3H]bumetanide binding to duck red cells. Correlation with inhibition of (Na + K + 2Cl) co-transport

Bumetanide is a potent inhibitor of cation-chloride co-transport systems in many cell types, including duck red cells. We studied equilibrium binding of [3H]bumetanide to intact duck red cells under a number of conditions known to affect (Na + K + 2Cl) co-transport in these cells. Saturable [3H]bumetanide binding to duck red cells is markedly stimulated by addition of norepinephrine or cell shrinkage, conditions which similarly stimulate co-transport. In the presence of norepinephrine and saturating concentrations of extracellular sodium, potassium, and chloride for the co-transporter, we found approximately 1000 [3H]bumetanide-binding sites/red cell, and measurement of 24Na+ influx on the same cells yielded a turnover number of approximately 4000/s for the co-transporter. 24Na+ influx was negatively correlated with the amount of bound [3H]bumetanide, and both saturable binding and inhibition of influx were half-maximal at approximately 10(-7) M [3H]bumetanide. Binding of [3H]bumetanide to duck red cells is stimulated in a saturable manner by increasing extracellular sodium and potassium. Chloride has a biphasic effect on [3H]bumetanide binding; increasing [Cl-]o (by replacement of methylsulfate) from 0 to 32 mM markedly enhances binding, whereas further increasing [Cl-]o to 160 mM inhibits binding. This behavior is similar to that reported for bumetanide inhibition of duck red cell (Na + K + 2Cl) co-transport (Haas, M., and McManus, T. J. (1983) Am. J. Physiol. 245, C235-C240; Haas, M., and McManus, T. J. (1982) Biophys. J. 37, 214a) and [3H]bumetanide binding to membranes from dog kidney outer medulla (Forbush, B. III, and Palfrey, H. C. (1983) J. Biol. Chem. 258, 11787-11792).     

3H]bumetanide binding to avian erythrocyte membranes. Correlation with activation and deactivation of Na/K/2Cl cotransport

The relationship between Na/K/2Cl cotransport activation in duck erythrocytes and binding of the diuretic [3H]bumetanide to isolated membranes from stimulated cells has been assessed. Cotransport was activated by either cAMP-dependent (norepinephrine) or -independent (fluoride, hypertonicity) pathways. Membranes isolated from unstimulated cells possessed no specific bumetanide binding. In the presence of norepinephrine, cotransport and saturable binding rose in parallel, reaching a maximum after 5-7 min. In membranes from maximally stimulated cells the K1/2 and Bmax for bumetanide binding were 100 nM and 1.7 pmol/mg protein, respectively. The diuretic binding properties of these membranes were characteristic of interactions of ligands with the Na/K/2Cl cotransporter: specific binding required the presence of all three cotransported ions (Na, K, and Cl), and the rank order of potency for diuretic competition with bumetanide for binding sites was benzmetanide greater than bumetanide greater than furosemide. The appearance of specific bumetanide binding was also seen in membranes from erythrocytes activated by non-cAMP-dependent stimuli, with an excellent temporal correlation between cotransport activation and diuretic binding. On removal of all stimuli both cotransport and bumetanide binding declined in parallel. Duck erythrocytes treated with norepinephrine in a solution containing 15 mM K+ swell to a new stable cell volume after 60 min, during which time cotransport becomes inoperative. Bumetanide binding to both whole cells and isolated membranes paralleled the decline in cotransport activity. It is concluded that bumetanide binding to isolated membranes faithfully reflects the state of activation of the Na/K/2Cl cotransporter in intact cells under a variety of conditions.     

