Ghrelin enhances tubular magnesium absorption in the kidney.

Osteoporosis after bariatric surgery is an increasing health concern as the rate of bariatric surgery has risen. In animal studies mimicking bariatric procedures, bone disease, together with decreased serum levels of Ca2+, Mg2+ and the gastric hormone Ghrelin were described. Ghrelin regulates metabolism by binding to and activating the growth hormone secretagogue receptor (GHSR) which is also expressed in the kidney. As calcium and magnesium are key components of bone, we tested the hypothesis that Ghrelin-deficiency contributes to osteoporosis via reduced upregulation of the renal calcium channel TRPV5 and the heteromeric magnesium channel TRPM6/7. We expressed GHSR with TRPV5 or TRPM6/7 channel in HEK293 cells and treated them with purified Ghrelin. Whole-cell current density was analyzed by patch-clamp recording. Nephron-specific gene expression was performed by tubular microdissection followed by qPCR in wild-type (WT) mice, and immunofluorescent imaging of GHSR-eGFP mice. Tubular magnesium homeostasis was analyzed in GHSR-null and WT mice at baseline and after caloric restriction. After Ghrelin exposure, whole-cell current density did not change for TRPV5 but increased for TRPM6/7 in a dose-dependent fashion. Applying the Ghrelin-mimetic (D-Trp7, Ala8,D-Phe10)-α-MSH (6-11) amide without and with the GHSR antagonist (D-Lys3)-GHRP6, we confirmed the stimulatory role of Ghrelin towards TRPM6/7. As GHSR initiates downstream signaling via protein kinase A (PKA), we found that the PKA inhibitor H89 abrogated TRPM6/7 stimulation by Ghrelin. Similarly, transfected Gαs, but not the Gαs mutant Q227L, nor Gαi2, Gαq, or Gα13 upregulated TRPM6/7 current density. In microdissected TALs and DCTs similar levels of GHSR mRNA were detected. In contrast, TRPM6 mRNA was expressed in the DCT and also detected in the TAL at 25% expression compared to DCT. Immunofluorescent studies using reporter GHSR-eGFP mice showed a strong eGFP signal in the TAL but surprisingly displayed no eGFP signal in the DCT. In 3-, 6-, and 9-month-old GHSR-null and WT mice, baseline serum magnesium was not significantly different, but 24-h urinary magnesium excretion was elevated in 9-month-old GHSR-null mice. In calorically restricted GHSR-null mice, we detected excess urinary magnesium excretion and reduced serum magnesium levels compared to WT mice. The kidneys from calorically restricted WT mice showed upregulated gene expression of magnesiotropic genes Hnf1b, Cldn-16, Cldn-19, Fxyd-2b, and Parvalbumin compared to GHSR-null mice. Our in vitro studies show that Ghrelin stimulates TRPM6/7 via GHSR and Gαs-PKA signaling. The murine studies are consistent with Ghrelin-GHSR signaling inducing reduced urinary magnesium excretion, particularly in calorically restricted mice when Ghrelin levels are elevated. This effect may be mediated by Ghrelin-upregulation of TRPM6 in the TAL and/or upregulation of other magnesiotropic genes. We postulate that rising Ghrelin levels with hunger contribute to increased renal Mg2+ reabsorption to compensate for lack of enteral Mg2+ uptake.

Sex differences in prenatal programming of hypertension by dexamethasone.

Prenatal dexamethasone has been shown to increase blood pressure in male offspring but the mechanism for the increase in blood pressure is unclear. The present study examined if prenatal programming by maternal injection of dexamethasone on days 15 and 16 of gestation affected the blood pressure comparably in female and male offspring. Our hypothesis was that males would be affected by prenatal dexamethasone to a greater extent than females and that either an increase in renal tubular transporter abundance or an increase in renin or aldosterone system would be associated with hypertension with prenatal programming. Prenatal dexamethasone increased blood pressure at two months and six months of age and resulted in proteinuria and albuminuria at six months in male but not female rat offspring. There was no effect of prenatal dexamethasone on blood pressure and proteinuria at one month in male and in female offspring. While prenatal dexamethasone increased male renal thick ascending limb sodium potassium two chloride cotransporter protein abundance at two months, prenatal dexamethasone on days 15 and 16 of gestation did not affect transporter abundance in males at other ages, nor did it affect proximal tubule sodium/hydrogen exchanger or distal convoluted tubule sodium chloride cotransporter protein abundance at any age. There was no difference in systemic renin or aldosterone in the prenatal dexamethasone group compared to same sex controls. In conclusion, male but not female offspring have an increase in blood pressure and urinary protein excretion with prenatal dexamethasone. The increase in blood pressure with prenatal programming was not associated with a consistent increase in renal tubular transporter protein abundance, nor plasma renin activity and serum aldosterone.

