Compensatory renal hypertrophy following uninephrectomy is calcineurin-independent

Experimental uninephrectomy associates with less parasympathetic modulation of heart rate and facilitates sodium-dependent arterial hypertension

Background

Blood pressure is known to be increased in kidney donors following living-donor kidney transplantation. However, the physiological underpinnings of the blood-pressure increase following uninephrectomy remain unclear. We hypothesized that changes in sympathetic tone or in parasympathetic modulation of sinus node function are involved in the blood-pressure increase following experimental kidney-mass reduction.

Methods

C57BL6N mice (6 to 11 per group) subjected to sham surgery (controls) or uninephrectomy with or without a one-week course of sodium chloride-enriched, taurine-deficient diet were studied. Uninephrectomized mice treated with a subcutaneous infusion of angiotensin-II over a period of one week were positive controls. A transfemoral aortic catheter with telemetry unit was implanted, readings of heart-rate and blood-pressure were recorded. Powerspectral analysis of heart rate and systolic blood pressure was performed to gain surrogate parameters of sympathetictone and parasympathetic modulation of sinus node function. Baroreflex sensitivity of heart rate was determined from awake, unrestrained mice using spontaneous baroreflex gain technique.

Results

Systolic arterial blood pressure, heart rate and baroreflex sensitivity were not different in uninephrectomized mice when compared to controls. Parasympathetic modulation of sinus node function was less in uninephrectomized mice in comparison to controls. Uninephrectomized mice of the high-angiotensin-II model or of the high-salt and taurine-deficiency model had an increased systolic arterial blood pressure.

Conclusions

Uninephrectomy associated with less parasympathetic modulation of sinus node function. The combination of uninephrectomy, taurine-deficiency and high-salt intake led to arterial hypertension.

Compensatory renal hypertrophy following nephrectomy: When and how?

Following surgical removal of one kidney, the other enlarges and increases its function. The mechanism for the sensing of this change and the growth is incompletely understood but begins within days and compensatory renal hypertrophy (CRH) is the dominant contributor to the growth. In many individuals undergoing nephrectomy for cancer or kidney donation this produces a substantial and helpful increase in renal function. Two main mechanisms have been proposed, one in which increased activity by the remaining kidney leads to hypertrophy, the second in which there is release of a kidney specific factor in response to a unilateral nephrectomy that initiates CRH. Whilst multiple growth factors and pathways such as the mTORC pathway have been implicated in experimental studies, their roles and the precise mechanism of CRH are not defined. Unrestrained hypoxia inducible factor activation in renal cancer promotes growth and may play an important role in driving CRH.

Validation of a clinically-relevant rodent model of statin-associated muscle symptoms for use in pharmacological studies

Various rodent models of statin-associated muscle symptoms (SAMS) have been used to investigate the aetiology of statin myotoxicity. Variability between these models, however, may be contributing to the ambiguity currently surrounding the pathogenesis of SAMS. Furthermore, few studies have assessed the reproducibility of these models. The aim of this study was to compare two established rodent models of statin myotoxicity, differing in treatment duration and dose, to determine which reproducibly caused changes characteristic of SAMS. Isolated skeletal muscle organ bath experiments, biochemical analyses, real-time quantitative-PCR and biometric assessments were used to compare changes in skeletal muscle and renal integrity in statin-treated animals and time-matched control groups. The SIM80 model (80 mg kg^-1 day^-1 simvastatin for 14 days) produced fibre-selective skeletal muscle damage characteristic of SAMS. Indeed, fast-twitch gastrocnemius muscles showed increased Atrogin-1 expression, reduced peak force of contraction and decreased Myh2 expression while slow-twitch soleus muscles were unaffected. Contrastingly, the SIM50 model (50 mg kg^-1 day^-1 simvastatin for 30 days) produced little evidence of significant skeletal muscle damage. Neither statin treatment protocol caused significant pathological changes to the kidney. The results of this study indicate that the SIM80 model induces a type of SAMS in rodents that resembles the presentation of statin-induced myalgia in humans. The findings support that the SIM80 model is reproducible and can thus be reliably used as a platform to assess the aetiology and treatment of this condition.

Kidney Mass Reduction Leads to l-Arginine Metabolism-Dependent Blood Pressure Increase in Mice

Background

Uninephrectomy (UNX) is performed for various reasons, including kidney cancer or donation. Kidneys being the main site of l‐arginine production in the body, we tested whether UNX mediated kidney mass reduction impacts l‐arginine metabolism and thereby nitric oxide production and blood pressure regulation in mice.

Methods and Results

In a first series of experiments, we observed a significant increase in arterial blood pressure 8 days post‐UNX in female and not in male mice. Further experimental series were performed in female mice, and the blood pressure increase was confirmed by telemetry. l‐citrulline, that is used in the kidney to produce l‐arginine, was elevated post‐UNX as was also asymmetric dimethylarginine, an inhibitor of nitric oxide synthase that competes with l‐arginine and is a marker for renal failure. Interestingly, the UNX‐induced blood pressure increase was prevented by supplementation of the diet with 5% of the l‐arginine precursor, l‐citrulline. Because l‐arginine is metabolized in the kidney and other peripheral tissues by arginase‐2, we tested whether the lack of this metabolic pathway also compensates for decreased l‐arginine production in the kidney and/or for local nitric oxide synthase inhibition and consecutive blood pressure increase. Indeed, upon uninephrectomy, arginase‐2 knockout mice (Arg‐2^−/−) neither displayed an increase in asymmetric dimethylarginine and l‐citrulline plasma levels nor a significant increase in blood pressure.

Conclusions

UNX leads to a small increase in blood pressure that is prevented by l‐citrulline supplementation or arginase deficiency, 2 measures that appear to compensate for the impact of kidney mass reduction on l‐arginine metabolism.

Compensatory Hypertrophy After Living Donor Nephrectomy

BACKGROUND

Previous studies have shown that kidney volume enhances the estimation of glomerular filtration rate (eGFR) in kidney donors. This study aimed to describe the phenomenon of compensatory hypertrophy after donor nephrectomy as measured on computerized tomographic (CT) scans.

METHODS

An institutional Domain Specific Review Board (DSRB)-approved study involved approaching kidney donors to have a follow up CT scan from 6 months to 1 year after surgery; 29 patients participated; 55% were female. Clinical chart review was performed, and the patient's remaining kidney volume was measured before and after surgery based on CT scans. eGFR was determined with the use of the Modification of Diet in Renal Disease equation.

RESULTS

Mean parenchymal volume of the remaining kidney for this population (mean age, 44.3 ± 8.5 y) was 204.7 ± 82.5 cc before surgery and 250.5 ± 113.3 cc after donor nephrectomy. Compensatory hypertrophy occurred in 79.3% of patients (n = 23). Mean increase in remaining kidney volume was 22.4 ± 23.2% after donor nephrectomy in healthy individuals. Over a median follow-up of 52.9 ± 19.8 months, mean eGFR was 68.9 ± 12.4 mL/min/1.73 m(2), with 24.1% of patients (n = 7) in chronic kidney disease grade 3. Absolute and relative change in kidney volume was not associated with sex, race, surgical approach, or background of hypertension (P = NS). There was a trend of decreased hypertrophy with increasing age (P = .5; Spearman correlation, -0.12).

CONCLUSIONS

In healthy kidney donors, compensatory hypertrophy of the remaining kidney occurs in 79.3% of the patients, with an average increment of about 22.4%. Older patients may have a blunted compensatory hypertrophy response after surgery.