The regulation of Na/K/2Cl cotransport and bumetanide binding in avian erythrocytes by protein phosphorylation and dephosphorylation. Effects of kinase inhibitors and okadaic acid

The Na/K/2Cl cotransport system in the avian erythrocyte can be activated by agents that raise intracellular cAMP suggesting the involvement of cAMP-dependent protein kinase (cAMP-PK) in its regulation. Another group of stimuli including fluoride and hypertonicity stimulate cotransport via cAMP-independent means. To further investigate the role of phosphorylation in these processes, we examined the effects of protein kinase inhibitors of 8 (p-Cl-phenylthio)-cAMP (cpt-cAMP), fluoride and hypertonic activation of cotransport in duck red cells, and [3H]bumetanide binding to isolated membranes. Preincubation of cells with the kinase inhibitors K-252a (Ki approximately 1.6 microM) and H-9 (Ki approximately 100 microM) blocked cpt-cAMP activation of bumetanide-sensitive 86Rb influx and bumetanide binding. These inhibitors also led to a rapid deactivation of cotransport and decrease in bumetanide binding when added to cells maximally stimulated by cpt-cAMP. K-252a and H-9 inhibited cotransport activation by cAMP-independent stimuli, but 10-fold higher concentrations were required, implying the involvement of a cAMP-independent phosphorylation process in the mechanism of action of these agents. Removal of stimuli that elevate cAMP leads to a rapid reversal of cotransport indicating the presence of active protein phosphatases in these cells. The protein phosphatase inhibitor okadaic acid (OA, EC50: 630 nM) stimulated both Na/K/2Cl cotransport and bumetanide binding to membranes. As with fluoride and hypertonic stimulation, the OA effect was inhibited only at relatively high concentrations of K-252a. Phosphorylation of the membrane skeletal protein goblin (Mr 230,000) at specific cAMP-dependent sites was used as an in situ marker for the state of activation of cAMP-PK. Goblin phosphorylation at these sites was increased by norepinephrine and cpt-cAMP and rapidly reversed by K-252a and H-9, confirming that both inhibitors do block cAMP-PK activity. While OA markedly increased overall phosphorylation of many erythrocyte membrane proteins, including goblin, it did not affect goblin phosphorylation at specific cAMP-dependent sites. These results implicate a cAMP-independent protein kinase in the mediation of the OA effect on cotransport and bumetanide binding. The bumetanide-binding component of the avian erythrocyte cotransporter, an Mr approximately 150,000 protein that can be photolabeled with the bumetanide analog [3H]4-benzoyl-5-sulfamoyl-3-(3-thenyloxy)-benzoic acid was found to be a phosphoprotein. These results strongly support the hypothesis that phosphorylation and dephosphorylation, possibly of the Na/K/2Cl cotransporter itself, regulates the activity of     

3H]bumetanide binding in vascular endothelial cells. Quantitation of Na-K-Cl cotransporters

Vascular endothelial cells previously have been shown to possess a prominent Na-K-Cl cotransport system which mediates a K+ influx of approximately 20 mumols/g of protein/min. Endothelial cell cotransport has also been shown to be regulated by a variety of vasoactive agents and their second messengers, suggesting that the transport system may have an important role in endothelial cell function. In the present study we investigated the possibility that the high level of cotransport in these cells is due to a large number of Na-K-Cl cotransporters in the plasma membrane. This was done by evaluating specific saturable binding of [3H]bumetanide to cultured bovine aortic endothelial cells. We found a maximal [3H]bumetanide binding of 0.83 pmol/mg protein with a dissociation constant of 0.13 microM. From these data, the number of [3H]bumetanide binding sites/endothelial cell was determined to be approximately 230,000, and the turnover number for cotransport activity was calculated to be 300 K+ ions/site/s. These findings indicate that endothelial cells do indeed exhibit a large number of Na-K-Cl cotransporters/cell relative to other cell types. We also investigated the effects on [3H]bumetanide binding of agents known to modulate Na-K-Cl cotransport activity. Saturable binding of [3H]bumetanide was found to be reduced significantly by treatment of the cells with 8-bromo-cyclic AMP, 8-bromo-cyclic GMP, phorbol esters, norepinephrine, or rat atriopeptin III, all of which have been shown to inhibit Na-K-Cl cotransport-mediated K+ influx.     

Identification of a regulated Na/K/Cl cotransport system in a distal nephron cell line.

