Effect of amiloride on the apical cell membrane cation channels of a sodium-absorbing, potassium-secreting renal epithelium

Physiology of a Forgotten Electrolyte-Magnesium Disorders

Magnesium (Mg²⁺) is the second most common intracellular cation and the fourth most abundant element on earth. However, Mg²⁺ is a frequently overlooked electrolyte and often not measured in patients. While hypomagnesemia is common in 15% of the general population, hypermagnesemia is typically only found in preeclamptic women after Mg²⁺ therapy and in patients with ESRD. Mild to moderate hypomagnesemia has been associated with hypertension, metabolic syndrome, type 2 diabetes mellitus, CKD, and cancer. Nutritional Mg²⁺ intake and enteral Mg²⁺ absorption are important for Mg²⁺ homeostasis, but the kidneys are the key regulators of Mg²⁺ homeostasis by limiting urinary excretion to less than 4% while the gastrointestinal tract loses over 50% of the Mg²⁺ intake in the feces. Here, we review the physiological relevance of Mg²⁺, the current knowledge of Mg²⁺ absorption in the kidneys and the gut, the different causes of hypomagnesemia, and a diagnostic approach on how to assess Mg²⁺ status. We highlight the latest discoveries of monogenetic conditions causing hypomagnesemia, which have enhanced our understanding of tubular Mg²⁺ absorption. We will also discuss external and iatrogenic causes of hypomagnesemia and advances in the treatment of hypomagnesemia.

L-WNK1 is required for BK channel activation in intercalated cells

Large-conductance K+ (BK) channels expressed in intercalated cells (ICs) in the aldosterone-sensitive distal nephron (ASDN) mediate flow-induced K+ secretion. In the ASDN of mice and rabbits, IC BK channel expression and activity increase with a high-K+ diet. In cell culture, the long isoform of with-no-lysine kinase 1 (L-WNK1) increases BK channel expression and activity. Apical L-WNK1 expression is selectively enhanced in ICs in the ASDN of rabbits on a high-K+ diet, suggesting that L-WNK1 contributes to BK channel regulation by dietary K+. We examined the role of IC L-WNK1 expression in enhancing BK channel activity in response to a high-K+ diet. Mice with IC-selective deletion of L-WNK1 (IC-L-WNK1-KO) and littermate control mice were placed on a high-K+ (5% K+, as KCl) diet for 10 or more days. IC-L-WNK1-KO mice exhibited reduced IC apical + subapical α-subunit expression and BK channel-dependent whole cell currents compared with controls. Six-hour urinary K+ excretion in response a saline load was similar in IC-L-WNK1-KO mice and controls. The observations that IC-L-WNK1-KO mice on a high-K+ diet have higher blood K+ concentration and reduced IC BK channel activity are consistent with impaired urinary K+ secretion, demonstrating that IC L-WNK1 has a role in the renal adaptation to a high-K+ diet.NEW & NOTEWORTHY When mice are placed on a high-K+ diet, genetic disruption of the long form of with no lysine kinase 1 (L-WNK1) in intercalated cells reduced relative apical + subapical localization of the large-conductance K+ channel, blunted large-conductance K+ channel currents in intercalated cells, and increased blood K+ concentration. These data confirm an in vivo role of L-WNK1 in intercalated cells in adaptation to a high-K+ diet.

Acidity and Acid-Sensing Ion Channels in the Normal and Alzheimer's Disease Brain

Alzheimer's disease prevalence has reached epidemic proportion with very few treatment options, which are associated with a multitude of side effects. A potential avenue of research for new therapies are protons, and their associated receptor: acid-sensing ion channels (ASIC). Protons are often overlooked neurotransmitters, and proton-gated currents have been identified in the brain. Furthermore, ASICs have been determined to be crucial for proper brain function. While there is more work to be done, this review is intended to highlight protons as neurotransmitters and their role along with the role of ASICs within physiological functioning of the brain. We will also cover the pathophysiological associations between ASICs and modulators of ASICs. Finally, this review will sum up how the studies of protons, ASICs and their modulators may generate new therapeutic molecules for Alzheimer's disease and other neurodegenerative diseases.

Relation between BK-α/β4-mediated potassium secretion and ENaC-mediated sodium reabsorption

