Potential osmoprotective roles of branchial heat shock proteins towards Na+, K+-ATPase in milkfish (Chanos chanos) exposed to hypotonic stress

In euryhaline teleosts, osmoregulatory mechanisms vary with osmotic stresses, and heat shock proteins (HSPs) play a central role in maintaining cellular homeostasis. The present study aimed to investigate the expression and potential roles of HSP70 and HSP90 in the gills of seawater (SW)- and freshwater (FW)-acclimated milkfish (Chanos chanos). Four HSP genes, including Cchsc70 (heat shock cognate 70), Cchsp70, Cchsp90α, and Cchsp90β, were analyzed in milkfish gills. Among these genes, only the mRNA abundance of branchial Cchsp90α was significantly lower in the FW-acclimated than in the SW-acclimated milkfish. Immunoblotting showed no significant difference in the relative protein abundance of branchial HSP70 and HSP90 between the two groups. The time-course experiments (from SW to FW) showed that the protein abundance of HSP70 and HSP90 at the 3 h and 6 h post-transfer and then declined gradually. To further illustrate the potential osmoregulatory roles of HSP70 and HSP90, their interaction with Na⁺, K⁺-ATPase (NKA, the primary driving force for osmoregulation) was analyzed using co-immunoprecipitation. The results showed the interaction between HSP70, HSP90 and NKA after acclimation to SW or FW increased within 3 h; and then returned to normal levels within 7 days. To our knowledge, the present study was the first to demonstrate that the interaction between HSP70, HSP90 and NKA changes with hypotonic stress in euryhaline teleosts. Before the transfer, no interaction was detected. When transferred to FW from SW, the interaction of HSP70 and HSP90 with NKA were detected. The results suggested that HSP70 and HSP90 participated in the acute responses of osmoregulatory mechanisms to protect branchial NKA from hypotonic stress in milkfish.

Fluoride Exposure Induces Inhibition of Sodium-and Potassium-Activated Adenosine Triphosphatase (Na+, K+-ATPase) Enzyme Activity: Molecular Mechanisms and Implications for Public Health

In this study, several lines of evidence are provided to show that Na + , K + -ATPase activity exerts vital roles in normal brain development and function and that loss of enzyme activity is implicated in neurodevelopmental, neuropsychiatric and neurodegenerative disorders, as well as increased risk of cancer, metabolic, pulmonary and cardiovascular disease. Evidence is presented to show that fluoride (F) inhibits Na + , K + -ATPase activity by altering biological pathways through modifying the expression of genes and the activity of glycolytic enzymes, metalloenzymes, hormones, proteins, neuropeptides and cytokines, as well as biological interface interactions that rely on the bioavailability of chemical elements magnesium and manganese to modulate ATP and Na + , K + -ATPase enzyme activity. Taken together, the findings of this study provide unprecedented insights into the molecular mechanisms and biological pathways by which F inhibits Na + , K + -ATPase activity and contributes to the etiology and pathophysiology of diseases associated with impairment of this essential enzyme. Moreover, the findings of this study further suggest that there are windows of susceptibility over the life course where chronic F exposure in pregnancy and early infancy may impair Na + , K + -ATPase activity with both short- and long-term implications for disease and inequalities in health. These findings would warrant considerable attention and potential intervention, not to mention additional research on the potential effects of F intake in contributing to chronic disease.

Progression of experimental autoimmune encephalomyelitis is associated with up-regulation of major sodium transporters in the mouse kidney cortex under a normal salt diet

Recent demonstrations of exacerbation of experimental autoimmune encephalomyelitis (EAE) by high salt diets prompted us to study whether EAE stimulated Na absorption by the renal cortex, a primary regulatory site for Na balance, even under a normal NaCl diet. We found that as EAE progressed from mild to severe symptoms, there were parallel increases in the protein abundance of NHE3 and αENaC and the Na,K-ATPase activity with an affiliated elevation of its β1-subunit protein. These effects are associated with increases in the protein levels of the well-known regulators SGK1 and scaffold NHERF2, and phosphorylation of ERK1/2. These effects of EAE could not be explained by reduction in water or food intake. We conclude that EAE progression is associated with up-regulation of major Na transporters, which is most likely driven by increased expression of SGK1 and NHERF2 and activation of ERK1/2. These data suggest that EAE progression increases Na absorption by the renal cortex.

