Feeding and swimming modulate iono-and-hormonal regulation differently in goldfish, Carassius auratus and common carp, Cyprinus carpio

Rh proteins and H+ transporters involved in ammonia excretion in Amur Ide (Leuciscus waleckii) under high alkali exposure

High alkaline environment can lead to respiratory alkalosis and ammonia toxification to freshwater fish. However, the Amur ide (Leuciscus waleckii), which inhabits an extremely alkaline lake in China with titratable alkalinity up to 53.57 mM (pH 9.6) has developed special physiological and molecular mechanisms to adapt to such an environment. Nevertheless, how the Amur ide can maintain acid-base balance and perform ammonia detoxification effectively remains unclear. Therefore, this study was designed to study the ammonia excretion rate (Tamm), total nitrogen accumulation in blood and tissues, including identification, expression, and localization of ammonia-related transporters in gills of both the alkali and freshwater forms of the Amur ide. The results showed that the freshwater form Amur ide does not have a perfect ammonia excretion mechanism exposed to high-alkaline condition. Nevertheless, the alkali form of Amur ide was able to excrete ammonia better than freshwater from Amur ide, which was facilitated by the ionocytes transporters (Rhbg, Rhcg1, Na+/H+ exchanger 2 (NHE2), and V-type H+ ATPase (VHA)) in the gills. Converting ammonia into urea served as an ammonia detoxication strategy to reduced endogenous ammonia accumulation under high-alkaline environment.

Female tilapia, Oreochromis sp. mobilised energy differently for growth and reproduction according to living environment

This study was conducted to investigate the energy mobilisation preference and ionoregulation pattern of female tilapia, Oreochromis sp. living in different environments. Three different treatments of tilapia as physiology compromising model were compared; tilapia cultured in recirculating aquaculture system (RAS as Treatment I-RAS), tilapia cultured in open water cage (Treatment II-Cage) and tilapia transferred from cage and cultured in RAS (Treatment III-Compensation). Results revealed that tilapia from Treatment I and III mobilised lipid to support gonadogenesis, whilst Treatment II tilapia mobilised glycogen as primary energy for daily exercise activity and reserved protein for growth. The gills and kidney Na+/K+ ATPase (NKA) activities remained relatively stable to maintain homeostasis with a stable Na+ and K+ levels. As a remark, this study revealed that tilapia strategized their energy mobilisation preference in accessing glycogen as an easy energy to support exercise metabolism and protein somatogenesis in cage culture condition, while tilapia cultured in RAS mobilised lipid for gonadagenesis purposes.

Prolonged cortisol elevation alters whole body and tissue metabolism in rainbow trout (Oncorhynchus mykiss)

Chronic elevation of circulating cortisol is known to have deleterious effects on fish, but information about the consequences of prolonged cortisol elevation on the metabolism of fish is scarce. To test the effects of chronic cortisol elevation on the aerobic performance of rainbow trout, we examined how two severities of chronically elevated plasma cortisol levels affected the oxygen uptake during rest and after exhaustive exercise using a high (HC) and a medium cortisol (MC) treatment. High cortisol doses significantly affected standard (SMR) and maximum metabolic rates (MMR) compared to control fish. In comparison, the medium cortisol treatment elevated maximum metabolic rates (MMR) but did not significantly influence SMR compared to a sham group (S) and control group (C). The medium cortisol treatment resulted in a significantly increased metabolic scope due to an elevation of MMR, an effect that was abolished in the HC group due to co-occuring elevations in SMR. The elevated SMR of the HC-treated fish could be explained by increased in vitro oxygen uptake rates (MO₂) of specific tissues, indicating that the raised basal metabolism was caused, in part, by an increase in oxygen demand of specific tissues. Haematological results indicated an increased reliance on anaerobic metabolic pathways in cortisol-treated fish under resting conditions.

