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

NCP activates chloroplast transcription by controlling phytochrome-dependent dual nuclear and plastidial switches

Phytochromes initiate chloroplast biogenesis by activating genes encoding the photosynthetic apparatus, including photosynthesis-associated plastid-encoded genes (PhAPGs). PhAPGs are transcribed by a bacterial-type RNA polymerase (PEP), but how phytochromes in the nucleus activate chloroplast gene expression remains enigmatic. We report here a forward genetic screen in Arabidopsis that identified NUCLEAR CONTROL OF PEP ACTIVITY (NCP) as a necessary component of phytochrome signaling for PhAPG activation. NCP is dual-targeted to plastids and the nucleus. While nuclear NCP mediates the degradation of two repressors of chloroplast biogenesis, PIF1 and PIF3, NCP in plastids promotes the assembly of the PEP complex for PhAPG transcription. NCP and its paralog RCB are non-catalytic thioredoxin-like proteins that diverged in seed plants to adopt nonredundant functions in phytochrome signaling. These results support a model in which phytochromes control PhAPG expression through light-dependent double nuclear and plastidial switches that are linked by evolutionarily conserved and dual-localized regulatory proteins.

Phytochrome signaling in the nucleus can activate expression of photosynthesis-associated genes in plastids. Here Yang et al. show that NCP is a dual-targeted protein that promotes phytochrome B localization to photobodies in the nucleus while facilitating PEP polymerase assembly in the plastids.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Effect of the Na-K-2Cl cotransporter NKCC1 on systemic blood pressure and smooth muscle tone

Studies in rat aorta have shown that the Na-K-2Cl cotransporter NKCC1 is activated by vasoconstrictors and inhibited by nitrovasodilators, contributes to smooth muscle tone in vitro, and is upregulated in hypertension. To determine the role of NKCC1 in systemic vascular resistance and hypertension, blood pressure was measured in rats before and after inhibition of NKCC1 with bumetanide. Intravenous infusion of bumetanide sufficient to yield a free plasma concentration above the IC(50) for NKCC1 produced an immediate drop in blood pressure of 5.2% (P < 0.001). The reduction was not prevented when the renal arteries were clamped, indicating that it was not due to a renal effect of bumetanide. Bumetanide did not alter blood pressure in NKCC1-null mice, demonstrating that it was acting specifically through NKCC1. In third-order mesenteric arteries, bumetanide-inhibitable efflux of (86)Rb was acutely stimulated 133% by phenylephrine, and bumetanide reduced the contractile response to phenylephrine, indicating that NKCC1 influences tone in resistance vessels. The hypotensive effect of bumetanide was proportionately greater in rats made hypertensive by a 7-day infusion of norepinephrine (12.7%, P < 0.001 vs. normotensive rats) but much less so when hypertension was produced by a fixed aortic coarctation (8.0%), again consistent with an effect of bumetanide on resistance vessels rather than other determinants of blood pressure. We conclude that NKCC1 influences blood pressure through effects on smooth muscle tone in resistance vessels and that this effect is augmented in hypertension.

Reduced hyperpolarization in endothelial cells of rabbit aortic valve following chronic nitroglycerine administration

This study was undertaken to determine whether long-term in vivo administration of nitroglycerine (NTG) downregulates the hyperpolarization induced by acetylcholine (ACh) in aortic valve endothelial cells (AVECs) of the rabbit and, if so, whether antioxidant agents can normalize this downregulated hyperpolarization. ACh (0.03-3 microM) induced a hyperpolarization through activations of both apamin- and charybdotoxin-sensitive Ca2+-activated K+ channels (K(Ca)) in rabbit AVECs. The intermediate-conductance K(Ca) channel (IK(Ca)) activator 1-ethyl-2-benzimidazolinone (1-EBIO, 0.3 mM) induced a hyperpolarization of the same magnitude as ACh (3 microM). The ACh-induced hyperpolarization was significantly weaker, although the ACh-induced [Ca2+]i increase was unchanged, in NTG-treated rabbits (versus NTG-untreated control rabbits). The hyperpolarization induced by 1-EBIO was also weaker in NTG-treated rabbits. The reduced ACh-induced hyperpolarization seen in NTG-treated rabbits was not modified by in vitro application of the superoxide scavengers Mn-TBAP, tiron or ascorbate, but it was normalized when ascorbate was coadministered with NTG in vivo. Superoxide production within the endothelial cell (estimated by ethidium fluorescence) was increased in NTG-treated rabbits and this increased production was normalized by in vivo coadministration of ascorbate with the NTG. It is suggested that long-term in vivo administration of NTG downregulates the ACh-induced hyperpolarization in rabbit AVECs, possibly through chronic actions mediated by superoxide.

