Cadmium uptake kinetics in rat hepatocytes: correction for albumin binding

The relationship between cytotoxicity and kinetics of cadmium uptake was investigated in primary rat hepatocyte cultures. Primary rat hepatocytes were exposed to cadmium concentrations ranging from 1.0 to 80 micro M in albumin-free buffer or 32 to 8,000 microM in buffer containing physiological concentrations of bovine serum albumin (600 micro M) for 1 h, and cellular toxicity was observed at 23 h postexposure. Hepatocytes exposed to cadmium in the presence of albumin appeared less sensitive to cadmium toxicity when compared to cells exposed in the absence of albumin. The experimentally derived 23-h postexposure EC(50)s for hepatocytes exposed to cadmium in both presence and absence of albumin was 65.5 +/- 2.4 and 14.3 +/- 3.9 microM, respectively. A Scatchard plot of cadmium binding to albumin suggested two high-affinity binding sites. The observed uptake of cadmium by hepatocytes in the absence and presence of albumin consisted of a composite fast uptake rate and cell membrane association (Component I), and a slow, sustained uptake rate (Component II). Cadmium uptake rates in hepatocytes, based on total medium cadmium concentrations, indicated that Component II uptake rates were four times faster under albumin-free exposure conditions. However, when uptake rates were evaluated, based on the calculated equilibrium concentration of free cadmium in the exposure buffer, uptake rates in hepatocytes exposed in the presence of albumin were two times as fast. This faster cadmium uptake in the presence of albumin may result from diffusion-limited, nonequilibrium conditions occurring at the cell surface.