Hyperosmotic and isosmotic shrinkage differentially affect protein phosphorylation and ion transport

In the present work, we compared the outcome of hyperosmotic and isosmotic shrinkage on ion transport and protein phosphorylation in C11-MDCK cells resembling intercalated cells from collecting ducts and in vascular smooth muscle cells (VSMC) from the rat aorta. Hyperosmotic shrinkage was triggered by cell exposure to hypertonic medium, whereas isosmotic shrinkage was evoked by cell transfer from an hypoosmotic to an isosmotic environment. Despite a similar cell volume decrease of 40%–50%, the consequences of hyperosmotic and isosmotic shrinkage on cellular functions were sharply different. In C11-MDCK and VSMC, hyperosmotic shrinkage completely inhibited Na+,K+-ATPase and Na+,Pi cotransport. In contrast, in both types of cells isosmotic shrinkage slightly increased rather than suppressed Na+,K+-ATPase and did not change Na+,Pi cotransport. In C11-MDCK cells, phosphorylation of JNK1/2 and Erk1/2 mitogen-activated protein kinases was augmented in hyperosmotically shrunken cells by ∼7- and 2-fold, respectively, but was not affected in cells subjected to isosmotic shrinkage. These results demonstrate that the data obtained in cells subjected to hyperosmotic shrinkage cannot be considered as sufficient proof implicating cell volume perturbations in the regulation of cellular functions under isosmotic conditions.

The stimulatory effect of angiotensin II on Na(+)-ATPase activity involves sequential activation of phospholipases and sustained PKC activity

Angiotensin II (Ang II) stimulates the proximal tubule Na(+)-ATPase through the AT(1) receptor/phosphoinositide phospholipase Cbeta (PI-PLCbeta)/protein kinase C (PKC) pathway. However, this pathway alone does not explain the sustained effect of Ang II on Na(+)-ATPase activity for 30 min. The aim of the present work was to elucidate the molecular mechanisms involved in the sustained effect of Ang II on Na(+)-ATPase activity. Ang II induced fast and correlated activation of Na(+)-ATPase and PKC activities with the maximal effect (115%) observed at 1 min and sustained for 30 min, indicating a pivotal role of PKC in the modulation of Na(+)-ATPase by Ang II. We observed that the sustained activation of PKC by Ang II depended on the sequential activation of phospholipase D and Ca(2+)-insensitive phospholipase A(2), forming phosphatidic acid and lysophosphatidic acid, respectively. The results indicate that PKC could be the final target and an integrator molecule of different signaling pathways triggered by Ang II, which could explain the sustained activation of Na(+)-ATPase by Ang II.

Intracellular monovalent ions as second messengers

It is generally accepted that electrochemical gradients of monovalent ions across the plasma membrane, created by the coupled function of pumps, carriers and channels, are involved in the maintenance of resting and action membrane potential, cell volume adjustment, intracellular Ca(2+ )handling and accumulation of glucose, amino acids, nucleotides and other precursors of macromolecular synthesis. In the present review, we summarize data showing that side-by-side with these classic functions, modulation of the intracellular concentration of monovalent ions in a physiologically reasonable range is sufficient to trigger numerous cellular responses, including changes in enzyme activity, gene expression, protein synthesis, cell proliferation and death. Importantly, the engagement of monovalent ions in regulation of the above-listed cellular responses occurs at steps upstream of Ca(2+) (i) and other key intermediates of intracellular signaling, which allows them to be considered as second messengers. With the exception of HCO (3) (-) -sensitive soluble adenylyl cyclase, the molecular origin of sensors involved in the function of monovalent ions as second messengers remains unknown.

Chronic PKC-beta activation in HT-29 Cl.19a colonocytes prevents cAMP-mediated ion secretion by inhibiting apical membrane current generation

