Shrinkage-induced activation of Na+/H+ exchange in rat renal mesangial cells

Role of Cl- -HCO3- exchanger AE3 in intracellular pH homeostasis in cultured murine hippocampal neurons, and in crosstalk to adjacent astrocytes

KEY POINTS

A polymorphism of human AE3 is associated with idiopathic generalized epilepsy. Knockout of AE3 in mice lowers the threshold for triggering epileptic seizures. The explanations for these effects are elusive. Comparisons of cells from wild-type vs. AE3^-/- mice show that AE3 (present in hippocampal neurons, not astrocytes; mediates HCO₃^- efflux) enhances intracellular pH (pH_i ) recovery (decrease) from alkali loads in neurons and, surprisingly, adjacent astrocytes. During metabolic acidosis (MAc), AE3 speeds initial acidification, but limits the extent of pH_i decrease in neurons and astrocytes. AE3 speeds re-alkalization after removal of MAc in neurons and astrocytes, and speeds neuronal pH_i recovery from an ammonium prepulse-induced acid load. We propose that neuronal AE3 indirectly increases acid extrusion in (a) neurons via Cl^- loading, and (b) astrocytes by somehow enhancing NBCe1 (major acid extruder). The latter would enhance depolarization-induced alkalinization of astrocytes, and extracellular acidification, and thereby reduce susceptibility to epileptic seizures.

ABSTRACT

The anion exchanger AE3, expressed in hippocampal (HC) neurons but not astrocytes, contributes to intracellular pH (pH_i ) regulation by facilitating the exchange of extracellular Cl^- for intracellular HCO₃^- . The human AE3 polymorphism A867D is associated with idiopathic generalized epilepsy. Moreover, AE3 knockout (AE3^-/- ) mice are more susceptible to epileptic seizure. The mechanism of these effects has been unclear because the starting pH_i in AE3^-/- and wild-type neurons is indistinguishable. The purpose of the present study was to use AE3^-/- mice to investigate the role of AE3 in pH_i homeostasis in HC neurons, co-cultured with astrocytes. We find that the presence of AE3 increases the acidification rate constant during pH_i recovery from intracellular alkaline loads imposed by reducing [CO₂ ]. The presence of AE3 also speeds intracellular acidification during the early phase of metabolic acidosis (MAc), not just in neurons but, surprisingly, in adjacent astrocytes. Additionally, AE3 contributes to braking the decrease in pH_i later during MAc in both neurons and astrocytes. Paradoxically, AE3 enhances intracellular re-alkalization after MAc removal in neurons and astrocytes, and pH_i recovery from an ammonium prepulse-induced acid load in neurons. The effects of AE3 knockout on astrocytic pH_i homeostasis in MAc-related assays require the presence of neurons, and are consistent with the hypothesis that the AE3 knockout reduces functional expression of astrocytic NBCe1. These findings suggest a new type of neuron-astrocyte communication, based on the expression of AE3 in neurons, which could explain how AE3 reduces seizure susceptibility.

Characterization of novel cannabinoid based T-type calcium channel blockers with analgesic effects

Low-voltage-activated (T-type) calcium channels are important regulators of the transmission of nociceptive information in the primary afferent pathway and finding ligands that modulate these channels is a key focus of the drug discovery field. Recently, we characterized a set of novel compounds with mixed cannabinoid receptor/T-type channel blocking activity and examined their analgesic effects in animal models of pain. Here, we have built on these previous findings and synthesized a new series of small organic compounds. We then screened them using whole-cell voltage clamp techniques to identify the most potent T-type calcium channel inhibitors. The two most potent blockers (compounds 9 and 10) were then characterized using radioligand binding assays to determine their affinity for CB1 and CB2 receptors. The structure–activity relationship and optimization studies have led to the discovery of a new T-type calcium channel blocker, compound 9. Compound 9 was efficacious in mediating analgesia in mouse models of acute inflammatory pain and in reducing tactile allodynia in the partial nerve ligation model. This compound was shown to be ineffective in Cav3.2 T-type calcium channel null mice at therapeutically relevant concentrations, and it caused no significant motor deficits in open field tests. Taken together, our data reveal a novel class of compounds whose physiological and therapeutic actions are mediated through block of Cav3.2 calcium channels.