Decreased blood pressure and vascular smooth muscle tone in mice lacking basolateral Na(+)-K(+)-2Cl(-) cotransporter

The basolateral Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) functions in the maintenance of cellular electrolyte and volume homeostasis. NKCC1-deficient (Nkcc1(-/-)) mice were used to examine its role in cardiac function and in the maintenance of blood pressure and vascular tone. Tail-cuff measurements demonstrated that awake Nkcc1(-/-) mice had significantly lower systolic blood pressure than wild-type (Nkcc1(+/+)) mice (114.5 +/- 2.2 and 131.8 +/- 2.5 mmHg, respectively). Serum aldosterone levels were normal, indicating that extracellular fluid-volume homeostasis was not impaired. Studies using pressure transducers in the femoral artery and left ventricle showed that anesthetized Nkcc1(-/-) mice have decreased mean arterial pressure and left ventricular pressure, whereas myocardial contraction parameters were not significantly different from those of Nkcc1(+/+) mice. When stimulated with phenylephrine, aortic smooth muscle from Nkcc1(+/+) and Nkcc1(-/-) mice exhibited no significant differences in maximum contractility and only moderate dose-response shifts. In phasic portal vein smooth muscle from Nkcc1(-/-) mice, however, a sharp reduction in mechanical force was noted. These results indicate that NKCC1 can be important for the maintenance of normal blood pressure and vascular tone.

JNK is a volume-sensitive kinase that phosphorylates the Na-K-2Cl cotransporter in vitro

Cell shrinkage phosphorylates and activates the Na-K-2Cl cotransporter (NKCC1), indicating the presence of a volume-sensitive protein kinase. To identify this kinase, extracts of normal and shrunken aortic endothelial cells were screened for phosphorylation of NKCC1 fusion proteins in an in-the-gel kinase assay. Hypertonic shrinkage activated a 46-kDa kinase that phosphorylated an NH2-terminal fusion protein, with weaker phosphorylation of a COOH-terminal fusion protein. This cytosolic kinase was activated by both hypertonic and isosmotic shrinkage, indicating regulation by cell volume rather than osmolarity. Subsequent studies identified this kinase as c-Jun NH2-terminal kinase (JNK). Immunoblotting revealed increased JNK activity in shrunken cells; there was volume-sensitive phosphorylation of NH2-terminal c-Jun fusion protein; immunoprecipitation of JNK from shrunken cells but not normal cells phosphorylated NKCC1 in gel kinase assays; and treatment of cells with tumor necrosis factor, a known activator of JNK, mimicked the effect of hypertonicity. We conclude that JNK is a volume-sensitive kinase in endothelial cells that phosphorylates NKCC1 in vitro. This is the first demonstration of a volume-sensitive protein kinase capable of phosphorylating a volume-regulatory transporter.

Vasoconstrictors and nitrovasodilators reciprocally regulate the Na+-K+-2Cl- cotransporter in rat aorta

