Modulation of Cd and BaP uptake rate during acute aqueous co-exposure in adult zebrafish (Danio rerio)

Cadmium and Benzo[a]pyrene are two toxicants of great environmental importance given their frequency and ability to cause extensive toxicity in aquatic organisms including fish. There is evidence that fish can modulate their respective uptake rate during simultaneous exposures, albeit the mechanism behind this is poorly understood. The present study aimed to examine this interaction by exposing adult zebrafish to either 89.3 nM Cd, 4.25 nM BaP or a combination of the two for 72 hrs prior to examining the uptake rate of either toxicant via short-term exposures (3-6 hrs) to radiotracers (109Cd and 14C-BaP). Our results showed that Cd uptake rate increased significantly in the gills when animals were pre-exposed to both toxicants simultaneously, resulting in an increased maximum uptake rate (Jmax). The increased Cd uptake rate did not correspond to increased expression of gill Cd transporters such as the epithelium calcium channel (ECaC) or the divalent metal transporter 1 (DMT1). Furthermore, BaP uptake rate increased significantly at the whole-body level when animals were exposed to both 5.03 nM 14C-BaP and 89.3 nM Cd concurrently. Additionally, we ran a time-course and observed BaP uptake rate is highest in the 6-12 hrs following the beginning of the exposure. Our results provide evidence that the increased bioaccumulation of Cd and BaP observed during co-exposures is at least in part due to an increase in uptake rate and is driven by separate mechanisms.