Effects of Bicarbonate Stress on Serum Ions and Gill Transporters in Alkali and Freshwater Forms of Amur Ide (Leuciscus waleckii)

The Amur ide (Leuciscus waleckii) is a fish in the Cyprinidae family. Compared with other Amur ide living in freshwater ecosystems, the Amur ide population in Lake Dali Nor of China is famous for its high tolerance to the alkaline conditions of 54 mM (pH 9.6). Yet, surprisingly, the ionoregulatory mechanism responsible for this remarkable alkaline adaptation remains unclear. Therefore, this study sought to investigate how bicarbonate affects the acid-base balancing and ionoregulatory responses of this animal. Here, using a comparative approach, the alkali form of Amur ide and its ancestral freshwater form living in other freshwater basins were each exposed to 50 mM (pH 9.59 ± 0.09), a level close to the alkalinity of Lake Dali Nor, and their physiological (AE1) adjustment of ions and acid-base regulation were investigated. This study highlighted differences in blood pH and serum ions (e.g., Na⁺, K⁺, Cl⁻, and Ca²⁺), Na⁺/K⁺ ATPase (NKA) activity and its mRNA level, and mRNA expression of gill transporters (Na⁺/H⁺ exchanger member 2 and/or 3, Na⁺/HCO3- cotransporter (NBC1), Cl⁻/HCO3- exchanger, Na⁺/Cl⁻ cotransporter (NCC), Na⁺/K⁺/2Cl⁻ (NKCC1), SLC26A5, and SLC26A6) for alkalinity adaptation between the two forms of Amur ide differing in alkalinity tolerance. Specifically, close relationships among the serum Na⁺ and mRNA levels of NCC, NKCC1, and NHE, and also NKA and NBC1, in addition to serum Cl⁻ and bicarbonate transporters (e.g., SLC26A5 and SLC26A6), characterized the alkali form of Amur ide. We propose that this ecotype can ensure its transepithelial Cl⁻ and Na⁺ uptake/base secretions are highly functional, by its basolateral NKA with NBC1 and apical ionic transporters, and especially NCC incorporated with other transporters (e.g., SLC26). This suggests an evolved strong ability to maintain an ion osmotic and acid-base balance for more effectively facilitating its adaptability to the high alkaline environment. This study provides new insights into the physiological responses of the alkaline form of the Amur ide fish for adapting to extreme alkaline conditions. This information could be used as a reference to cultivating alkaline-tolerant fish species in abandoned alkaline waters.

Common carp, Cyprinus carpio, prefer branchial ionoregulation at high feeding rates and kidney ionoregulation when food supply is limited: additional effects of cortisol and exercise

This study aims to examine ionoregulatory parameters during exercise and cortisol elevation in common carp fed different food rations. Fish subjected to two different feeding regimes (0.5 or 3.0% body mass (BM) daily) received no implant or an intraperitoneal cortisol implant (250 mg/kg BM) or sham, and were monitored over a 168-h post-implant (PI) period under resting, low aerobic swimming or exhaustive swimming conditions. Plasma osmolality was maintained at relatively stable levels without much influence of feeding, swimming or cortisol, especially in low feeding groups. Nevertheless, a transient hyponatremia was observed in all low feeding fish implanted with cortisol. The hyponatremia was more pronounced in fish swum to exhaustion but even in this group, Na⁺ levels returned to control levels as cortisol levels recovered (168 h-PI). Cortisol-implanted fish also had lower plasma Cl^- levels, and this loss of plasma Cl^- was more prominent in fish fed a high ration during exhaustive swimming (recovered at 168 h-PI). Cortisol stimulated branchial NKA and H⁺ ATPase activities, especially in high ration fish. In contrast, low ration fish upregulated kidney NKA and H⁺ ATPase activities when experiencing elevated levels of cortisol. In conclusion, low feeding fish experience an ionoregulatory disturbance in response to cortisol implantation especially when swum to exhaustion in contrast to high feeding fish.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

Kidney activity increases in copper exposed goldfish (Carassius auratus auratus)

In the present study, the effect of copper was examined in the common goldfish (Carassius auratus auratus). Fish were fasted and exposed to either a high (0.84μM), a low (0.34μM) or a control copper concentration (0.05μM) for 1 and 7days. Swimming performance was not affected by either fasting or copper exposure. Food deprivation alone had no effect on ionoregulation, but low plasma osmolality levels and plasma Na⁺ were noticed in fasted fish exposed to Cu for 7days. Both gill Na⁺/K⁺-ATPase and H⁺-ATPase activities were undisturbed, while both kidney ATPase activities were up-regulated when challenged with the high Cu levels. Up-regulated kidney ATPase activities likely acted as compensatory strategy to enhance Na⁺ reabsorption. However, this up-regulation was not sufficient to restore Na⁺ to control levels in the highest exposure group.