The large conductance, calcium-activated BK-α/β4 potassium channel, localized to the intercalated cells of the distal nephron, mediates potassium secretion during high potassium, alkaline diets. Here we determine whether BK-α/β4-mediated potassium transport is dependent on epithelial sodium channel (ENaC)-mediated sodium reabsorption. We maximized sodium-potassium exchange in the distal nephron by feeding mice a low sodium, high potassium diet. Wild type and BK-β4 knockout mice were maintained on low sodium, high potassium, alkaline diet or a low sodium, high potassium, acidic diet for 7–10 days. Wild type mice maintained potassium homeostasis on the alkaline but not acid diet. BK-β4 knockout mice could not maintain potassium homeostasis on either diet. During the last 12 hours of diet, wild type mice on either a regular, alkaline or an acid diet, or knockout mice on an alkaline diet were administered amiloride (an ENaC inhibitor). Amiloride enhanced sodium excretion in all wild type and knockout groups to similar values; however, amiloride diminished potassium excretion by 59% in wild type but only by 33% in knockout mice on an alkaline diet. Similarly, amiloride decreased the transtubular potassium gradient by 68% in wild type but only by 42% in knockout mice on an alkaline diet. Amiloride treatment equally enhanced sodium excretion and diminished potassium secretion in knockout mice on an alkaline diet and wild type mice on an acid diet. Thus, the enhanced effect of amiloride on potassium secretion in wild type compared to knockout mice on the alkaline diet, clarify a BK- α/β4-mediated potassium secretory pathway in intercalated cells driven by ENaC-mediated sodium reabsorption linked to bicarbonate secretion.

Mechanisms of tubular sodium chloride transport

Extracellular fluid volume is determined by sodium and its accompanying anions. There are control mechanisms which regulate sodium balance in the body. These include high and low pressure baroreceptors, intrarenal baroreceptors, renal autoregulation, tubuloglomerular feedback, aldosterone, and numerous other physical and hormonal factors. Sodium transport by the nephron involves active and passive processes which occur in several different nephron segments. Mechanisms of cotransport, Na(+)-H+ exchange, antiporters and ion-specific channels are all utilized by the nephron to maintain sodium balance. These regulatory factors and transport mechanisms for sodium in the kidney will he discussed in detail.

Aldosterone regulation of sodium channel gamma-subunit mRNA in cortical collecting duct cells

Specific regulatory mechanisms of aldosterone-stimulated Na+ reabsorption through the apical amiloride-sensitive channel are unknown. In this study, we examined the effects of aldosterone on Na+ channel gamma-subunit mRNA levels in cultured rabbit cortical collecting duct cells. With the use of reverse transcriptase-polymerase chain reaction (RT-PCR) with RNA isolated from aldosterone-treated cells and degenerate primers, a 446-base pair (bp) PCR product was amplified and further characterized by nested PCR and sequencing. The nested PCR yielded a predicted 164-bp product. Sequencing of the 446-bp PCR product revealed 83% nucleotide and 91% amino acid identity to the rat colonic Na+ channel gamma-subunit. The relative abundance of Na+ channel mRNA was determined by quantitative PCR after a 24-h aldosterone treatment. The results demonstrate that Na+ channel gamma-subunit mRNA levels were significantly higher (2.6 +/- 0.42) in aldosterone-treated cultures vs. the controls. This increase, however, is less than the aldosterone-induced increase (3.2 +/- 2.0) in the amiloride-sensitive short-circuit current. These results indicate that Na+ channel gamma-subunit mRNA levels are increased by aldosterone and that this increase is likely to be responsible, at least in part, for the aldosterone-induced Na+ current in the kidney.

Amiloride-sensitive sodium channels in confluent M-1 mouse cortical collecting duct cells

Confluent M-1 cells show electrogenic Na+ absorption and possess an amiloride-sensitive Na(+)-conductance (Korbmacher et al., J. Gen. Physiol. 102:761-793, 1993). In the present study, we further characterized this conductance and identified the underlying single channels using conventional patch clamp technique. Moreover, we isolated poly(A)+ RNA from M-1 cells to express the channels in Xenopus laevis oocytes, and to check for the presence of transcripts related to the epithelial Na+ channel recently cloned from rat colon (Canessa et al., Nature 361:467-470, 1993). Patch clamp experiments were performed in 6-13-day-old confluent M-1 cells at 37 degrees C. In whole-cell experiments application of 10(-5) M amiloride caused a hyperpolarization of 24.9, SEM +/- 2.2 mV (n = 35) and a reduction of the inward current by 107 +/- 10 pA (n = 51) at a holding potential of -60 mV. Complete removal of bath Na+ had similar effects, indicating that the amiloride-sensitive component of the inward current is a Na+ current. The effect of amiloride was concentration-dependent with half-inhibition at 0.22 microM. The Na+ current saturated with increasing extracellular Na+ concentrations with an apparent Km of 24 mM. Na+ replacement for Li+ demonstrated a higher apical membrane conductance for Li+ than for Na+. In excised inside-out (i/o) or outside-out (o/o) patches from the apical membrane, we observed single-channels which showed slow kinetics and were reversibly inhibited by amiloride. Their average conductance for Na+ was 6.8 +/- 0.5 pS (n = 15) and for Li+ 11.2 +/- 1.0 pS (n = 14). They had no measurable conductance for K+. In o/o patches, channel activity was slightly voltage dependent with an open probability (NPo) of 0.46 +/- 0.14 and 0.16 +/- 0.05 at a holding potential of -100 and 0 mV, respectively (n = 8, P < 0.05). Using the two-microelectrode voltage-clamp technique, we assayed defolliculated stage V-VI Xenopus oocytes for an amiloride-sensitive inward current 1-6 days after injection with H2O or with 20-50 ng of M-1 poly(A)+ RNA. In poly(A)+ RNA-injected oocytes held at -60 or -100 mV application of amiloride (2 microM) reduced the Na-inward current by 25.5 +/- 4.6 nA (n = 25) while it had no effect in H2O-injected oocytes (n = 19). Northern blot analysis of M-1 poly(A+) RNA revealed the presence of transcripts related to the three known subunits of the rat colon Na+ channel (Canessa et al., Nature 367:463-467, 1994). We conclude that the channel in M-1 cells is closely related to the amiloride-sensitive epithelial Na+ channel in the rat colon and that the M-1 cell line provides a useful tool to investigate the biophysical and molecular properties of the corresponding channel in the cortical collecting duct.

