Osmoregulation in the renal papilla: membranes, messengers and molecules

Accelerated NaCl-induced hypertension in taurine-deficient rat: Role of renal function

Taurine modulates blood pressure and renal function. As the kidney plays a pivotal role in long-term control of arterial pressure, we tested the hypothesis that taurine-deficient rats display maladaptive renal and blood pressure responses to uninephrectomy. Control and taurine-deficient (i.e., beta-alanine-treated) rats with either one or two remaining kidneys were fed diets containing basal or high (8%) NaCl diet. Urine osmolality was greater in the taurine-deficient than controls fed a normal NaCl diet; proteinuria and blood pressure were unaffected by uninephrectomy. Following 6 weeks on an 8% NaCl diet, the uninephrectomized (UNX) animals developed significant hypertension, which was more severe in the taurine-deficient group; baroreflex function was unaffected. However, the UNX taurine-deficient rats displayed impaired ability to dispose of an acute isotonic saline volume load before a switchover to a high NaCl diet. Nonetheless, a more protracted exposure (i.e., 14 weeks) to dietary NaCl excess eliminated the blood pressure differential between the two groups; at this stage, renal excretory responses to an acute saline volume load or to atrial natriuretic peptide were similar in the two groups. Nonetheless, hypertensive taurine-deficient rats displayed greater proteinuria, although both groups excreted proteins of similar molecular weights ( approximately 15-66 kDa). Further, taurine-deficient kidney specimens displayed periarterial mononuclear cell infiltrates with strong immunoreactivity to the histiocyte marker CD68, suggestive of increased phagocytic activity. In conclusion, taurine deficiency modulates renal adaptation to combined uninephrectomy and dietary NaCl excess, resulting in an accelerated development of hypertension.

Effects of dietary salt and fat on taurine excretion in healthy and diseased rats

Taurine modulates renal and cardiovascular function. Although the kidney regulates body taurine status, the impact of renal and cardiovascular risk factors, such as dietary intake of excess NaCl and saturated fat, on renal handling of taurine is less clear. One would predict that the kidney would modulate taurine excretion during dietary NaCl excess to insure adequate osmotic homeostasis. Similarly, fat feeding would be expected to affect taurine homeostasis, as taurine is involved in bile acid conjugation and therefore fat emulsification. To examine these aspects, male rats were divided into four groups: basal fat diet (control), high fat diet (FAT), basal fat and high salt diet (SALT) and a combination of a high fat and salt diet (FATSALT). While the control, FAT and SALT groups excreted similar amounts of taurine; the SALTFAT group excreted significantly more taurine than the other 3 groups. Although all of the dietary regimens increased renal tissue content of taurine, the increases were greatest in the two SALT groups. In a subsequent study, we examined the effect of excess dietary fat on taurine handling by the hypertensive (H) and hypertensive-glucose intolerant (HGI) rat. When fed a basal fat diet, the HGI group excreted more taurine than the H group, an effect likely related to increased endogenous taurine biosynthesis, alterations in renal function or a combination of the two effects. While excess fat intake increased urinary taurine excretion in the H group, it reduced taurine excretion in the HGI group. Nonetheless, kidney taurine content was similar in the 4 groups. Taken together, the data suggest that dietary constituents and preexisting systemic disorders are important modulators of renal handling of taurine.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Potentiation of the osmosensitive taurine release and cell volume regulation by cytosolic Ca2+rise in cultured cerebellar astrocytes