Aldosterone and the cardiovascular system: a dangerous association

Initial studies have focussed on the actions of aldosterone in renal electrolyte handling and, as a consequence, blood pressure control. More recently, attention has primarily been focussed on its actions on the heart and vascular system, where it is locally produced. Aldosterone by binding mineralocorticoid receptors causes oxidative stress, fibrosis and triggers an inflammatory response in the cardiovascular system. All these effects could be underlying the role of aldo-sterone on cardiac and vascular remodelling associated with different pathological situations. At the vascular level, aldo-sterone affects endothelial function because administration of aldosterone to rats impaired endothelium-dependent relaxations. In addition, the administration of mineralocorticoid receptor antagonists ameliorates endothelium-dependent relaxation in models of both hypertension and atherosclerosis, and in patients with heart failure. Several mechanisms can participate in this effect, including production of vasoconstrictor factors and a reduction in nitric oxide levels. This reduction can involve both a decrease in its production as well as an increase in its degradation by reactive oxygen species. Aldosterone can produce oxidative stress by the activation of transcription factors such as the NF-κB system, which can also trigger an inflammatory process through the production of different cytokines. At cardiac level, high levels of aldosterone can also adversely impact heart function by producing cardiac hypertrophy, diastolic dysfunction and electrical remodelling through changes in ionic channels. All these effects can explain the beneficial effect of mineralocorticoid blockade in the cardiovascular system.

Effects of acute hypoxia on intracellular-pH regulation in astrocytes cultured from rat hippocampus

We used the pH-sensitive dye BCECF to evaluate the effect of acute (5-10 min) hypoxia (approximately 3% O(2)) on the regulation of intracellular pH (pH(i)) in astrocyte populations cultured from rat hippocampus. For cells in the nominal absence of CO(2)/HCO(3)(-) at an extracellular pH of 7.40 (37 degrees C), acute hypoxia caused a small (0.05) decrease in steady-state pH(i), but increased the pH(i) recovery rate from an acid load during all but the late phase of the pH(i) recovery. During such pH(i) recoveries, the total acid extrusion rate (phi(E), the product of dpH(i)/dt and proton buffering power) decreased with increasing pH(i). Hypoxia alkali shifted the plot of phi(E) vs. pH(i); over the upper approximately 85% of the phi(E) range, this shift was 0.15-0.30. Hypoxia also stimulated the pH(i) recovery rate from an alkali load. Under normoxic conditions, switching the extracellular buffer to 5% CO(2)/22 mM HCO(-)(3) also alkali shifted the phi(E)-pH(i) plot (upper approximately 85%) by 0.4-0.5. Superimposing hypoxia on CO(2)/HCO(-)(3) further alkali shifted the phi(E)-pH(i) plot (upper approximately 85% of the phi(E) range) by 0.05-0.15. The SITS-insensitive component of phi(E) was alkali shifted by 0.20-0.30, whereas the SITS-sensitive component of phi(E) was depressed in the low pH(i) range. Thus, in the nominal absence of CO(2)/HCO(3)(-), acute hypoxia has little effect on steady-state pH(i) but stimulates acid extrusion and acid loading, whereas in the presence of CO(2)/HCO(-)(3), hypoxia stimulates the SITS-insensitive but inhibits the SITS-sensitive acid extrusion.

