Elimination and biological half-time of cadmium in kidney

Introduction
Cadmium (Cd) is an occupational and environmental contaminant which can potentially affect human health at relatively low concentrations. Sources of exposure in the general population are mainly diet and smoking. Cd accumulates in the kidney where it has a long biological half-time (T1/2). The aim of this study was to investigate the T1/2 of Cd in kidney.

Methods
In total 152 healthy kidney donors were recruited to the study. A biopsy from the kidney cortex was available for Cd analysis in 109 donors. A 24h urine sample was also collected and Cd concentrations of the kidney biopsies (K-Cd) and the urine samples (U-Cd) were analysed using ICP-MS. The dry weight metal concentration of the kidney was transformed to wet weight concentration (K-Cdconc) by multiplying by 0.18. The kidney weight for each subject was estimated from body surface area. To obtain the estimated total amount of Cd in the kidney (K-Cdtot), the estimated kidney weights were multiplied by the K-Cdconc and divided with 1.25 (higher kidney Cd in cortex then in remaining part). The Cd elimination coefficient (k) and T1/2 were calculated from the association between U-Cd/24h and K-Cdtot. The half-time of Cd in kidney was assessed using the equation T1/2=ln(2)/k assuming a one-compartment model.

Results
The elimination coefficient in the kidney was estimated to 0.00011 assuming a straight line equation without intercept. Thus the portion of K-Cdtot excreted in urine per 24h was 0.011%. The calculated T1/2 of Cd in the kidney was 17 years. There was however a significant difference between the slopes for K-Cdconc<15 µg/g and ≥15 µg/g (median concentration), and separate calculations of these half-times showed 10.2 and 19.2 years, respectively.

Discussion
This is the first time T1/2 has been calculated using living kidney donors representative for the healthy general population.