We investigated the effects of PKC-stimulating 12-deoxyphorbol 13-phenylacetate 20-acetate (DOPPA) and phorbol 12-myristate 13-acetate (PMA) phorbol esters on cAMP-dependent, forskolin (FSK)-stimulated, short-circuit Cl- current (ISC-cAMP) generation by colonocyte monolayers. These agonists elicited different actions depending on their dose and incubation time; PMA effects at the onset (<5 min) were independent of cAMP agonist and were characterized by transient anion-dependent transcellular and apical membrane ISC generation. DOPPA failed to elicit similar responses. Whereas chronic (24 h) exposure to both agents inhibited FSK-stimulated transcellular and apical membrane ISC-cAMP, the effects of DOPPA were more complex: this conventional PKC-beta-specific agonist also stimulated Ba2+-sensitive basolateral membrane-dependent facilitation of transcellular ISC-cAMP. PMA did not elicit a similar phenomenon. Prolonged exposure to high-dose PMA but not DOPPA led to apical membrane ISC-cAMP recovery. Changes in PKC alpha-, beta1-, gamma-, and epsilon-isoform membrane partitioning and expression correlated with these findings. PMA-induced transcellular ISC correlated with PKC-alpha membrane association, whereas low doses of both agents inhibited transcellular and apical membrane ISC-cAMP, increased PKC-beta1, decreased PKC-beta2 membrane association, and caused reciprocal changes in isoform mass. During the apical membrane ISC-cAMP recovery after prolonged high-dose PMA exposure, an almost-complete depletion of cellular PKC-beta1 and a significant reduction in PKC-epsilon mass occurred. Thus activated PKC-beta1 and/or PKC-epsilon prevented, whereas activated PKC-alpha facilitated, apical membrane ISC-cAMP. PKC-beta-dependent augmentation of transcellular ISC-cAMP at the level of the basolateral membrane demonstrated that transport events with geographically distinct subcellular membranes can be independently regulated by the PKC beta-isoform.

Effect of adenosine triphosphate on phosphate uptake in renal proximal tubule cells: involvement of PKC and p38 MAPK

ATP has been known to act as an extracellular signal and to be involved in various functions of kidney. Renal proximal tubular reabsorption of phosphate (Pi) contributes to the maintenance of phosphate homeostasis, which is regulated by Na+/Pi cotransporter. However, the effects of ATP on Na+/Pi cotransporters were not elucidated in proximal tubule cells (PTCs). Thus, the effects of ATP on Na+/Pi cotransporter and its related signal pathways are examined in the primary cultured renal PTCs. In the present study, ATP inhibited Pi uptake in a time (> 1 h) and dose (>10(-6)M) dependent manner. ATP-induced inhibition of Pi uptake was correlated with the decrease of type II Na+/Pi cotransporter mRNA. ATP-induced inhibition of Pi uptake may be mediated by P2Y receptor activation, since suramin (non-specific P2 receptor antagonist) and RB-2 (P2Y receptor antagonist) blocked it. ATP-induced inhibition of Pi uptake was blocked by neomycin, U73122 (phospholipase C (PLC) inhibitors), bisindolylmaleimide I, H-7, and staurosporine (protein kinase C (PKC) inhibitors), suggesting the role of PLC/PKC pathway. ATP also increased inositol phosphates (IPs) formation and induced PKC translocation from cytosolic fraction to membrane fraction. In addition, ATP-induced inhibition of Pi uptake was blocked by SB 203580 [a p38 mitogen activated protein kinase (MAPK) inhibitor], but not by PD 98059 (a p44/42 MAPK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK, which was not blocked by PKC inhibitor. In conclusion, ATP inhibited Pi uptake via PLC/PKC as well as p38 MAPK in renal PTCs.

Nitric oxide-cyclic GMP contributes to abnormal activation of Na+-K+-ATPase in the aorta from rats with endotoxic shock

We examined pharmacologically the influence of nitric oxide (NO), guanosine 3':5'-cyclic monophosphate (cyclic GMP), adenine 3':5'-cyclic monophosphate (cyclic AMP), and protein kinase C-linked signaling pathways on relaxation to potassium in aortic segments isolated from rats treated for 6 h with bacterial endotoxin (lipopolysaccharide). Endotoxemia for 6 h was associated with a severe hypotension and vascular hyporeactivity to norepinephrine (NE), and an increase in plasma NO in vivo and aortic NO ex vivo. The NE-induced contraction was attenuated and the potassium-induced relaxation was accentuated in the aorta of rats with endotoxic shock. Ouabain inhibited the potassium-induced relaxation in aortae from normal and endotoxemic rats. 8-Bromo-cyclic GMP significantly enhanced the potassium-induced relaxation in control aortae, whereas 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) abolished this difference between normal and endotoxemic rats. In contrast, inhibition of potassium-induced relaxation was observed in aortae from normal and endotoxemic rats treated with 8-bromo-cyclic AMP or phorbol 12-myristate 13-acetate. Individually, inhibitors of protein kinase A or protein kinase C did not significantly alter relaxation to potassium; however, in combination, these inhibitors significantly potentiated relaxation in aortae from control rats. These results suggest that activity of Na(+)-K(+)-ATPase is enhanced in the vascular bed of animals with endotoxic shock and that this elevation in activity is mediated by NO-cyclic GMP, but not by cyclic AMP-protein kinase A or protein kinase C.