Signalling pathways involved in hypertonicity- and acidification-induced activation of Na+/H+ exchange in trout hepatocytes

In trout hepatocytes, hypertonicity and cytosolic acidification are known to stimulate Na+/H+ exchanger (NHE) activity, which contributes to recovery of cell volume and intracellular pH (pHi),respectively. The present study investigated the signalling mechanisms underlying NHE activation under these conditions. Exposing trout hepatocytes to cariporide, a specific inhibitor of NHE-1, decreased baseline pHi,completely blocked the hypertonicity-induced increase of pHi and reduced the hypertonicity-induced proton secretion by 80%. Changing extracellular pH (pHe)above and below normal values, and allowing cells to adjust pHi accordingly,significantly delayed alkalinization during hypertonic exposure, whereas following an acid load an enhanced pHi recovery with increasing pHe was seen. Chelating Ca2+, and thereby preventing the hypertonicity-induced increase in intracellular Ca2+ ([Ca2+]i), significantly diminished hypertonic elevation of pHi, indicating that Ca2+signalling might be involved in NHE activation. A reduction in alkalinization and proton secretion was also observed in the presence of the protein kinase A(PKA) inhibitor H-89 or the calmodulin (CaM) inhibitor calmidazolium. A complete inhibition of hypertonic- and acidification-induced changes of pHi concurrent with an increase in hypertonically induced proton efflux was seen with the protein kinase C (PKC) inhibitor chelerythrine. Recovery of pHi following sodium propionate addition was reduced by more than 60% in the presence of cariporide, was sensitive to PKA inhibition, and tended to be reduced by CaM inhibition. In conclusion, we showed that NHE-1 is the main acid secretion mechanism during hypertonicity and recovery following acid loading. In addition, Ca2+-, PKA- and CaM-dependent pathways are involved in NHE-1 activation for recovery of cell volume and pHi. On the other hand, PKC appeared to have an impact on NHE-independent pathways affecting intracellular acid-base homeostasis.

The hydroosmotic response of frog urinary bladder to serosal hypertonicity is dependent on adenylate cyclase for its maintenance and affected by [Cl−]ochanges

The role of adenylate cyclase (AC) in the maintenance of the hydroosmotic response to serosal hypertonicity (SH) in anuran urinary bladder is disputed. In this study, norepinephrine (NE) significantly reversed the hydroosmotic response of Rana temporaria bladders in hypertonic medium (330 mosmol/kgH2O). The reversal was inhibited by yohimbine but was unaffected by prazosin and propranolol, indicating that NE action was mediated via α2-adrenergic receptors. Preincubation of bladders with indomethacin did not interfere with the inhibitory action of NE, contraindicating a role for prostaglandins. The SH hydroosmotic response was abolished in the presence of 5- n-ethyl- N-isopropyl amiloride (EIPA), but the antidiuretic hormone (ADH) hydroosmotic response was not. EIPA inhibits Na+/H+, known to be activated by cell shrinkage. An investigation of the anionic requirement of the SH hydroosmotic response revealed that replacement of bath Cl−with the nonpermeable anion gluconate reversibly abolished this response. In contrast, the hydroosmotic response to ADH was unaffected by Cl−removal; however, when Cl−was absent, it was no longer augmented in hypertonic bath. The SH response was inhibited by the Cl−channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate but not by the Na/K/2Cl inhibitor bumetanide. Our results show that not only the onset but also the maintenance of the SH hydroosmotic response is dependent on AC activity and does not differ in this respect to the ADH hydroosmotic response. The effect of modifying extracellular Cl−concentration, suggests that this anion, possibly functionally linked with Na+/H+activity, may be involved in invoking the SH hydroosmotic response in anuran urinary bladder.

Hyperosmolarity Causes BKCa-Dependent Vasodilatations in Rat Skeletal Muscle Arteries

PURPOSE

The release of different metabolites during skeletal muscle contraction causes a pronounced increase in extracellular osmolarity (hyperosmolarity (HO)). HO has been considered as a possible mediator of the exercise hyperemia. In the present study, we investigated the vasodilatory effect of physiologically relevant increases in the extracellular osmolarity in isolated rat gluteal muscle arterioles. In addition, we analyzed the underlying mechanisms of the HO-induced vasodilatations.

METHODS

Rat gluteal arteries were isolated and mounted in an organ bath for isometric tension recording. After precontraction with norepinephrine, 20, 40, or 60 mmol x L(-1) sucrose, mannitol, or urea was added in control conditions, after removal of the endothelium or in the presence of inhibitors.