Effect of sex on glomerular filtration rate in programmed rats by prenatal dexamethasone.

We have previously demonstrated that dexamethasone administered to pregnant rats during specific times during gestation results in a reduction in glomerular number and hypertension in offspring at 2 and 6 months of age. In this study, we examined the effect of prenatal dexamethasone administered daily on days 15 and 16 of gestation in male and female offspring after 1 year of age on glomerular filtration rate. The prenatal dexamethasone male group had a higher systolic blood pressure than the vehicle male group. Females had lower systolic blood pressures than the males and prenatal dexamethasone did not affect blood pressure in female offspring. Prenatal dexamethasone resulted in a reduction in glomerular filtration rate in male but not in female rats. When corrected for body weight, the control male rats had a lower glomerular filtration rate than the control female rats. Males had greater protein excretion than females and prenatal dexamethasone increased the protein excretion only in male rats. Glomerulosclerosis was also greater in male rats than females but was not affected by prenatal dexamethasone. In summary, male rats appear to have evidence of a decline in glomerular filtration rate after 1 year of age and prenatal dexamethasone programs an accelerated decline in glomerular filtration rate in male but not in female offspring.

Prevalence of hyperfiltration among US youth/young adults with overweight and obesity: A population-based association study.

OBJECTIVE: Determine prevalence of hyperfiltration (high estimated glomerular filtration rate "eGFR" >95th percentile for age/sex) among youth and association with BMI classification. METHODS: With the use of 1999 to 2016 National Health and Nutrition Examination Survey data from 12- to 29-year-olds, data for serum creatinine and thresholds for high eGFR were normed using a metabolically healthy subsample (no albuminuria, healthy weights, normal blood pressures, blood glucoses, lipids, and liver enzymes). Logistic regression examined the association of BMI classification (healthy weight, overweight, and obesity classes 1-3) with hyperfiltration (eGFR > 95th percentile for age/sex), adjusted for diabetes and other covariates. RESULTS: Of 12- to 29-year-olds (N = 18 698), 27.4% (n = 5493) met criteria for entry into the "healthy subsample" and contributed data to derive normative values for serum creatinine/hyperfiltration thresholds. In the full sample, hyperfiltration prevalence in 12- to 29-year-olds classified as healthy-weight, overweight, and obesity classes 1 to 3 was 4.9%, 4.7%, 6.5%, 8.7%, and 11.8%, respectively (P < .001). In multivariable analysis, obesity classes 2 and 3 were associated with greater likelihood of hyperfiltration (adjusted ORs for class 2: 1.5, 95% CI, 1.1-2.1; and for class 3, 2.1, 95% CI, 1.5-2.9). Diabetes also was associated with hyperfiltration (AOR, 4.0; 95% CI, 2.2-7.4). CONCLUSION: Obesity classes 2 to 3 are associated with hyperfiltration in youth. Age/sex-specific norms for creatinine and hyperfiltration thresholds may aid recognition of kidney dysfunction early.

Role of renal sympathetic nerve activity in prenatal programming of hypertension.

Prenatal insults, such as maternal dietary protein deprivation and uteroplacental insufficiency, lead to small for gestational age (SGA) neonates. Epidemiological studies from many different parts of the world have shown that SGA neonates are at increased risk for hypertension and early death from cardiovascular disease as adults. Animal models, including prenatal administration of dexamethasone, uterine artery ligation and maternal dietary protein restriction, result in SGA neonates with fewer nephrons than controls. These models are discussed in this educational review, which provides evidence that prenatal insults lead to altered sodium transport in multiple nephron segments. The factors that could result in increased sodium transport are discussed, focusing on new information that there is increased renal sympathetic nerve activity that may be responsible for augmented renal tubular sodium transport. Renal denervation abrogates the hypertension in programmed rats but has no effect on control rats. Other potential factors that could cause hypertension in programmed rats, such as the renin-angiotensin system, are also discussed.

Suppression of microRNA Activity in Kidney Collecting Ducts Induces Partial Loss of Epithelial Phenotype and Renal Fibrosis.

microRNAs (miRNAs) are sequence-specific inhibitors of post-transcriptional gene expression. The physiologic function of these noncoding RNAs in postnatal renal tubules still remains unclear. Surprisingly, they appear to be dispensable for mammalian proximal tubule (PT) function. Here, we examined the effects of miRNA suppression in collecting ducts (CDs). To conclusively evaluate the role of miRNAs, we generated three mouse models with CD-specific inactivation of key miRNA pathway genes Dicer, Dgcr8, and the entire Argonaute gene family (Ago1, 2, 3, and 4). Characterization of these three mouse models revealed that inhibition of miRNAs in CDs spontaneously evokes a renal tubule injury-like response, which culminates in progressive tubulointerstitial fibrosis (TIF) and renal failure. Global miRNA profiling of microdissected renal tubules showed that miRNAs exhibit segmental distribution along the nephron and CDs. In particular, the expression of miR-200c is nearly 70-fold higher in CDs compared with PTs. Accordingly, miR-200s are downregulated in Dicer-KO CDs, its direct target genes Zeb1, Zeb2, and Snail2 are upregulated, and miRNA-depleted CDs undergo partial epithelial-to-mesenchymal transition (EMT). Thus, miRNAs are essential for CD homeostasis. Downregulation of CD-enriched miRNAs and the subsequent induction of partial EMT may be a new mechanism for TIF progression.

