Effects of prolonged immobilization on sequential changes in mineral and bone disease parameters

Early Biomarkers of Altered Renal Function and Orthostatic Intolerance During 10-day Bedrest

Exposure to actual or simulated microgravity results in alterations of renal function, fluid redistribution, and bone loss, which is coupled to a rise of urinary calcium excretion. We provided evidence that high calcium delivery to the collecting duct reduces local Aquaporin 2 (AQP2)-mediated water reabsorption under vasopressin action, thus limiting the maximal urinary concentration to reduce calcium saturation. To investigate early renal adaptation into simulated microgravity, we investigated the effects of 10 days of strict bedrest in 10 healthy volunteers. We report here that 10 days of inactivity are associated with a transient, significant decrease (day 5) in vasopressin (copeptin) paralleled by a decrease in AQP2 excretion, consistent with an increased central volume to the heart, resulting in reduced water reabsorption. Moreover, bedrest caused a significant increase in calciuria secondary to bone demineralization paralleled by a decrease in PTH. Urinary osteopontin, a glycoprotein exerting a protective effect on stone formation, was significantly reduced during bedrest. Moreover, a significant increase in adrenomedullin (day 5), a peptide with vasodepressor properties, was observed at day 5, which may contribute to the known reduced orthostatic capacity post-bedrest. We conclude that renal function is altered in simulated microgravity and is associated with an early increase in the risk of stone formation and reduced orthostatic capacity post-bedrest within a few days of inactivity.

Salivary levels of phosphorus and urea as indices of their plasma levels in nephropathic patients

BACKGROUND

Phosphorus and urea are measurable in saliva. Measurements of saliva phosphorus (S-Pho) and saliva urea (S-Urea) could be useful because of low invasivity. Data are limited to saliva tests methodology and to correlations between plasma and saliva compositions. S-Pho and S-Urea were investigated focusing on blind duplicates, differences between collection sites, differences between collection times, freezing-thawing effects, and plasma-saliva correlations.

METHODS

Tests were performed using fresh saliva collected by synthetic swap early morning after overnight fast (standard). Methodology was investigated in fifteen healthy volunteers. Plasma-saliva correlations were investigated in thirty nephropathic outpatients.

RESULTS

S-Pho and S-Urea in all measurements ranged above detection limits (0.3 mmol/L). In healthy volunteers, S-Pho and S-Urea were similar in duplicates (results for S-Pho and S-Urea: % difference between samples ≤ 4.85%; R between samples ≥ .976, P < .001), in samples from different mouth sites (≤4.24%; R ≥ .887, P < .001), and in samples of different days (≤5.61%; R ≥ .606, P < .01) but, compared to standard, were substantially lower in after-breakfast samples (-28.0% and -21.3%; R ≥ .786, P < .001) and slightly lower in frozen-thawed samples (-12.4% and -5.92%; R ≥ .742, P < .001). In nephropathic patients, S-Pho was higher than but correlated with plasma phosphorus (saliva/plasma ratio 4.80; R = .686, P < .001), whereas S-Urea and plasma urea were similar and correlated with each other (saliva/plasma ratio 0.96; R = .944, P < .001). Post-dialysis changes in S-Pho and S-Urea paralleled post-dialysis changes in plasma phosphorus and urea.

CONCLUSION

S-Pho and S-Urea reflect plasma phosphorus and plasma urea. Early morning fasting fresh samples are advisable because collection time and freezing-thawing affect saliva tests.

Analysis of the kinetics of the parathyroid hormone, and of associated patient outcomes, in a cohort of haemodialysis patients

Background

Observational studies have recently associated a decrease in serum parathyroid hormone (PTH) level with a higher rate of mortality among hemodialysis (HD) patients. Decreases in PTH level can result from medical intervention (MPD) and surgical parathyroidectomy (PTX), or may occur spontaneously, usually associated with an underlying malnutrition-inflammation syndrome (SPD). The aim of our study was to prospectively identify the incidence of decreases in PTH level in a cohort of HD patients and the frequency distribution of the different causes (MPD, PTX and SPD), as well as to evaluate the survival outcomes for each PTH group (MPD, PTX and SPD) compared to patients who did not experience a PTH decrease over the first 36 months of the study (NPD).

Methods

The 197 patients receiving HD at our center in January 2010, and meeting our eligibility criteria, were enrolled in our prospective study, and were observed for a period of 60 months. A decrease in PTH level >50 % between two successive PTH measurements obtained within an interval <3 months was defined as a significant event. MPD referred to a decrease in PTH due to an increased oral calcium intake, increased dialysate calcium concentration (DCC), increased alfacalcidol use, or use of cinacalcet therapy. A surgical 7/8 PTX was performed in young patients or in patients in whom cinacalcet therapy failed. SPD referred to a decrease in PTH related to a medical or surgical event. Baseline characteristics among patients in each group (MPD, PTX, SPD, and NPD) were evaluated using Fisher’s exact test. The 60-month survival was evaluated using Kaplan-Meier and Cox multivariable proportional hazards models. Univariate and multivariate Cox analyzes were used identify variables with mortality. The relative risk of mortality was expressed as a hazard ratio (HR).