Lack of an adequate cell model has limited investigation of Na/K/Cl cotransporter regulation in the kidney. Using A6 cells, an amphibian distal renal cell line, we observed that 63% of rubidium uptake in confluent A6 monolayers was ouabain-insensitive. Ouabain-insensitive rubidium uptake was inhibited in a dose-dependent fashion by furosemide (IC50 6.6 microM) or bumetanide (IC50 1.7 microM). Kinetic studies confirmed that furosemide-sensitive rubidium uptake had features consistent with cotransporter activity in other cell lines. Furthermore, specific binding of [3H]bumetanide occurred with a capacity of 8.6 pmol/mg protein and a Kd of 1.6 microM bumetanide. Finally, furosemide-sensitive rubidium uptake was rapidly regulated by a calcium ionophore, the phorbol ester PDBu, forskolin, and adenosine. These data demonstrate an Na/K/Cl cotransport system in the A6 cell which will serve as a useful model for studying cotransporter regulation by endogenous signaling pathways.     

Time-dependent biphasic regulation of Na+/K+/Cl- cotransport in rat glomerular mesangial cells

Time-dependent regulation of loop diuretic-sensitive Na+/K+/Cl- cotransport and [3H]bumetanide binding was investigated in cultured rat glomerular mesangial cells. Angiotensin II or epidermal growth factor induced stimulation of Na+/K+/Cl- cotransport within 5 min, with a return to the control values by 30 min. Treatment of cells with phorbol 12-myristate 13-acetate (0.1 microM) (PMA), the calcium ionophore A23187 (1 microM), or the combination of 5 mM NaF and 10 microM AlCl3 produced a transient stimulation of Na+/K+/Cl- cotransport in 5-10 min to 148, 135, and 163% of control, respectively, which was followed by a progressive decrease to 34, 64, and 20% of the base-line activity, respectively, by 60 min. Exposure to cyclic 8-bromo-AMP (0.1 mM) or to forskolin (1 microM) and isobutylmethylxanthine (0.1 mM) caused a maximal inhibition of the cotransport in 5 min to 79 and 60% of control, respectively, with a subsequent gradual increase to 137 and 164% of the base-line activity, respectively, by 60 min. The effects of PMA, forskolin, and cyclic 8-bromo-AMP were concentration-dependent. In order to characterize further the alterations in the cotransport activity, binding of [3H]bumetanide was determined. Saturation binding analyses showed that the late inhibition of the cotransport by PMA and stimulation by forskolin were associated with a significant decrease and increase, respectively, in Bmax, with no significant changes in binding affinity. Correlations between changes in the cotransport activity and [3H]bumetanide binding were also observed in cells treated with cyclic 8-bromo-AMP or with NaF and AlCl3. Incubation of cells in Cl- or Na+ free solution greater than or equal to 60 min resulted in an increase in both the cotransport activity and [3H]bumetanide binding. These observations indicate that, in glomerular mesangial cells, persistent stimulation of second messengers that regulate the cotransporter induces a time-dependent, biphasic regulation of Na+/K+/Cl- cotransport and that the regulation occurring after greater than or equal to 60 min of treatment is primarily due to changes in the number of the active cotransport sites. Because long term removal of the transported ions also increases the number of active cotransport sites, these results suggest that alterations in intracellular ionic homeostasis may also mediate cotransport activity.     

Cytosolic protein concentration is the primary volume signal for swelling-induced [K-Cl] cotransport in dog red cells.

Chloride-dependent K transport ([K-Cl] cotransport) in dog red cells is activated by cell swelling. Whether the volume signal is generated by a change in cell configuration or by the dilution of some cytosolic constituent is not known. To differentiate between these two alternatives we prepared resealed ghosts that, compared with intact red cells, had the same surface area and similar hemoglobin concentration, but a greatly diminished volume. Swelling-induced [K-Cl] cotransport was activated in the ghosts at a volume (20 fl) well below the activation volume for intact cells (70 fl), but at a similar hemoglobin concentration (30-35 g dry solids per 100 g wet weight). Ghosts made to contain 40% albumin and 60% hemoglobin showed activation of [K-Cl] cotransport at a concentration of cell solids similar to intact cells or ghosts containing only hemoglobin. [K-Cl] cotransport in the resealed ghosts became quiescent at a dry solid concentration close to that at which shrinkage-induced Na/H exchange became activated. These results support the notion that the primary volume sensor in dog red cells is cytosolic protein concentration. We speculate that macromolecular crowding is the mechanism by which cells initiate responses to volume perturbation.     

