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Studies on the mechanisms of branchial calcium transport in teleost fish, with special reference to a calcium-stimulated ATPase in the gill plasma membranes Ma, Stephanie W. Y.

Abstract

Although mechanisms of calcium regulation in teleost fish are still poorly understood, there is indirect evidence that environmental Ca²⁺ may be involved. The gill, being the respiratory organ where blood comes into intimate contact with the external water, is likely to be a major site for ion transport. The objective of this project was to study the mechanisms of calcium transport in teleost fish, with special reference to branchial calcium-uptake, properties of a gill Ca²⁺-ATPase and the possible role of two calcium-active hormones, calcitonin from the ultimobranchial glands and the active principle(s) of the corpuscles of Stannius. A method was developed to measure Ca²⁺ transport across the isolated and perfused gill of the American eel, Anguilla rostrata. This preparation demonstrated a net Ca²⁺-uptake across the gill epithelium against a chemical and electrical gradient. This uptake was inhibited by 2,4 dinitrophenol, suggesting active transport. The branchial Ca²⁺ transport system was sensitive to changes in environmental temperature, internal and external Ca²⁺ concentrations and pH. Ca²⁺-uptake was stimulated by salmon calcitonin, but inhibited by extracts of the corpuscles of Stannius. A Ca²⁺-stimulated ATPase, independent of Na⁺ and K⁺ and insensitive to ouabain, was identified in the gill plasma membranes of the American eel. The enzyme was also stimulated by Mg²⁺, but the V[sub max] and affinity of the enzyme were higher for Ca²⁺ than for Mg²⁺. Ca²⁺ and Mg²⁺ were found to act on the same site. ATP was the preferential substrate for the enzyme. The pH optimum for enzyme activation was 7.9-8.1. Salmon calcitonin had no effect on the Ca²⁺-ATPase activity, however, the enzyme activity was markedly inhibited by extracts of the Stannius corpuscles. The existence of a similar Ca²⁺-ATPase was demonstrated in the gill plasma membranes of a variety of teleost fish from both freshwater and marine habitats. The gill Ca²⁺-ATPase activity was not altered by sea water adaptation or by starvation, but was significantly elevated during sexual maturation and spawning. This series of studies has revealed the existence of an active branchial calcium uptake system in the fresh- water teleost, possibly mediated via a membrane-bound Ca²⁺-ATPase. Calcitonin and the corpuscles of Stannius "factor" were demonstrated to play a regulatory role in this Ca²⁺ transport system, suggesting the gill as an important site of hormonal action in teleostean calcium homeostasis.

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