Stimulation of calcium transport by amiloride in mouse distal convoluted tubule cells

This study examined the mechanism by which amiloride dissociates Na and Ca transport in distal convoluted tubules. Control rates of Na uptake averaged 288 nmol/(min mg protein) and were inhibited 39% by microM amiloride. Amiloride had no effect on Cl uptake. Resting membrane voltage, measured with the voltage-sensitive dye DiOC6 (3), averaged -70 mV. Amiloride hyperpolarized cells in a reversible manner by 18 mV. Control rates of Ca uptake averaged 2.8 nmol/(min mg protein) and increased by 39% in the presence of amiloride. Alterations of intracellular Ca activity were measured in single cells loaded with Fura2-AM. Control intracellular Ca activity averaged 100 nM. Amiloride increased intracellular Ca activity in a concentration-dependent manner to a maximum of 330 nM at microM amiloride. Amiloride analogues ethylisopropyl amiloride (EIPA) and dimethylbenzamil (DMB), which preferentially block Na/H and Na/Ca exchange, respectively, had no effect on Na or Ca influx or on intracellular Ca activity. The dihydropyridine Ca channel blocker nifedipine inhibited amiloride-stimulated Ca uptake and the rise of intracellular Ca activity but had no effect on membrane voltage. It is concluded that amiloride blocks Na entry mediated by Na channels. Inhibition of Na entry results in membrane hyperpolarization, which activates Ca entry by dihydropyridine-sensitive Ca channels.

Amoxicillin intestinal absorption reduction by amiloride: possible role of the Na(+) -H+ exchanger

Intestinal absorption of beta-lactam antibiotics has been shown to use the dipeptide carrier system. In vitro experiments have established that the efficiency of uptake by enterocytes depends on an inwardly directed proton gradient--dipeptides and beta-lactam antibiotics being cotransported along with hydrogen ion. This gradient is thought to result from the sodium-hydrogen (Na(+)-H+) exchanger located on the brush-border membrane. The aim of the present study was to assess the in vivo relevance of these data in humans by examining the effect of amiloride, a well-known inhibitor of the Na(+) -H+ exchanger, on the bioavailability of amoxicillin in eight healthy volunteers. The results show that amiloride reduces significantly amoxicillin absorption rate (mean time to maximum concentration increases from 1.0 to 1.6 hours, p < 0.05) and absolute bioavailability (by 27%, p < 0.01) and that amiloride-induced inhibition of the intestinal Na(+) -H+ exchange could be associated with an additional inhibitory effect on (Na/K)-ATPase activity. The present data seem to confirm the role of Na(+) -H+ exchange in the uptake of beta-lactams by the intestine and to support the indirect sodium dependence of this carrier system in vivo.

Mouse cortical collecting duct cells show nonselective cation channel activity and express a gene related to the cGMP-gated rod photoreceptor channel

Apical nonselective cation channels with an average single-channel conductance of 34 +/- 2.3 pS were found in M-1 mouse cortical collecting duct cells. Channel activity is increased by depolarization and abolished by cytoplasmic calcium removal. Cytoplasmic application of 0.1 mM cGMP decreases channel open probability by 27%. cDNAs corresponding to approximately 40% of the coding region of the photoreceptor channel were isolated by the polymerase chain reaction from M-1 cells and a rat kidney cDNA library. The rat kidney-derived sequence differs by a single base, and the M-1-cell-derived sequence differs by only two bases, from the photoreceptor sequence. A second clone from M-1 cells differs by 20 out of 426 bases from the photoreceptor sequence. In all three clones, the deduced amino acid sequence is identical to that of the rat photoreceptor channel. Northern blot analysis of poly(A)+ RNA from M-1 cells reveals the presence of a 3.2-kilobase band hybridizing with a retinal cGMP-gated cation channel probe. The results suggest the expression in M-1 cells of more than one gene coding for nonselective cation channels or channel subunits, one of which is identical to the cGMP-gated cation channel gene of rod photoreceptors.

