Renal functional reserve in cyclosporin-treated recipients of kidney transplant

The physiological basis of renal nuclear medicine

Renal physiology underpins renal nuclear medicine, both academic and clinical. Clearance, an important concept in renal physiology, comprises tissue uptake rate of tracer (tissue clearance), disappearance rate from plasma (plasma clearance), appearance rate in urine (urinary clearance) and disappearance rate from tissue. In clinical research, steady-state plasma clearances of para-amino-hippurate and inulin have been widely used to measure renal blood flow (RBF) and glomerular filtration rate (GFR), respectively. Routinely, GFR is measured at non-steady state as plasma clearance of a filtration agent, such as technetium-99m diethylenetriaminepentaacetic acid. Scaled to three-dimensional whole body metrics rather than body surface area, GFR in women is higher than in men but declines faster with age. Age-related decline is predominantly from nephron loss. Tubular function determines parenchymal transit time, which is important in renography, and the route of uptake of technetium-99m dimercaptosuccinic acid, which is via filtration. Resistance to flow is defined according to the pressure-flow relationship but in renography, only transit time can be measured, which, being equal to urine flow divided by collecting system volume, introduces further uncertainty because the volume is also unmeasurable. Tubuloglomerular feedback governs RBF and GFR, is regulated by the macula densa, mediated by adenosine and renin, and can be manipulated with proximal tubular sodium-glucose cotransporter-2 inhibitors. Other determinants of renal haemodynamics include prostaglandins, nitric oxide and dopamine, while protein meal and amino acid infusion are used to measure renal functional reserve. In conclusion, for measuring renal responses to exogenous agents, steady-state para-amino-hippurate and inulin clearances should be replaced with rubidium-82 and gallium-68 EDTA for measuring RBF and GFR.

Functional Reserve of the Kidney

An exploration of the normal limits of physiologic responses and how these responses are lost when the kidney is injured rarely occurs in clinical practice. However, the differences between "resting" and "stressed" responses identify an adaptive reactiveness that is diminished before baseline function is impaired. This functional reserve is important in the evaluation of prognosis and progression of kidney disease. Here, we discuss stress tests that examine protein-induced hyperfiltration, proximal tubular secretion, urea-selective concentration defects, and acid retention. We discuss diseases in which these tests have been used to diagnose subclinical injury. The study and follow-up of abnormal functional reserve may add considerable understanding to the natural history of CKD.

Cyclosporine-a-based immunosuppression and renal functional reserve in organ-transplant patients

The glomerular filtration rate (GFR) may increase after oral protein overload or after an amino acid infusion. Renal functional reserve (RFR) is defined as the difference between the poststimulation and the baseline glomerular filtration rate. RFR has been studied in transplant patients. The results have been conflicting because RFR may be affected by donor age, the time from transplantation, the type of immunosuppression, the basal GFR, the body surface area, and the presence of calcium channel blocker therapy. We observed that during the first year posttransplantation renal recipients maintained on cyclosporine (CsA) therapy retain a level of RFR that represents approximately 30% of the baseline GFR. Moreover, hypertensive cyclosporine treated renal transplant patients on calcium channel blocker therapy do not exhibit permanent glomerular hyperfiltration until 8 months posttransplantation. Finally, both renal and heart transplant patients on cyclosporine therapy possess renal reserve, but the single renal graft in renal transplant patients shows a proportionally higher baseline GFR and a better ability to exhibit a RFR than the two native kidneys in heart transplant patients. Many studies, although not all, have documented a positive RFR in both children and adult cyclosporine-treated kidney graft recipients, demonstrating that hyperfiltration is not the rule following single kidney transplantation.

Structural and functional correlations in stable renal allografts

BACKGROUND

Renal functional reserve (RFR) has been proposed as a surrogate marker of renal mass, but its significance in well-functioning renal transplants is controversial. Thus, we used early protocol biopsies to analyze structural and functional correlations in stable grafts.