RESULTS

Application of sucrose or mannitol induced large and fast concentration-dependent vasodilatations (up to 46.15% with 60 mmol x L(-1) sucrose). Removal of the vascular endothelium had no effect on this relaxation. Inhibition of the Na+/K+ pumps with ouabain, the Kir IR channels with Ba2+ and the K ATP channels with glibenclamide did not alter the HO-induced relaxations. Incubation with the KCa channel blockers charybdotoxin and apamin significantly inhibited sucrose-induced vasodilatations. In addition, application of the specific BK Ca channel blocker iberiotoxin significantly decreased the HO-induced vasodilatations.

CONCLUSION

The present study shows that an increase in the extracellular osmolarity elicits strong, fast, and long-lasting relaxations of rat skeletal muscle arterioles, suggesting an important role both at the onset and during the steady-state phase of an exercise bout. Vascular smooth muscle BK Ca channels seem to play a crucial role in the HO-induced vasorelaxations.

Antiarrhythmic drug target choices and screening

Most antiarrhythmic drugs are ion channel blockers, and to date, those tested in large randomized placebo-controlled clinical trials have shown no decrease in mortality outcome. This apparent lack of survival benefit may result from the significant liabilities associated with these agents that offset any long-term benefit. Despite the current success of implantable defibrillators and the future promise of gene therapy, there is still a pressing need for new antiarrhythmic drugs. An improved understanding of cardiac ion channels and novel approaches to target selection and compound screening will provide new opportunities for drug discovery in the near future. Here, we briefly review the multiple mechanisms of arrhythmia, the history of drug failures, and the possibilities that evolving technologies may provide in the search for more efficacious and safer antiarrhythmic drugs.

Regulatory volume increase is associated with p38 kinase-dependent actin cytoskeleton remodeling in rat kidney MTAL

The kidney medulla is physiologically exposed to variations in extracellular osmolality. In response to hypertonic cell shrinkage, cells of the rat kidney medullary thick ascending limb of Henle's loop undergo p38 kinase-dependent regulatory volume increase (RVI). In the present study, we investigated the role of actin cytoskeleton reorganization in this process. Addition of hyperosmotic NaCl or sucrose, which activates MAP kinases and reduces cellular volume, induced a sustained actin polymerization occurring after 10 min and concurrently with RVI. In contrast, hyperosmotic urea, which does not modify MAP kinase activity and cellular volume, did not induce sustained actin polymerization. Fluorescence microscopy revealed that hyperosmotic NaCl and sucrose, but not urea, induced the redistribution of F-actin from a dense cortical ring to a diffuse network of actin bundles. Stabilization of actin filaments by jasplakinolide and inhibition of the generation of new actin filaments by swinholide A prevented RVI, whereas depolymerization of actin filaments by latrunculin B attenuated cell shrinkage and enhanced RVI. These actin-interfering drugs did not alter extracellular regulated kinase and p38 kinase activation under hypertonic conditions. Similar to swinholide A, inhibiting p38 kinase with SB-203580 abolished sustained actin polymerization, actin redistribution, and decreased RVI efficacy. We therefore propose that in rat kidney the medullary thick ascending limb of Henle's loop exposed to extracellular hypertonicity, p38 kinase activation induces depolymerization of the F-actin cortical ring and polymerization of a dense diffuse F-actin network that both contribute to increase RVI efficacy.