Regulation of Na+/K+/Cl- cotransport and [3H]bumetanide binding site density by phorbol esters in HT29 cells

The involvement of protein kinase C in the regulation of Na+/K+/Cl- cotransport was investigated in cultured HT29 human colonic adenocarcinoma cells. We have demonstrated previously the presence of a Na+/K+/Cl- cotransport pathway in HT29 cells (Kim, H.D., Tsai, Y-S., Franklin, C.C., and Turner, J.T. (1989) Biochim. Biophys. Acta 946, 397-404). Treatment of cells with the phorbol esters phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu) caused an increase in membrane-associated protein kinase C activity that was accompanied by a concomitant decrease in cytosolic protein kinase C activity. PMA also produced a rapid transient increase in cotransport to 137% of control values by 5 min followed by a progressive decrease to 19% of control values by 2 h. To determine the underlying mechanism for the reduction in Na+/K+/Cl- cotransport, changes in cotransporter number and/or affinity were determined in radioligand binding studies using [3H]bumetanide. PMA and PDBu produced essentially identical time- and dose-dependent decreases in specific [3H]bumetanide binding that were similar to the observed decreases in cotransport. Analysis of saturation and competition binding data indicated that the decrease in binding was due to a lowered Bmax with no change in affinity. Both the decrease in binding and the changes in cotransport elicited by PMA were prevented by the protein kinase inhibitor H7. These findings suggest that phorbol esters cause a decrease in the number of cotransporters in HT29 cells, resulting in a reduction in Na+/K+/Cl- cotransport activity.     

3H]bumetanide binding to membranes isolated from dog kidney outer medulla. Relationship to the Na,K,Cl co-transport system

We have synthesized the radiolabeled "loop" diuretics [3H]bumetanide and [3H]benzmetanide (3-benzylamino-4-phenoxy-5-sulfamoylbenzoic acid) and have tested their potential as reversible labels of the Na,K,Cl co-transport system. These compounds bind with high affinity (Kd less than or equal to 30 nM, under optimal conditions) to membranes isolated from dog kidney; we found approximately 2 pmol/mg of sites in crude membranes from the outer medulla, and less than or equal to 0.5 pmol/mg in a similar preparation from kidney cortex. On sucrose gradient centrifugation, a peak of [3H]bumetanide binding activity (30 pmol/mg) is obtained at 37% (w/v) sucrose, distinct from the basolateral membranes in outer medulla and from brush borders in proximal tubule; our hypothesis is that this peak contains luminal membranes from the thick ascending limb of the loop of Henle. [3H]Bumetanide is displaced from its binding sites by various unlabeled loop diuretics at concentrations that have previously been shown to inhibit co-transport. Na+, K+, and Cl- (K1/2 congruent to 2, 1, and 1 mM, respectively) are required for [3H]bumetanide binding, and Cl- inhibits at higher concentrations. We interpret these data to demonstrate that the Na,K,Cl cotransport system is the site involved in [3H]bumetanide binding in kidney membranes.     

Interaction of (Na + K + 2 Cl) cotransport and the Na/K pump in cultured chick cardiac myocytes

We have recently reported the presence of an electroneutral (Na + K + 2 Cl) cotransport mechanism that is bumetanide-sensitive and maintains Cl_i above its electrochemical equilibrium in cultured chick heart cells. In steady state, (Na + K + 2 Cl) cotransport is inwardly directed and so contributes to the Na influx that must be counterbalanced by the activity of the Na/K pump to maintain Na_i homeostasis. We now show that manipulating (Na + K + 2 Cl) cotransport by restoring Cl_o to a Cl-free solution indirectly influences Na/K pump activity because the bumetanide-sensitive recovery of a _infNa ^supi to its control level and the accompanying hyperpolarization could be blocked by 10^–4M ouabain. In another protocol, when the Na/K pump was reactivated by restoring K_o (from 0.5 mM to 5.4 mM) and removing ouabain, the recovery of a_Na was attenuated by 10^–4M bumetanide. The relatively slow rate of ouabain dissociation coupled with the activation of Na influx by (Na + K + 2 Cl) cotransport clearly establishes the interaction of these transport mechanisms in regulating Na_i. Although (Na + K + 2 Cl) cotransport is electroneutral, secondary consequences of its activity can indirectly affect the electrophysiological properties of cardiac cells.     

