Hormones, ionic regulation and kidney function in fishes.

Renal osmoregulatory mechanisms in the context of hormones is considered in three types of fish: the Agnatha, the Chondrichthyes and the Osteichthyes. Particular reference is made to endocrine status and hormonal interplay in renal homeostatic mechanisms. Among Agnatha, hagfishes display atypical osmoregulatory characteristics and their endocrine repertoire is poorly understood. Hormonal actions are unclear although the kidney appears to act as a regulator of extracellular fluid volume. Lampreys show many similarities with teleost fish with respect to osmoregulation, but again their endocrine system requires further definition. Chondricthyean fishes have a number of unique hormones, among them 1-alpha-hydroxycorticosterone from the adrenocortical homologue (interrenal gland). Their complex kidneys have not been extensively studied with respect to hormonal regulation, but a key role is certainly the maintenance of high plasma levels of urea and trimethylamine oxide. The importance of the ratio of these two compounds with respect to urea tolerance is discussed. Evidence is presented and discussed that points to 1-alpha-hydroxycorticosterone playing a role in osmoregulation, although its sites and mechanisms of action are not known. The presence of a non-hypophysial control of interrenal function (a renin-angiotensin system) is indicated. The largest group of fishes, the Teleostei, are considered with respect to renal mechanisms involved in euryhalinity. Highly selective reference is made to the renin-angiotensin system and arginine vasotocin. In fresh water eels a clear negative feedback relationship exists between angiotensin II and arginine vasotocin, while in seawater-adapted animals the interplay is less clear. It is suggested that the observed increases in both arginine vasotocin and angiotensin II in eels adapted to environments hyperosmotic to their extracellular fluid in some way affects the "setting" of the feedback between the two. The possible interactions with other hormones is considered in outline.