Negative impact of a cadmium concentration considered environmentally safe in Brazil on the cardiac performance of bullfrog tadpoles

Abstract

A drastic amphibian decline has been observed worldwide, which can be attributed (among other factors) to exposure to pollutants. Considering that cadmium corresponds to the most rapidly increasing trace metal in the environment, the aim of this work was to evaluate whether the exposure (2 and 16 days) of bullfrog tadpoles to this trace metal, at the concentration currently considered environmentally safe (at 1ppb) in class 1 and 2 waters by the Brazilian Environmental Council, can affect the cardiac performance of these animals. The acute exposure (2 days) of tadpoles to cadmium resulted in a marked bradycardic response, which was correlated with an incomplete cardiac relaxation, without any compensation by improved cardiac twitch force (Fc) or contraction velocity (TPT), nor even by cardiac hypertrophy. Indeed, after 16 days of exposure, the cardiac function of tadpoles became even more depressed due to a marked decrease in Fc, a prolongation of TPT, and also incomplete relaxation (i.e. increases in the ventricle resting tension), without changes in ventricle relative mass. Altogether, the cardiodepressive effects of cadmium (especially after more prolonged exposure periods) impose negative alterations on a tadpole׳s development and also impede adequate homeostatic adjustments to respond appropriately to the exposure to cadmium with increase in energetic demand to counteract the deleterious effects of the xenobiotic. These disturbances can impair tadpoles׳ growth, development and reproduction. It is a fact that allows us to strongly suggest that cadmium concentrations, which are currently considered environmentally safe in Brazil, should be revised.

Keywords: Brazilian Environmental Council Resolutions; Cadmium chloride; Cardiac performance; Lithobates catesbeianus; North American bullfrog tadpoles.