Na+, K+, and Cl- transport in resting pancreatic acinar cells

To understand the role of Na+, K+, and Cl- transporters in fluid and electrolyte secretion by pancreatic acinar cells, we studied the relationship between them in resting and stimulated cells. Measurements of [Cl-]i in resting cells showed that in HCO3(-)-buffered medium [Cl- ]i and Cl- fluxes are dominated by the Cl-/HCO3- exchanger. In the absence of HCO3-, [Cl-]i is regulated by NaCl and NaK2Cl cotransport systems. Measurements of [Na+]i showed that the Na(+)-coupled Cl- transporters contributed to the regulation of [Na+]i, but the major Na+ influx pathway in resting pancreatic acinar cells is the Na+/H+ exchanger. 86Rb influx measurements revealed that > 95% of K+ influx is mediated by the Na+ pump and the NaK2Cl cotransporter. In resting cells, the two transporters appear to be coupled through [K+]i in that inhibition of either transporter had small effect on 86Rb uptake, but inhibition of both transporters largely prevented 86Rb uptake. Another form of coupling occurs between the Na+ influx transporters and the Na+ pump. Thus, inhibition of NaK2Cl cotransport increased Na+ influx by the Na+/H+ exchanger to fuel the Na+ pump. Similarly, inhibition of Na+/H+ exchange increased the activity of the NaK2Cl cotransporter. The combined measurements of [Na+]i and 86Rb influx indicate that the Na+/H+ exchanger contributes twice more than the NaK2Cl cotransporter and three times more than the NaCl cotransporter and a tetraethylammonium-sensitive channel to Na+ influx in resting cells. These findings were used to develop a model for the relationship between the transporters in resting pancreatic acinar cells.     

Effect of bradykinin on Na-K-2Cl cotransport and bumetanide binding in aortic endothelial cells

Simultaneous measurements of potassium influx and binding of [3H]bumetanide were performed in endothelial cells cultured from bovine aortas to determine how bradykinin regulates Na-K-2Cl cotransport. [3H]Bumetanide displayed saturable binding and was displaced by low concentrations of unlabeled bumetanide. All three transported ions were required for binding and high concentrations of chloride inhibited binding, consistent with binding of bumetanide to the second chloride site of the transporter. Scatchard analysis of binding under maximal conditions (100 mM sodium, 30 mM potassium, 30 mM chloride) revealed a single class of binding sites with a binding constant of 112 nM and a density of 22 fmol/cm2 or approximately 122,000 sites/cells. Na-K-2Cl cotransport, measured as bumetanide-sensitive potassium influx, was stimulated 118 +/- 30% by bradykinin (p less than 0.01) at physiologic ion concentrations. Stimulation was inhibited by increased potassium or decreased external chloride concentrations and was not seen in conditions required for maximal binding of bumetanide. Simultaneous measurement of the binding of tracer [3H]bumetanide and its inhibition of potassium influx in medium containing 10 mM potassium and 130 mM chloride revealed a turnover number for the cotransporter of 293 +/- 68 s-1 which increased to 687 +/- 105 s-1 with bradykinin (p less than 0.001). There was no change in cell volume and only a 5.6 mM increase in intracellular sodium concentration associated with this stimulation. Bradykinin also increased the affinity of the cotransporter for bumetanide as indicated by a decrease in the Ki for potassium influx from 464 +/- 46 nM to 219 +/- 19 nM (p less than 0.005). Our results show that [3H]bumetanide can be used to quantitate Na-K-2Cl cotransporter sites in aortic endothelial cells and to determine the mechanism by which cotransport is regulated. The stimulation of cotransport in aortic endothelial cells by bradykinin is due to an increase in the activity of existing transporters rather than to an increase in the number of transporters. This, together with the increased affinity for bumetanide, strongly suggests that a change in cotransporter structure is occurring in response to bradykinin.     