Metabolic and ionic responses of trout hepatocytes to anisosmotic exposure

Trout hepatocytes exposed to hypo- or hyperosmotic conditions respond by swelling and shrinking, respectively, followed by regulatory volume changes that almost, although not completely, restore cell volume. These anisosmotic conditions have a significant impact on metabolic functions. In hyposmotic medium, oxygen consumption (.VO2) and glucose production rates were significantly reduced, whereas lactate accumulation was not significantly affected. By contrast, hyperosmotic conditions did not affect .VO2 and lactate production but caused a sustained reduction in glucose production. Volume changes were also accompanied by alterations in intracellular free calcium ([Ca2+](i)). At the cell population level, hyposmotic exposure evoked a moderate and slowly developing increase in [Ca2+](i), whereas hyperosmolarity caused a pronounced and sustained increase, which peaked at the time of maximum cell shrinkage but clearly exceeded a mere concentration effect due to volume reduction. Responses of individual cells were highly variable in hyposmotic medium, with only 60% showing a clear increase in [Ca2+](i), while in hyperosmotic conditions all cells displayed elevated [Ca2+](i) levels. A decrease in intracellular pH (pHi) observed in hyposmotic medium was insensitive to EIPA, an inhibitor of Na(+)/H(+) exchange, and SITS, an inhibitor of Cl(-)/HCO(3)(-) exchange, but was prevented in Cl(-)-free medium. In hyperosmotic medium, pHi increased. This alkalinization did not occur under conditions of blocked Na(+)/H(+) exchange and was significantly diminished upon inhibition of Cl(-)/HCO(3)(-) exchange, suggesting an important role of these ion transporters in regulatory volume increase of trout hepatocytes.

Intracellular pH changes in isolated bovine articular chondrocytes during the loading and removal of cryoprotective agents

The addition and removal of a cryoprotective agent (CPA) are necessary steps in the cryopreservation of natural or engineered tissue products. However, the introduction and removal of CPAs induces dramatic chemical changes inside tissues and cells and these could cause irreversible damage. This study examined the effect of CPA loading and removal on the intracellular pH of isolated bovine articular chondrocytes using a fluorimetric technique. Chondrocytes that had been isolated from bovine articular cartilage were loaded with the pH-sensitive fluorophore 2('),7(')-bis(carboxyethyl)-5(6)-carboxyfluorescein. After removal of the extracellular fluorophore, the intensity of fluorescence was used to measure the intracellular pH according to a pre-determined calibration curve. Changes of intracellular pH in chondrocytes were measured following their exposure to dimethyl sulfoxide (Me(2)SO) and glycerol at concentrations of 0.6, 0.9, and 1.2M and later to the isotonic or hypertonic solutions that were used to remove the CPA. The effect of the presence of NaCl on the intracellular pH during CPA removal was also examined. The temperature was maintained at 37 degrees C. Trypan blue exclusion was used to quantify cell membrane integrity after the addition and removal of CPA. It was found that when the cells were exposed to CPA, the intracellular pH decreased quickly and recovered gradually later. During CPA removal, the intracellular pH rose following exposure to isotonic Hepes-buffered medium, but the opposite was observed if the Hepes buffer solution contained no NaCl; this was ascribed to the role of NaCl in cell membrane transport. It was noted that the change in intracellular pH correlated with the cell volume excursion, which could be estimated by the Kedem-Katchalsky model, and was linked to cell survival. The resulting alteration of pH inside the cells might contribute to cell damage and loss of function after cryopreservation.

Effect of T-type selective calcium antagonist on renal microcirculation: studies in the isolated perfused hydronephrotic kidney

Although calcium antagonists exert preferential vasodilation of renal afferent arterioles, we have recently demonstrated that nilvadipine and efonidipine, possessing both L-type and T-type calcium channel blocking action, reverse the angiotensin (Ang) II-induced afferent and efferent arteriolar constriction. In the present study, we investigated the role of T-type calcium channels in mediating the Ang II-induced efferent arteriolar tone using the selective T-type calcium channel blocker mibefradil. Isolated perfused hydronephrotic rat kidneys were used for direct visualization of renal microcirculation. Administration of Ang II (0.3 nmol/L) caused marked constriction of afferent (from 13.5+/-0.6 to 9.2+/-0.6 microm, P<0.01, n=6) and efferent (from 11.5+/-1.0 to 7.4+/-0.7 microm, P<0.01, n=5) arterioles. Mibefradil (1 micromol/L) dilated both vessels, with 82+/-11% and 72+/-7% reversal of afferent and efferent arterioles, respectively. Similarly, nickel chloride (100 micromol/L) caused dilation of both arterioles, similar in magnitude in afferent (68+/-10%, n=7) and efferent (80+/-7%, n=7) arterioles. To eliminate the possibility that the mibefradil-induced dilation was mediated by L-type channel blockade, mibefradil was administered in the presence of nifedipine (1 micromol/L). Thus, nifedipine caused modest efferent arteriolar dilation (30+/-6% reversal, n=9), and subsequent addition of mibefradil elicited further dilation of this vessel (80+/-4%, P<0.01 versus nifedipine). Furthermore, mibefradil reversed the Ang II-induced efferent arteriolar constriction even in the presence of nifedipine and phentolamine. These findings demonstrate that T-type calcium antagonists markedly dilate the Ang II-induced efferent arteriolar constriction, but the action is not mediated by inhibition of catecholamine release. This potent activity would contribute to the efferent arteriolar response to nilvadipine and efonidipine and may offer benefit in light of glomerular hemodynamics.

