Role of calcium in insulin-stimulated NaC1 transport in medullary thick ascending limb

Metabolic Syndrome and Its Components Are Associated with New-Onset Hyperuricemia in a Large Taiwanese Population Follow-Up Study

The prevalence rate of hyperuricemia remains high in Taiwan, at 21.6% in men and 9.57% in women. Both metabolic syndrome (MetS) and hyperuricemia can cause many complications; however, few studies have evaluated the correlation between MetS and hyperuricemia. Therefore, in this observational cohort study, we explored associations between metabolic syndrome (MetS) and its components and new-onset hyperuricemia. Of 27,033 individuals in the Taiwan Biobank who had complete follow-up data, we excluded those with hyperuricemia at baseline (n = 4871), those with gout at baseline (n = 1043), those with no data on baseline uric acid (n = 18), and those with no data on follow-up uric acid (n = 71). The remaining 21,030 participants (mean age 50.8 ± 10.3 years) were enrolled. We found a significant association between new-onset hyperuricemia with MetS and the components of MetS (hypertriglyceridemia, abdominal obesity, low high-density lipoprotein cholesterol, hyperglycemia, and high blood pressure). Furthermore, compared to those without any MetS components, those with one MetS component (OR = 1.816), two MetS components (OR = 2.727), three MetS components (OR = 3.208), four MetS components (OR = 4.256), and five MetS components (OR = 5.282) were significantly associated with new-onset hyperuricemia (all p < 0.001). MetS and its five components were associated with new-onset hyperuricemia in the enrolled participants. Further, an increase in the number of MetS components was associated with an increase in the incidence rate of new-onset hyperuricemia.

P2Y2 receptor knock-out mice display normal NaCl absorption in medullary thick ascending limb

Local purinergic signals modulate renal tubular transport. Acute activation of renal epithelial P2 receptors causes inhibition of epithelial transport and thus, should favor increased water and salt excretion by the kidney. So far only a few studies have addressed the effects of extracellular nucleotides on ion transport in the thick ascending limb (TAL). In the medullary thick ascending limb (mTAL), basolateral P2X receptors markedly (~25%) inhibit NaCl absorption. Although this segment does express both apical and basolateral P2Y₂ receptors, acute activation of the basolateral P2Y₂ receptors had no apparent effect on transepithelial ion transport. Here we studied, if the absence of the P2Y₂ receptor causes chronic alterations in mTAL NaCl absorption by comparing basal and AVP-stimulated transepithelial transport rates. We used perfused mouse mTALs to electrically measure NaCl absorption in juvenile (<35 days) and adult (>35 days) male mice. Using microelectrodes, we determined the transepithelial voltage (V_te) and the transepithelial resistance (R_te) and thus, transepithelial NaCl absorption (equivalent short circuit current, I'_sc). We find that mTALs from adult wild type (WT) mice have significantly lower NaCl absorption rates when compared to mTALs from juvenile WT mice. This could be attributed to significantly higher R_tevalues in mTALs from adult WT mice. This pattern was not observed in mTALs from P2Y₂ receptor knockout (KO) mice. In addition, adult P2Y₂ receptor KO mTALs have significantly lower V_tevalues compared to the juvenile. No difference in absolute I'_sc was observed when comparing mTALs from WT and KO mice. AVP stimulated the mTALs to similar increases of NaCl absorption irrespective of the absence of the P2Y₂ receptor. No difference was observed in the medullary expression level of NKCC2 in between the genotypes. These data indicate that the lack of P2Y₂ receptors does not cause substantial differences in resting and AVP-stimulated NaCl absorption in mouse mTAL.

Insulin's impact on renal sodium transport and blood pressure in health, obesity, and diabetes

Insulin has been shown to have antinatriuretic actions in humans and animal models. Moreover, endogenous hyperinsulinemia and insulin infusion have been correlated to increased blood pressure in some models. In this review, we present the current state of understanding with regard to the regulation of the major renal sodium transporters by insulin in the kidney. Several groups, using primarily cell culture, have demonstrated that insulin can directly increase activity of the epithelial sodium channel, the sodium-phosphate cotransporter, the sodium-hydrogen exchanger type III, and Na-K-ATPase. We and others have demonstrated alterations in the expression at the protein level of many of these same proteins with insulin infusion or in hyperinsulinemic models. We also discuss how this regulation is perturbed in type I and type II diabetes mellitus. Finally, we discuss a potential role for regulation of insulin receptor signaling in the kidney in contributing to sodium balance and blood pressure.

Acute growth hormone administration induces antidiuretic and antinatriuretic effects and increases phosphorylation of NKCC2

Growth hormone (GH) has antidiuretic and antinatriuretic effects in rats and humans, but the molecular mechanisms responsible for these effects are unknown. The aim of this study was to investigate the mechanisms behind the acute renal effects of GH in rats. Female rats received rat (r)GH (2.8 mg/kg sc) or saline and were placed in metabolic cages for 5 h. Urinary excretion of electrolytes and urinary volume were reduced after rGH injection, while urine osmolality was increased. Creatinine and lithium clearance remained unchanged, suggesting that rGH increases reabsorption in segments distal to the proximal tubule. Total plasma insulin-like growth factor I (IGF-I) levels did not change, while cortical IGF-I mRNA abundance was increased. The relative abundance of total and Ser(256)-phosphorylated aquaporin 2 was found to be unchanged by immunoblotting, whereas a significant increase of Thr(96) and Thr(101)-phosphorylated NKCC2 (renal Na(+), K(+), 2Cl(-) cotransporter) was found in the inner stripe of outer medulla thick ascending limbs (mTAL). Additionally, an increased NKCC2 expression was observed in the cortical region. Immunohistochemistry confirmed these findings. The density of NKCC2 molecules in the apical membrane of mTAL cells appeared to be unchanged after rGH injection evaluated by immunoelectron microscopy. Basolateral addition of rGH or IGF-I to microperfused rat mTAL segments did not change transepithelial voltage. In conclusion, GH appears to exert its acute antinatriuretic and antidiuretic effects through indirect activation of NKCC2 in the mTAL.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.