consequence, to better understand the physiology of the kidney, methods are necessary that allow insights on the function of these diverse structures in the physiological context of the intact kidney. The renal micropuncture technique allows direct access to study superficial nephrons in vivo. In this review, the application of micropuncture techniques on the single nephron level is outlined as an approach to better understand aspects of glomerular filtration, tubular transport, and tubulo-glomerular communication. Studies from the author's lab, including experiments in gene-targeted mice, are briefly presented to illustrate some of the approaches and show how they can further advance our understanding of the molecular mechanisms involved in the regulation of kidney function.

The Na(+)-HCO3- cotransporter operates with a coupling ratio of 2 HCO3- to 1 Na+ in isolated rabbit renal proximal tubule

All the relevant literature reports indicate that net rates of salt and water absorption and cell membrane potentials (Vb) are lower, but intracellular Na+ concentration is higher in rabbit renal proximal tubule in vitro than in rat proximal tubule in vivo. Since the different driving forces should influence basolateral Na(+)-HCO3- cotransport we have studied the operation of the cotransporter in isolated rabbit renal proximal tubule in vitro with special emphasis on the stoichiometry of flux coupling (q). Using conventional and ion-selective intracellular microelectrodes three series of experiments were performed: (a) we determined the Vb response to a 2:1 reduction of bath HCO3- or Na+ concentration, (b) we determined initial efflux rates of HCO3- or Na+ ions in response to a sudden 10:1 reduction of bath HCO3- concentration, and (c) we collapsed the tubules and determined electrochemical driving forces of Na+ and HCO3- across the basolateral cell membrane under conditions approaching zero net flux in the control state in the presence of Ba2+- and in Cl(-)-free solutions. All measurements concurrently yielded a coupling ratio of approximately two HCO3- ions to one Na+ ion (q = 2). This result contrasts with the ratio q = 3, which we have previously observed in similar experiments on rat renal proximal tubule in vivo [Yoshitomi et al. (1985) Pflügers Arch 405:360] and which was also observed on rabbit renal basolateral cell membrane vesicles in vitro [Soleimani et al. (1987) J Clin Invest 79:1276].(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiology of ammonia transport in renal straight proximal tubules

To test for electrogenic transport of ammonium ions in straight proximal renal tubules, isolated perfused tubules have been exposed to peritubular ammonium ions during continuous recording of cell membrane potential. As a result, 20 mmol/liter NH4+ leads to a rapid, reversible depolarization of the cell membrane by 9.0 +/- 0.3 mV (N = 86). This depolarization is not significantly affected by 10 mmol/liter barium or 0.1 mmol/liter amiloride on both sides of the epithelium, but is significantly blunted by omission of extracellular bicarbonate and CO2 (3.8 +/- 0.4 mV, N = 9), by 1 mmol/liter acetazolamide (4.3 +/- 0.3 mV, N = 11), by 1 mmol/liter peritubular amiloride (4.3 +/- 1.1 mV, N = 7), by 1 mmol/liter SITS (5.7 +/- 0.4 mV, N = 6), and by replacement of extracellular sodium with choline (4.7 +/- 0.5 mV, N = 8). In the presence of both amiloride (1 mmol/liter) and acetazolamide (1 mmol/liter) in the bath, the NH4+ induced depolarization is completely abolished. Furthermore, the combined omission of bicarbonate and addition of 10 mmol/liter barium eliminates the NH4+ induced depolarization. About 50% of the depolarization can be explained by enhanced electrogenic bicarbonate exit due to the intracellular alkalosis. The other 50% is explained by amiloride and barium sensitive electrogenic entry of NH4+ into the cell.

Electrophysiological responses to non-electrolytes in lingual nerve of rat and in lingual epithelia of dog

