As animals evolved, the extracellular fluids became compartmentalized into the intravascular (blood) and interstitial (between the cells) fluids. Blood pressure regulation and cardiovascular function is intimately dependent upon blood volume and, because fluids can move between blood and interstitial compartments, the interstitial fluid volume also influences blood pressure. Due to technical problems, it is impossible to experimentally manipulate these fluids in terrestrial vertebrates without drugs or surgery. The proposed research will employ a different animal model, the rainbow trout, to examine the interrelationships between fluid volumes and cardiovascular function. Trout are ideal models for these studies because they naturally live in a variety of environments that are either dehydrating or hydrating, and salt loading or salt depleting. Fluid compartments will also me measured in other, more primitive fish such as the hagfish and shark. While these fish do not tolerate manipulation of the environment as well as trout, they provide an evolutionary window into the phylogenetic origin of the relationships between fluid balance and the vertebrate cardiovascular system and the factors that shaped their development. In the proposed research, the primary fluid compartments, blood, interstitial, and intracellular, will be examined in hagfish and sharks and in rainbow trout living in controlled environments in which water, salt, and osmotic load are independently controlled. These environments include freshwater, saltwater, freshwater plus high salt diet, and high-osmolarity salt-free water. In addition, the nature of a unique fluid compartment, the secondary circulation, will be examined. This circulation, possibly the evolutionary prototype of the mammalian lymphatic system, has not been accurately measured in any fish. Fluid compartments will be measured by indicator dilution methods using specific radiolabelled markers. Once the relationships between fluid compartments and water and salt balance have been established, the role of pressure-volume regulatory systems, the renin angiotensin system, sympathetic nervous system, antidiuretic hormones (arginine vasotocin), and natriuretic peptides will be examined in the context of altered water and salt balance. These experiments will provide the first complete quantitative description of fluid compartments in any fish. They are also the first experiments, in any vertebrate, to independently examine how salt and water balance are regulated and how these parameters are integrated into cardiovascular function. These studies are not possible with other animal or non-animal models. The information obtained from the project is crucial in all areas of cardiovascular, renal and endocrine physiology and it is applicable to all vertebrates, including mammals. The results will also be of practical benefit by providing base-line information for development of less stressful environments for fish culture.