D5 dopamine receptor knockout mice and hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. All of the five dopamine receptor genes (D1, D2, D3, D4, and D5) expressed in mammals and some of their regulators are in loci linked to hypertension in humans and in rodents. Under normal conditions, D1-like receptors (D1 and D5) inhibit sodium transport in the kidney and the intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats, and humans with essential hypertension, the D1-like receptor-mediated inhibition of sodium transport is impaired because of an uncoupling of the D1-like receptor from its G protein/effector complex. The uncoupling is genetic, and receptor-, organ-, and nephron segment-specific. In human essential hypertension, the uncoupling of the D1 receptor from its G protein/effector complex is caused by an agonist-independent serine phosphorylation/desensitization by constitutively active variants of the G protein-coupled receptor kinase type 4. The D5 receptor is also important in blood pressure regulation. Disruption of the D5 or the D1 receptor gene in mice increases blood pressure. However, unlike the D1 receptor, the hypertension in D5 receptor null mice is caused by increased activity of the sympathetic nervous system, apparently due to activation of oxytocin, V1 vasopressin, and non-N-methyl D-aspartate receptors in the central nervous system. The cause of the activation of these receptors remains to be determined.

Inhibition of bicarbonate reabsorption in the rat proximal tubule by activation of luminal P2Y1 receptors

The present study used a stationary microperfusion technique to investigate in vivo the effect of P2Y1 receptor activation on bicarbonate reabsorption in the rat proximal tubule. Proximal tubules were perfused with a bicarbonate Ringer solution before flow was stopped by means of an oil block. The recovery of lumen pH from the initial value (pH 8.0) to stationary values (pH approximately 6.7) was recorded by a H+-sensitive microelectrode inserted downstream of the perfusion pipette and oil block. The stationary pH value and the t(1/2) of pH recovery were used to calculate bicarbonate reabsorption (JHCO3). Both EIPA and bafilomycin A1 caused significant reductions in proximal tubule JHCO3, consistent with the established contributions of Na/H exchange and H+-ATPase to proximal tubule HCO3 reabsorption. The nucleotides ADP and, to a lesser extent, ATP reduced JHCO3 but AMP and UTP were without effect. 2MeSADP, a highly selective agonist of the P2Y1 receptor, reduced JHCO3 in a dose-dependent manner. MRS-2179, a P2Y1 receptor-specific antagonist, abolished the effect of 2MeSADP, whereas theophylline, an antagonist of adenosine (P1) receptors, did not. The inhibitory action of 2MeSADP was blocked by inhibition of protein kinase C and reduced by inhibition of protein kinase A. The effects of EIPA and 2MeSADP were not additive. The data provide functional evidence for P2Y1 receptors in the apical membrane of the rat proximal tubule: receptor activation impairs acidification in this nephron segment.

Evidence for a role of protein kinase C-alpha in urine concentration

In mouse kidney, the conventional protein kinase C (PKC) isoenzyme alpha is expressed in glomeruli, the cortical collecting duct (intercalated cells only), and medullary collecting duct. To get insights on its function, PKC-alpha knockout (-/-) and wild-type (+/+) mice were studied. When provided free access to water, PKC-alpha -/- mice showed approximately 50% greater urine flow rate and lower urinary osmolality in 24-h metabolic cage experiments despite a greater urinary vasopressin-to-creatinine ratio vs. PKC-alpha +/+ mice. Renal albumin excretion was not different. Clearance experiments under inactin/ketamine anesthesia revealed a modestly reduced glomerular filtration rate and showed a reduced absolute and fractional renal fluid reabsorption in PKC-alpha -/- mice. The sodium-restricting response to a low-sodium diet was unaffected in PKC-alpha -/- mice. Urinary osmolality was reduced to similar hypotonic levels in PKC-alpha -/- and +/+ mice during acute oral water loading or application of the vasopressin V(2)-receptor antagonist SR-121463. In comparison, the lower urinary osmolality observed in PKC-alpha -/- mice vs. wild-type mice under basal conditions persisted during water restriction for 36 h. In conclusion, PKC-alpha appears not to play a major role in renal sodium reabsorption but, consistent with its expression in the medullary collecting duct, contributes to urinary concentration in mice. Considering that PKC-beta I and -beta II are coexpressed with PKC-alpha in mouse medullary collecting duct, the present results indicate that conventional PKC isoenzymes cannot fully compensate for each other.

