Origin of the spin Seebeck effect in compensated ferrimagnets.

Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.

The effect of maintenance immunosuppression medication on the change in kidney allograft function.

BACKGROUND: The optimum maintenance immunosuppression regimen for kidney transplant recipients is uncertain. In this study we determined the effect of maintenance immunosuppression medications on the rate of kidney allograft function loss defined by the annualized change in glomerular filtration rate (GFR). METHODS: We studied 40,963 first kidney only transplant recipients between 1987 and 1996 with allograft survival of at least two years in the United States Renal Data System. Linear regression methods were applied to serial GFR estimates after transplantation to determine the annualized change in GFR. Patients were classified according to the type of maintenance calcineurin and purine metabolism inhibitor received after transplantation. Multiple linear regression was used to determine the independent effect of maintenance immunosuppression medications on the annualized change in GFR (mL/min/1.73m2/year). RESULTS: Compared to patients who received cyclosporine microemulsion (Neoral), a slower decline in GFR was observed in tacrolimus-treated patients (1.60 mL/min/1.73m2/year, 95% CI 1.22-1.97, P < 0.001) and patients who did not receive calcineurin inhibitors (0.82 mL/min/1.73m2/year, 95% CI 0.08-1.56, P= 0.03). In contrast, compared to compared to patients who received Neoral, a faster decline in GFR was observed in patients who received the original oil-based formulation of cyclosporine (Sandimmune) (-0.16 mL/min/1.73m2/year, 95% CI -0.003 to -0.32, P= 0.04) and patients with unknown calcinuerin inhibitor exposure (-2.11 mL/min/1.73m2/year, 95% CI -2.27 to -1.95, P < 0.001). Compared to patients who received azathioprine, patients who received mycophenolate mofetil (MMF) had a slower decline in GFR (0.61 mL/min/1.73m2/year, 95% CI 0.14-1.08, P= 0.01) and patients with unknown purine metabolism inhibitor exposure had a faster decline in GFR (-0.61 mL/min/1.73m2/year, 95% CI -0.75 to -0.47, P < 0.001.) In a subgroup analysis of patients who received a transplant after 1993, the decline in GFR was slower for tacrolimus compared to neoral treated patients (1.64 mL/min/1.73m2/year, 95% CI 1.15-2.14, P < 0.001) but was not different for MMF compared to azathioprine-treated patients (0.24 mL/min/1.73m2/year, 95% CI -0.38-0.85, P= 0.45). CONCLUSION: Tacrolimus and MMF were the calcineurin inhibitor and purine metabolism inhibitor associated with the most favorable effects on rates of change in allograft function. Because most transplant recipients establish a low baseline level of allograft function, the effect of immunosuppression medication on GFR decline should be considered when selecting a maintenance immunosuppression regimen.

The change in allograft function among long-term kidney transplant recipients.

Long-term kidney allograft survival continues to remain an elusive goal. Kidney transplant recipients are believed to be at high risk for loss of allograft function, and new, potentially non-nephrotoxic immunosuppressive medications are advocated to improve long-term allograft survival. To evaluate the efficacy of such therapeutic interventions, information regarding the change in GFR among kidney transplant recipients with long-term allograft survival is needed. We studied 40,963 transplant recipients between 1987 and 1996 with allograft survival of at least 2 yr in the United States Renal Data System. Linear regression methods were applied to serial GFR estimates after transplantation. The baseline mean GFR at 6 mo after transplantation was 49.6 +/- 15.4 ml/min per 1.73 m(2). During the mean follow-up of 5.7 +/- 2.3 yr, the mean +/- standard error of the change in GFR was -1.66 +/- 6.51 ml/min per 1.73 m(2) per year (median, -0.94 L/min per 1.73 m(2) per year). A total of 12,583 (30%) of patients had improvement in GFR, 8133 (20%) patients had no change in GFR, and 20,247 (50%) patients had decline in GFR. It is concluded that, although most patients had significant impairment of GFR at baseline, the decline in GFR was slow and many patients had either no change or improvement in GFR. Strategies to improve long-term kidney allograft survival that increase baseline allograft function may be more effective than strategies to slow the decline in GFR.