Amiloride and its analogs as tools to inhibit Na+ transport via the Na+ channel, the Na+/H+ antiport and the Na+/Ca2+ exchanger

Amiloride analogs inhibit a number of transmembrane Na+ transport systems: 1) the epithelium Na+ channel, 2) the Na+/H+ exchange system and 3) the Na+/Ca2+ exchange system. Structure--activity relationships using amiloride derivatives with selected modification of each of the functional groups of the molecule indicate that the 3 Na+ transporting systems have distinct pharmacological profiles. 5-N Disubstituted derivatives of amiloride, such as ethylisopropylamiloride are the most potent inhibitors of the Na+/H+ exchange system. Conversely, amiloride derivatives that are substituted on the guanidino moiety, such as phenamil, are potent inhibitors of the epithelium Na+ channel. It is thus possible, by using selected amiloride derivatives to inhibit selectively one or another of the Na+ transport systems.

Effect of amiloride on sodium transport in the proximal, distal, and entire human colon in vivo

In vitro studies under short-circuited conditions suggest that amiloride, a diuretic agent which is thought to block apical membrane sodium entry, has significant effects on sodium absorption by the human colon. To evaluate this in vivo, we studied the effect of amiloride applied in concentrations of 10(-5) and 10(-4) M on sodium transport and potential difference (PD) in human colon during steady-state perfusion. Net sodium absorption was reduced 25% by amiloride and chloride absorption by 15%; potassium and bicarbonate secretion rates were enhanced. In other studies the colon was divided into a proximal and distal test segment by endoscopic introduction of a collection channel to the descending colon-sigmoid junction. Comparison of tritiated water absorption by the two segments indicated that the distal segment comprised approximately 20% of the total colon surface area. However, the distal test segment only accounted for 5-7% of total sodium, chloride, or water absorption; in contrast, 17-20% of total potassium or bicarbonate secretion occurred there. In the proximal test segment, amiloride reduced net sodium absorption by almost one third from 21.0 to 14.4 mmol/hr (P less than 0.02) but had no significant effect on PD. In the distal test segment, amiloride produced a 25% reduction in mean sodium absorption from 1.2 to 0.9 mmol/hr, but this reduction was not statistically significant; however, potential difference was significantly reduced by 33% (P less than 0.02). These results suggest that most sodium absorption in normal human colon in vivo is mediated by transport processes which are insensitive to these doses of amiloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupled Na+/H+ exchange in rat parotid basolateral membrane vesicles

pH gradient-dependent sodium transport in highly purified rat parotid basolateral membrane vesicles was studied under voltage-clamped conditions. In the presence of an outwardly directed H+ gradient (pHin = 6.0, pHout = 8.0) 22Na uptake was approximately ten times greater than uptake measured at pH equilibrium (pHin = pHout = 6.0). More than 90% of this sodium flux was inhibited by the potassium-sparing diuretic drug amiloride (K1 = 1.6 microM) while the transport inhibitors furosemide (1 mM), bumetanide (1 mM), SITS (0.5 mM) and DIDS (0.1 mM) were without effect. This transport activity copurified with the basolateral membrane marker K+-stimulated p-nitrophenyl phosphatase. In addition, 22Na uptake into the vesicles could be driven against a concentration gradient by an outwardly directed H+ gradient. pH gradient-dependent sodium flux exhibited a simple Michaelis-Menten-type dependence on sodium concentration consistent with the existence of a single transport system with KM = 8.0 mM at 23 degrees C. A component of pH gradient-dependent, amiloride-sensitive sodium flux was also observed in rabbit parotid basolateral membrane vesicles. These results provide strong evidence for the existence of a Na+/H+ antiport in rat and rabbit parotid acinar basolateral membranes and extend earlier less direct studies which suggested that such a transporter was present in salivary acinar cells and might play a significant role in salivary fluid secretion.

Ultrastructure of the kidney of a South American caecilian, Typhlonectes compressicaudus (Amphibia, Gymnophiona). II. Distal tubule, connecting tubule, collecting duct and Wolffian duct

The ultrastructure of the distal nephron, the collecting duct and the Wolffian duct was studied in a South American caecilian, Typhlonectes compressicaudus (Amphibia, Gymnophiona) by transmission and scanning electron microscopy (TEM, SEM). The distal tubule (DT) is made up of one type of cell that has a well-developed membrane labyrinth established both by interdigitating processes and by interlocking ramifications. The processes contain large mitochondria, the ramifications do not. The tight junction is shallow and elongated by a meandering course. The connecting tubule (CNT) is composed of CNT cells proper and intercalated cells, both of which are cuboidal in shape. The CNT cells are characterized by many lateral interlocking folds. The intercalated cells have a dark cytoplasm densely filled with mitochondria. Their apical cell membrane is typically amplified by microplicae beneath which a layer of globular particles (studs) is found. The collecting duct (CD) is composed of principal cells and intercalated cells, again both cuboidal in shape. The CD epithelium is characterized by dilated intercellular spaces, which are often filled with lateral microfolds projecting from adjacent principal cells. The apical membrane is covered by a prominent glycocalyx. The intercalated cells in the CD are similar to those in the CNT. The Wolffian duct (WD) has a tall pseudostratified epithelium established by WD cells proper, intercalated cells and basal cells. The WD cells contain irregular-shaped dense granules located beneath the apical cell membrane. The intercalated cells of the WD have a dark cytoplasm with many mitochondria; their nuclei display a dense chromatin pattern.

