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Biol Open
2016 Jul 15;57:955-61. doi: 10.1242/bio.016543.
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Metabolic cost of osmoregulation in a hypertonic environment in the invasive African clawed frog Xenopus laevis.
Peña-Villalobos I
,
Narváez C
,
Sabat P
.
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Studies of aquatic invertebrates reveal that salinity affects feeding and growth rates, reproduction, survival, and diversity. Little is known, however, about how salinity impacts the energy budget of vertebrates and amphibians in particular. The few studies focused on this topic in vertebrates suggest that the ingestion of salts and the resulting osmoregulatory activity is energetically expensive. We analyzed the effect of saline acclimation on standard metabolic rates (SMR) and the activities of metabolic enzymes of internal organs and osmoregulatory variables (plasma osmolality and urea plasma level) in females of Xenopus laevis by means of acclimating individuals to an isosmotic (235 mOsm NaCl; ISO group) and hyper-osmotic (340 mOsm NaCl; HYP group) environment for 40 days. After acclimation, we found that total and mass-specific SMR was approximately 80% higher in the HYP group than those found in the ISO group. These changes were accompanied by higher citrate synthase activities in liver and heart in the HYP group than in the ISO group. Furthermore, we found a significant and positive correlation between metabolic rates and plasma urea, and citrate synthase activity in liver and heart. These results support the notion that the cost of osmoregulation is probably common in most animal species and suggest the existence of a functional association between metabolic rates and the adjustments in osmoregulatory physiology, such as blood distribution and urea synthesis.
Fig. 1. Relationship between physiological parameters in Xenopus laevis acclimated to a medium with 235 mOsm (n=6, open circles) and 340 mOsm (n=5, closed circles), for 40 days. (A) Correlation between residuals of citrate synthase activity of heart and residuals of plasma urea. (B) Correlation between residuals of citrate synthase activity of heart and residuals of standard metabolic rate. The P-value was obtained by permutation test (see text for details).
Fig. 2. Effects of salt acclimation on metabolic enzymes. The activity of the enzyme citrate synthase and cytochrome c oxidase in liver of Xenopus laevis increased when frogs were acclimated for 40 days to an hyperosmotic medium (HYP, 340 mOsm, n=5) compared to frogs acclimated to an isosmotic environment (ISO, 235 mOsm, n=6). The enzyme activity was not affected by treatment in cardiac tissue. Data represented as mean±1 s.d.; *P<0.05.
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