Approach to the patient with hypomagnesaemia

The kidneys play a central role in the maintenance of a normal magnesium balance. The distal renal tubule, especially the thick ascending limb of the loop of Henle, and the distal convoluted tubule mediate the regulation of a normal magnesium balance. The clinical assessment of magnesium deficiency is described. Genetic causes are described in detail. Acquired causes are more common and often related to drug therapy or toxicity, or to metabolic acidosis or phosphate depletion.

Acute inhibition of NCC does not activate distal electrogenic Na+ reabsorption or kaliuresis

Na+ reabsorption from the distal renal tubule involves electroneutral and electrogenic pathways, with the latter promoting K+ excretion. The relative activities of these two pathways are tightly controlled, participating in the minute-to-minute regulation of systemic K+ balance. The pathways are interdependent: the activity of the NaCl cotransporter (NCC) in the distal convoluted tubule influences the activity of the epithelial Na+ channel (ENaC) downstream. This effect might be mediated by changes in distal Na+ delivery per se or by molecular and structural adaptations in the connecting tubule and collecting ducts. We hypothesized that acute inhibition of NCC activity would cause an immediate increase in Na+ flux through ENaC, with a concomitant increase in renal K+ excretion. We tested this using renal clearance methodology in anesthetized mice, by the administration of hydrochlorothiazide (HCTZ) and/or benzamil (BZM) to exert specific blockade of NCC and ENaC, respectively. Bolus HCTZ elicited a natriuresis that was sustained for up to 110 min; urinary K+ excretion was not affected. Furthermore, the magnitude of the natriuresis was no greater during concomitant BZM administration. This suggests that ENaC-mediated Na+ reabsorption was not normally limited by Na+ delivery, accounting for the absence of thiazide-induced kaliuresis. After dietary Na+ restriction, HCTZ elicited a kaliuresis, but the natiuretic effect of HCTZ was not enhanced by BZM. Our findings support a model in which inhibition of NCC activity does not increase Na+ reabsorption through ENaC solely by increasing distal Na+ delivery but rather by inducing a molecular and structural adaptation in downstream nephron segments.

Direct inhibition of basolateral Kir4.1/5.1 and Kir4.1 channels in the cortical collecting duct by dopamine

It is recognized that dopamine promotes natriuresis by inhibiting multiple transporting systems in the proximal tubule. In contrast, less is known about the molecular targets of dopamine actions on water-electrolyte transport in the cortical collecting duct (CCD). Epithelial cells in the CCD are exposed to dopamine, which is synthesized locally or secreted from sympathetic nerve endings. Basolateral K+ channels in the distal renal tubule are critical for K+ recycling and controlling basolateral membrane potential to establish the driving force for Na+ reabsorption. Here, we demonstrate that Kir4.1 and Kir5.1 are highly expressed in the mouse kidney cortex and are localized to the basolateral membrane of the CCD. Using patch-clamp electrophysiology in freshly isolated CCDs, we detected highly abundant 40-pS and scarce 20-pS single channel conductances, most likely representing Kir4.1/5.1 and Kir4.1 channels, respectively. Dopamine reversibly decreased the open probability of both channels, with a relatively greater action on the Kir4.1/5.1 heterodimer. This effect was mediated by D2-like but not D1-like dopamine receptors. PKC blockade abolished the inhibition of basolateral K+ channels by dopamine. Importantly, dopamine significantly decreased the amplitude of Kir4.1/5.1 and Kir4.1 unitary currents. Consistently, dopamine induced an acute depolarization of basolateral membrane potential, as directly monitored using current-clamp mode in isolated CCDs. Therefore, we demonstrate that dopamine inhibits basolateral Kir4.1/5.1 and Kir4.1 channels in CCD cells via stimulation of D2-like receptors and subsequently PKC. This leads to depolarization of the basolateral membrane and a decreased driving force for Na+ reabsorption in the distal renal tubule.

Functional specificity of Sgk1 and Akt1 on ENaC activity

Reabsorption of sodium by the epithelial sodium channel (ENaC) is essential for maintaining the volume of the extracellular compartment and blood pressure. The function of ENaC is regulated primarily by aldosterone, antidiuretic hormone [arginine vasopressin (AVP)], and insulin, but the molecular mechanisms that increase channel activity are still poorly understood. It has been proposed that the related serine/threonine kinases serum- and glucocorticoid-induced kinase (Sgk1) and protein kinase B (Akt) mediate activation of ENaC. Here, we addressed the question of whether there is functional specificity of these kinases for the activation of ENaC in epithelial cells of the distal renal tubule. We demonstrate that Akt does not increase ENaC function under basal conditions or after stimulation with aldosterone, insulin, or AVP. In contrast, under the same experimental conditions, Sgk1 increases ENaC activity by 10-fold. The effect of Sgk1 is additive to that of aldosterone, whereas, in the presence of active Sgk1, cells do not further respond to insulin or AVP. We conclude that, in cells expressing both kinases, modulation of ENaC activity is mediated by Sgk1 but not by Akt1.

Thrombospondin-1 mediates distal tubule hypertrophy induced by glycated albumin

Diabetic nephropathy is characterized by early hypertrophy in both glomerular and tubuloepithelial elements. However, no studies to date have established a direct causal link between hyperglycaemia and renal hypertrophy. Our previous studies have found that high glucose does not induce cellular hypertrophy or expression of TGF-β1 (transforming growth factor-β1) in distal renal tubule cells [Yang, Guh, Yang, Lai, Tsai, Hung, Chang and Chuang (1998) J. Am. Soc. Nephrol. 9, 182–193]. In the present study, we used AGEs (advanced glycation end-products) to mimic long-term hyperglycaemia. Similar to glucose, AGEs did not induce TGF-β1 mRNA in distal renal tubule cells [MDCK (Madin–Darby canine kidney) cells]; however, TGF-β1 bioactivity was increased significantly. This result indicated post-translational regulation. Since TSP-1 (thrombospondin-1) has been demonstrated to activate latent TGF-β1 in a variety of systems, the following experiments were performed. We found that AGEs dose-dependently increased both intracellular and extracellular levels of TSP-1. Purified TSP-1, like AGEs, increased the cellular protein content. Furthermore, anti-TSP-1 neutralizing antibodies attenuated the AGE-induced increase in TGF-β1 bioactivity and hypertrophy. Thus TSP-1 might mediate AGE-induced distal renal tubule hypertrophy. In addition, we observed several putative transcription factor binding sites in the TSP-1 promoter, including those for AP-1 (activator protein-1), CREB (cAMP response element binding protein), NF-κB (nuclear factor-κB), SRF (serum response factor) and HSF (heat-shock factor), by sequence mapping. We used an enhancer assay to screen possible transcription factors involved. We showed that AP-1 and CREB were specifically induced by AGEs; furthermore, TFD (transcription factor decoy) for AP-1 could attenuate the AGE-induced increases in TSP-1 levels and cellular hypertrophy. Thus regulation of TSP-1 might be critical for hyperglycaemic distal tubule hypertrophy. Furthermore, TSP-1 TFD might be a potential approach to ameliorate diabetic renal hypertrophy.