Rapid, nongenomic effects of aldosterone in the heart mediated by epsilon protein kinase C

Aldosterone elevates Na+/K+/2Cl- cotransporter activity in rabbit cardiomyocytes within 15 min, an effect blocked by K-canrenoate and thus putatively mineralocorticoid receptor mediated. Increased cotransporter activity raises intracellular [Na+] sufficient to produce a secondary increase in Na+-K+ pump activity; when this increase in intracellular [Na+] is prevented, a rapid effect of aldosterone to lower pump activity is seen. Addition of transcription inhibitor actinomycin D did not change basal or aldosterone-induced lowered pump activity, indicating a direct, nongenomic action of aldosterone. We examined a possible role for protein kinase C (PKC) in the rapid nongenomic effects of aldosterone. Single ventricular myocytes and pipette solutions containing 10 mm intracellular [Na+] were used in patch clamp studies to measure Na+-K+ pump activity. Aldosterone lowered pump current, an effect abolished by epsilon PKC (epsilonPKC) inhibition but neither alphaPKC nor scrambled epsilonPKC; addition of epsilonPKC activator peptide mimicked the rapid aldosterone effect. In rabbits chronically infused with aldosterone, the lowered pump current in cardiomyocytes was acutely (< or =15 min) restored by epsilonPKC inhibition. These studies show that rapid effects of aldosterone on Na+-K+ pump activity are nongenomic and specifically epsilonPKC mediated; in addition, such effects may be prolonged (7 d) and long-lived ( approximately 4 h isolated cardiomyocyte preparation time). The rapid, prolonged, long-lived effects can be rapidly (< or =15 min) reversed by epsilonPKC blockade, suggesting a hitherto unrecognized complexity of aldosterone action in the heart and perhaps by extension other tissues.

Relative molecular similarity in selected chemical carcinogens and the nucleoside triphosphate chain

Several markers of cell toxicity are useful as screening tests for epigenetic carcinogens. The direct effects of chemicals on ATPase and GTPase function are pertinent to the early stages of carcinogenesis. Interference with triphosphate-diphosphate exchange mechanisms may result from the interaction of carcinogens with the substrate triphosphate chain. To investigate this hypothesis, a computational chemistry programme is used in this study to investigate molecular similarity in ATPase inhibitors, carcinogens and tumour promoters, in relation to the nucleoside triphosphate chain. The results show that atoms in the investigated molecular structures superimpose on sets of oxygen atoms in the triphosphate chain with interatomic distances < 0.3A. Relative molecular similarity to the substrate triphosphate chain is discussed in terms of the established inhibitory properties of carcinogens/tumour promoters on ATPase function, the carcinogen/ tumour promoting properties of ATPase inhibitors and the prediction of carcinogenic activity from chemical structure.

Elevated renal cortical calmodulin-dependent protein kinase activity and blood pressure

The spontaneously hypertensive rat (SHR) exhibits not only hypertension but also behavioral hyperactivity which are not genetically linked. Two strains of rats, one hypertensive but normoactive (WKHT) and another, hyperactive but normotensive (WKHA), have been generated from SHR. We have reported that in renal proximal tubules, the linkage between D1-like receptors an adenylyl cyclase was impaired in SHR and WKHT but intact in WKHA. The impaired renal D1-like receptor function in the SHR was associated with increased phosphorylation of the D1 receptor, presumably caused by increased phosphorylation by G protein-coupled receptor kinases (GRK) or decreased dephosphorylation by protein phosphatase 2A. Because calmodulin kinase (CaMK) can regulate GRK activity, CaMK activity in renal cortical membranes of WKHA and WKHT were studied. We found that CaMK-dependent phosphorylation was two-fold higher in WKHA than in WKHT. In addition, serine phosphorylation of a 36 KDa and a 24 KDa protein was 5-fold and 3-fold greater in WKHA than in WKHT. We hypothesize that the increased CaMK activity in the renal cortical membrane may serve to inhibit GRK activity in WKHA and prevent the development of hypertension.

