Impact of an environmental relevant concentration of 17α-ethinylestradiol on the cardiac function of bullfrog tadpoles

This study evaluated if a concentration of 17α-ethinylestradiol (EE2 – 10 ng L−1 for 96 h) normally found in Brazilian surface waters exerts any impact on cardiac function of bullfrog tadpoles (25 Gosner stage), Lithobates catesbeianus. During exposure, the animals' activity level (AL -% of active individuals) was monitored twice a day. Then, the in loco heart rate (fH – bpm) was determined, as well as the relative ventricular mass (RVM - % of body mass). Afterwards, cardiac ventricles were mounted for isometric force recordings (CS – mN mm−2), and determination of the cardiac pumping capacity (CPC – mN mm−2 min−1). EE2 did not affect tadpoles' AL, although it resulted in a tachycardia in animals exposed to EE2 (fH = 66 bpm) when compared to controls (fH = 52 bpm), suggesting that EE2 acts directly on the cardiac muscle of tadpoles, rather than being a result of an increased cardiac demand due to a higher activity level (i.e., avoidance response). Additionally, EE2 exerted a positive inotropic response, which resulted in a higher CPC, which occurred independently of an increase in the number of myofibrils of EE2-exposed animals, since RVM remained similar between experimental groups. Thus, the increase on cardiac demand induced by the exposure to EE2 elevates considerably the animal energy expenditure, diverting a large amount of energy that tadpoles could use for their growth and development. These alterations can make amphibians more susceptible to predators and reduce the likelihood to reach reproductive stage. •Environmental relevant 17α-ethinylestradiol (EE2) concentrations alter the heart function of bullfrog tadpoles.•EE2 induced tachycardia, but did not change tadpoles' activity level, pointing to its direct effect on the heart.•EE2 increased the cardiac force, which seems to be due to a PKA-dependent increase of transarcolemmal Ca2+ influx.•EE2 elevated the cardiac energetic demand, reducing energy availability to other morphogenetic processes.•The lethargy can make the tadpoles more vulnerable to predators, as well less competitive for obtaining food.