Epithelial and neural mechanisms underlying the trigeminal chemoreception of non-electrolytes were investigated in whole-nerve recordings from lingual nerve and in Ussing-chamber studies of isolated lingual epithelia. The non-electrolytes included menthol, amyl acetate, phenethyl alcohol, toluene, methanol, ethanol, propanol, butanol, hexanol and octanol. They produced different lingual nerve responses: methanol and ethanol only increased ongoing activity; longer-chain alcohols initially increased but then suppressed activity below baseline; phenethyl alcohol and toluene only suppressed activity. Their threshold concentrations for lingual nerve responses, with the exception of menthol, were proportional to the octanol:water partition coefficients of the stimuli. The threshold concentration for menthol was significantly lower than predicted by this coefficient. Calculation of the free energy of transfer from the threshold concentrations for the n-alcohols suggests that they undergo partition into a hydrophobic environment such as is found in lipid bilayers. Lanthanum chloride, which inhibited lingual nerve responses to hydrophilic compounds, presumably by blocking their diffusion across tight junctions, did not inhibit responses to these non-electrolytes. At high concentrations, hexanol acted as an anaesthetic in that the lingual nerve no longer responded to thermal and chemical stimuli whereas ethanol, which only increased lingual nerve activity, did not inhibit those responses. Epithelial transport, as indicated by the short-circuit current (Isc) measured across tongues bathed in symmetrical solutions of Krebs-Henseleit buffer, was reversibly inhibited by ethanol, hexanol, octanol, phenyl ethanol and menthol. The stimulus concentration necessary to inhibit 50% of the Isc decreased with increasing octanol:water partition coefficient.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical properties of cultured renal tubular cells (OK) grown in confluent monolayers

OK cells grown to confluent monolayers were investigated by microelectrode techniques and microinjection. Cell membrane potential difference (PDm) in bicarbonate-free solution is -61.8 +/- 0.6 mV (n = 208), cell membrane resistance (Rm) amounts to 1.4 +/- 0.2 k omega. cm2 (n = 8). The apparent transference number for potassium (t'k+) is 71 +/- 3% (n = 28) and can be reduced by 3 mmol/l BaCl2 to 7.5 +/- 4.0%; (n = 8). In the presence of extracellular CO2 and HCO3- (pH 7.4) the cells acidify by 0.34 +/- 0.05 pH units (n = 12). This leads to a depolarization of PDm by 8.4 +/- 1.8 mV (n = 8), an increase in Rm by 49 +/- 10% (n = 10), and a reduction of K+-conductance to 63 +/- 5% (n = 13). Intracellular acidification by the NH4Cl-prepulse technique also inhibits K+-conductance and depolarizes the membrane. Recovery from an intracellular acid load is reflected by cell membrane repolarization. This recovery can be inhibited by amiloride (10(-3) mol/l). Na+- and Cl- -conductances could not be detected. The transepithelial resistance (Rte) of OK cell monolayers 1 day after plating is 41 +/- 6 omega.cm2 and decreases with time after plating. Intercellular communication (electrical or dye coupling) was not observed.

CONCLUSIONS

1. The membrane potential of OK cells is largely determined by a pH-sensitive, barium-blockable K+-conductance. 2. Amiloride-blockable Na+/H+-exchange is reflected by membrane potential changes via this K+-conductance. 3. Monolayers of OK cells are electrically leaky.

Electrophysiological characterization of rabbit distal convoluted tubule cell

The distal convoluted tubule (DCT) from rabbit kidney were perfused in vitro to study the conductive properties of the cell membranes by using electrophysiological methods. When the lumen and the bath were perfused with a bicarbonate free solution buffered with HEPES, the transepithelial voltage (VT) averaged -2.8 +/- 0.6 mV (n = 20), lumen negative. The basolateral membrane voltage (VB) averaged -77.8 +/- 1.1 mV (n = 33) obtained by intracellular impalement of microelectrodes. Cable analysis performed by injecting a current from perfusion pipette revealed that the transepithelial resistance was 21.8 +/- 1.7 omega.cm2 and the fractional resistance of the luminal membrane was 0.78 +/- 0.03 (n = 8), indicating the existence of ionic conductances in the luminal membrane. Addition of amiloride (10(-5) mol/l) to the luminal perfusate or Na+ removal from the lumen abolished the lumen negative VT and hyperpolarized the apical membrane. An increase in luminal K+ concentration from 5 to 50 mmol/l reduced the apical membrane potential (VA) by 37.5 +/- 2.6 mV (n = 7), whereas a reduction of Cl- in the luminal perfusate did not change VA significantly (0.5 +/- 0.5 mV, n = 4). Addition of Ba2+ to the lumen reduced VA by 42.6 +/- 1.0 mV (n = 4). When the bathing fluid was perfused with 50 mmol/l K+ solution, the basolateral membrane voltage (VB) fell from -76.8 +/- 1.5 to -31.0 +/- 1.3 mV (n = 18), and addition of Ba2+ to the bath reduced VB by 18.3 +/- 4.8 mV (n = 7).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for conductive Cl- pathways across the cell membranes of the thin ascending limb of Henle's loop