Beta-adrenergic control of cell volume and chloride transport in an established rat submandibular cell line

Rat submandibular cells treated with methylcholanthrene are able to be propagated in continuous culture while retaining beta-adrenergic responsiveness. A specific clone, RSMT-A5, has been isolated and studied in detail. RSMT-A5 cells possess beta-adrenergic receptors (BARS) as judged by [3H]-dihydroalprenolol ([3H]-DHA) binding studies. [3H]-DHA binds to RSMT-A5 membranes in a specific and saturable manner with respect to time and [3H]-DHA concentration. Specific binding is saturable within three min of incubation, and a Scatchard analysis reveals a single class of high affinity binding sites with an equilibrium dissociation constant of 0.62 +/- 0.03 nM and a receptor density of 101 +/- 4 fmole/mg protein. Antagonist competition studies indicate that the BARs are primarily of the beta 2-subtype. The BARs are functional since isoproterenol stimulation results in an increased intracellular cAMP content, marked morphological change, and decreased cell volume and chloride content. These same responses can be evoked by treating RSMT-A5 cells with 8-bromo-cAMP. Ion transport inhibitors such as bumetanide (an inhibitor of Na/K/Cl cotransport), SITS and DIDS (inhibitors of chloride-bicarbonate exchange), amiloride (an inhibitor of Na-H exchange), ouabain (an inhibitor of Na/K-ATPase), and dipyridamole and 9-anthracene carboxylic acid (chloride channel blockers) fail to inhibit the isoproterenol-stimulated change in chloride content. The effects of either isoproterenol or 8-bromo-cAMP on both chloride content and cell volume can be inhibited by the chloride channel blocker N-phenylanthranilic acid, however. Taken together, our results indicate that RSMT-A5 cells possess a beta-adrenergic receptor system which controls intracellular volume and chloride content by modulating transport processes that are 1) cAMP-responsive and 2) inhibitable by the putative chloride channel blocker N-phenylanthranilic acid.     

Regulation of Na/K/Cl cotransport in vascular smooth muscle cells

The regulation of Na/K/Cl cotransport was investigated in vascular smooth muscle cells. That a Na/K/Cl cotransport system exists was established by the finding that the ouabain insensitive K influx was sensitive to the "loop" diuretic bumetanide. Furthermore, bumetanide sensitive K influx was dependent upon the presence of both Na and Cl in the extracellular milieu. Bumetanide sensitive K influx was inhibited by agents which elevate cellular cyclic AMP levels, and to a lesser extent by agents which elevate cellular cyclic GMP levels. When serum, EGF or TPA was added, bumetanide sensitive K influx was enhanced. These results suggest that vascular smooth muscle cells have a ouabain insensitive, bumetanide sensitive Na/K/Cl cotransport system which is stimulated by serum, EGF or TPA and inhibited by cAMP or cGMP.     

Inhibition of hepatic proteolysis by insulin. Role of hormone-induced alterations of the cellular K+ balance