Hyposmolarity (-30%) in cultured cerebellar astrocytes raised cytosolic Ca2+ concentration ([Ca2+]i) from 160 to 400 nM and activated the osmosensitive taurine release (OTR) pathway. Although OTR is essentially [Ca2+]i-independent, further increase in [Ca2+]i by ionomycin strongly enhanced OTR, with a more robust effect at low and mild osmolarity reductions. Ionomycin did not affect isosmotic taurine efflux. OTR was decreased by tyrphostin A25 and increased by ortho-vanadate, suggesting a modulation by tyrosine kinase or phosphorylation state. Inhibition of phosphatidylinositol-3-kinase activity by wortmannin markedly decreased OTR and the ionomycin increase. Conversely, OTR and the ionomycin effect were independent of ERK1/ERK2 activation. OTR and its potentiation by ionomycin differed in their sensitivity to CaM and CaMK blockers and in the requirement of an intact cytoskeleton for the ionomycin effect, but not for normal OTR. Changes in the actin cytoskeleton organization elicited by hyposmolarity were not observed in ionomycin-treated cells, which may permit the operation of CaM/CaMK pathways involved in the OTR potentiation by [Ca2+]i rise. OTR potentiation by [Ca2+]i requires the previous or simultaneous activation/operation of the taurine release mechanism and is not modifying its set point, but rather increasing the effectiveness of the pathway, resulting in a more efficient volume regulation. This may have a beneficial effect in pathological situations with concurrent swelling and [Ca2+]i elevation in astrocytes.

Urea inhibition of renal (NA+ + K+)ATPase activity is reversed by cAMP

In the present work we studied the modulation of the effect of urea on the renal (Na+ + K+)ATPase by cAMP. We observed that urea inhibits the (NA+ + K+)ATPase activity in a dose-dependent manner, reaching 60% of inhibition at the concentration of 1M. This effect was completely reversed by dibutyryl-cAMP (dBcAMP) at 5 x 10(-4)M. The effect of dBcAMP was mimicked by 50 units of the catalytic subunit of protein kinase A and completely abolished by 5 x 10(-7)M H89, an inhibitor of protein kinase A. Addition of 1M urea decreases basal phosphorylation of the immunoprecipitated (NA+ + K+)ATPase in 50%, with this effect completely reversed by 5 x 10(-4)M dBcAMP. Furthermore, 5 x 10(-4)M dBcAMP by itself induced (NA+ + K+)ATPase phosphorylation. Taken together these data indicate that cAMP could be, in addition to the organic solutes already known, an important physiological modulator of the deleterious effect of urea on enzyme activity.

Organic osmolytes betaine, sorbitol and inositol are potent inhibitors of erythrocyte membrane ATPases

Organic osmolytes are used in animal and plant cells to adapt to hyper- and hypoosmolar stress. We used our RBC-membrane model to investigate the effects of the osmolytes betaine, sorbitol and myo-inositol on Na(+)/K(+)-ATPase, Ca(2+)-ATPase and calmodulin-stimulated Ca(2+)-ATPase (CaM). Our results show that betaine inhibited ATPases by more than 61%: Na(+)/K(+)-ATPase (75 +/- 5.9 vs 27 +/- 2.2), Ca(2+)-ATPase (236 +/- 18.9 vs 62 +/- 4.9), and CaM (450 +/- 18 vs 174 +/- 6.9) (microM pi/min/mg protein, control (0 microM betaine) vs 100 micromol/L betaine). Sorbitol (100 micromol/L) inhibited the Ca(2+)-ATPases by 41% (126 +/- 7.6 vs 74 +/- 4.4) and CaM by 42% (253 +/- 17.7 vs 147 +/- 10.3). Inositol (100 micromol/L) inhibited Na(+)/K(+)-ATPase strongest (37 +/- 1.9 vs 20 +/- 1.0; 47% inhibition) while it showed a lesser effect on the Ca(2+)-ATPases (136 +/- 6.8 vs 102 +/- 5.1; 25% inhibition). All osmolytes inhibited RBC membrane ATPases at concentrations above 50 micromol/L, which corresponds to high normal physiologic range for organic osmolytes in serum. Furthermore, the presence of osmolytes (250 micromol/L) decreased hypoosmotic stress induced hemolysis by 42%. Together these data indicate an important regulatory role of organic osmolytes on human RBC membrane ATPases and a protective function of osmolytes in RBCs against hypoosmotic stress.

