Mechanisms for pesticide-induced cardiotoxicity in fish: An electrophysiological study using the zebrafish (Danio rerio) as a model

Fish kills are a common phenomenon in Prince Edward Island (Canada) and their cause often remains unexplained. I hypothesized that fish exposure to selected environmental toxicants leads to increased mortality and morbidity by depressing cardiac function, precipitated by suppression of cardiac electrical activity and rhythm disturbances. These rhythm disturbances could result from toxicant-induced changes in the function of specific cardiac ion channels. Recently, the zebrafish heart has been shown to hold promise as a suitable model to study cardiac cellular electrophysiology. The purpose of the present study was to evaluate the effects of selected environmental toxicants on cardiac action potential morphology. More specifically, the present work focused on evaluating the effects on atrial action potential in adult zebrafish of: 1) acetylcholine (1-10 μM); 2) a documented acetylcholinesterase inhibitor, physostigmine (50 μM); 3) pesticides with acetylcholinesterase inhibition properties that are commonly used in Prince Edward Island (mancozeb and phorate). Zebrafish hearts were isolated under anesthesia and action potentials were recorded in vitro using a standard single microelectrode technique. Action potential morphology was then assessed by measuring parameters including action potential duration (APD) and action potential amplitude (APA). Results revealed that in vitro exposures to high concentrations of acetylcholine and physostigmine led to suppression of cardiac electrical activity by significantly reducing APD and APA, a phenomenon known as cholinergic non-excitability. Experiments evaluating the effects of commercial pesticides (mancozeb and phorate) did not reveal similar effects. This study supports the possibility that environmental toxicants containing acetylcholinesterase inhibitors may participate in fish kills by suppressing cardiac electrical activity. However, the specific pesticides mancozeb and phorate did not demonstrate such dramatic cardiotoxic effects, even at high concentrations.