A novel cell isolation technique was used to characterize cadmium and calcium uptake in distinct populations of gill cells from the adult rainbow trout (Oncorhynchus mykiss). A specific population of mitochondria-rich (MR) cell, termed the PNA+ MR cell (PNA is peanut lectin agglutinin), was found to accumulate over threefold more 109Cd than did PNA- MR cells, pavement cells (PV cells), and mucous cells during a 1-h in vivo exposure at 2.4 microg/l 109Cd. In vitro 109Cd exposures, performed in standard PBS and Cl- -free PBS, at concentrations from 1 to 16 microg/l 109Cd, were also carried out to further characterize Cd2+ uptake kinetics. As observed during in vivo experiments, PNA+ MR cells accumulated significantly more 109Cd than did other cell types when exposures were performed by an in vitro procedure in PBS. Under such conditions, Cd2+ accumulation kinetics in all cell types could be described with Michaelis-Menten relationships, with Km values of approximately 3.0 microg/l Cd (27 nM) for both MR cell subtypes and 8.6 microg/l Cd (77 nM) for PV cells. In similar experiments performed in Cl- -free conditions, a significant reduction in 109Cd accumulation in PNA+ MR cells was seen but not in PNA- MR or in PV cells. In vitro 45Ca fluxes were also performed to determine the cellular localization of Ca2+ transport in these functionally distinct populations of gill cells. 45Ca uptake was most pronounced in PNA+ MR cells, with levels over threefold higher than those found in either PNA(-) MR or in PV cells. Results from the present study suggest that the PNA+ MR cell type is a high-affinity and high-capacity site for apical entry of Cd2+ and Ca2+ in the gill epithelium of rainbow trout.