Cortical and medullary betaine-GPC modulated by osmolality independently of oxygen in the intact kidney

Renal osmolyte concentrations are reduced during reflow following ischemia. Osmolyte decreases may follow oxygen depletion or loss of extracellular osmolality in the medulla. Image-guided volume-localized magnetic resonance (MR) microspectroscopy was used to monitor regional osmolytes during hyposmotic shock and hypoxia in the intact rat kidney. Alternate spectra were acquired from 24-microl voxels in cortex and medulla of the isolated perfused kidney. There was a progressive decrease in the combined betaine-glycerophosphorylcholine (GPC) peak intensity of 21% in cortex and 35% in medulla of normoxic kidneys between 60 and 160 min after commencing perfusion. Hypoxia had no significant effect on the betaine-GPC peak intensity in cortex or medulla, despite a dramatic reduction in tubular sodium, potassium, and water reabsorption. The results suggest that cortical and medullary intracellular osmolyte concentrations depend on osmotically regulated channels that are insensitive to oxygen and dissociated from the oxygen-dependent parameters of renal function, the fractional excretion of sodium, the fractional excretion of potassium, and urine-to-plasma inulin concentration ratio.

Regulation of sorbitol content in cultured porcine urinary bladder epithelial cells

Sorbitol content was determined in porcine urinary bladder epithelial cells immediately after death of the animals and after primary culture of the cells at different osmolalities. In both instances, sorbitol content increased with urine and medium osmolality, respectively. For example, at 300 mosmol/kg the cultured cells contained 0.84 +/- 0.02 nmol/mg protein, at 600 mosmol/kg contained 21.7 +/- 0.95 nmol/mg protein, and at 900 mosmol/kg contained 59.5 +/- 2.8 nmol/mg protein. Similarly, aldose reductase activity rose from 0.27 +/- 0.04 mumol.h-1.mg protein-1 at 300 mosmol/kg to 1.81 +/- 0.16 at 600 mosmol/kg and to 3.02 +/- 0.33 at 900 mosmol/kg. These changes were, however, only observed when NaCl but not when urea was used to augment the medium osmolality, since urea equilibrated across the cell membrane. In contrast, sorbitol release from cells cultured at 900 mosmol/kg was slowest into a 900 mosmol/kg medium and fastest into a 300 mosmol/kg medium (63 +/- 16 nmol/10 min compared with 389 +/- 52 nmol/10 min). These studies demonstrate that the sorbitol content of porcine urinary bladder epithelium is regulated by changes both in sorbitol synthesis and sorbitol release. Thus the regulatory mechanisms in the urinary bladder seem to be similar to those present in the embryological related collecting duct.

Role of Na+ conductance, Na(+)-H+ exchange, and Na(+)-K(+)-2Cl- symport in the regulatory volume increase of rat hepatocytes

1. In rat hepatocytes under hypertonic stress, the entry of Na+ (which is thereafter exchanged for K+ via Na(+)-K(+)-ATPase) plays the key role in regulatory volume increase (RVI). 2. In the present study, the contributions of Na+ conductance, Na(+)-H+ exchange and Na(+)-K(+)-2Cl- symport to this process were quantified in confluent primary cultures by means of intracellular microelectrodes and cable analysis, microfluorometric determinations of cell pH and buffer capacity, and measurements of frusemide (furosemide)/bumetanide-sensitive 86Rb+ uptake, respectively. Osmolarity was increased from 300 to 400 mosmol l-1 by addition of sucrose. 3. The experiments indicate a relative contribution of approximately 4:1:1 to hypertonicity-induced Na+ entry for the above-mentioned transporters and the overall Na+ yield equalled 51 mmol l-1 (10 min)-1. 4. This Na+ gain is in good agreement with the stimulation of Na+ extrusion via Na(+)-K(+)-ATPase plus the actual increase in cell Na+, namely 55 mmol l-1 (10 min)-1, as we determined on the basis of ouabain-sensitive 86Rb+ uptake and by means of Na(+)-sensitive microelectrodes, respectively. 5. The overall increase in Na+ and K+ activity plus the expected concomitant increase in cell Cl- equalled 68 mmol l-1, which fits well with the increase in osmotic activity expected to occur from an initial cell shrinkage to 87.5% and a RVI to 92.6% of control, namely 53 mosmol l-1. 6. The prominent role of Na+ conductance in the RVI of rat hepatocytes could be confirmed on the basis of the pharmacological profile of this process, which was characterized by means of confocal laser-scanning microscopy.

