THE MODES OF ACTION OF TOXICANTS ON THE CARDIAC PHYSIOLOGY OF THE BLUE MUSSEL, MYTILUS EDULIS, AND THE COMMON SHORE CRAB, CARCINUS MAENAS
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The primary aim of this thesis was to establish the mode of action of the heavy metal, copper, on the cardiac physiology of the blue mussel, Mytilus edulis. Exposure of specimens of M.edulis to dissolved copper led to a decrease in heart rate and an increase in heart rate variability (HRV). The concentration of copper causing a 50% reduction in heart rate was found to be 0.8µM, while the concentration causing a 50% increase in HRV was 0.06µM (48 h EC50s). Simultaneous measurements of valve activity indicated that the observed bradycardia was not caused by valve closure. Subsequently, it was considered that copper might directly affect cardiac physiology by disrupting important cellular functions of the heart. Four different ionic currents were identified and characterised in M.edulis ventricular myocytes: two outward potassium currents, a sodium current and a calcium conductance. Copper ions had no effect on the ionic currents of M.edulis heart cells at concentrations shown to inhibit the cardiac activity of whole animals. Clearly, the bradycardia measured in whole animals was not due to a change in the configuration of the ventricular action potential. It was recognised that copper could still directly affect cardiac physiology in mussels by altering excitation-contraction coupling, contractile protein function or myocardial energy production. To determine whether this was the case, recordings of heart contractions from isolated ventricular strips were made using an isometric force transducer. Using isolated strips, inhibition of cardiac activity was only induced by exposure to copper concentrations ≥1 mM. Thus, the fall in heart rate measured in the whole animals dosed with copper could not be attributed to direct cardiomyopathy. Control of the. beating of M. edulis heart is known to be exercised by nerves from the visceral ganglion (VG) that contains both excitatory and inhibitory fibres. Following the removal of the VG (in vivo), exposure to copper had no effect on the heart rate of whole animals as occurred in the initial experiments. This suggests that copper affects the heart rate in M.edulis via a neuronal pathway. The principal cardioexcitatory and cardioinhibitory transmitters in molluscs are thought to be serotonin and acetylcholine, respectively, The effect of copper on the heart rate of M.edulis could not be abolished by depletion of the monoamine content of the animal using reserpine. However, pre-treatment of mussels with α-bungarotoxin considerably reduced the sensitivity of the heart to copper. These results indicated that the influence of copper on the heart of M.edulis might be mediated by a change in the activity of cholinergic nerves to the heart. Acetylcholine is known to have a biphasic action on the heart of M.edulis, low doses depress and high doses excite (the endpoint of both responses resulting in a cessation of the heart beat). In the final experiments of this series, mussels were injected with either benzoquinonium or D-tubocurarine, prior to copper exposure, in an attempt to selectively block the inhibitory or excitatory cholinoreceptors of the heart. Only benzoquinoniuin decreased the susceptibility of the heart to copper, suggesting that copper affects the cardiac activity of blue mussels by stimulating inhibitory cholinergic nerves to the heart. It is suggested that there may be a chemosensory mechanism present in mussels which responds to increased levels of metals in seawater leading to changes in a number of physiological functions. The last result chapter of this thesis examined the effects of the organophosphorous pesticide, dimethoate, on cardiac and acetylcholinesterase (AChE) activity in the common shore crab Carcinus maenas. Cardiac activity was measured non-invasively before and during dimethoate exposure. Heart rates decreased in a concentration-dependent manner. Serial measurements of AChE activity in haemolymph samples taken from crabs before and after exposure indicated that 2 mg 1ˉ¹ dimethoate also significantly reduced AChE activity. The percentage inhibition in AChE activity was correlated with the percentage reduction in heart rate following dimethoate exposure. This suggests that organophosphates may directly affect neuronal control of the heart. These experiments indicate that non-destructive, serial measurements of cardiac activity and AChE activity are valuable biomarkers of organophosphate exposure and adverse effects.
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