Effects of amiloride in the medullary collecting duct of rat kidney

The in vivo microcatheterization technique was used to study amiloride-induced transport alterations in the inner medullary collecting duct. Amiloride treated rats (0.1 mg/hr) had significant diuresis and natriuresis, as well as antikaliuresis, compared to untreated controls. The relative decrease in potassium excretion was associated with a significant rise in plasma potassium concentration. Net sodium transport in the duct was decreased from 83 + 3 to 46 + 6 per cent of delivered load, as a result of amiloride treatment. Smaller, but statistically significant, reductions (P less than 0.01) were seen for fluid and chloride reabsorptions (from 66 + 3 to 51 + 4%, and from 72 + 4 to 52 + 5%, respectively). Potassium reabsorption increased from 15 + 8 to 61 + 6% of delivered load. The data indicated that amiloride natriuresis is determined primarily by inhibition of sodium reabsorption in the medullary collecting duct, probably due to blockade of a specific Na channel. The antikaliuresis, on the other hand, appears to be due to inhibition of secretion both in upstream nephron segments and in the duct itself.

Evolution of diuretics and ACE inhibitors, their renal and antihypertensive actions--parallels and contrasts

The emergence of diuretic drugs and angiotensin converting enzyme (ACE) inhibitors ranks amongst the major therapeutic advances of modern medicine. The discovery of these drug groups arose largely by chance, yet each has dramatically influenced the treatment of congestive cardiac failure and arterial hypertension. The central role which diuretics have had in the management of both oedema and hypertension hinges on their ability to induce a net renal excretion of solute and water by selective interference with either active or passive ion transport processes in different segments of the nephron. Irrespective of sites of action, the continued antihypertensive action of diuretics is characterized by a reduction in plasma volume and extracellular fluid (ECF) volume that lasts for as long as the diuretic is given. The mechanism of this effect remains unclear but may involve autoregulatory reactions that leave cardiac output unaltered but maintain a sustained reduction in total peripheral resistance. ACE inhibitors also lower blood pressure by decreasing total peripheral resistance, leaving cardiac output, plasma volume and ECF volume unchanged. The detailed way these haemodynamic changes are achieved remains unknown but inhibition of converting enzyme present not only in the kidney but also in many extrarenal tissue sites, appears important. In both hypertension and cardiac failure, however, the kidney acts as a key target organ for ACE inhibitors. The increased renal vascular resistance and inappropriate renal salt excretion are reversed with enhanced renal blood flow and saluresis. Both angiotensin II (AII) and vasopressin-mediated contraction of glomerular mesangial cells is inhibited, making glomerular filtration more efficient. Reduced aldosterone secondary to blockade of AII formation contributes to saluresis whilst encouraging positive potassium balance. ACE inhibition also impairs breakdown of kinins which may contribute to intrarenal and peripheral vasodilation either on their own or via release of prostaglandins and other vasoactive substances. The hypotensive actions of diuretics are potentiated by ACE inhibition primarily through blockade of AII formation and prevention of secondary aldosteronism. In combination, these drugs permit low doses to be used because of their synergistic effects. Caution has to be exercised whenever ACE inhibition is used, without and especially with diuretics, in the management of renovascular hypertension and other low-perfusion states. In these circumstances, AII plays an important autoregulatory role in preserving glomerular filtration through an increase in post-glomerular resistance.(ABSTRACT TRUNCATED AT 400 WORDS)

Amiloride-sensitive Na channels from the apical membrane of the rat cortical collecting tubule

Currents through individual Na channels in the apical membrane of the rat cortical collecting tubule were resolved by using the patch-clamp technique. In cell-attached patches, the channels had a conductance of 5 pS with 140 mM NaCl in the pipet. The conductance was a saturable function of external Na, with a maximal value of about 8 pS and a half saturation at about 75 mM Na. In excised inside-out patches, the selectivity of the channels for Na over K was estimated from reversal potentials to be at least 10:1. The channels underwent spontaneous transitions between open and closed states. Both states had mean lifetimes of 3-4 sec. Amiloride (0.5 microM) added to the pipet induced more frequent closures and openings of the channels and a reduction in the mean open time. These channels are presumed to mediate Na reabsorption by this nephron segment in vivo.