Results

The distribution of the 197 patients forming our four study groups was 34 % in the NPD group, 35 % in the SPD group, 25 % in the MSD group and 6 % in the PTX group. Among patients in the SPD group, the main acute comorbid conditions were peripheral vascular and cardiac complications, sepsis, fractures, and cancers with an increase in serum CRP level (from 14.3 ± 18 to 132 ± 90 mg/L) and a decrease in serum albumin (from 33 ± 4.5 to 28.6 ± 4 g/L). In the MPD group, the main therapeutic change was an increase in DCC, either independently or in association with cinacalcet therapy. The median survival rate among patients was 10 months for SPD, compared to 22 months among patients in the MPD group (p < 0.001). Using multivariable Cox model and taking the NPD group as reference, the risk of mortality was lower among patients in the MPD group (HR, 0.42[0.2-0.87] p = 0.01), with survival being comparable for the SPD and NPD groups (HR, 1.3 [0.75-2.2]). No mortality was observed in the PTX group.

Conclusion

The poor outcomes associated with SPD, related to acute comorbid conditions, should not lead to undertreat secondary hyperparathyroidism whose appropriate medical or surgical therapies are associated with better outcomes.

Effects of bed-rest on urea and creatinine: correlation with changes in fat-free mass

Background

Bed-rest experiments are designed for investigation on catabolic effects of hypokinetic conditions and/or for microgravity simulation in on-ground aerospace research. Bed-rest effects include a reduction in fat-free mass and muscle mass. Urea and creatinine are catabolites of endogenous protein and of muscular energetic metabolism which are excreted mainly by the kidney. The study investigated on urea, creatinine, and kidney function during bed-rest.

Methods

Twenty healthy young men underwent a 7-day adaptation period (day-6 to day-0) and a 35-day bed-rest experiment (day1 to day35) during normocaloric diet. Urine were collected from day-3 to day0 (baseline) and from day1 to day35. Blood samples and anthropometrical data were collected at day0 (baseline) and bed-rest days 7, 14, 21, 28, and 35.

Results

Bed-rest reduced plasma volume, weight, fat-free mass, and muscle mass (P<0.001). During bed-rest there was a transient increase in plasma and urinary urea, a decrease in plasma creatinine, and no change in urinary creatinine. The overall integral of changes from day0 to day35 was on average +101.7 mg/dL for plasma urea (95%CI = +43.4/+159.9), +82.2 g/24 h for urinary urea (95%CI = +55.8/+108.7), −2.5 mg/dL for plasma creatinine (95%CI = −3.1/−1.9). Bed-rest reduced plasma cistatyn C also, which was used as mass-independent marker of glomerular filtration rate (−13.1%, P<0.05). Correlations with final reduction in fat-free mass and muscle mass were significant for the overall integral of changes in urinary urea from day0 to day35 (R = 0.706, P<0.001) and for early changes in urinary urea and plasma urea from day0 to day7 (R = 0.566, P = 0.009 and R = 0.715, P<0.001, respectively).

Conclusions

Study results shows that urea is a marker of catabolic conditions secondary to hypokinetic conditions.

A decrease in aquaporin 2 excretion is associated with bed rest induced high calciuria

Background

Exposure to microgravity or immobilization results in alterations of renal function, fluid redistribution and bone loss, which couples to a rise of urinary calcium excretion. We recently demonstrated that high calcium delivery to the collecting duct reduces local Aquaporin-2 (AQP2) mediated water reabsorption under vasopressin action, thus limiting the maximal urinary concentration and reducing calcium saturation. To investigate renal water balance adaptation during bed rest, a model to mimic the effects of microgravity on earth, the effect of changes in urinary calcium on urinary AQP2 excretion were assessed.

Methods

Ten healthy men (aged 21-28 years) participated in the experiment. Study design included 7 days of adaptation and 35 days of continuous bed rest (days -6 to 0 and 1 to 35, respectively) under controlled diet. Food records and 24-hour urine samples were collected daily from day -3 to 35. Changes in blood hematocrit were used as an indirect index of plasma volume changes. AQP2 excretion was measured by ELISA.

Results

Bed rest induced bone demineralization and a transient increase in urinary calcium followed by transient decrease in AQP2 excretion, which can reduce the urine concentrating ability causing plasma volume reduction. The return of calciuria to baseline was followed by a recovery of AQP2 excretion, which allows for a partial restoration of plasma volume.

Conclusions

These results further support the view that urinary calcium can modulate the vasopressin-dependent urine concentration through a down-regulation of AQP2 expression/trafficking. This mechanism could have a key role in the prevention of urine super-saturation due to hypercalciuria.