METHODS

We studied 32 cyclosporine (CsA)-treated stable cadaveric transplants at 5 months. Biopsies were evaluated according to Banff criteria and histomorphometry. Inulin and p-aminohippurate clearances were used to calculate glomerular filtration rate (GFR) and effective renal plasma flow (ERPF). RFR after an amino acid infusion (RFR-AA) and after a combined amino acid and dopamine infusion (RFR-AA-DOPA) was evaluated.

RESULTS

Baseline GFR was 54 +/- 16 mL/min/1.73 m2, and ERPF was 219 +/- 55 mL/min/1.73 m2. RFR-AA was 9% +/- 13%, and RFR-AA-DOPA was 22% +/- 20%. RFR-AA correlated with CsA dose (R = 0.39; P = 0.02), whereas RFR-AA-DOPA correlated with CsA dose (R = 0.36; P = 0.04) and CsA levels (R = 0.40; P = 0.02). The only histological parameter associated with RFR was the presence of arteriolar hyalinosis (AH). Patients showing an AH score of 1 or greater (n = 7) had lower RFR-AA (0% +/- 9% versus 11% +/- 13%; P = 0.02) and lower RFR-AA-DOPA (9% +/- 17% versus 26% +/- 19%; P = 0.03). Multivariate analysis showed that an AH score of 1 or greater, but not CsA dose or levels, was associated with RFR-AA (R = 0.42; P = 0.01). RFR-AA-DOPA was associated with hyaline arteriolar damage (R = 0.43; P = 0.01), as well as CsA levels (R = 0.54; P = 0.006).

CONCLUSION

The presence of AH is the only histological parameter associated with impaired RFR in well-functioning grafts.

Effects of glycine on glomerular filtration rate and segmental tubular handling of sodium in conscious rats

1. Infusion of the amino acid glycine leads to an increase in effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) by a mechanism that possibly involves stimulation of nitric oxide (NO). Because NO also increases proximal tubular fluid output (Vprox) by inhibition of proximal tubular Na+ reabsorption and modulation of the tubuloglomerular feedback system, we hypothesized that glycine would increase Vprox as measured by lithium clearance (CLi). 2. In the first series of experiments, the effect of glycine infusion (4 mg/min) was examined in conscious, unstressed, chronically catheterized rats. In an additional series of experiments, the effect of glycine was examined under similar conditions in rats pretreated with a NO synthase (NOS) inhibitor (NG-nitro-L-arginine methyl ester (L-NAME), 2.5 microg/min). 3. Glycine significantly increased ERPF (from 3268 to 4018 microL/min per 100 g bodyweight (BW)), GFR (from 874 to 1009 microL/min per 100 g BW), CLi (from 275 to 461 microL/min per 100 g BW) and Na+ clearance (CNa; from 2.9 to 14.0 microL/min per 100 g BW). Fractional excretion of lithium (FELi; from 32 to 46%) and CNa/CLi (from 0.99 to 2.99%) also rose, indicating inhibition of proximal and distal nephron Na+ reabsorption, respectively. In the rats pretreated with L-NAME, similar haemodynamic and tubular responses to glycine infusion were seen, suggesting that the effects were not mediated by NO. 4. We conclude, that glycine increases ERPF and GFR and it also inhibits proximal and distal nephron Na+ reabsorption leading to an increase in CLi and CNa. There was no indication that any of these effects were mediated by NO.

Transient renal effects of sildenafil in male kidney transplant recipients

BACKGROUND

Sildenafil (Viagra) improves erection by sustaining Guanosine 3', 5'-cyclic monophosphate (cGMP)-mediated smooth muscle relaxation in the corpus cavernosum. It also induces systemic vasodilation, resulting in a minor decrease in blood pressure. We evaluated the effect of one dose of sildenafil on graft function and hemodynamics in impotent male transplant recipients.

METHODS

Two sets of combined lithium, inulin, and p-amino hippurate clearance studies were conducted, with and without sildenafil (100 mg orally) in 11 male kidney transplant recipients (KTRs).