To examine whether Cl- is transported via transcellular pathways in the thin ascending limb of Henle's loop (TAL), conventional microelectrode technique was applied in isolated TAL segments of hamsters perfused in vitro. The average basolateral membrane voltage (VB) was -24.5 +/- 1.5 mV (n = 18). Ouabain (10(-4) M) had no effect on VB. Sudden reduction of basolateral Cl- concentration from 165 to 5 mmol/liter caused a large depolarizing spike (+49.1 +/- 2.7 mV, n = 18), while the transepithelial potential (VT) showed lumen positive deflection by 33.4 +/- 1.2 mV, which indicates that a large Cl- conductance exists in the basolateral membrane. Reduction of luminal Cl- concentration caused sustained depolarization of luminal cell membrane from +24.5 +/- 2.1 to -9.7 +/- 3.4 mV (n = 6), which indicates that there is also a Cl- conductance in the luminal membrane. Since we have previously shown that acidification of ambient solution suppresses the transmural Cl- permeability, we tested whether acid pH also inhibits the Cl- conductance of the basolateral membrane. When pH of the bathing fluid was lowered to 5.8, the depolarizing spike of VB and the change of VT upon sudden reduction of basolateral Cl- were almost completely abolished. From these results we conclude: (a) both the luminal and the basolateral membrane of hamster TAL segments have Cl- conductances, and (b) Cl- transport in the TAL takes place, at least in part, via a transcellular route when a transepithelial Cl- gradient is present.

Ionic conductances of cultured principal cell epithelium of renal collecting duct

The ionic conductive properties were studied of epithelia of collecting duct principal cells which had been grown in primary tissue culture from renal cortex/capsule explants. When pretreated with aldosterone (10(-6) mol/l) and bathed on either surface with isotonic HCO3(-)-free Ringer's solution, the transepithelial voltage, Vte, varied between -21 and -72 mV (apical surface negative) while the transepithelial resistance, Rte, ranged from 0.4 to 1.5 k omega cm2. By 10:1 step-changes in Na+ concentration the apical cell membrane was shown to have a high conductivity for sodium, inhibitable by amiloride, 10(-6) mol/l. However, contrary to observations in natural collecting duct under control conditions, amiloride never reversed the polarity of Vte even at 10(-4) mol/l. Both the apical and the basolateral cell membranes were conductive for potassium and both conductivities were inhibitable by Ba2+ (5 mmol/l). 10:1 reduction of apical Cl- concentration strongly hyperpolarized Vte with a monophasic time course suggesting the presence of a paracellular shunt conductance for Cl-. In addition there may be a small Cl- conductance present in the apical cell membrane since apical application of the chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPAB) at 10(-7) mol/l produced a minute but significant hyperpolarization. On the other hand, 10:1 reduction of basolateral Cl- concentration caused a biphasic change in Vte (initial depolarization, followed by repolarization) which indicates the presence of a large Cl- conductance in the basolateral cell membrane. The latter was not inhibitable by 10(-7) mol/l NPPAB. Higher concentrations of this and of an other Cl-channel blocker produced non-specific effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for rheogenic sodium bicarbonate cotransport in the basolateral membrane of oxyntic cells of frog gastric fundus

Ionic conductance properties of the basolateral cell membrane of oxyntic cells were studied in frog gastric fundus in vitro. After mounting the fundus in a modified Ussing chamber the serosal connective tissue was dissected off and individual oxyntic cells were punctured from the serosal surface with microelectrodes. Under resting conditions the membrane potential averaged -56.9, SD +/- 9.5 mV (n = 63), cytoplasm negative. Lowering or raising serosal HCO-3 concentration from 17.8 to 6 or 36 mmol/l respectively at constant PCO2 depolarized or hyperpolarized the cell membrane by +16.7 or -18.2 mV respectively. Sudden removal of serosal Na+ also depolarized the cell membrane (anomalous Nernst response). Since both the HCO-3 dependent and the Na+ dependent potential changes were strongly depressed by the disulfonic stilbene SITS and since the potential response to HCO-3 was virtually abolished in Na+-free solution we conclude that a rheogenic Na+ (HCO-3)n-cotransport system (n greater than 1) is present in the basolateral cell membrane of oxyntic cells. Its possible role in base transfer during HCl-secretion or HCO-3 secretion remains to be elucidated.