Mineralocorticoid effect on K+ permeability of the rabbit cortical collecting tubule

Mineralocorticoid hormones stimulate Na+ absorption and K+ secretion by the cortical collecting tubule. There is good evidence that this stimulation involves increasing luminal membrane Na+ permeability and the turnover rate (or number) of the Na+-K+ pumps. These experiments were designed to examine whether mineralocorticoid hormones also increase cell K+ permeability. Using 42K tracer measurements in tubules treated with amiloride to inhibit active Na+ and K+ transport, passive K+ permeation increased with increasing mineralocorticoid effect. Net Na+ absorption and the (passive) K+ efflux rate coefficient (KK) showed a linear relationship. The stimulatory effect was evident in vitro since 0.2 microM aldosterone added to the bath of tubules harvested from NaCl-loaded rabbits increased KK at 3 hrs while time controls showed no change. Since these tubules were also treated with amiloride, this increase in KK was not dependent on increasing Na+ absorption. The results indicate that in addition to the well-described effects of aldosterone on Na+ permeability and cell metabolism, the mineralcorticoid effect includes an increase in cellular K+ permeability.

Tubular action of diuretics: distal effects on electrolyte transport and acidification

We used clearance and free-flow micropuncture techniques to evaluate the influence of several diuretic agents, given both individually and in various combinations, on transport of sodium, potassium, and fluid, and on acidification and ammonium transport, within the distal tubule of the rat kidney. The loop diuretics, furosemide and piretanide, sharply increased fractional delivery of fluid, sodium, and potassium into the distal tubule, and, as a result, sodium reabsorption and potassium secretion were enhanced in this nephron segment. These two drugs also stimulated urinary acidification and increased urinary phosphate, titratable acid, and ammonium excretion. These effects took place both within the loop of Henle and along the distal tubule. Amiloride and triamterene alone inhibited distal tubular sodium reabsorption and potassium secretion, and, when given with one of the loop diuretics, suppressed both the kaliuresis and the increased acid and ammonium excretion induced by the latter agents. Hydrochlorothiazide and tizolemide inhibited sodium reabsorption within the distal tubule, and were associated with a stimulation of potassium secretion within this segment. Addition of one of these two latter distally acting agents to either of the loop diuretics led to a further augmentation of sodium excretion, but to a reduction of potassium excretion, compared to the responses seen after the loop diuretics alone.

Cortical collecting tubule potassium secretion: effect of amiloride, ouabain, and luminal sodium concentration

The present study examined the effect of sodium transport inhibition by amiloride, ouabain, and luminal sodium removal on potassium secretion in isolated cortical collecting tubules from adrenalectomized and DOCA-stimulated rabbits. Collecting tubules from adrenalectomized rabbits had a mean potassium secretion of 3.62 +/- 0.37 pmoles X mm-1 X min-1, which significantly decreased to 1.52 +/- 0.21 pmoles X mm-1 X min-1 after addition of amiloride, but no additional effect was observed after the addition of ouabain. The transepithelial voltage (VT) became less positive after exposure to amiloride. Cortical collecting tubules from DOCA-treated animals exhibited significantly greater potassium secretion (28.6 +/- 9.4 pmoles X mm-1 X mm-1). Amiloride totally inhibited potassium secretion, and VT reversed polarity in these tubules. In tubules from adrenalectomized rabbits the removal of luminal sodium inhibited potassium secretion by approximately 44% but had no effect on VT. There remained, however, a substantial amount of potassium secretion in the absence of transepithelial sodium flux. Thus, potassium secretion in the cortical collecting tubule is highly dependent on sodium reabsorption under conditions of mineralocorticoid stimulation but significantly less so in adrenalectomized animals. Potassium secretion in the cortical collecting tubule of adrenalectomized rabbits is inhibited independent of VT and occurs, in part, by an apparent electroneutral process. Chronic exposure to mineralocorticoids appears to stimulate electrogenic sodium reabsorption and potassium secretion.

Origin of positive transepithelial potential difference in early distal segments of rat kidney

Previous studies from our laboratory indicate that early distal segments of the rat kidney have a positive transepithelial potential difference (PD). The present study investigates the origin of the positive PD. PDs were measured in early distal segments using a technique which allowed simultaneous microperfusion and PD measurement through a single pipette (3 to 6 micron O.D.). Microperfusion with artificial plasma ultrafiltrate resulted in a significantly negative mean PD of -4.9 +/- 0.7 mV (N = 17), in contrast to a positive free-flow PD of +5.7 +/- 1.1 mV (N = 174) (P less than 0.001). Addition of amiloride 10(-4) M to plasma ultrafiltrate changed the PD to +1.7 +/- 0.2 mV (N = 25, P less than 0.001). In contrast, furosemide 10(-4) M had no effect on the perfusion PD. Removal of sodium from the luminal perfusate abolished any effect of amiloride on the perfusion PD. Perfusion with artificial early distal fluid yielded a positive PD of +4.2 +/- 0.2 mV (N = 19). Amiloride increased this PD to +8.3 +/- 0.7 mV (N = 21, P less than 0.001). Subsequent experiments in which the sodium and potassium concentrations of the perfusates were varied indicated that concentration gradients for these ions across the early distal tubule could generate substantial diffusion PDs and that potassium was much more permeant than sodium.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of amiloride analogues with the Na+/H+ exchanger in kidney medulla microsomes

