Intracellular potassium activity in mammalian proximal tubule: effect of perturbations in transepithelial sodium transport

A model of mitochondrial O2 consumption and ATP generation in rat proximal tubule cells

Oxygen tension in the kidney is mostly determined by O₂ consumption (Qo₂), which is, in turn, closely linked to tubular Na⁺ reabsorption. The objective of the present study was to develop a model of mitochondrial function in the proximal tubule (PT) cells of the rat renal cortex to gain more insight into the coupling between Qo₂, ATP formation (G_ATP), ATP hydrolysis (Q_ATP), and Na⁺ transport in the PT. The present model correctly predicts in vitro and in vivo measurements of Qo₂, G_ATP, and ATP and P_i concentrations in PT cells. Our simulations suggest that O₂ levels are not rate limiting in the proximal convoluted tubule, absent large metabolic perturbations. The model predicts that the rate of ATP hydrolysis and cytoplasmic pH each substantially regulate the G_ATP-to-Qo₂ ratio, a key determinant of the number of Na⁺ moles actively reabsorbed per mole of O₂ consumed. An isolated increase in Q_ATP or in cytoplasmic pH raises the G_ATP-to-Qo₂ ratio. Thus, variations in Na⁺ reabsorption and pH along the PT may, per se, generate axial heterogeneities in the efficiency of mitochondrial metabolism and Na⁺ transport. Our results also indicate that the G_ATP-to-Qo₂ ratio is strongly impacted not only by H⁺ leak permeability, which reflects mitochondrial uncoupling, but also by K⁺ leak pathways. Simulations suggest that the negative impact of increased uncoupling in the diabetic kidney on mitochondrial metabolic efficiency is partly counterbalanced by increased rates of Na⁺ transport and ATP consumption. This model provides a framework to investigate the role of mitochondrial dysfunction in acute and chronic renal diseases.

Diuretic effect and mechanism of action of parsley

This work provides substantial evidence for the advocated diuretic effect of parsley in folk medicine and determines the mechanism of action of the herb. Rats offered an aqueous parsley seed extract to drink, eliminated a significantly larger volume of urine per 24 h as compared to when they were drinking water. These findings were supported by the results of other experiments using an in situ kidney perfusion technique which demonstrated also a significant increase in urine flow rate with parsley seed extract. This effect was still apparent in presence of amiloride, furosemide and in the absence of sodium, but not in the absence of potassium, suggesting that the diuretic effect of the herb is mediated through an increase in K+ retention in the lumen. Parsley extract, was shown on the other hand, to reduce the activity of the Na+-K+ ATPase in both cortex and medulla homogenates. Such an inhibition would decrease apical cellular Na+ reabsorption, lower K+ secretion, increase K+ concentration in the intercellular space and consequently would inhibit passive K+ influx across the tight junctions. The mechanism of action of parsley seems to be mediated through an inhibition of the Na+-K+ pump that would lead to a reduction in Na+ and K+ reabsorption leading thus to an osmotic water flow into the lumen, and diuresis.

Na+/H+-exchange activity and immunolocalization of NHE3 in rat epididymis

An acidic luminal pH in the epididymis and vas deferens (VD) helps maintain mature sperm in an immotile state during storage. We have previously shown that the majority of proton secretion in the VD is due to the activity of the vacuolar H+-ATPase. Acidification is dependent on luminal sodium in more proximal regions of the epididymis, and we examined the distribution of the Na+/H+ exchanger, NHE3, by immunofluorescence and measured Na+/H+ exchange (NHE) activity in isolated epididymal tubules. NHE3 was detected in the apical pole of nonciliated cells of the efferent ducts and principal cells (PC) of the epididymis. No staining was seen in the distal cauda epididymidis and the VD. Isolated tubules from the distal initial segment (DIS) and proximal cauda epididymidis were perfused in vitro and loaded with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6')-carboxyfluorescein. Ethylisopropyl amiloride (EIPA) (50 microM) reduced the initial rate of intracellular pH recovery (dpH(i)/dt), in response to an acute acid load, by 51% and 45% in the DIS and cauda epididymidis, respectively. In the DIS, removal of luminal sodium reduced dpH(i)/dt by 52%. HOE694 (50 microM) inhibited all EIPA-sensitive dpH(i)/dt in the DIS, despite the previously reported absence of NHE2 in this region (Cheng Chew SB, Leung GPH, Leung PY, Tse CM, and Wong PYD, Biol Reprod 62: 755-758, 2000). These data indicate that HOE694- and EIPA-sensitive Na+/H+ exchange may participate, together with the H+-ATPase, in luminal acidification in the male excurrent duct.

