Interrenal function in larval Ambystoma tigrinum. II. Control of aldosterone secretion and electrolyte balance by ACTH

Preparatory Mechanisms for Salinity Tolerance in Two Congeneric Anuran Species Inhabiting Distinct Osmotic Habitats

Anurans occupy a wide variety of habitats of diverse salinities, and their osmoregulatory ability is strongly regulated by hormones. In this study, we compared the adaptability and hormonal responses to osmotic stress between two kajika frogs, Buergeria japonica (B.j.) and B. buergeri, (B.b.), which inhabit coastal brackish waters (BW) in the Ryukyu Islands and freshwater (FW) in the Honshu, respectively. Both hematocrit and plasma Na⁺ concentration were significantly higher in B.j. than in B.b. when both were kept in FW. After transfer to one-third seawater (simulating the natural BW environment), which is slightly hypertonic to their body fluids, their body mass decreased and plasma Na concentration increased significantly in both species. After transfer, plasma Na⁺ concentration increased significantly in both species. We examined the gene expression of two major osmoregulatory hormones, arginine vasotocin (AVT) and atrial natriuretic peptide (ANP), after partial cloning of their cDNAs. ANP mRNA levels were more than 10-fold higher in B.j. than in B.b. in FW, but no significant difference was observed for AVT mRNA levels due to high variability, although the mean value of B.j. was twice that of B.b. Both AVT and ANP mRNA levels increased significantly after transfer to BW in B.b. but not in B.j., probably because of the high levels in FW. These results suggest that B.j. maintains high plasma Na⁺ concentration and anp gene expression to prepare for the future encounter of the high salinity. The unique preparatory mechanism may allow B.j. wide distribution in oceanic islands.

Host stress response is important for the pathogenesis of the deadly amphibian disease, Chytridiomycosis, in Litoria caerulea

Chytridiomycosis, a disease caused by Batrachochytrium dendrobatidis, has contributed to worldwide amphibian population declines; however, the pathogenesis of this disease is still somewhat unclear. Previous studies suggest that infection disrupts cutaneous sodium transport, which leads to hyponatremia and cardiac failure. However, infection is also correlated with unexplained effects on appetite, skin shedding, and white blood cell profiles. Glucocorticoid hormones may be the biochemical connection between these disparate effects, because they regulate ion homeostasis and can also influence appetite, skin shedding, and white blood cells. During a laboratory outbreak of B. dendrobatidis in Australian Green Tree Frogs, Litoria caerulea, we compared frogs showing clinical signs of chytridiomycosis to infected frogs showing no signs of disease and determined that diseased frogs had elevated baseline corticosterone, decreased plasma sodium and potassium, and altered WBC profiles. Diseased frogs also showed evidence of poorer body condition and elevated metabolic rates compared with frogs showing no signs of disease. Prior to displaying signs of disease, we also observed changes in appetite, body mass, and the presence of shed skin associated with infected but not yet diseased frogs. Collectively, these results suggest that elevated baseline corticosterone is associated with chytridiomycosis and correlates with some of the deleterious effects observed during disease development.

Interrenal activity during metamorphosis of the tiger salamander, Ambystoma tigrinum

Plasma corticosterone concentrations were low in premetamorphic tiger salamander larvae (Norman Stage I; M. F. Norman (1985) Anat. Rec. 211, 102-109). Corticosterone levels were significantly elevated at midmetamorphosis (Norman Stage IV) but decreased at the end of metamorphosis (Norman Stage VII). Corticosterone levels remained low 2 weeks after metamorphosis. Interrenal 3 beta-hydroxysteroid dehydrogenase activity was low in premetamorphic larvae (Norman Stage I) but was significantly elevated by midmetamorphosis (Norman Stage IV) and remained elevated at the end of metamorphosis (Norman Stage VII). There were no significant changes in interrenal cell nuclear size during metamorphosis. There was a significant decrease in body weight as well as a significant increase in hematocrit accompanying metamorphosis. The increase in plasma corticosterone concentration seen during metamorphosis of the tiger salamander is accompanied by an increase in interrenal steroidogenesis.