A mechanistic approach to acute lead toxicity in the rainbow trout: Investigations of lead-induced ionoregulatory disruption

Abstract

Relative to other metals, little is known about lead toxicity in fish. The use of predictive models such as the biotic ligand model (BLM) has been limited, a situation that is at least partially due to the lack of understanding of lead's acute toxic mechanism and characterization of key binding sites involved in this toxicity. Using the rainbow trout as a model species, the acute toxic mechanism for lead was found to be ionoregulatory disruption. While having no apparent respiratory or acid/base effects, Pb exposure resulted in significant ionoregulatory impacts that affected Ca2+ homeostasis, as well as Na+ and Cl- balance. Active Ca2+ uptake by the gills obeyed typical Michaelis-Menten kinetics, and Pb interacted in a competitive fashion with the uptake process. Exposure to increasing waterborne Pb concentrations resulted in significant increases in Km value while Jmax showed little or no change. A slower, non-competitive interaction occurred after prolonged Pb-exposure, evidenced by a significant reduction of high-affinity Ca2+ -ATPase activity that correlated well with branchial Pb accumulation. Conversely, calcium had a protective effect against branchial Pb accumulation, this relationship being predominately competitive in nature. Voltage-independent calcium channel blockers La3+, Cd, and Zn significantly reduced gill Pb burden while the voltage-dependent, L-type calcium channel blockers, nifedipine and verapamil, did not, suggesting Pb enters fish by a similar mechanism to that of Ca2+. Stimulated stanniocalcin release by CaCl2 injection also significantly reduced branchial Pb accumulation. Based on the evidence presented in this thesis, it is apparent that acute Pb toxicity occurs by ionoregulatory disruption. It is likely that Pb shares a similar uptake pathway as that for Ca2+ and that resulting accumulation results in disruption of Ca2+ influx as well as Na+ and Cl- balance. This study has provided data essential to the characterization of key binding sites involved in Pb toxicity, and ultimately, validates the development and application of predictive models such as the BLM.