The effects of ten amiloride analogues on Na+-H+ exchange in rabbit kidney medulla microsomes have been examined. Most of the analogues appeared to inhibit Na+ uptake into the microsomes more effectively than did amiloride either in the presence or absence of a pH gradient. However, the analogues were also capable of stimulating Na+ efflux from the microsomes at concentrations somewhat higher than the concentrations at which they inhibited Na+ influx. The concentrations at which the analogues stimulated Na+ efflux were about 2-4-times higher than the concentrations at which they blocked influx. This suggested that the two processes were related. The analogues that stimulated efflux most effectively (the 5-N-benzyl-amino analogue of amiloride and the 5-N-butyl-N-methylamino analogue) were shown to induce completely reversible effects. These analogues did not stimulate L-[3H]glucose efflux from medulla microsomes which ruled out nonspecific vesicle destruction or reversible detergent effects. These analogues also induced Na+ efflux from microsomes in the presence of high concentrations of added buffer, which ruled out weak-base uncoupling effects. The possibility exists that these analogues are carried into the microsomes via the Na+-H+ exchange protein and that this permits them to both block Na+ influx into the microsomes and stimulate Na+ efflux as well.

Electrophysiological properties of cellular and paracellular conductive pathways of the rabbit cortical collecting duct

Microelectrode techniques were applied to the rabbit isolated perfused cortical collecting duct to provide an initial quantitation and characterization of the cell membrane and tight junction conductances. Initial studies demonstrated that the fractional resistance (ratio of the resistance of the apical cell membrane to the sum of the resistances of the apical and basolateral membranes) was usually independent of the point along the tubule of microelectrode impalement--implicating little cell-to-cell coupling--supporting the application of quantitative techniques to the cortical collecting duct. It was demonstrated that in the presence of amiloride, either reduction in the luminal pH or the addition of barium to the perfusate selectively reduced the apical membrane potassium conductance. From the changes in Gte and fractional resistance upon reducing the luminal pH or addition of barium to the perfusate, the transepithelial, apical membrane, basolateral membrane and tight junction conductances were estimated to be 9.3, 6.7, 8.1 and 6.0 mS cm-2, respectively. Ninety to ninety-five percent of the apical membrane conductance reflected the barium-sensitive potassium conductance in the presence of amiloride with an estimated potassium permeability of 1.1 X 10(-4) cm sec-1. Reduction in the perfusate pH to 4.0 caused a 70% decrease in the apical membrane potassium conductance, implying a blocking site with an acidic group having a pKa near 4.4. It is concluded that both the transcellular and paracellular pathways of the cortical collecting tubule have high ionic conductances, and that the apical membrane conductance primarily reflects a high potassium conductance. Furthermore, both reduction in the perfusate pH and addition of barium to the perfusate selectively block the apical potassium channels, although the site of inhibition likely differs since the two ions display markedly different voltage-dependent blocks of the channel.

Control of potassium transport by turtle colon: role of membrane potential

To more clearly define the role of the transepithelial electrical potential difference (V m----s), potassium permeability, and sodium-potassium pump rate in transcellular potassium transport by isolated turtle colon, we measured transmural potassium fluxes under open-circuit conditions in the presence and absence of putative blockers of potassium transport: amiloride and barium. The results were consistent with the notion that V m----s is a major determinant of cellular potassium secretion, whereas active potassium absorption is insensitive to changes in V m----s. These observations suggest that "coupling" between colonic sodium absorption and potassium secretion in vivo could be due primarily to the effect of the lumen negative V m----s on transcellular secretory potassium flow. Amiloride-induced inhibition of potassium secretion appeared to be due to the reductions in V m----s and sodium-potassium pump rate that accompanied the inhibition of active sodium absorption.

Reconstituted amiloride-inhibited sodium transporter from rabbit kidney medulla is responsible for Na+-H+ exchange

Microsomes formed from rabbit kidney medulla and reconstituted proteoliposomes formed from these microsomes were capable of amiloride-inhibited Na+ transport that was insensitive to valinomycin either with or without K+. This indicated that the Na+ transport process was electroneutral. This Na+ transport process was insensitive to extravesicular Cl- or HCO-3 and not stimulated by high intravesicular gradients of K+, Ca2+ or Mg2+, which indicated that the process did not require NaCl or NaHCO3 co-transport or Na+/K+, Na+/Ca2+ or Na+/Mg2+ counter-transport. Na+ uptake into microsomes or proteoliposomes was inhibited by extravesicular K+, Ca2+, Mg2+ or La3+, which indicated that these ions interacted with the Na+-binding site on the transport protein. Na+ uptake into microsomes was stimulated by intravesicular protons and inhibited by extravesicular protons. This suggested that microsomes were capable of Na+-H+ exchange and this was confirmed when Na+ was shown to stimulate H+ efflux from microsomes. The amiloride-inhibited Na+ transporter from medulla microsomes which has been reconstituted into proteoliposomes is most likely a Na+-H+ exchanger. This Na+ transporter was totally insensitive to the uncoupler 1799, either in the presence or absence of valinomycin plus K+ and less sensitive to NH3 than to amiloride. This indicated that amiloride inhibited Na+ transport not merely by acting as a weak-base uncoupler but by directly interacting with the protein responsible for Na+-H+ exchange.