RESULTS

Sildenafil increased glomerular filtration rate by 14+/-4 from the baseline value of 55+/-7 ml x min(-1) x 1.73 m2(-1) (P<0.01), whereas calculated renal vascular resistances decreased by 40+/-18 from the baseline value of 247+/-29 mmHg/L x min(-1) x 1.73 m2-1 (P<0.05).

CONCLUSIONS

The oral administration of sildenafil in KTRs did not impair the function of the graft. In terms of renal physiology, the observed modifications did not warrant any specific precautions when offering sildenafil to KTRs suffering from erectile dysfunction.

Renal response to a protein load persists during long-term follow-up of children after renal transplantation

BACKGROUND

Kidney donors and transplant recipients may be at risk of complications from glomerular hyperfiltration of the single kidney. It has been assumed that tests of the existence of renal functional reserve [delta glomerular filtration rate (deltaGFR), delta effective renal plasma flow (deltaERPF)] can be used to demonstrate hyperfiltration. It would therefore be of interest to evaluate the response of the kidney graft to a protein load. i.e., testing the renal reserve and to find out whether a reduction in baseline GFR is preceded by a loss of deltaGFR.

METHODS

We repeatedly studied the change in GFR and renal plasma flow (ERPF) after an oral protein load in 30 children after renal transplantation (Tx). Follow-up time was 1.0-8.0 years. Renal function was evaluated with the clearances of inulin and para-aminohippuric acid (PAH). Seven recipient/donor pairs were examined twice (median 0.3 and 4 years, after Tx).

RESULTS

The baseline GFR and ERPF remained stable throughout the follow-up and the increase after stimulation (deltaGFR and deltaERPF) did not change in the whole group of Tx children over the years. However, a reduction in the baseline GFR from the first to the last investigation occurred in 23 of 30 children. In the 23 patients whose baseline GFR decreased, deltaGFR was still preserved. In the recipient/donor pairs, the baseline GFR and ERPF were the same, but on the second investigation, donors showed higher deltaGFR.

CONCLUSION

Despite fairly low baseline GFR and ERPF values in the Tx children, no change occurs in the capacity to increase GFR and ERPF after a protein load during follow-up, which suggests that they are not maximally hyperfiltrating.

Mycophenolate mofetil-based, cyclosporine-free induction and maintenance immunosuppression: first-3-months analysis of efficacy and safety in two cohorts of renal allograft recipients

BACKGROUND

The currently used macrolide immunosuppressants, i.e., cyclosporine and tacrolimus, exert considerable nephrotoxicity. We aimed to avoid the nephrotoxic effects by applying a cyclosporine-free regimen for the induction as well as for the maintenance treatment of renal allograft recipients using mycophenolate mofetil (MMF) as the primary immunosuppressant.

METHODS

Thirteen patients were converted from cyclosporine (CsA) to MMF monotherapy. For 4 weeks, MMF (2 g/day) was added to the CsA treatment, before CsA was tapered by weekly steps of 25 mg/day and without "safeguard treatment" with additional immunosuppressants. In a second approach, 12 patients older than 50 years, and receiving a renal graft from a donor older than 50 years, were treated primarily with MMF combined with steroids and an induction therapy using antithymocyte globulin, and without the addition of CsA.

RESULTS

Thirteen long-term renal transplant patients could be converted from CsA to MMF monotherapy. Conversion resulted in an immediate and long-lasting improvement of their median creatinine values by 20%. No serious adverse events occurred. In the second cohort of 12 patients, MMF was used as the primary immunosuppressant. All patients are alive and no grafts were lost after 4 months (n= 12) and after 6 months (n=7). The median creatinine values achieved after 4 and 6 months were 1.16+/-0.25 and 1.30+/-0.21 mg/dl, respectively. One patient was converted to CsA because of a reversible rejection episode (8.3%), and another patient was converted because of cytomegalovirus disease. Major complications consisted of wound-healing disturbances (16.6%) and cytomegalovirus infections (41.6%).

CONCLUSION

MMF monotherapy can be safely applied as long-term maintenance immunosuppression with improvement of renal function. Steroids are not required as an adjunct to MMF. MMF monotherapy, in the absence of drug-related nephrotoxicity, is particularly beneficial for grafts derived from marginal donors, such as donors of advanced age.

