Aldosterone and other corticosteroids from chondrichthyean interrenal glands

Ancient fishes and the functional evolution of the corticosteroid stress response in vertebrates

The neuroendocrine mechanism underlying stress responses in vertebrates is hypothesized to be highly conserved and evolutionarily ancient. Indeed, elements of this mechanism, from the brain to steroidogenic tissue, are present in all vertebrate groups; yet, evidence of the function and even identity of some elements of the hypothalamus-pituitary-adrenal/interrenal (HPA/I) axis is equivocal among the most basal vertebrates. The purpose of this review is to discuss the functional evolution of the HPA/I axis in vertebrates with a focus on our understanding of this neuroendocrine mechanism in the most ancient vertebrates: the agnathan (i.e., hagfish and lamprey) and chondrichthyan fishes (i.e., sharks, rays, and chimeras). A review of the current literature presents evidence of a conserved HPA/I axis in jawed vertebrates (i.e., gnathostomes); yet, available data in jawless (i.e., agnathan) and chondrichthyan fishes are limited. Neuroendocrine regulation of corticosteroidogenesis in agnathans and chondrichthyans appears to function through similar pathways as in bony fishes and tetrapods; however, key elements have yet to be identified and the involvement of melanotropins and gonadotropin-releasing hormone in the stress axis in these ancient fishes warrants further investigation. Further, the identities of physiological glucocorticoids are uncertain in hagfishes, chondrichthyans, and even coelacanths. Resolving these and other knowledge gaps in the stress response of ancient fishes will be significant for advancing knowledge of the evolutionary origins of the vertebrate stress response.

Lack of renal effects of DOCA, ACTH, spironolactone, and angiotensin II in Squalus acanthias

Angiotensin II was infused intravenously in spiny dogfish sharks (Squalus acanthias). There were no significant effects on arterial blood pressure, glomerular filtration rate, urine flow, or Na excretion either in comparison with pre- and postinfusion values or in comparison with values measured in a control group of fish given elasmobranch saline intravenously. In other dogfish, glomerular filtration rate, urine flow, and Na and K excretory rates were measured for 3 days following implantation of desoxycorticosterone (DOCA), adrenocorticotropin (ACTH), or spironolactone; a control group was given no drug. There were no significant differences between these four groups of fish with respect to any of the measured parameters. These results suggest that the dogfish kidney is not a target organ for several substances known to affect renal function, either directly or indirectly, in other animals.

Hormones and endocrine glands of fishes. Studies of fish endocrinology reveal major physiologic and evolutionary problems

The structures discussed above illustrate some of the evolutionary knowledge to be obtained from studies of the comparative biology of the endocrine system among the fishes (see also 17, 78). The interrenal (adrenocortical) gland is an example of an endocrine structure which shows great morphologic variation in the vertebrate series, and among the fishes themselves. The structural variability implies little selective value for any particular pattern, and, in fact, biochemically the interrenal tissue would seem to be involved in much the same kind of steroidogenesis in the vertebrates generally. However, the hormone aldosterone may be a tetrapod novelty. The caudal neurosecretory system is a good example of a ubiquitous endocrine apparatus, among at least teleostean and elasmo-branch fishes, for which a function has yet to be elucidated and which continues to challenge the comparative physiologist. This system, along with the Stannius corpuscles, is lacking in the tetrapods. The existence of these structures makes it clear that the endocrine biology of fishes cannot be tacitly summarized as being essentially similar to that of the tetrapods, only less well developed. The prolactin situation illustrates the existence of a gland-the pituitary-present among fishes as among tetrapods, which secretes a product only partly related to the tetrapod hormone and having a very different functional significance. In this case it is clear that "the hormone and the uses to which it is put" have undergone evolutionary change during vertebrate phylogeny.