Effect of renal denervation on urine angiotensinogen excretion in prenatally programmed rats.

Prenatal programming results in an increase in blood pressure in adult offspring. We have shown that compared to control adult offspring whose mothers were fed a 20% protein diet, programmed adults whose mothers were fed a 6% protein diet during the last half of pregnancy have an increase in renal sympathetic nerve activity and urinary angiotensinogen/creatinine levels. We hypothesized that the increase in urinary angiotensinogen was mediated by renal sympathetic nerve activity in programmed rats. In this study performed in 3 month old rats, renal denervation resulted in normalization of blood pressure in the 6% programmed group (150 ± 3 Hg in 6% sham vs. 121 ± 4 Hg in 6% denervated, P < 0.001), and a reduction in blood pressure in the 20% group (126 ± 2 Hg 20% sham vs. 113 ± 4 Hg 20% denervated (P < 0.05). We confirm that the intrarenal renin-angiotensin system assessed by urinary angiotensinogen/creatinine is upregulated in offspring of rats fed a 6% protein diet rats compared to 20% controls. To determine if sympathetic nerve activity was mediating the increase in urinary angiotensinogen in programmed rats, we compared denervated to sham-operated control and programmed rats. Renal denervation had no effect on urinary angiotensinogen/creatinine ratio in the 20% group and no effect on the increased urinary angiotensinogen/creatinine ratio found in programmed rats. This study demonstrates that the increase in urinary angiotensinogen in programmed rats is not mediated by renal sympathetic nerve activity.

Transient enalapril attenuates the reduction in glomerular filtration rate in prenatally programmed rats.

A maternal low-protein diet has been shown to program hypertension and a reduction in glomerular filtration rate in adult offspring. This study examined the effect of continuous administration of enalapril in the drinking water and transient administration of enalapril administered from 21 to 42 days of age on blood pressure and glomerular filtration rate (GFR) in male rats whose mothers were fed a 20% protein diet (control) or a 6% protein diet (programmed) during the last half of pregnancy. After birth all rats were fed a 20% protein diet. Programmed rats (maternal 6% protein diet) were hypertensive at 15 months of age compared to control rats and both continuous and transient administration of enalapril had no effect on blood pressure on control offspring, but normalized the blood pressure of programmed offspring. GFR was 3.2 ± 0.1 mL/min in the control group and 1.7 ± 0.1 mL/min in the programmed rats at 17 months of age (P < 0.001). The GFR was 3.0 ± 0.1 mL/min in the control and 2.7 ± 0.1 mL/min in the programmed group that received continuous enalapril in their drinking water showing that enalapril can prevent the decrease in GFR in programmed rats. Transient administration of enalapril had no effect on GFR in the control group (3.2 ± 0.1 mL/min) and prevented the decrease in GFR in the programmed group (2.9 ± 0.1 mL/min). In conclusion, transient exposure to enalapril for 3 weeks after weaning can prevent the hypertension and decrease in GFR in prenatal programmed rats.

Use of continuous renal replacement therapy in salicylate toxicity: A case report and review of the literature.

OBJECTIVE: To report a case of salicylate toxicity treated with continuous venovenous hemodiafiltration (CVVHDF) and review the literature regarding the use of continuous renal replacement therapy (CRRT) for salicylate toxicity. CASE: A 16-year-old male presented after ingesting 1901 mg/kg of enteric coated aspirin. Salicylate level was 92 mg/dl 4 h after ingestion. Sequele included seizure, acute kidney injury, pulmonary edema, and prolonged QTc. He received 5.5 h of hemodialysis followed by CVVHDF to continue to augment clearance. His aspirin level fell to 37.4 mg/dl after HD and then to 11.3 mg/dl after nearly 10 h of CVVHDF. DISCUSSION: Cited reasons for the use of CRRT for salicylate toxicity primarily have been hypotension or desire for ongoing augmentation of salicylate clearance in the setting of multiorgan toxicity. CVVHDF may have a role in severe salicylate toxicity to enhance ongoing clearance after an initial round of HD in order to prevent significant rebound.