Electrophysiological heterogeneity of proximal convoluted tubules in Amphiuma kidney

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Potassium-selective channels in the basolateral membrane of single proximal tubule cells of frog kidney

The membrane potential of proximal tubule cells is dominated by the potassium conductance of the basolateral membrane. In the present paper the nature of this conductance is investigated by the patch-clamp technique. Only one type of K channel was found in the basolateral membranes of freshly isolated proximal cells. In cell-attached patches, the current/voltage relationship is markedly non-linear with much larger inward (30 pS) than outward (approximately 6 pS) conductances, even in the presence of roughly symmetrical K concentrations. Thus the channels show inward rectification. The determination of the conductance for outward current flow is complicated since the current/voltage curves show an area of negative conductance. Nevertheless, taking the conductance for outward current flow and the density of the channels it is possible to account for all of the previously reported potassium conductance of amphibian proximal tubule cells. The open probability of the channels was found not to depend upon the membrane potential. However, the non-linearity of the current/voltage relationships will confer upon the channel the same voltage dependence as that seen in intact proximal tubules, i.e. the conductance decreases with depolarisation. Incubation of cells in Ringer with no substrates or in the presence of alanine and/or glucose showed no change in the activity of the channels. These findings suggest that, although these channels may represent the basolateral conductance of frog proximal tubule cells, they are not involved in the well-established coupling between transport rate and potassium conductance.

Influence of mepacrine, indomethacin, and nordihydroguaiaretic acid on the electrical properties of frog renal proximal tubules

In proximal renal tubules of the frog kidney, stimulation of sodium-coupled transport leads to a depolarization of the peritubular cell membrane, followed by partial repolarization. These alterations of the potential difference across the peritubular cell membrane (PDpt) are in part the result of altered peritubular potassium conductance. The repolarization has been blunted by the phospholipase A2 inhibitor mepacrine, but not by the cyclooxygenase inhibitor indomethacin. In the present study the effect of mepacrine, indomethacin and the lipoxygenase inhibitor nordihydroguaiaretic acid on the electrical properties of proximal renal tubules has been tested in the presence and absence of stimulated sodium-coupled transport. In the absence of inhibitors, addition of 10 mmol/l phenylalanine to the luminal perfusate leads to a rapid depolarization and partial repolarization of the peritubular cell membrane, a decrease of the luminal cell membrane resistance (Ra) and a small increase of the cellular core resistance (Rc). Removal of phenylalanine leads to rapid hyperpolarization, increase of Ra and decline Rc. Mepacrine (100 mumols/l) depolarizes the cell membrane and increases the peritubular cell membrane resistance (Rb), Rc and the intracellular pH. In the presence of mepacrine, phenylalanine leads to a sustained depolarization and a transient decrease of Ra. Indomethacin (10 mumol/l) does not significantly modify PDpt, the lumped resistance of both cell membranes (Rm) or Rc in the presence or absence of phenylalanine. Nordihydroguaiaretic acid (50 mumols/l) does not alter significantly PDpt, Ra, Rb or Rc prior to phenylalanine.(ABSTRACT TRUNCATED AT 250 WORDS)

Base induced hyperpolarization of the cell potential in HCO3- free perfused Necturus renal proximal tubules

Short-term peritubular alkalinization from 7.5 to 8.5 hyperpolarized (-8.8 mV) the basolateral membrane potential (V1) in HCO3- free Herpes buffered Necturus renal proximal tubule cells. This sustained base induced hyperpolarization (BIH) was associated with an increase in the peritubular apparent transference number for potassium (tK+). The apparent transference number for potassium (tK+) was estimated at pH 7.5 and 8.5 by raising peritubular K+ from 2.5 to 10 mmol/l. tK+ increased linearly as V1 hyperpolarized, whereas tK+ measured in the presence of peritubular Ba2+ at pH 7.5 and 8.5 was nearly zero. However, the BIH persisted in the presence of barium at the peritubular, luminal or both sides of the epithelium. Moreover this BIH was also accompanied by a small hyperpolarization (-0.4 mV) of the transepithelial membrane potential (V3) in the absence or presence of peritubular and/or luminal Ba2+. Therefore we conclude that BIH must originate from additional mechanisms other than an increase in peritubular or luminal potassium conductance.

Fused cells of frog proximal tubule: I. Basic membrane properties

Proximal tubular cells of the frog (Rana esculenta) kidney were fused within an isolated tubule portion to giant cells according to the polyethylene-glycol fusion method. Cell membrane potentials (Vm) were measured while cells were superfused with various experimental solutions. Rapid concentration step-changes of different ions allowed to calculate the respective transference numbers (tion). In some experiments the specific cell membrane resistances (Rm) were evaluated by measuring Vm induced by short current pulses injected into the cell with a second electrode. The experiments reveal: i) Fused cells of the proximal tubule exhibit a Vm of -49.5 +/- 1.6 mV (n = 65). ii) Addition of glucose to the perfusate yields a transient depolarization, consistent with a rheogenic Na/glucose cotransport system. iii) In absence of organic substrates the whole cell membrane conductance is made up of K+ and HCO3-. iv) There is a positive relationship between Vm and tK+ and a negative relationship between Vm and tHCO3-. v) HCO3--induced Vm changes are attenuated or abolished when Na+ is replaced with choline+, consistent with a rheogenic Na+/HCO3- cotransport system. vi) Replacement of Na+ by choline+ depolarizes Vm and increases Rm by about 50%; addition of 3 mmol/liter Ba2+ to the Na+-free perfusate increases Rm by about 58% compared to the initial control value. vii) There is no measurable cell membrane Cl- conductance. We conclude that fused cells of proximal tubule exert both luminal and peritubular membrane properties. In absence of organic substrates the cell membrane potential is determined by the HCO3- and K+ transport systems.