Is heart rate in fish a sensitive indicator to evaluate acute effects of β-blockers in surface water?

We have investigated if propranolol, a non-selective β-blocker present in sewage effluents, affects heart rate in rainbow trout. During a 48h exposure to a very high concentration of propranolol (70.9μg/L) no effects on heart rate were found. After a subsequent intravenous injection of propranolol, heart rate remained unaffected in pre-exposed fish but was significantly lowered in naïve fish. Other studies have reported effects on the reproduction of fish by propranolol dissolved in water at much lower concentrations. The present study suggests that physiological systems under homeostatic control, like heart rate, may not be particularly sensitive despite being direct targets.