Little is known about the function and regulation of the Na+-K+-2Cl- cotransporter NKCC1 in vascular smooth muscle. The activity of NKCC1 was measured as the bumetanide-sensitive efflux of 86Rb+ from intact smooth muscle of the rat aorta. Hypertonic shrinkage (440 mosmol/kgH2O) rapidly doubled cotransporter activity, consistent with its volume-regulatory function. NKCC1 was also acutely activated by the vasoconstrictors ANG II (52%), phenylephrine (50%), endothelin (53%), and 30 mM KCl (54%). Both nitric oxide and nitroprusside inhibited basal NKCC1 activity (39 and 34%, respectively), and nitroprusside completely reversed the stimulation by phenylephrine. The phosphorylation of NKCC1 was increased by hypertonic shrinkage, phenylephrine, and KCl and was reduced by nitroprusside. The inhibition of NKCC1 significantly reduced the contraction of rat aorta induced by phenylephrine (63% at 10 nM, 26% at 30 nM) but not by KCl. We conclude that the Na+-K+-2Cl- cotransporter in vascular smooth muscle is reciprocally regulated by vasoconstrictors and nitrovasodilators and contributes to smooth muscle contraction, indicating that alterations in NKCC1 could influence vascular smooth muscle tone in vivo.

Acetylcholine-induced membrane potential changes in endothelial cells of rabbit aortic valve

1. Using a microelectrode technique, acetylcholine (ACh)-induced membrane potential changes were characterized using various types of inhibitors of K+ and Cl- channels in rabbit aortic valve endothelial cells (RAVEC). 2. ACh produced transient then sustained membrane hyperpolarizations. Withdrawal of ACh evoked a transient depolarization. 3. High K+ blocked and low K+ potentiated the two ACh-induced hyperpolarizations. Charybdotoxin (ChTX) attenuated the ACh-induced transient and sustained hyperpolarizations; apamin inhibited only the sustained hyperpolarization. In the combined presence of ChTX and apamin, ACh produced a depolarization. 4. In Ca2+-free solution or in the presence of Co2+ or Ni2+, ACh produced a transient hyperpolarization followed by a depolarization. In BAPTA-AM-treated cells, ACh produced only a depolarization. 5. A low concentration of A23187 attenuated the ACh-induced transient, but not the sustained, hyperpolarization. In the presence of cyclopiazonic acid, the hyperpolarization induced by ACh was maintained after ACh removal; this maintained hyperpolarization was blocked by Co2+. 6. Both NPPB and hypertonic solution inhibited the membrane depolarization seen after ACh washout. Bumetanide also attenuated this depolarization. 7. It is concluded that in RAVEC, ACh produces a two-component hyperpolarization followed by a depolarization. It is suggested that ACh-induced Ca2+ release from the storage sites causes a transient hyperpolarization due to activation of ChTX-sensitive K+ channels and that ACh-activated Ca2+ influx causes a sustained hyperpolarization by activating both ChTX- and apamin-sensitive K+ channels. Both volume-sensitive Cl- channels and the Na+-K+-Cl- cotransporter probably contribute to the ACh-induced depolarization.

Prostaglandin E2 inhibits Na-K-2Cl cotransport in medullary thick ascending limb cells

Prostaglandin E2 (PGE2) is known to inhibit transepithelial Cl transport in medullary thick ascending limb (mTAL), but the mechanism of inhibition or the transport pathway affected has not been identified. We undertook this study to examine the effect of PGE2 on Na-K-2Cl cotransport in mouse mTAL cells in culture. In nanomolar concentrations, PGE2 inhibited the Na- and Cl-dependent, bumetanide-sensitive K influx by 45%, and this inhibition was also observed in the presence of 3 mM ouabain. Although PGE2 also inhibited ouabain-sensitive K flux, that inhibition was abolished in the presence of apical nystatin, suggesting that the pump inhibition was secondary to diminished Na entry into the cells. The effect of PGE2 was concentration dependent. Inhibition was observed at a concentration of < 1 nM, and half-maximal effect was observed at 2.5 nM. The effect of PGE2 was not mediated by an action on cytosolic Ca because cytosolic Ca was unchanged after the addition of PGE2. PGE2 reduced the maximal velocity for the cotransporter but had no effect on the affinity of the cotransporter for external Na, K, or Cl. Specific [3H]bumetanide binding was reduced in the presence of PGE2, suggesting that PGE2 affected bumetanide-sensitive K influx by downregulating the number of functioning Na-K-2Cl cotransporters. These results suggest that Na-K-2Cl cotransport in the mTAL cells may be under tonic inhibitory control of PGE2.