Functional glomerular reserve in recipients of en bloc pediatric transplant kidneys

BACKGROUND

The transplantation of an adequate renal mass is increasingly recognized to be of importance. The improved graft survival is probably due to a lesser risk of developing hyperfiltration-associated lesions.

METHODS

We have reviewed the glomerular reserve in our recipients of en bloc pediatric transplant kidneys after an intravenous amino acid overload and compared them to single adult kidney transplant recipients.

RESULTS

En bloc transplants evidenced increased glomerular filtration rate as compared with baseline as from the second hour of amino acid infusion (from 71+/-14 to 84.9+/-17 ml/min, 1.73 m2, P<0.05) and increased renal plasma flow as from the third hour (from 335+/-116 to 402+/-155 ml/min, 1.73 m2, P<0.05). In the single adult kidney recipient group, no change was seen either in the glomerular filtration rate (from 62.5+/-13 to 58.1+/-13 ml/min, 1.73 m2, P=NS) nor in renal plasma flow (from 354+/-125 to 304+/-98 ml/min, 1.73 m2, P=NS).

CONCLUSIONS

These results show that patients receiving en bloc pediatric kidney transplantations have a greater renal functional reserve and show a lesser risk of hyperfiltration.

Renal hemodynamics in renal transplant recipients. The role of reduced kidney mass and cyclosporine administration

It has been hypothesized that both the cyclosporine (CsA) treatment and the reduction of renal mass may affect the renal hemodynamic regulation in kidney transplant recipients. To address this question, we evaluated the renal hemodynamic response to hyperaminoacidemia (i.v. mixed amino acid infusion 3.3 mg/kg/minute for 150 minutes) in four study groups: (1) 16 renal transplant recipients (Tx), (2) 6 uninephrectomized (Nx) subjects, (3) 7 subjects treated with CsA for chronic uveitis (CsA), and (4) 9 normal controls (NC). In response to amino acid administration (AA), glomerular filtration rate (GFR) rose significantly in NC subjects (80 +/- 6 vs. 91 +/- 6 ml/minute; P<0.01) and Nx patients (57 +/- 3 vs. 68 +/- 7 ml/minute; P<0.01) and failed to increase in Tx recipients (39 +/- 3 vs. 37 +/- 3 ml/minute) and CsA-treated patients (58 +/- 3 vs. 53 +/- 4 ml/minute). Renal plasma flow (RPF) did not change in Tx recipients (243 +/- 27 vs. 235 +/- 25 ml/minute) but rose significantly in all other groups (257 +/- 17 vs. 344 +/- 33 in NX, 364 +/- 6l vs. 441 +/- 55 in CsA, 412 +/- 49 vs. 472 +/- 72 ml/min in NC subjects; P<0.05 vs. basal). Basal renal vascular resistances were significantly higher in Tx (0.29 +/- 0.04 mmHg/mlxmin; P<0.01 vs. all other groups) than in Nx (0.21 +/- 0.01 mmHg/mlxmin), CsA (0.23 +/- 0.04 mmHg/mlxmin) (both P<0.01 vs. NC subjects), and NC subjects (0.13 +/- 0.02 mmHg/mlxmin). Renal vascular resistance failed to decline in Tx (0.31 +/- 0.04 mmHg/mlxmin) during AA infusion but declined significantly in all other groups. In Tx, basal GFR was positively correlated to renal allograft volume (r=0.547, P<0.03); however, no relationship was found between the latter and basal RPF or the AA induced changes in GFR. In summary, the present study demonstrates that in kidney transplant recipients and in CsA-treated subjects, the renal functional reserve to hyperaminoacidemia is impaired. This is at variance to what is observed in normal controls and uninephrectomized subjects. In renal transplant recipients, basal but not amino acid stimulated GFR correlates with renal allograft volume. We conclude that basal GFR is related to renal volume in Tx and that the response to hyperaminoacidemia seems to be affected by chronic CsA administration.