The cardiovascular physiology of winter flounder (Pseudopleuronectes americanus): control mechanisms and environmental influences

Abstract

In this thesis, I used a multi-level approach (whole animal, organ/tissue and cellular) to determine the functional relationships between maximum cardiac performance, heart morphology, and adrenergic capacity of the winter flounder (Pseudopleuronectes americanus) heart. Further, I studied the capacity of the flounder cardiovascular system to respond to two ecologically relevant environmental stressors: hypoxia and an acute temperature increase. In the first series of experiments, an in situ heart preparation was developed for this species, and in situ cardiac function and in vitro pressure-volume curves were determined. This research showed that maximum stroke volume per gram of ventricle is the highest reported for teleosts, and that this elevated stroke volume is related to a pronounced and extended Starling curve, more compliant heart chambers, and a high bulbus:ventricle mass ratio. -- In the second part of my thesis I determined how flounder in vivo cardiovascular function is regulated by nervous and humoral mechanisms. Although resting cholinergic tonus (26%) was comparable to other teleosts, bretylium (an adrenergic nerve blocker) injection resulted in an increase in heart rate, and there was little evidence that catecholamines support cardiac function at rest or following an exhaustive chase. Further, the high myocardial β-adrenoreceptor density (252.8 fmol mg⁻¹ protein), yet low affinity (1.02 nM), measured for cardiac β-adrenoreceptors raises the possibility that the flounder heart is populated primarily by β₃-adrenoreceptors, not β₁- or β₂-adrenoreceptors as has been reported for most other teleosts. Finally, although flounder acclimated to 8°C underwent a typical reflex bradycardia when exposed to hypoxia, and this species' cardiac response to an acute elevation in water temperature to critical thermal maximum was in general typical of that shown for teleosts, a number of observations do not fit with the pattern established for other species. Specifically, the onset of bradycardia at 8°C occurred earlier than expected for an inactive and hypoxia-tolerant species (60% water O₂ saturation), resting cardiac output was similar in flounder acclimated to 8 and 15°C, and hypoxic bradycardia was absent at 15°C. These observations raise the possibility that behaviour is an important component of this species' response to environmental changes.