Volume-sensitive myosin phosphorylation in vascular endothelial cells: correlation with Na-K-2Cl cotransport

To identify protein kinases that are regulated by cell volume, we examined protein phosphorylation in hypertonically shrunken aortic endothelial cells. Shrinkage reversibly increased, and swelling decreased, phosphorylation of a 19-kDa cytoskeletal protein identified as myosin light chain (MLC) by immune precipitation and immunoblotting. Shrinkage also increased MLC phosphorylation in human umbilical vein endothelial cells, rat aortic smooth muscle cells, and human dermal fibroblasts. Phosphorylation was blocked by ML-7, an inhibitor of MLC kinase (MLCK). Neither inhibition of protein kinase C nor inhibition of myosin phosphatase (with calyculin) altered MLC phosphorylation. Peptide mapping of MLC indicated phosphorylation by MLCK. Na-K-2Cl cotransport activation paralleled MLC phosphorylation in hypertonic medium. Na-K-2Cl was stimulated by low concentrations of ML-7 with no further stimulation by hypertonic shrinkage and was inhibited by higher concentrations, paralleling inhibition of MLC phosphorylation. Shrinkage-induced phosphorylation of the cotransporter was not blocked by ML-7. We conclude that cell volume regulates MLC phosphorylation by MLCK. MLCK influences Na-K-2Cl cotransport but independently of cotransporter phosphorylation. These data suggest an important link between cell volume, volume-regulatory transporters, and the contractile state of the cytoskeleton.

Functional coupling of Na(+)-K(+)-2Cl- cotransport and Ca(2+)-dependent K+ channels in vascular endothelial cells

To determine whether the activation of Na(+)-K(+)-2Cl- cotransport by Ca(2+)-mobilizing agonists is a direct effect of Ca2+ or is secondary to activation of Ca(2+)-dependent K+ channels [via cell shrinkage or decreased intracellular Cl- concentration ([Cl-]), we measured K+ fluxes in aortic endothelial cells in response to ATP and bradykinin. With either agonist there was an immediate bumetanide-insensitive efflux inhibitable by the K+ channel blockers tetrabutylammonium (TBA, 23 mM) and quinidine (1 mM), followed several minutes later by increased bumetanide-sensitive efflux or influx (Na(+)-K(+)-2Cl- cotransport). ATP induced a loss of cell K+ that was prevented by TBA and augmented by bumetanide. Both TBA and quinidine prevented the stimulation of cotransport by agonists but not by hypertonic shrinkage. Raising medium [K+] to prevent K+ loss also blocked activation of cotransport by agonists. The results indicate that the stimulation of Na(+)-K(+)-2Cl- cotransport by Ca2+ is not direct but instead is indirect via activation of Ca(2+)-dependent K+ channels and a resulting decrease in cell volume and intracellular [Cl-]. This suggests that at least one role of Na(+)-K(+)-2Cl- cotransport in endothelial cells is to maintain cell volume and intracellular [Cl-] during agonist stimulation.

Characterization of the proteins of the intestinal Na(+)-K(+)-2Cl- cotransporter

Absorptive intestinal epithelia, such as that of the winter flounder, absorb salt via a bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport mechanism on the brush-border membrane (BBM). The present study demonstrates the first molecular characterization of the intestinal Na(+)-K(+)-2Cl- cotransporter and its unique regulation. The photoaffinity bumetanide analogue, 4-[3H]benzoyl-5-sulfamoyl-3- (3-thenyloxy)benzoic acid, specifically labeled three groups of proteins in flounder intestinal microsomal membranes (MM): a approximately 180-kDa peptide, prominently labeled, and diffuse bands at approximately 110-70 and 50 kDa, less intensely labeled. Subcellular fractionation revealed a single prominently labeled protein of approximately 170 kDa in BBM but not in basolateral membranes (BLM) and little or no labeling of proteins of approximately 110-70 or 50 kDa. Polyclonal antiserum raised against the Ehrlich ascites cell cotransporter identified a 180-kDa peptide in MM and a 175-kDa peptide (pI approximately 5.4) in BBM but none in BLM or in the cytosol of flounder intestine. As predicted from the regulation of cotransport in this tissue, phosphorylation of this protein is increased by guanosine 3',5'-cyclic monophosphate (cGMP)-dependent but not by adenosine 3',5'-cyclic monophosphate-dependent protein kinase. In addition, phosphorylation of the protein is not increased by protein kinase C or Ca2+/calmodulin-dependent protein kinase but is increased by the phosphatase inhibitor calyculin A. Finally, calyculin A preserves the inhibitory effect of cGMP on ion transport, even in the absence of the nucleotide, suggesting that phosphorylation-dephosphorylation mechanisms are crucial in cotransporter regulation. Thus the flounder intestinal cotransporter is a approximately 175-kDa BBM protein that can be regulated by phosphorylation.

