Glycosphingolipids modulate renal phosphate transport in potassium deficiency

BACKGROUND

Potassium (K) deficiency (KD) and/or hypokalemia have been associated with disturbances of phosphate metabolism. The purpose of the present study was to determine the cellular mechanisms that mediate the impairment of renal proximal tubular Na/Pi cotransport in a model of K deficiency in the rat.

METHODS

K deficiency in the rat was achieved by feeding rats a K-deficient diet for seven days, which resulted in a marked decrease in serum and tissue K content.

RESULTS

K deficiency resulted in a marked increase in urinary Pi excretion and a decrease in the V(max) of brush-border membrane (BBM) Na/Pi cotransport activity (1943 +/- 95 in control vs. 1184 +/- 99 pmol/5 sec/mg BBM protein in K deficiency, P < 0.02). Surprisingly, the decrease in Na/Pi cotransport activity was associated with increases in the abundance of type I (NaPi-1), and type II (NaPi-2) and type III (Glvr-1) Na/Pi protein. The decrease in Na/Pi transport was associated with significant alterations in BBM lipid composition, including increases in sphingomyelin, glucosylceramide, and ganglioside GM3 content and a decrease in BBM lipid fluidity. Inhibition of glucosylceramide synthesis resulted in increases in BBM Na/Pi cotransport activity in control and K-deficient rats. The resultant Na/Pi cotransport activity in K-deficient rats was the same as in control rats (1148 +/- 52 in control + PDMP vs. 1152 +/- 61 pmol/5 sec/mg BBM protein in K deficiency + PDMP). These changes in transport activity occurred independent of further changes in BBM NaPi-2 protein or renal cortical NaPi-2 mRNA abundance.

CONCLUSION

K deficiency in the rat causes inhibition of renal Na/Pi cotransport activity by post-translational mechanisms that are mediated in part through alterations in glucosylceramide content and membrane lipid dynamics.

Gentamicin causes endocytosis of Na/Pi cotransporter protein (NaPi-2)

BACKGROUND

Renal toxicity is a major side-effect of aminoglycoside antibiotics and is characterized by an early impairment in proximal tubular function. In a previous study, we have shown that gentamicin administration to the rat causes an early impairment in sodium gradient-dependent phosphate (Na/Pi) cotransport activity. The purpose of our current study was to determine the molecular mechanisms of the impairment in Na/Pi cotransport activity, specifically the role of the proximal tubular type II Na/Pi cotransporter.

METHODS

Rats were treated for one, two, and three days with two daily injections of 30 mg/kg body weight gentamicin or the vehicle.

RESULTS

Gentamicin caused a progressive decrease in superficial cortical apical brush-border membrane (SC-BBM) Na/Pi cotransporter activity (856 +/- 93 in control vs. 545 +/- 87 pmol/mg BBM protein in 3-day gentamicin, P < 0.01). Western blot analysis showed a parallel and progressive decrease in SC-BBM Na/Pi cotransporter protein abundance, a 50% decrease after one day of treatment, a 63% decrease after two days of treatment, and an 83% decrease after three days treatment with gentamicin. In contrast, gentamicin treatment had no effect on Na/Pi cotransport activity or Na/Pi cotransporter protein abundance in BBM isolated from the juxtamedullary cortex (JMC-BBM). Immunofluorescence microscopy showed a major decrease in the expression of Na/Pi cotransporter protein in the apical membrane of the proximal convoluted tubule, with progressive intracellular accumulation of Na/Pi protein. Colocalization studies showed that in gentamicin-treated rats, Na/Pi protein was colocalized in the early endosomes and especially in the lysosomes. Northern blot analysis of cortical RNA interestingly showed no reduction in Na/Pi cotransporter mRNA abundance even after three days of gentamicin treatment.

CONCLUSION

We conclude that gentamicin inhibits Na/Pi cotransport activity by causing a decrease in the expression of the type II Na/Pi cotransport protein at the level of the proximal tubular apical BBM and that inhibition of Na/Pi cotransport activity is most likely mediated by post-transcriptional mechanisms.

