ATP is a coupling modulator of parallel Na,K-ATPase-K-channel activity in the renal proximal tubule

An electrophysiological study of angiotensin II regulation of Na-HCO3 cotransport and K conductance in renal proximal tubules. I. Effect of picomolar concentrations

The effect of picomolar concentrations of angiotensin II (AII) was investigated in isolated perfused rabbit renal proximal tubules using conventional or pH-sensitive intracellular microelectrodes. Under control conditions cell membrane potential (Vb) and cell pH (pHi) averaged -53.8 +/- 1.9 mV (mean +/- SEM, n = 49) and 7.24 +/- 0.01 (n = 10), respectively. AII (at 10(-11) mol/l), when applied from the bath (but not when applied from the lumen perfusate), produced the following effects: approximately 85% of the viable tubules responded with a small depolarization (+5.5 +/- 0.4 mV, n = 43) which was accompanied in half of the pHi measurements by a slow acidification (delta pHi = -0.03 +/- 0.01, n = 5). The remaining 15% responded with a small hyperpolarization (delta Vb = -3.1 +/- 0.4 mV, n = 6). All changes were fully reversible and repeatable. Experiments with fast changes in bath HCO3 or K concentrations, as well as measurements of the basolateral voltage divider fraction in response to transepithelial current flow, explain these observations as stimulation of a basolateral Na-HCO3 cotransporter and of a basolateral K conductance. Both counteract in their effect on Vb, but can be individuated by blocker experiments with 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and barium. Both the stimulation of Na-HCO3 cotransport and the stimulation of the K conductance may result from down-regulation of the level of cyclic adenosine monophosphate in the cell.

Metabolic support of Na+ transport by the rabbit CCD: analysis of the use of equivalent current

The role of metabolism in the support of ion transport by the cortical collecting duct (CCD) is being increasingly recognized as a complex process involving energy supply to the Na+/K+ pump and maintenance of cellular conductive pathways. In order to assess both of these processes, we measured the metabolic support of Na+ transport using transepithelial electrical measurements and, in some cases, simultaneous determination of lumen-to-bath Na+ flux. Analysis of the calculated equivalent current (Ieq), the product of the transepithelial voltage and conductance, showed a predicted (and a measured) discrepancy between this value and the magnitude of active Na+ transport. Under conditions of this study, the change in Ieq in a single tubule was a reasonable index of the change in Na+ transport. The majority of the support of Na+ transport appears to come from oxidative metabolism. Glucose supports transport better than the other substrates tested, but lactate, pyruvate, and some acids provide near maximal support. We found some conditions where large changes in Na+ transport occurred without significant changes in conductance. Conductance could also be altered without producing major changes in transport. These results demonstrate complex and possibly independent influences of metabolism in the regulation of Na+ transport and cell conductive pathways.

Identification and regulation of whole-cell Cl- and Ca(2+)-activated K+ currents in cultured medullary thick ascending limb cells

The whole-cell patch-clamp technique has been used to study membrane currents in cultured rabbit medullary thick ascending limb (MTAL) epithelial cells. A Ca(2+)-activated K+ current was characterized by its voltage-dependent and Ca(2+)-dependent properties. When the extracellular K+ ion concentration was increased from 2 to 140 mM, the re-reversal potential (Ek) was shifted from -85 to 0 mV with a slope of 46 mV per e-fold change. The Ca(2+)-activated K+ current is blocked by charybdotoxin (CTX) in a manner similar to the apical membrane Ca(2+)-activated K+ channel studied with the single channel patch-clamp technique. The results suggest that the Ca(2+)-activated K+ current is the predominant, large conductance and Ca(2+)-dependent K+ pathway in the cultured MTAL cell apical membrane. The biophysical properties and physiological regulation of a Cl- current were also investigated. This current was activated by stimulation of intracellular cAMP using forskolin and isobutyl-1-methylxanthine (IBMX). The current-voltage (I-V) relationship of the Cl- current showed an outward-rectifying pattern in symmetrical Cl- solution. The Cl- selectivity of the whole-cell current was confirmed by tail current analysis in different Cl- concentration bath solutions. Several Cl- channel blockers were found to be effective in blocking the outward-rectifying Cl- current in MTAL cells. The cAMP-dependent Cl- transport in MTAL cells was further confirmed by measuring changes in the intensity of Cl- sensitive dye using fluorescence microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