Two types of K(+) channels at the basolateral membrane of proximal tubule: inhibitory effect of taurine

The cell-attached configuration of the patch-clamp technique was used to investigate the effects of taurine on the basolateral potassium channels of rabbit proximal convoluted tubule. In the absence of taurine, the previously reported ATP-blockable channel, K(ATP), was observed in 51% of patches. It is characterized by an inwardly rectifying current-voltage curve with an inward slope conductance of 49 +/- 5 pS (n = 15) and an outward slope conductance of 13 +/- 6 pS (n = 15). The K(ATP) channel open probability (P(o)) is low, 0.15 +/- 0.06 (n = 15) at a -V(p) = -100 mV (V(p) is the pipette potential), and increases slightly with depolarization. The gating kinetics are characterized by one open time constant (tau(o) = 5.0 +/- 1.9 ms, n = 6) and two closed time constants (tau(C1) = 5. 2 +/- 1.5 ms, tau(C2) = 140 +/- 40 ms; n = 6). In 34% of patches, a second type of potassium channel, sK, with distinct properties was recorded. Its current-voltage curve is characterized by a sigmoidal shape, with an inward slope conductance of 12 +/- 2 pS (n = 4). Its P(o) is voltage independent and averages 0.67 +/- 0.03 (n = 4) at -V(p) = -80 mV. Both its open time and closed time distributions are described by a single time constant (tau(o) = 96 +/- 19 ms, tau(C) = 10.5 +/- 3.6 ms; n = 4). Extracellular perfusion of 40 mM taurine fails to affect sK channels, whereas K(ATP) channel P(o) decreases by 75% (from 0.17 +/- 0.06 to 0.04 +/- 0.02, n = 7, P < 0.05). In conclusion, the absolute basolateral potassium conductance of rabbit proximal tubules is the resulting combination of, at least, two types of potassium channels of roughly equal importance: a high-conductance low-open probability K(ATP) channel and a low-conductance high-open probability sK channel. The previously described decrease in the basolateral absolute potassium conductance by taurine is, however, mediated by a single type of K channel: the ATP-blockable K channel.

Cross-talk and the role of KATP channels in the proximal tubule

Over the last few years it has become evident that an assortment of functionally-related, but diverse, KATP channels provide an important and physiologically-regulated determinant of the K conductive pathways in many, if not all, epithelial cells expressed along the nephron. As such, KATP plays central roles in regulating and maintaining a number of transport processes in concert with physiological demands of the kidney. In the renal proximal tubule, KATP channels and changes in the hydrolytic activity of the Na,K-ATPase permit ATP to act as a coupling modulator of parallel Na,K-ATPase-K recycling. The response insures that cell membrane potential, intracellular K activity and cell volume are protected in the face of physiological variations in transcellular ion transport. In addition to demonstrating the physiological relevance of KATP in renal epithelial, these studies have provided a long awaited answer to the underlying mechanism of pump-leak coupling, a universal and essential homeostatic mechanism observed in nearly all salt translocating epithelia.

Change of apparent stoichiometry of proximal-tubule Na(+)-HCO3- cotransport upon experimental reversal of its orientation

Electrogenic cotransport of Na+ with HCO3- has been reported in numerous tissues. It has always been shown with a net transfer of negative charge, but in some situations achieves net outward transport of both species with a stoichiometry of at least three HCO3- ions per Na+ ion (3:1), and in other situations achieves net inward transport of both species and has a stoichiometry of at most two HCO3- ions per Na+ ion (2:1). This suggests either that there may be more than one protein responsible for Na(+)-HCO3- cotransport in different tissues or that if there is a single protein, its stoichiometry may differ depending on the orientation of net transport. The present study, using conventional or double-barreled ion-selective microelectrodes to follow basolateral membrane potential and intracellular pH or Na+ activity in Necturus proximal convoluted tubule in vivo, shows that the orientation of the basolateral Na(+)-HCO3- cotransporter can be reversed upon switching from a perfusate simulating normal acid-base conditions to one that imposes peritubular isohydric hypercapnia. Moreover, accompanying the reversal of orientation is a change of apparent stoichiometry from 3:1 to 2:1. Given that the observed change of orientation and accompanying change of apparent stoichiometry occur within seconds and in the same preparation, these results suggest that a single transport protein is responsible for both types of behavior.

Regulation of basolateral membrane potential after stimulation of Na+ transport in proximal tubules

We have previously shown that stimulation of apical Na-coupled glucose and alanine transport produces a transient depolarization of basolateral membrane potential (Vbl) in rabbit proximal convoluted tubule (PCT, S1 segment). The present study is aimed at understanding the origin of the membrane repolarization following the initial effect of addition of luminal cotransported solutes. Luminal addition of 10-15 mM L-alanine produced a rapid and highly significant depolarization of Vbl (20.3 +/- 1.1 mV, n = 15) which was transient and associated with an increase in the fractional K+ conductance of the basolateral membrane (tK) from 8 to 29% (P less than 0.01, n = 6). Despite the significant increase in tK, the repolarization was only slightly reduced by the presence of basolateral Ba2+ (2 mM, n = 6) or quinine (0.5 mM, n = 5). The repolarization was greatly reduced in the presence of 0.1 mM 4-acetamino-4'isothiocyamostilbene-2,2'-disulfonic acid (SITS) and blunted by bicarbonate-free solutions. Intracellular pH (pHi) determined with the fluorescent dye 2',7'-bis-2-carboxyethyl-5(and -6)-carboxyfluorescein (BCECF), averaged 7.39 +/- 0.02 in control solution (n = 9) and increased to 7.50 +/- 0.03 in the first 15 sec after the luminal application of alanine. This was followed by a significant acidification averaging 0.16 +/- 0.01 pH unit in the next 3 min. In conclusion, we believe that, contrary to other leaky epithelia, rabbit PCT can regulate its basolateral membrane potential not only through an increase in K+ conductance but also through a cellular acidification reducing the basolateral HCO3- exit through the electrogenic Na-3(HCO3) cotransport mechanism.