Replacement of (alpha)1-Na-K-ATPase of Dahl rats by Milan rats lowers blood pressure but does not affect its activity

Both linkage and use of congenic strains have shown that a chromosome region near the gene for the Na-K-ATPase alpha(1)-subunit (Atp1a1) contained a quantitative trait locus (QTL) for blood pressure (BP). Currently, two congenic strains, designated S.M5 and S.M6, were made by replacing a segment of the Dahl salt-sensitive SS/Jr (S) rat by the homologous region of the Milan normotensive rat (MNS). In S.M5, the gene for Atp1a1 is from the MNS strain; whereas in S.M6, Atp1a1 is from the S strain. The baseline activity of the alpha(1)-Na-K-ATPase and its stoichiometry were evaluated by an assay of ouabain-sensitive inwardly and outwardly directed (86)Rb and (22)Na fluxes in erythrocytes. The two congenic strains showed a similar BP, but both had a BP lower than that of S rats (P < 0.0001). Neither the alpha(1)-Na-K-ATPase activity nor its stoichiometry was affected by the substitution of the Atp1a1 alleles of S by those of MNS. Thus the BP-lowering effects observed in S.M5 and S.M6 could not be attributed to the alpha(1)-Na-K-ATPase activity or its stoichiometry. Atp1a1 is not supported as a candidate to be a BP QTL.

Parathyroid hormone stimulates translocation of protein kinase C isozymes in UMR-106 osteoblastic osteosarcoma cells

Studies with antagonists have provided evidence that protein kinase C (PKC) is involved in several of the actions of parathyroid hormone (PTH) on bone. PTH increases total PKC activity in bone and bone cells. The current studies investigated whether PTH can activate specific PKC isozymes, as demonstrated by translocation of these isozymes from cytosolic to membrane fractions. The isozymes selected for study, alpha, betaI, delta, epsilon, and zeta, were shown previously by us to be present in normal osteoblasts and several osteosarcoma-derived osteoblastic cells. UMR-106 cells, a widely used osteoblastic cell line, were selected for the current study. PKC isozymes in whole cell lysates and cell fractions were visualized by western blotting; isozyme distribution was also visualized by immunofluorescence. The total amounts of the isozymes and their relative distribution between membrane and cytosolic fractions in untreated cells were stable over a range of passages (5-20 from initial plating). In untreated cells, the concentrations of PKC alpha, betaI, and zeta were higher in the cytosol, and PKC delta and epsilon were higher in the membrane fraction. Treatment with 1 or 10 nmol/L PTH for 1 or 5 min stimulated translocation of PKC alpha and betaI, with variable effects on the other isozymes. Treatment with phorbol-12,13-dibutyrate (PDBu), 1 micromol/L for 5 min, elicited similar effects to those of PTH on PKC alpha and betaI. Treatment with PDBu for 48 h resulted in a downregulation of PKC alpha, whereas a 48 h treatment with PTH did not cause downregulation. The results indicate that PTH can affect specific PKC isozymes, providing a mechanism for differential regulation of cellular actions through this pathway.

Regulation of epithelial transport and barrier function by distinct protein kinase C isoforms

The phorbol ester phorbol 12-myristate 13-acetate (PMA) inhibits Cl(-) secretion (short-circuit current, I(sc)) and decreases barrier function (transepithelial resistance, TER) in T84 epithelia. To elucidate the role of specific protein kinase C (PKC) isoenzymes in this response, we compared PMA with two non-phorbol activators of PKC (bryostatin-1 and carbachol) and utilized three PKC inhibitors (Gö-6850, Gö-6976, and rottlerin) with different isozyme selectivity profiles. PMA sequentially inhibited cAMP-stimulated I(sc) and decreased TER, as measured by voltage-current clamp. By subcellular fractionation and Western blot, PMA (100 nM) induced sequential membrane translocation of the novel PKC epsilon followed by the conventional PKC alpha and activated both isozymes by in vitro kinase assay. PKC delta was activated by PMA but did not translocate. By immunofluorescence, PKC epsilon redistributed to the basolateral domain in response to PMA, whereas PKC alpha moved apically. Inhibition of I(sc) by PMA was prevented by the conventional and novel PKC inhibitor Gö-6850 (5 microM) but not the conventional isoform inhibitor Gö-6976 (5 microM) or the PKC delta inhibitor rottlerin (10 microM), implicating PKC epsilon in inhibition of Cl(-) secretion. In contrast, both Gö-6976 and Gö-6850 prevented the decline of TER, suggesting involvement of PKC alpha. Bryostatin-1 (100 nM) translocated PKC epsilon and PKC alpha and inhibited cAMP-elicited I(sc). However, unlike PMA, bryostatin-1 downregulated PKC alpha protein, and the decrease in TER was only transient. Carbachol (100 microM) translocated only PKC epsilon and inhibited I(sc) with no effect on TER. Gö-6850 but not Gö-6976 or rottlerin blocked bryostatin-1 and carbachol inhibition of I(sc). We conclude that basolateral translocation of PKC epsilon inhibits Cl(-) secretion, while apical translocation of PKC alpha decreases TER. These data suggest that epithelial transport and barrier function can be modulated by distinct PKC isoforms.