1. Proteolysis was measured as [3H]leucine release from isolated perfused livers from rats, which had been labeled in vivo by an intraperitoneal injection of [3H]leucine about 16 h prior to the perfusion experiment. In livers from fed rats, insulin (35 nM) inhibited [3H]leucine release by 24.5 +/- 1.3% (n = 15) and led to an amiloride-sensitive, bumetanide-sensitive and furosemide-sensitive net K+ uptake of 5.53 +/- 0.31 mumol.g-1 (n = 15). Both the insulin effects on net K+ uptake and on [3H]leucine release were diminished by about 65% or 55% in presence of furosemide (0.1 mM) or bumetanide (5 microM), respectively. The insulin-induced net K+ uptake was virtually abolished in the presence of amiloride (1 mM) plus furosemide (0.1 mM). 2. In perfused livers from 24-h-starved rats, both the insulin-stimulated net K+ uptake and the insulin-induced inhibition of [3H]leucine release were about 80% lower than observed in experiments with livers from fed rats. The insulin effects on K+ balance and [3H]leucine release were not significantly influenced in the presence of glycine (2 mM), although glycine itself inhibited [3H]leucine release by 30.3 +/- 0.3% (n = 4) and 13.8 +/- 1.2% (n = 5) in livers from starved and fed rats, respectively. When livers from fed rats were preswollen by hypoosmotic perfusion (225 mOsmol.l-1), both the insulin-induced net K+ uptake and the inhibition of [3H]leucine release were diminished by 50-60%. 3. During inhibition of [3H]leucine release by insulin, further addition of glucagon (100 nM) led to a marked net K+ release from the liver (3.82 +/- 0.24 mumol.g-1), which was accompanied by stimulation of [3H]leucine release by 16.4 +/- 4.6% (n = 4). 4. Ba2+ (1 mM) infusion led to a net K+ uptake by the liver of 3.2 +/- 0.2 mumol.g-1 (n = 4) and simultaneously inhibited [3H]leucine release by 12.4 +/- 1.7% (n = 4). 5. There was a close relationship between the Ba2+ or insulin-induced net K+ uptake and the degree of inhibition of [3H]leucine release, even when the K+ response to insulin was modulated by bumetanide, furosemide, glucagon, hypotonic or glycine-induced cell swelling or the nutritional state. 6. The data suggest that the insulin-induced net K+ uptake involves activation of both NaCl/KCl cotransport and Na+/H+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)     

Selective inhibitors of KCl cotransport in human red cells

Two analogues of the loop diuretics furosemide and bumetanide have been identified as differential inhibitors of KCl and NaKCl cotransport systems, assayed by measuring K+ influx in 'young' human red cells. H25 inhibited both NaKCl and KCl cotransport, with I50% values of 0.03 and 30 microM respectively; H74 had no effect on NaKCl cotransport, even at 0.3 mM, but inhibited KCl cotransport with an I50% of 75 microM. These compounds are therefore useful for resolving the two transport systems.     

Comparative study on sodium transport and Na+,K+-ATPase activity in Caco-2 and rat jejunal epithelial cells: effects of dopamine

The present study reports on the effects of dopamine on sodium transepithelial transport and Na+,K+-ATPase activity in Caco-2 cells, a human epithelial intestinal cell line which undergoes enterocyte differentiation in culture, and jejunal epithelial cells from 20 day old Wistar rats. Addition of amphotericin B to the mucosal side stimulated Isc in a concentration dependent manner (Caco-2 cells, EC50=0.9 [0.5, 1.7] microM; rat jejunum, EC50=7.4 [0.8; 70.1] microM). The presence of 1 microM dopamine did not change the effect of amphotericin B in Caco-2 cells, but produced a significant (P<0.05) decrease in the maximal effect of amphotericin B in the rat jejunum. Dopamine (1 microM), added to the serosal side, did not change the Isc profile in Caco-2 cells, but produced a significant increase in the rat jejunum. This effect was antagonized by SKF 83566 (1 microM), but not S-sulpiride (1 microM), and was mimicked by SKF 38393 (10 nM), but not by quinerolane (10 nM). Basal Na+,K+-ATPase activity (in nmol Pi mg protein(-1) min(-1)) in Caco-2 cells (49.5+/-0.2) was similar to that observed in isolated rat jejunal epithelial cells (52.3+/-3.4). Dopamine (1 microM) significantly (P<0.05) decreased Na+,K+-ATPase activity in rat jejunal epithelial cells, but failed to inhibit Na+,K+-ATPase in Caco-2 cells. This effect of dopamine was antagonized by SKF 83566 (1 microM), but not S-sulpiride (1 microM), and was mimicked by SKF 38393 (10 nM), but not by quinerolane (10 nM). The specific binding of [3H]-Sch 23390 to the rat intestinal mucosa was saturable with an apparent dissociation constant (KD) of 2.4 (0.4; 4.5) nM and maximum receptor density of 259.8+/-32.6 fmol/mg protein. No significant specific binding of [3H]-Sch 23390 was observed in membranes from Caco-2 cells. In conclusion, the results obtained show that D1-like receptor mediated effects of dopamine in the rat jejunum on sodium absorption are absent in Caco-2 cells, most probably because this cell line does not express D1-like dopamine receptors, which ultimately are responsible for the inhibitory effect of the amine upon intestinal Na+,K+-ATPase.     

