Zebrafish as a model organism to study cadmium and calcium kinetics in developing embryos

Cadmium (Cd(2+)) is extremely toxic to freshwater fish, with LC50's in the sub-micromolar range in ion-depleted waters. Cd(2+) is a biologically non-essential metal, which directly interferes with active calcium (Ca(2+)) uptake in the fish gill. The gill epithelium, which is in intimate contact with the water, represents the primary route of Ca(2+) uptake from the environment. Despite our understanding of Cd(2+) toxicity in juvenile and adult fishes, little is known of the effects of Cd(2+) during early development, when Ca(2+) transport is elevated for optimal growth. This study utilized a standard radioisotopic flux technique to measure the unidirectional uptake of Ca(2+) and Cd(2+) in zebrafish embryos from 24 to 96 hours post fertilization (hpf). Ca(2+) burdens were highest in embryos at 24 hpf. However, over 70% of the Ca(2+) burden at this developmental stage was eliminated following proteolytic removal of the chorion. Post-hatch (72 hpf) zebrafish had significantly lower Ca(2+) burdens compared to embryos at 24 hpf, although levels were again increased in animals by 96 hpf. This result further supported the role of the chorion as a sink for Ca(2+). Cd(2+) transport also changed dramatically during the first 4 days of development. Time-course experiments showed high Cd(2+), contents in embryos at 24 and 96) hpf, compared to the low levels measured at 72 hpf. Embryos at 24 hpf showed linear, non-saturable Cd(2+) kinetics, up to a waterborne Cd(2+) of 100 nM. However, by 72 hpf, zebrafish started demonstrating saturable Cd" kinetics, supporting carrier-mediated Cd(2+) transport. Michaelis-Menten analyses of these kinetic relationships suggested that by 96 hpf, these larval fish were similar to adult zebrafish with regards to Cd(2+) transport. The current studies support the hypothesis that Ca(2+) and Cd(2+) share a common uptake pathway in zebrafish.