Down-regulation of Na-K-Cl cotransport by protein kinase C is mediated by protein phosphatase 1 in pigmented ciliary epithelial cells

The role of protein phosphatases in the regulation of Na-K-Cl cotransport was examined in human pigmented ciliary epithelial (PE) cells. Both a 37 kDa form and a 72 kDa form of protein phosphatase 1 (PP1) could be immunologically detected. The protein phosphatase inhibitor calyculin A stimulated Na-K-Cl cotransport by 89 +/- 12% at 10 n M, whereas okadaic acid had no effect at concentrations less than 100 n M. Calyculin A had no significant effect on either Na-K ATPase or ouabain-insensitive, bumetanide-insensitive 86Rb+uptake. These data suggest that PP1 plays a role in the inhibition of Na-K-Cl cotransport in PE cells. Treatment of cells with phorbol 12-myristate, 13-acetate (PMA), a protein kinase C (PKC) activator caused an 82% inhibition of Na-K-Cl cotransport. When cells were first treated for 5 min with PMA, 10 n M calyculin A stimulated Na-K-Cl cotransport by 53% compared to 101% by calyculin A alone. Treatment of cells with PMA after stimulation of Na-K-Cl cotransport by calyculin A resulted in a prompt 56% drop in cotransport activity. These data suggest that maximal inhibition of Na-K-Cl cotransport by PKC requires PP1 activity, but that a part of PKCs inhibitory effect is independent of PP1. The effect of PKC activation on PP1 was further examined by determining PP1 activity in cells pretreated with PMA. PP1 activity increased 38+/-8% in cells exposed to 1 microM PMA for 5 min. This stimulation was blocked by 100 n M staurosporine or 1 microM bisindolylmaleimide, two PKC inhibitors. An isomer which does not activate PKC (4 alpha phorbol didecanoate), did not stimulate PP1 activity. Thus PKC activation leads to an increase in PP1 activity in PE cells. Pretreatment of cells with the protein kinase A (PKA) inhibitor PHI 14-22 resulted in a partial reduction in calyculin A stimulation of cotransport, suggesting that PP1 and PKA function in a kinase-phosphatase regulatory loop. To determine whether other protein kinases might also be involved, several protein kinase inhibitors were tested, including KT5823 (protein kinase G, type II-specific), KN62 (calmodulin activated kinase-specific) and ML7 (myosin light chain kinase-specific). None prevented activation of Na-K-Cl cotransport by calyculin A, suggesting that these kinases are not involved in the activation of Na-K-Cl cotransport.

Identification of a sea urchin Na(+)/K(+)/2Cl(−) cotransporter (NKCC): microfilament-dependent surface expression is mediated by hypotonic shock and cyclic AMP