Tolbutamide-sensitive potassium conductance in the basolateral membrane of A6 cells

K+ channels sensitive to intracellular ATP (KATP channels) have been described in a number of cell types and are selectively inhibited by sulfonylurea drugs. To look for the presence of this type of K+ channel in the basolateral membrane of tight epithelia, we have used an amphibian renal cell line, the A6 cells, grown on filters. After the selective permeabilization of the apical membrane with amphotericin B, the basolateral conductance was studied under voltage-clamp conditions. Tolbutamide inhibited 65.8 +/- 6.3% of the barium-sensitive current. The tolbutamide-sensitive conductance had an equilibrium potential of -83 +/- 1 mV and was inward rectifying in spite of the outwardly directed K+ gradient. Similar results were obtained with glibenclamide. The half-inhibition constants were 25.7 +/- 3.0 microM and 0.114 +/- 0.018 microM for tolbutamide and glibenclamide, respectively. To study the relation between cellular ATP and the activity of this conductance, A6 cells were treated with glucose (5 mM) and insulin (250 microU/ml). This maneuver significantly increased the cellular ATP and abolished the tolbutamide-sensitive conductance. A sulfonylurea-sensitive K+ conductance is present and active in the basolateral membrane of A6 cells. This conductance appears to be modulated by physiological changes of intracellular ATP.

Mechanism of apical K+ channel modulation in principal renal tubule cells. Effect of inhibition of basolateral Na(+)-K(+)-ATPase

The effects of inhibition of the basolateral Na(+)-K(+)-ATPase (pump) on the apical low-conductance K+ channel of principal cells in rat cortical collecting duct (CCD) were studied with patch-clamp techniques. Inhibition of pump activity by removal of K+ from the bath solution or addition of strophanthidin reversibly reduced K+ channel activity in cell-attached patches to 36% of the control value. The effect of pump inhibition on K+ channel activity was dependent on the presence of extracellular Ca2+, since removal of Ca2+ in the bath solution abolished the inhibitory effect of 0 mM K+ bath. The intracellular [Ca2+] (measured with fura-2) was significantly increased, from 125 nM (control) to 335 nM (0 mM K+ bath) or 408 nM (0.2 mM strophanthidin), during inhibition of pump activity. In contrast, cell pH decreased only moderately, from 7.45 to 7.35. Raising intracellular Ca2+ by addition of 2 microM ionomycin mimicked the effect of pump inhibition on K+ channel activity. 0.1 mM amiloride also significantly reduced the inhibitory effect of the K+ removal. Because the apical low-conductance K channel in inside-out patches is not sensitive to Ca2+ (Wang, W., A. Schwab, and G. Giebisch, 1990. American Journal of Physiology. 259:F494-F502), it is suggested that the inhibitory effect of Ca2+ is mediated by a Ca(2+)-dependent signal transduction pathway. This view was supported in experiments in which application of 200 nM staurosporine, a potent inhibitor of Ca(2+)-dependent protein kinase C (PKC), markedly diminished the effect of the pump inhibition on channel activity. We conclude that a Ca(2+)-dependent protein kinase such as PKC plays a key role in the downregulation of apical low-conductance K+ channel activity during inhibition of the basolateral Na(+)-K(+)-ATPase.

Activation of K+ Channels in the Plasma Membrane of Arabidopsis by ATP Produced Photosynthetically

Light activates a K+ channel and transiently depolarizes the plasma membrane of Arabidopsis mesophyll cells. Genetically or chemically impairing photosynthesis abolished this electrical response to light. These results indicate that illuminated chloroplasts produce a factor that activated K+ channels in the plasma membrane. By patch clamping at the single-channel level, we have obtained evidence that ATP is one such factor. Application of 0.2 to 2 mM ATP to the cytoplasmic side of excised patches of membrane reversibly activated the type of channel that was activated by light in cell-attached patches. In addition, an outward-rectifying K+ channel and an outward-rectifying nonselective cation channel were similarly activated by ATP, whereas a nonselective stretch-activated channel was unaffected by this treatment. This novel mechanism for controlling the permeability of the plasma membrane to K+ may be important to photosynthetic metabolism.