Abstract
The occurrence, characteristics and response to changes in environmental salinity of Na+-K+ ATPase and levamisole-sensitive alkaline phosphatase (AP) activities were studied in chela muscle of the euryhaline crab Cyrtograpsus angulatus. Chela muscle exhibited an Na+-K+ ATPase activity which was strongly dependent on ATP concentration, pH and temperature of the reaction mixture. Maximal activity was found at 1 mM ATP, 30-37°C and pH 7.4. Levamisole-sensitive AP activity was characterised at physiological pH 7.4 and at pH 8.0. I50 for levamisole-sensitive AP activity was 8.8 mM and 8.0 mM at pH 7.4 and 8.0, respectively. At both pH levels, levamisole-sensitive AP activity exhibited Michaelis-Menten kinetics (Km=3.451 mM and 6.906 mM at pH 7.4 and 8.0, respectively). Levamisole-sensitive AP activities were strongly affected by temperature, exhibiting a peak at 37oC. In crabs acclimated to low salinity (10; hyperegulating conditions), Na+-K+ ATPase activity and levamisole-sensitive AP activity at the physiological pH were higher than in 35 psu (osmoconforming conditions). The response to low salinity suggests that both activities could be components of muscle regulatory mechanisms at the biochemical level secondary to hyperegulation of C. angulatus. The study of these activities under hyperegulating conditions contributes to a better understanding of the complexity of biochemical mechanisms underlying the adaptive process of euryhaline crabs.
Highlights
Estuarine crabs have to cope with a variety of challenges, including frequent and abrupt changes in environmental salinity
We have recently demonstrated the occurrence of a levamisole-insensitive alkaline phosphatase (AP) activity with optimal pH 7.7, which decreased upon acclimation to low salinity in chela muscle of C. granulatus, whereas levamisole-sensitive AP activity appeared not to be affected
As part of our integrative studies on the identification of enzyme activities involved in biochemical adaptations to environmental salinity in estuarine crabs, the aim of this work was to determine the occurrence, characteristics and response to low salinity at physiological pH of AP activity and Na+-K+ ATPase activity in muscle of C. angulatus from Mar Chiquita coastal lagoon
Summary
Estuarine crabs have to cope with a variety of challenges, including frequent and abrupt changes in environmental salinity. Environmental fluctuations in salinity can trigger adjustments at different levels (biochemical, physiological, morphological and/or behavioural) to control movements of water and ions between animals and their medium (Kirschner, 1991). In low salinities, hyperregulating crabs maintain the hemolymph osmotic concentration above that of the external medium by absorbing both sodium and chloride from the environment. Posterior gills are considered to be the main site of the biochemical adaptations of hyperregulation (reviewed by Lucu and Towle, 2003; Kirschner, 2004). Branchial Na+-K+ ATPase activity appears to be a central component of the ionorregulatory process at the biochemical level (reviewed by Towle, 1997; Lucu and Towle, 2003). Little is known about the occurrence and characteristics of Na+-K+ ATPase in other tissues of hyperregulating crabs
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