Sodium transport from blood to brain: inhibition by furosemide and amiloride

Brain sodium uptake in vivo was studied using a modified intracarotid bolus injection technique in which the uptake of 22Na+ was compared with that of the relatively impermeable molecule, [3H]L-glucose. At a Na+ concentration of 1.4 mM, Na+ uptake was 1.74 +/- 0.07 times greater than L-glucose uptake. This decreased to 1.34 +/- 0.04 at 140 mM Na+, indicating saturable Na+ uptake. Relative Na+ extraction was not affected by pH but was inhibited by amiloride (Ki = 3 X 10(-7) M) and by 1 mM furosemide. The effects of these two inhibitors were additive. Brain uptake of 86Rb+, a K+ analogue, was measured to study interaction of K+ with Na+ transport systems. Relative 86Rb+ extraction was also inhibited by amiloride; however, it was not inhibited by furosemide. The results suggest the presence of two distinct transport systems that allow Na+ to cross the luminal membrane of the brain capillary endothelial cell. These transport systems could play an important role in the movement of Na+ from blood to brain.

Amiloride-inhibited Na+ uptake into toad bladder microsomes is Na+-H+ exchange

Amiloride-inhibited Na+ transport into toad urinary bladder microsomes is sensitive to a pH gradient across the vesicular membrane. The magnitude of the gradient was measured directly with acridine orange. Also Na+ could stimulate amiloride-sensitive proton efflux from the microsomes. These results indicated that the transport process was Na+-H+ exchange.

Mechanisms of K+ transport in isolated turtle urinary bladder. Induction of active K+ secretion in a K+-absorbing epithelium

Transepithelial K(+) movement was studied in vitro in the short-circuited turtle bladder by increasing luminal K(+) permeability and by inhibiting the basolateral Na/K pump. Luminal addition of amphotericin B caused net K(+) secretion (180+/-52 nmol/h) compared with net K(+) absorption (42+/-6 nmol/h) in control bladders. Serosal ouabain and luminal amiloride abolished K(+) secretion in amphotericin-treated bladders; ouabain restored net absorption (45+/-16 nmol/h). The direction and rate of net K(+) transport are controlled by the relative K(+) permeabilities and the Na/K pump sites at the two cell membranes of the epithelium.

Distal tubular segments of the rabbit kidney after adaptation to altered Na- and K-intake. I. Structural changes

The baso-lateral cell-membrane area in kidney tubules appears to be associated with the capacity for electrolyte transport; in the rabbit, it decreases from the distal convoluted tubule (DCT-cells) over the connecting tubule (CNT-cells) to the cortical collecting duct (principal cells). Adaptation to low Na-, high K-intake changes this pattern: CNT-cells at the beginning of the connecting tubule have the highest membrane area, which decreases along the segment, but remains two-fold higher than in controls. Principal cells have a four-fold higher membrane area than in controls. Simultaneous treatment with the antimineralocorticoid canrenoate-K inhibits the structural changes in CNT-cells only in end-portions of the connecting tubule and in principal cells. After prolonged high Na-, low K-intake DCT-cells display a two-fold higher membrane area than controls, while CNT-cells and principal cells are not affected. Simultaneous treatment with DOCA does not affect the DCT-cells but provokes a moderate increase in membrane area in CNT-cells, and a 5.5-fold increase in principal cells. The data provide evidence that DCT-, CNT- and principal cells are functionally different cell types. The baso-lateral cell-membrane area, associated with electrolyte-transport capacity, appears to be influenced in DCT-cells mainly by Na-intake, in CNT-cells mainly by K-intake and in part also by mineralocorticoids, and in principal cells mainly by mineralocorticoids.

Inhibition by amiloride of sodium transport into rabbit kidney medulla microsomes

Sodium transport into rabbit kidney medullar microsomes was 50% inhibited by amiloride. This Na+ uptake was shown to represent transport when the uptake process was reserved by the ionophore nigericin. The transport was complete within 60 min and proportional to the microsomal protein concentration. The effect of amiloride on transport was specific since the similar compound sulfaguanidine failed to affect microsomal Na+ transport. Amiloride-sensitive Na+ transport into microsomes was inhibited 70% by decreasing the pH (from 7.0 to 5.9), but was unaffected by the presence of a pH gradient. The kinetics of Na+ transport could be explained by a simple model, assuming that amiloride lowered the rate of Na+ entrance into the vesicles but had not effect on the rate of efflux. The failure of amiloride to effect efflux from the vesicles was also demonstrated directly.