We report the identification of an invertebrate Na(+)/K(+)/2Cl(−) cotransporter, NKCC. As a model system, we used the immune cells (coelomocytes) of the Mediterranean sea urchin Paracentrotus lividus. These cells are particularly interesting because they can be activated to undergo a rapid and dynamic change in cell shape. We demonstrate that forskolin, a cyclic AMP agonist known to regulate NKCC, induced coelomocyte transformation at doses of 10 micromol l(−)(1) and greater. Using two distinct monoclonal antibodies (T4 and T9) raised against the human intestinal epithelial NKCC, we have identified a high-molecular-mass (195 kDa) protein in coelomocyte extracts. We propose a novel method for the isolation of NKCC in one step by using bumetanide-Sepharose affinity chromatography under low-[Cl(−)] conditions. This method was successful in isolating coelomocyte 195 kDa NKCC. The T4 monoclonal antibody was used in immunocytochemical experiments to localize NKCC in resting and activated coelomocytes. In petalloid coelomocytes, a punctate, cytoplasmic distribution was observed in close proximity to actin filament bundles; in transformed coelomocytes, the immunofluorescence was distributed along the length of the filopodia and uniformly throughout the perinuclear region. The change in subcellular distribution of NKCC between the resting and the activated state was further investigated by using cell surface biotinylation followed by immunoprecipitation. These studies revealed an upregulation of NKCC at the plasma membrane upon activation, a process that was blocked by the F-actin-stabilizing drug phalloidin. These studies identify a novel model system in which to investigate a newly identified invertebrate Na(+)/K(+)/2Cl(−) cotransporter.

Renal dopamine and sodium homeostasis

During the past decade, it has become evident that dopamine plays an important role in the regulation of fluid and electrolyte balance and blood pressure. Dopamine exerts its actions through two families of dopamine receptors, designated D1-like and D2-like, which are identical in the brain and in peripheral tissues. The two D1-like receptors--D1 and D5 receptors--expressed in mammals are linked to stimulation of adenylyl cyclase. The three D2-like receptors--D2, D3, and D4,--are linked to inhibition of adenylyl cyclase. Dopamine affects fluid and electrolyte balance by regulation of renal excretion of electrolytes and water through actions on renal hemodynamics and tubular epithelial transport and by modulation of the secretion and/or action of vasopressin, renin, aldosterone, catecholamines, and endothelin B receptors (ETB) receptors. It also affects fluid and sodium intake by way of "appetite" centers in the brain and alterations of gastrointestinal tract transport. The production of dopamine in neural and non-neural tissues and the presence of receptors in these tissues suggest that dopamine can act in an autocrine or paracrine fashion. This renal autocrine-paracrine function, which becomes most evident during extracellular fluid volume expansion, is lost in essential hypertension and in some animal models of genetic hypertension. This deficit may be caused by abnormalities in renal dopamine production and polymorphisms or abnormal post-translational modification and regulation of dopamine receptor subtypes.

Heat stress preconditioning does not protect renal epithelial Na(+),K(+),Cl(-) and Na(+),P(i) cotransporters from their modulation by severe heat stress

This study compares the effects of heat and osmotic stress on heat stress protein (HSP) production while examining the putative protective action of HSPs on modulation of Na(+),K(+),Cl(-) and Na(+),P(i) cotransporters in Madin-Darby canine kidney (MDCK) epithelial cells by severe heat stress (46 degrees C, 15 min). Preconditioning heat stress (43 degrees C, 20 min) followed by 4 h recovery at 37 degrees C led to a 35-fold increase of HSP70 mRNA expression measured by Northern blot analysis. The protein content of HSP70 and HSP27, assessed by Western blots, was augmented by 5- and 2-fold, respectively, after 6 h of recovery. In contrast to preconditioning heat stress, hyperosmotic stress (520 vs. 320 mosm) elevated HSP70 mRNA content only by 7-fold and did not significantly affect the protein content of HSP70 or HSP27. Neither cell survival, assessed as lactate dehydrogenase (LDH) release, nor the basal activities of the ion transporters and their modulation by protein kinase C, P(2)-purinoceptor and cell volume were altered by preconditioning heat stress. Severe heat stress increased extracellular LDH content from 3+/-2 to 23+/-5% and enhanced Na(+),K(+),Cl(-) and Na(+),P(i) cotransport activity by 2-3-fold. The volume- and protein kinase C-dependent regulation of these carriers was abolished by severe heat stress while regulation by P(2)-purinoceptors was preserved. Preconditioning heat stress diminished severe heat stress-induced LDH release to 11+/-4% but did not protect Na(+),K(+),Cl(-) and Na(+),P(i) cotransporters from activation by severe heat stress and did not prevent severe heat stress-induced inactivation of protein kinase C- and volume-dependent signaling pathways. These results show that in MDCK cells, preconditioning heat stress-induced HSPs are not involved in the regulation of Na(+),K(+),Cl(-) and Na(+),P(i) cotransporters and do not protect them from modulation by severe heat stress.