Graduate Thesis Or Dissertation
 

Calcium metabolism and regulation in larval Rana catesbeiana

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  • Calcium concentration and its regulation was examined in tissues of larval Rana catesbiana. Considerable variability was found in tissue calcium that could not be explained by stage of development. A seasonal effect was observed, with plasma calcium decreasing and muscle calcium increasing during winter. Total plasma calcium concentration reflects, within certain limits, environmental calcium concentrations, Animals kept in calcium-free medium have a lower plasma calcium concentration than animals kept in a high calcium medium ( 1.03 mM compared to 1.26 mM), But plasma calcium does not continue to decrease or increase with time (at least for up to six weeks) in calcium-free or high-calcium medium, and controls (0. 8 mM external calcium) maintain a constant plasma calcium concentration. The skin, which is an important amphibian calcium storage site, also shows some tendency to calcium "load" and "deplete" in response to high calcium and calcium-free medium, although this tendency does not continue with time, indicating the presence of some regulation. Calcium fluxes between the animal and the bathing medium were variable between batches of animals and with time, decreasing for about the first four hours with net flux inward to little or no net flux after four hours. Influx and efflux after about four hours are usually about 0. 1 μmoles Ca/hour-10 10 g animal. Analysis of the characteristics indicate the presence of more than one calcium pool in this animal. The presence of carrier-mediated transport was examined by flux ratio analysis, competitive inhibition, saturation kinetics and temperature effects. The calculated flux-ratio using estimates of ionic calcium concentration inside concentration inside and potential difference between animal and bath are about three times smaller than the observed flux ratio. The presence of magnesium, strontium, or barium in the medium inhibits calcium influx, indicating competitive inhibition. Influx increases with increasing bath calcium concentration up to about 0. 5 mM calcium, showing apparent saturation kinetics. There is no effect of temperature on calcium exchange in this animal. The flux ratio analysis, competitive inhibition, and saturation kinetics thus indicate that there is a carrier-mediated contribution to the observed calcium exchange. Evidence suggests that this exchange occurs primarily at the gills and gastrointestinal tract with little movement across the skin. The possibility of hormonal regulation of calcium in the tadpole was examined by calcitonin, parathyroid hormone, and thyroxine treatment, and by hypophysectomy. Calcitonin and parathyroid hormone both produce a typical vertebrate response, i. e. hypocalcemia and hypercalcimia, respectively. The bath to animal calcium exchanges are correspondingly increased and decreased, for both influx and efflux, Thyroxine produces hypocalcemia and reduced calcium influx and efflux. The calcium changes in response to thyroxine could be a secondary result from metamorphic changes in skin, intestine, and gills. Plasma calcium concentrations are apparently regulated at least in part by hormonal regulation of exchange between internal calcium stores and the plasma calcium pool. Relatively early phylogenetic and ontogenetic importance of calcitonin and parathyroid hormone in calcium regulation is indicated by the response of these larval anurans.
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