The absorption of sugars and sodium in vitro by Tilapia mossambica

Typescript.Thesis (Ph. D.)--University of Hawaii, 1967.Bibliography: leaves 96-104.xi, 104 l illus., tablesThe absorption of 14C sugars and 22Na has been studied in the cichlid Tilapia mossambica and the surgeonfish Acanthurus sandvicensis using the everted sac technique of Wilson and Wiseman (1954). Some of the parameters of glucose transport were investigated and it was found that the amount of glucose transport is directly related to the length of time of incubation, the amount of gut tissue, and the level of endogenous tissue glucose. The fresh or salt water environment in which the fish were raised or the diet which the fish were fed did not affect glucose transport. Surgeonfish transported glucose faster than tilapia, indicating that differences in glucose transport between fish are species specific. The possibility of a common carrier for sodium and glucose transport was investigated in tilapia, and a direct relationship between the sodium concentration of the medium and glucose transport was found when the sodium level of the medium was below 100 meq/L. Countertransport of endogenous glucose was inversely related to the sodium concentration below 100 meq/L and occurred before a reversal of the sodium gradient was possible. The sodium and osmotic pressure levels of the blood and intestinal fluids were about the same, and the sodium level was slightly higher than the level required for maximal transport of glucose in vitro. Different areas of the gut transported glucose and sodium at different rates. The poisons KCN and ouabain were found to inhibit glucose transport, but they did not seem to affect sodium transport. An increase in the transport of glucose, but not sodium, occurred with increasing mucosal glucose concentrations. The possibility of a 1:1 stochiometry for glucose and sodium transport could not be eliminated, however, because of the high level of sodium transport regardless of the presence or absence of glucose. It was concluded that in fish glucose and sodium do not share the same transport mechanism as prop9sed by Crane (1965). The results of the study of the relationship between glucose transport and enzyme systems in tilapia indicate that sugar transport is very specific and that enzymes are probably involved. D-Glucose and D-galactose were rapidly transported while D-zylose, D-fructose, D-mannose, L-fucose, D-glucuronic acid, D-gluconic acid, and D-mannitol did not enter the serosal sac faster than could be accounted for by diffusion. Galactose was found to be a non-competitive inhibitor of glucose transport only when in the mucosal fluid, indicating that the rate limiting part of the enzyme system is on the luminal or mucosal surface. The rate of glucose transport was affected by the pH of the medium, which also indicates that enzymes of the mucosal surface may be involved in glucose transport. In conclusion, a model of glucose transport is presented where the first step in transport is the binding of glucose to enzymes on the mucosal surface and where at least a part of the energy necessary for glucose transport is derived from the metabolism of glucose itself