Coordinated regulation of shrinkage-induced Na/H exchange and swelling- induced [K-Cl] cotransport in dog red cells. Further evidence from activation kinetics and phosphatase inhibition

Hypertonic shrinkage of dog red cells caused rapid activation of Na/H exchange and rapid deactivation of [K-Cl] cotransport. Hypotonic swelling caused delayed deactivation of Na/H exchange and delayed activation of [K-Cl] cotransport. Okadaic acid stimulated shrinkage-induced Na/H exchange and inhibited swelling-induced [K-Cl] cotransport. The data are compatible with the kinetic model of Jennings and Al-Rohil (1990. J. Gen. Physiol. 95:1021-1040) for volume regulation of [K-Cl] cotransport in rabbit red cells and suggest that in dog red cells Na/H exchange and [K-Cl] cotransport are controlled by a common regulatory system. The proposal of Jennings and Schulz (1991. J. Gen. Physiol. 96:799-817) that activation/deactivation of volume-sensitive transport involves phosphorylation/dephosphorylation of a regulatory protein is supported by these observations.     

Hemolysate increases calcium-inhibition of the Na+,K+ pump of resealed human red cell ghosts

The Na+,K+ pump of resealed human red cell ghosts is more sensitive to inhibition by intracellular Ca (Cai) when they contain diluted hemolysate compared to ghosts without hemolysate. The activity of the Na+,K+ pump was assessed by measuring ouabain-sensitive 22Na efflux in ghosts that, in addition to the presence or absence of hemolysate, also contained arsenazo III to measure free Cai and a regenerating system to maintain a constant concentration of ATP. Incorporating hemolysate diluted 20-fold compared to in situ conditions doubled the inhibitory effects of 1-50 microM free Cai on the Na+,K+ pump and caused 50% inhibition to occur between 5 and 10 microM free Cai. Increased inhibition in the presence of the hemolysate was not due to a cytoplasm-induced decrease in the ATP content of the ghosts. These findings are consistent with the suggestion that the cytoplasm of human red cells contains a factor which increases the sensitivity of the Na+,K+ pump to inhibition by Cai.     

Growth factors activate the bumetanide-sensitive Na+/K+/Cl-cotransport in hamster fibroblasts

alpha-Thrombin, a potent mitogen for the hamster fibroblast cell line CCL 39, stimulates by approximately 3-fold 86Rb+ uptake in a mutant lacking the Na+/H+ antiport activity (PS 120). The major component of this stimulated 86Rb+ (K+) uptake is a bumetanide-sensitive flux (IC50 = 0.4 microM), which accounts for 50% of total K+ uptake in Go-arrested cells and is approximately 4-fold stimulated by maximal thrombin concentrations (EC50 = 5 X 10(-4) units/ml). This bumetanide-sensitive 86Rb+ uptake represents a Na+/K+/Cl- cotransport, as indicated by its dependence on extracellular Na+ and Cl- and by the existence in PS 120 cells of a bumetanide-sensitive K+-dependent 22Na+ uptake. The stimulation reaches its maximum within 2 min, is reduced at acidic intracellular pH values (half-maximal at pHi = 6.8), and can also be induced, to a lesser extent, by EGF and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, the effects of which are nearly additive. In contrast, preincubation with 12-O-tetradecanoylphorbol 13-acetate results in inhibition of thrombin- and EGF-induced stimulations, suggesting that activated protein kinase C might exert a feedback inhibitory control. This study clearly demonstrates that the growth factor-induced activation of the Na+/K+/Cl- cotransport is separated from the activation of the Na+/H+ antiport. However, activation of this cotransporter does not seem to play a major role in the mitogenic signaling pathway since its complete inhibition with bumetanide reduces only by 25-30% reinitiation of DNA synthesis.