Dept of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, VA 24061, USA.
Dept of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA.
Gen Comp Endocrinol. 2020 Jan 1;285:113267. doi: 10.1016/j.ygcen.2019.113267. Epub 2019 Sep 3.
Corticosterone is widely regarded to be the predominant glucocorticoid produced in amphibians. However, we recently described unusually low baseline and stress-induced corticosterone profiles in eastern hellbenders (Cryptobranchus alleganiensis alleganiensis), a giant, fully aquatic salamander. Here, we hypothesized that hellbenders might also produce cortisol, the predominant glucocorticoid used by fishes and non-rodent mammals. To test our hypothesis, we collected plasma samples in two field experiments and analyzed them using multiple analytical techniques to determine how plasma concentrations of cortisol and corticosterone co-varied after 1) physical restraint and 2) injection with adrenocorticotropic hormone (ACTH), the pituitary hormone responsible for triggering the release of glucocorticoids from amphibian interrenal glands. Using liquid chromatography-mass spectrometry, we found that baseline and restraint-induced plasma concentrations of cortisol were more than five times those of corticosterone. We then demonstrated that plasma concentrations of both glucocorticoids increased in response to ACTH in a dose-dependent manner, but cortisol concentrations were consistently higher (up to 10-fold) than corticosterone. Cortisol and corticosterone concentrations were not correlated with one another at basal or induced conditions. The extremely low plasma concentrations of corticosterone in hellbenders suggests that corticosterone could simply be a byproduct of cortisol production, and raises questions as to whether corticosterone has any distinct physiological function in hellbenders. Our results indicate that hellbenders produce cortisol as their predominant glucocorticoid, supporting a small and inconclusive body of literature indicating that some other amphibians may produce appreciable quantities of cortisol. We hypothesize that the use of cortisol by hellbenders could be an adaptation to their fully aquatic life history due to cortisol's ability to fulfill both mineralocorticoid and glucocorticoid functions, similar to its functions in fishes. Given the large number of amphibian species that are fully aquatic or have aquatic life stages, we suggest that the broadly held assumption that corticosterone is the predominant glucocorticoid in all amphibians requires further scrutiny. Ultimately, multi-species tests of this assumption will reveal the ecological factors that influenced the evolution of endocrine adaptations among amphibian lineages, and may provide insight into convergent evolution of endocrine traits in paedomorphic species.
皮质酮被广泛认为是两栖动物中产生的主要糖皮质激素。然而，我们最近描述了东部美洲大鲵（Cryptobranchus alleganiensis alleganiensis）异常低的基线和应激诱导的皮质酮谱，东部美洲大鲵是一种巨大的、完全水生的蝾螈。在这里，我们假设大鲵也可能产生皮质醇，这是鱼类和非啮齿类哺乳动物使用的主要糖皮质激素。为了验证我们的假设，我们在两个野外实验中收集了血浆样本，并使用多种分析技术对其进行了分析，以确定皮质醇和皮质酮的血浆浓度在以下情况下如何共同变化：1）物理束缚和 2）注射促肾上腺皮质激素（ACTH），ACTH 是一种垂体激素，负责触发两栖动物间肾上皮质腺释放糖皮质激素。使用液相色谱-质谱法，我们发现皮质醇的基线和束缚诱导的血浆浓度是皮质酮的五倍以上。然后，我们证明，这两种糖皮质激素的血浆浓度都以剂量依赖性的方式对 ACTH 作出反应而增加，但皮质醇浓度始终高于皮质酮（高达 10 倍）。在基础或诱导条件下，皮质醇和皮质酮浓度彼此之间没有相关性。大鲵中极低的皮质酮血浆浓度表明，皮质酮可能只是皮质醇产生的副产物，并提出了皮质酮在大鲵中是否具有任何独特的生理功能的问题。我们的结果表明，大鲵产生皮质醇作为其主要的糖皮质激素，这支持了一小部分文献的观点，即其他一些两栖动物可能会产生相当数量的皮质醇。我们假设，由于皮质醇能够满足盐皮质激素和糖皮质激素的功能，类似于其在鱼类中的功能，因此大鲵使用皮质醇可能是其完全水生生活史的一种适应。鉴于有大量的两栖动物是完全水生的或具有水生生活阶段，我们建议，皮质酮是所有两栖动物中主要糖皮质激素的广泛假设需要进一步审查。最终，对这一假设的多物种测试将揭示影响两栖动物谱系内分泌适应进化的生态因素，并可能为幼态成熟物种内分泌特征的趋同进化提供启示。