Na+/K+/2Cl- cotransport in medullary thick ascending limb cells: kinetics and bumetanide binding

We examined the properties of Na+/K+/2Cl- cotransport in cultured mouse mTAL cells with respect to its kinetics, the contribution of K/K exchange to K fluxes mediated by the cotransporter, and [3H]bumetanide binding and turnover numbers in media with varying osmolality. The addition of bumetanide, the replacement of external Na+ or the replacement of external Cl- resulted in an almost identical (approx. 50%) decrease in K+ influx, suggesting that Na(+)-dependent, Cl(-)-dependent, BS K+ influx was a measure of Na+/K+/2Cl- cotransport. The kinetics of the BS K+ influx revealed a high affinity for external Na+ (apparent Km 7 mM) and external K+ (apparent Km 1.3 mM), but a very low affinity for external Cl- (apparent Km 67 mM with a two-site model). Of interest was the finding that none of the K+ (86Rb+) efflux was sensitive to bumetanide, suggesting the absence of cotransport mediated K/K exchange in this cell type. Specific [3H]bumetanide binding was a saturable function of free bumetanide concentration with a Kd of 0.20 microM and maximum binding (Bmax) of 0.63 pmol/mg, or about 53,000 sites per cell. Simultaneous transport and bumetanide binding assays yielded a turnover number of 255 min-1. The omission of external Na+, K+ or Cl- reduced specific [3H]bumetanide binding to values indistinguishable from zero. Changing medium osmolarity resulted in a co-ordinate change in BS K+ influx and bumetanide binding, with a monotonic increase in both transport and bumetanide binding with increase in osmolality from 200 to 400 mosmol/kg. About 85% of the cotransporter sites were located on the apical side, as in the intact mTAL tubule. The simultaneous measurement of BS ion transport and [3H]bumetanide binding in the mTAL cell may provide valuable insights into the regulation of Na+/K+/2Cl- cotransport in this nephron segment.

Na+, K+, Cl- cotransport and its regulation in Ehrlich ascites tumor cells. Ca2+/calmodulin and protein kinase C dependent pathways

Net Cl- uptake as well as unidirectional 36Cl influx during regulatory volume increase (RVI) require external K+. Half-maximal rate of bumetanide-sensitive 36Cl uptake is attained at about 3.3 mM external K+. The bumetanide-sensitive K+ influx found during RVI is strongly dependent on both Na+ and Cl-. The bumetanide-sensitive unidirectional Na+ influx during RVI is dependent on K+ as well as on Cl-. The cotransporter activated during RVI in Ehrlich cells, therefore, seems to transport Na+, K+ and Cl-. In the presence of ouabain and Ba+ the stoichiometry of the bumetanide-sensitive net fluxes can be measured at 1.0 Na+, 0.8 K+, 2.0 Cl- or approximately 1:Na, 1:K, 2:Cl. Under these circumstances the K+ and Cl- flux ratios (influx/efflux) for the bumetanide-sensitive component were estimated at 1.34 +/- 0.08 and 1.82 +/- 0.15 which should be compared to the gradient for the Na+, K+, 2Cl- cotransport system at 1.75 +/- 0.24. Addition of sucrose to hypertonicity causes the Ehrlich cells to shrink with no signs of RVI, whereas shrinkage with hypertonic standard medium (all extracellular ion concentrations increased) results in a RVI response towards the original cell volume. Under both conditions a bumetanide-sensitive unidirectional K+ influx is activated. During hypotonic conditions a small bumetanide-sensitive K+ influx is observed, indicating that the cotransport system is already activated. The cotransport is activated 10-15 fold by bradykinin, an agonist which stimulates phospholipase C resulting in release of internal Ca2+ and activation of protein kinase C. The anti-calmodulin drug pimozide inhibits most of the bumetanide-sensitive K+ influx during RVI. The cotransporter can be activated by the phorbol ester TPA. These results indicate that the stimulation of the Na+, K+, Cl- cotransport involves both Ca2+/calmodulin and protein kinase C.