Metabolic acidosis regulates rat renal Na-Si cotransport activity

Recently, we cloned a cDNA (NaSi-1) localized to rat renal proximal tubules and encoding the brush-border membrane (BBM) Na gradient-dependent inorganic sulfate (Si) transport protein (Na-Si cotransporter). The purpose of the present study was to determine the effect of metabolic acidosis (MA) on Na-Si cotransport activity and NaSi-1 protein and mRNA expression. In rats with MA for 24 h (but not 6 or 12 h), there was a significant increase in the fractional excretion of Si, which was associated with a 2.4-fold decrease in BBM Na-Si cotransport activity. The decrease in Na-Si cotransport correlated with a 2.8-fold decrease in BBM NaSi-1 protein abundance and a 2.2-fold decrease in cortical NaSi-1 mRNA abundance. The inhibitory effect of MA on BBM Na-Si cotransport was also sustained in rats with chronic (10 days) MA. In addition, in Xenopus laevis oocytes injected with mRNA from kidney cortex, there was a significant reduction in the induced Na-Si cotransport in rats with MA compared with control rats, suggesting that MA causes a decrease in the abundance of functional mRNA encoding the NaSi-1 cotransporter. These findings indicate that MA reduces Si reabsorption by causing decreases in BBM Na-Si cotransport activity and that decreases in the expression of NaSi-1 protein and mRNA abundance, at least in part, play an important role in the inhibition of Na-Si cotransport activity during MA.

Chronic K depletion inhibits renal brush border membrane Na/sulfate cotransport

BACKGROUND

The purpose of this study was to determine if dietary potassium (K) deficiency regulates renal proximal tubular sodium gradient-dependent sulfate transport (Na/Si cotransport) in the rat and, furthermore, determine if the regulation takes place at the level of the recently cloned Na/Si cotransport system (NaSi-1). Methods and Results. Rats treated chronically (seven days) with a K-deficient diet had a significant decrease in serum Si levels and an increase in fractional excretion of ultrafilterable Si, which paralleled a significant decrease in brush border membrane (BBM) Na/Si cotransport activity. The decrease in BBM Na/Si cotransport activity was associated with decreases in BBM NaSi-1 protein and renal cortical NaSi-1 mRNA abundance. In addition, in Xenopus oocytes injected with mRNA from kidney cortex slices of K-deficient rats, there was a significant reduction in the induced Na/Si cotransport, whereas there was no alteration in l-leucine uptake, suggesting that in K-deficient rats, there is a specific decrease in functional mRNA encoding the NaSi-1 mRNA.

CONCLUSION

These findings indicate that chronic K deficiency leads to a reduction in serum Si levels and an increase fractional excretion of Si, and reduces Si reabsorption by down-regulating the expression of the proximal tubular Na/Si-1 cotransporter protein and mRNA.

Regulation of renal phosphate transport by acute and chronic metabolic acidosis in the rat

Metabolic acidosis results in impaired renal tubular phosphate reabsorption and proximal tubular apical brush border membrane (BBM) sodium gradient-dependent phosphate transport (Na/Pi cotransport) activity. In the present study we investigated the cellular mechanisms responsible for decreased Na/Pi cotransport activity following six hours to 10 days of metabolic acidosis induced by ingestion of NH4Cl. Urinary Pi excretion was significantly increased and BBM Na/Pi cotransport activity was progressively and significantly decreased by 18% at six hours, 24% at 12 hours, 32% at 24 hours, and 61% after 10 days of metabolic acidosis. The progressive and time-dependent decreases in BBM cotransport activity were associated with progressive decreases in BBM NaPi-2 protein (43% at 12 hr, 54% at 24 hr and 66% at 10 days) and cortical NaPi-2 mRNA (22% at 12 hr, 54% at 24 hr and 56% at 10 days) abundance. Interestingly, following six hours of metabolic acidosis, there was a significant 29% decrease in BBM NaPi-2 protein abundance that was not associated with decreases in either cortical homogenate NaPi-2 protein or cortical NaPi-2 mRNA abundance. In additional studies we found that the effects of chronic metabolic acidosis on Na/Pi cotransport activity were independent of endogenous parathyroid hormone activity, but were somewhat dependent on dietary Pi intake. In rats fed a high or a normal Pi diet metabolic acidosis caused significant decreases in Na/Pi cotransport activity, NaPi-2 protein and NaPi-2 mRNA abundance, however, in rats fed a low Pi diet the inhibitory effect of metabolic acidosis on Na/Pi cotransport were minimal and not significant. These results indicate that in chronic (> or = 12 hr) metabolic acidosis the progressive decrease in BBM Na/Pi cotransport activity is most likely mediated by decrease in BBM NaPi-2 protein and cortical mRNA abundance. In contrast, in acute (< or = 6 hr) metabolic acidosis the decrease in BBM Na/Pi cotransport activity is likely mediated by changes in the trafficking of the NaPi-2 protein that is, enhanced internalization from and/or impaired delivery of the NaPi-2 protein to the apical BBM.