Physiological and Reproductive Effects of Beta Adrenergic Receptor Antagonists in Daphnia magna Edward M. Dzialowski, 1 Philip K. Turner, 1 Bryan W. Brooks 2 1 Department of Biological Sciences, University of North Texas, Denton, TX 76203, USA 2 Department of Environmental Studies, Baylor University, Waco, TX 76798, USA Received: 13 May 2005 /Accepted: 22 August 2005 Abstract. Although pharmaceuticals are increasingly found in surface waters, environmental levels of many of these com- pounds are not acutely toxic to model test organisms. Prior to conducting appropriate ecological risk assessments of phar- maceuticals, the mode of action-based biomarkers needs to be developed for non-target species. To evaluate toxicity of the b-adrenergic blockers propranolol and metoprolol on non- target biota, we assessed standard ecotoxicological endpoints after 9 days of subchronic exposure in a transgenerational study with Daphnia magna. On day 9 of exposure, the lowest observed effects concentration (LOEC) for growth were 0.44 mg L )1 for propranolol and 12 mg L )1 for metoprolol. Total fecundity LOECs for the 9-day exposure were 0.11 mg L )1 and 6 mg L )1 for propranolol and metoprolol, respectively. In addition to typical ecotoxicological endpoints, we evaluated the utility of employing the physiological biomarkers of heart rate and metabolic rate following subchronic and acute expo- sures to propranolol and metoprolol. Propranolol and meto- prolol LOECs for heart rate were 0.055 mg L )1 and 3.1 mg L )1 , significantly lower than the LOECs for growth and reproduction. At these concentrations, heart rate was signifi- cantly lower than the control values for both compounds. Daphnia magna endpoint sensitivity to chronic propranolol and metoprolol exposure was mortality < growth < fecundity < heart rate. Second generation D. magna were less sensitive to both compounds. Responses of the physiological biomarkers heart rate and metabolic activity suggest that propranolol and metoprolol exerts sublethal toxicity to D. magna at lower concentrations than observed in the classical endpoints. As medical treatment in society has improved through the use of pharmaceuticals, the presence of these pharmaceuticals in the aquatic environment has increased (Huggett et al. 2003a,b; Kolpin et al. 2002). In addition, advances in analytical chemistry techniques such as mass spectrometry have allowed for the quantification of target pharmaceuticals at low ng L )1 concentrations in a variety of matrices. The primary source of pharmaceutical introduction to the environment is the incom- plete metabolism of pharmaceuticals by humans and sub- sequent excretion to wastewater. Subsequently, a portion of the pharmaceuticals in influent wastewater passes through the wastewater treatment process and is released to aquatic sys- tems from effluent discharges. In a now classic example, ng L )1 levels of estrogenic drugs from birth control or hormone replacement have been found in wastewater effluent through- out the Americas and Europe (Belfroid et al. 1999; Huggett et al. 2003a; Kolpin et al. 2002; Kozak et al. 1999). One class of environmental pharmaceuticals that has re- ceived recent attention is the b-adrenergic receptor antagonists (b-blockers). The b-blockers propranolol, metoprolol, and nadolol have been detected in European effluent discharges (Ternes 1998). More recently, b-adrenergic receptor antago- nists were measured in wastewater effluent samples from Texas, New York, and Mississippi at levels as high as 1.9 lgL )1 (Huggett et al. 2003b). b-adrenergic antagonists are typically indicated in humans for the treatment of angina, hypertension, and post-traumatic stress syndrome treatment. b-blockers act on b-adrenergic receptors, a class of receptors critical to normal function of the sympathetic branch of the vertebrate autonomic nervous system. Metoprolol is specific for the b 1 -receptor, while propranolol is a non-specific antagonist blocking both b 1 and b 2 -receptors. In addition to b-receptor blockade, propranolol acts as a serotonin receptor antagonist (Alexander and Wood 1987), as well as a potent membrane-stabilizing agent (Kim et al. 2003). Although the potential influence of these b-blockers on aquatic systems is currently not fully understood, the response of aquatic popu- lations to the presence of b-blocker must be considered (Huggett et al. 2002). Previous investigators identified the importance of under- standing the mode of action (MOA) of environmental phar- maceuticals in non-target organisms (Daughton and Ternes 1999; Brooks et al. 2003). Because b-blockers target specific pathways and may subsequently influence physiological function of aquatic organisms, b-blockers could have greater impacts on sublethal endpoints assessed in chronic exposures. In fact, MOA-specific biomarkers have been recommended for Correspondence to: Edward M. Dzialowski; email: edzial@unt.edu Arch. Environ. Contam. Toxicol. 50, 503–510 (2006) DOI: 10.1007/s00244-005-0121-9