Role of dietary potassium in the hyperaldosteronism and hypertension of the remnant kidney model

The remnant kidney model of progressive renal disease is marked by arterial hypertension, especially when produced by nephrectomy and partial infarction. Hyperaldosteronism sustains much of the hypertension, but the stimuli to the increased aldosterone levels are uncertain. It is hypothesized that the hyperaldosteronism attending this model stems from the combination of fixed dietary potassium load in the face of reduced filtration on the one hand, and persistent renin secretion from the scarred remnant kidney on the other. This hypothesis predicted that dietary potassium restriction would lower aldosterone and BP in this model. To test this prediction, two groups of rats with a remnant kidney were studied. Group 1 consumed 0.4 +/- 0.06 mEq (mean +/- SD) of potassium chloride daily, and group 2 ate 4.8 +/- 1.0 mEq daily. Two sham-operated groups with intact kidneys also were studied. Group 3 consumed 1.7 +/- 0.2 mEq daily and group 4 ate 15.2 +/- 1.4 mEq daily. These levels of intake were designed to provide at least as much potassium per liter of GFR in the sham groups as in the remnant kidney rats. Systolic BP (SBP), 24-h protein excretion, plasma aldosterone levels, 24-h urinary aldosterone excretion, and plasma renin activity (PRA) were determined in all groups at 2 wk. At 4 wk, after SBP and protein excretion measurements, remnant kidneys were perfusion-fixed for morphometric analysis. SBP was normal in both sham-operated groups and was not different between the groups (113 +/- 13 versus 117 +/- 2 mmHg, group 3 versus group 4). In the remnant animals, SBP at 2 wk followed potassium intake: Group 1 had a lower SBP than group 2 (140 +/- 26 versus 170 +/- 34 mmHg, P = 0.005). The same SBP pattern persisted at 4 wk (153 +/- 25 versus 197 +/- 27 mmHg, group 1 versus group 2, P = 0.0006). However, 24-h urinary protein excretion was not different between the two groups with remnant kidneys at either 2 or 4 wk. Both plasma and 24-h urinary aldosterone excretion at 2 wk followed potassium intake (120 +/- 124 versus 580 +/- 442 pg/ml for plasma aldosterone, group 1 versus group 2, P = 0.03, and 2.6 +/- 1.8 versus 23.2 +/-9.8 ng/d for urinary aldosterone, group 1 versus group 2, P = 0.0001). PRA, however, followed a reverse pattern in which dietary potassium restriction resulted in higher levels (16 +/- 6 versus 6 +/- 3 ng angiotensin I/ml per h, group 1 versus group 2, P = 0.01). A similar pattern for PRA and aldosterone excretion was also observed in the sham groups, in which lower potassium intake also resulted in a significantly higher PRA and lower aldosterone excretion. The constancy of BP in the sham groups likely reflects their lack of nephron reduction and greater sodium excretory capacity. Morphometric analysis in remnant animals revealed no significant difference between the two dietary groups in the prevalence of glomerular sclerosis, glomerular volume, or interstitial volume. It is concluded that dietary potassium is a potent determinant of hypertension in the remnant kidney model probably through the actions of aldosterone and that the high aldosterone secretion in this model is a function of the dietary potassium load. In this model, reduction in nephron number is also critical in promoting hypertension in conjunction with hyperaldosteronism.