Intracellular calcium, currents, and stimulus-response coupling in endothelial cells

Vascular endothelium appears to be a unique organ. It not only responds to numerous hormonal and chemical signals but also senses changes in physical parameters such as shear stress, producing mediators that modulate the responses of numerous cells, including vascular smooth muscle, platelets, and leukocytes. In many cases, the initial response of endothelial cells to these diverse signals involves elevation of cytosolic Ca2+ and activation of Ca(2+)-dependent enzymes, including nitric oxide synthase and phospholipase A2. Both the release of Ca2+ from intracellular stores, most likely the endoplasmic reticulum, and the influx of Ca2+ from the extracellular space contribute to the [Ca2+]i increase. The most important trigger for Ca2+ release is inositol 1,4,5-trisphosphate, which is generated by the action of phospholipase C, a plasmalemmal enzyme activated in many cases by the receptor-G protein cascade. Ca2+ influx appears to be related to the activity of receptor-G protein-enzyme complex and to the degree of fullness of the endoplasmic reticulum but does not involve voltage-gated Ca2+ channels. The magnitude of the Ca2+ influx depends on the electrochemical gradient, which is modulated by the membrane potential, Vm. Under basal conditions, Vm is dominated by a large inward rectifier K+ current. Some stimuli, e.g., acetylcholine, have been shown to hyperpolarize Vm, thus increasing the electrochemical gradient for Ca2+, which appears to be modulated by activation of Ca(2+)-dependent K+ and Cl- currents. However, the lack of potent and specific blockers for many of the described or postulated channels (e.g., nonselective cation channel, Ca(2+)-activated Cl- channel) makes an estimation of their effect on endothelial cell function rather difficult. Possible future directions of research and clinical implications are discussed.

Regulation of vascular endothelial cell volume by Na-K-2Cl cotransport

The relationship between cell volume and Na-K-2Cl cotransport was studied in cultured bovine aortic endothelial cells. Hypertonic cell shrinkage increased bumetanide-sensitive, Na- or Cl-dependent K influx without altering bumetanide-insensitive influx. Greater stimulation of cotransport was observed in cells shrunken isosmotically either by preincubation in K-free and Na-free medium or by preincubation in hypotonic medium. Cell swelling, produced by preincubation in isotonic high-K medium, inhibited bumetanide-sensitive K influx. Simultaneous measurements of [3H]bumetanide binding and K influx revealed an increased number of binding sites without an increased influx per binding site in shrunken cells. Bumetanide did not alter the volume or ion content of cells in isotonic or hypertonic medium, indicating that no net influx of ions occurs through cotransport under these conditions. In isosmotically shrunken cells, there was greater stimulation of bumetanide-sensitive influx than of bumetanide-sensitive efflux, resulting in net bumetanide-sensitive influx. Rapid recovery of cell K, Na, and water occurred over 10-20 min and was inhibited by bumetanide or by the removal of external Na or Cl. These data demonstrate that Na-K-2Cl cotransport in aortic endothelial cells is regulated by cell volume, possibly through changes in the number of functional cotransporters, and mediates a brisk regulatory volume increase in isosmotically shrunken cells. Although thermodynamically favored, no net influx occurs through Na-K-2Cl cotransport in cells of normal volume or in hypertonically shrunken cells. This suggests additional regulation of cotransport, perhaps through trans-inhibition by intracellular Cl. Regulation of cell volume by Na-K-2Cl cotransport may be important in maintaining endothelial integrity.