Hypertonicity-induced alkalinization of rat hepatocytes is not involved in activation of Na+ conductance or Na+,K+-ATPase

We investigated whether cell alkalinization via activation of Na+/H+ exchange is involved in the stimulation of Na+ conductance and Na+,K+-ATPase in rat hepatocytes under hypertonic stress. Osmolarity was increased from 300 to 400 mOsm/l at constant extracellular pH (7.4), whereas osmotically induced cell alkalinization (0.3 pH units in HCO3-free solutions) was mimicked by increasing extracellular pH from 7.4 to 7.8 in normosmotic solutions. In intracellular recordings with conventional and ion-sensitive microelectrodes, hypertonic stress led to a transient shift in the voltage response to low Na+ solutions (95% in exchange for choline) by -4.3 +/- 0.8 mV and a continuous increase in cell Na+ from 13.7 +/- 1.8 to 18.6 +/- 3.0 mmol/l within 8 min. In the presence of 10(-5) mol/l amiloride, these effects were reduced by 80 and 90%, respectively. In contrast, increasing pH did not change the voltage responses to low Na+ or cell Na+ concentrations significantly. In addition, application of 2 mmol/l Ba2+ pulses revealed that a sustained membrane hyperpolarization of 15.6 +/- 1.4 mV following intracellular alkalinization exclusively reflects an increase in K+ conductance. Increasing osmolarity at pH 7.4 augmented ouabain-sensitive 86Rb+ uptake from 5.5 +/- 1.1 to 8.5 +/- 1.6 nmol mg protein(-1) min(-1). In normosmotic solution at pH 7.8, 86Rb+ uptake equalled 4.9 +/- 1.6 nmol mg protein(-1) min(-1), which is not significantly different from control. We conclude that, in rat hepatocytes, cell alkalinization under hypertonic stress is not responsible for the activation of Na+ conductance and probably does not participate in the stimulation of Na+,K+-ATPase.

Nordihydroguaiaretic acid depletes ATP and inhibits a swelling-activated, ATP-sensitive taurine channel

The mechanism by which nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, prevents swelling-activated organic osmolyte efflux was examined in the human hepatoma cell line Hep G2. When swollen in hypotonic medium, Hep G2 cell exhibited a regulatory volume decrease that was associated with the release of intracellular taurine, an amino acid found at a concentrations of 22.0 +/- 2.5 nmol/mg protein (approximately 5 mM) in these cells. Rate coefficients for swelling-activated [3H]taurine uptake and efflux were unaffected when extracellular taurine was increased from 0.1 to 25 mM, indicating that taurine is released via a channel. Taurine efflux was rapidly activated after cell swelling and immediately inactivated when cells were returned to normal size by restoration of isotonicity. Swelling-activated taurine efflux was not altered by replacement of extracellular Na+ with choline+ or K+ but was inhibited when cellular ATP levels were decreased with a variety of chemical agents, consistent with an ATP-regulated channel previously described in other cell types. NDGA inhibited swelling-activated [3H]taurine efflux in Hep G2 cells at concentrations of 50-150 microM; however, these same concentrations of NDGA also lowered cell ATP levels. Likewise, ketoconazole, an inhibitor of cytochrome P-450 monoxygenases, inhibited [3H]taurine efflux only at concentrations at which cell ATP levels were also lowered. In contrast, other inhibitors of cyclooxygenase (indomethacin, 100 microM) or of lipoxygenases (caffeic acid, 100 microM), as well as arachidonic acid itself (100 microM), had no effect on either taurine efflux or cell ATP. The present findings characterize a swelling-activated, ATP-sensitive osmolyte channel in Hep G2 cells and demonstrate that inactivation of the channel by NDGA is related to the ability of this drug to deplete cellular ATP.