Thomas W. Moon

Cortisol in teleosts: dynamics, mechanisms of action, and metabolic regulation

Cortisol is the principal corticosteriod in teleost fishes and its plasma concentrations rise dramatically during stress. The relationship between this cortisol increase and its metabolic consequences are subject to extensive debate. Much of this debate arises from the different responses of the many species used, the diversity of approaches to manipulate cortisol levels, and the sampling techniques and duration. Given the extreme differences in experimental approach, it is not surprising that inconsistencies exist within the literature. This review attempts to delineate common themes on the physiological and metabolic roles of cortisol in teleost fishes and to suggest new approaches that might overcome some of the inconsistencies on the role of this multifaceted hormone. We detail the dynamics of cortisol, especially the exogenous and endogenous factors modulating production, clearance and tissue availability of the hormone. We focus on the mechanisms of action, the biochemical and physiological impact, and the interaction with other hormones so as to provide a conceptual framework for cortisol under resting and/or stressed states. Interpretation of interactions between cortisol and other glucoregulatory hormones is hampered by the absence of adequate hormone quantification, resulting in correlative rather than causal relationships.The use of mammalian paradigms to explain the teleost situation is generally inappropriate. The absence of a unique mineralocorticoid and likely minor importance of glucose in fishes means that cortisol serves both glucocorticoid and mineralocorticoid roles; the unusual structure of the fish glucocorticoid receptor may be a direct consequence of this duality. Cortisol affects the metabolism of carbohydrates, protein and lipid. Generally cortisol is hyperglycaemic, primarily as a result of increases in hepatic gluconeogenesis initiated as a result of peripheral proteolysis. The increased plasma fatty acid levels during hypercortisolaemia may assist to fuel the enhanced metabolic rates noted for a number of fish species. Cortisol is an essential component of the stress response in fish, but also plays a significant role in osmoregulation, growth and reproduction. Interactions between cortisol and toxicants may be the key to the physiology of this hormone, although cortisols many important housekeeping functions must not be ignored. Combining molecular approaches with isolated cell systems and the whole fish will lead to an improved understanding of the many faces of this complex hormone in an evolutionary and environmental framework.

Glucose intolerance in teleost fish: fact or fiction?

Teleost fish are generally considered to be glucose intolerant. This mini-review examines some of the background and the possible mechanistic bases for this statement. Glucose intolerance is a clinical mammalian term meaning that a glucose load results in persistent hyperglycemia. Teleost fish show persistent hyperglycemia that is generally coincident with transient hyperinsulinemia. The fact that teleost generally have high plasma insulin compared with mammals implies insulin-deficiency is not a suitable explanation for this persistent hyperglycemia. Instead, peripheral utilization of glucose is probably the principle cause of hyperglycemia. Recent evidence for muscle insulin receptors, glucose transporters and hexokinase/glucokinase is reviewed and future experimental directions are suggested. If by altering peripheral glucose utilization fish could become more glucose tolerant, costs to the aquaculture industry may be substantially reduced.

Glucose metabolism in fish: a review

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

Seasonal variation in carbohydrate and lipid metabolism of yellow perch (Perca flavescens) chronically exposed to metals in the field

The effects of heavy metals on growth, intermediary metabolism and enzyme activities were investigated in yellow perch (Perca flavescens), sampled in summer and fall from lakes situated along a contamination gradient of Cd, Zn and Cu in the mining region of Rouyn-Noranda, Québec. An exposure-dependent decrease in condition factor was observed in both seasons. Liver glycogen and triglyceride reserves were higher in summer than in fall in fish from the reference lake, while the seasonal pattern was different in fish from the contaminated lakes. Plasma free fatty acids (FFA) levels were also influenced by season and contamination. Activities of malic enzyme (ME) and glucose 6-phosphate dehydrogenase (G6PDH) in the liver were higher in the summer than in the fall in reference lakes whereas no seasonal variations were detected in fish from contaminated lakes. Activities of pyruvate kinase (PyK), aspartate transaminase (AST), phosphoenolpyruvate carboxykinase (PEPCK) and malate dehydrogenase (MDH), were higher in fish from contaminated lakes in fall but not in summer. Chronic exposure of yellow perch to sublethal levels of heavy metals impairs growth and alters the seasonal cycling of liver glycogen and triglycerides as well as the activities of metabolic enzymes.

Endurance exercise training in the fast and slow muscles of a teleost fish (Pollachius virens)

Summary1.The recruitment of muscle fibre types has been investigated in the coalfish (Pollachius virens) using electromyography. Red trunk muscles were active at all swimming speeds examined (0.25–3.6 lengths/s). Interestingly, white fibres were recruited at 0.8–2.0 lengths/s providing evidence that this muscle type is also used during sustained activity.2.The effect of endurance exercise training on muscle fibre size and enzymes of energy metabolism has also been investigated. Fish were exercised continuously at 2.1 lengths/s for a period of three weeks in an experimental swimming chamber. This swimming speed represents a significant increase in work load relative to non-exercised fish as evidenced by muscle fibre hypertrophy and an increase in creatine kinase activities in both red (184%) and white (260%) muscles.3.Glycogen storage levels increased to a greater extent in red (+520%) than white (+200%) muscles. Phosphofructokinase activity was eight times higher in the red muscle of exercised fish. In contrast, there was only a small increase in citrate synthetase (+30%) and no change in either hexokinase or cytochrome oxidase activities in the red muscle of trained fish.4.Increased hydroxyacyl CoA dehydrogenase activities in both muscle types indicate an enhanced capacity for fatty acid catabolism with training.5.White muscle phosphofructokinase activities were not significantly different in trained and untrained fish. It is likely that the maximum potential of white muscle for anaerobic glycogenolysis is already sufficient to meet all its energy requirements at this swimming speed.6.The results suggest that the capacity of coalfish red muscle to do aerobic work remains essentially unchanged by endurance exercise training and that any increase in the ability to produce ATP must be met anaerobically.

The role of circulating catecholamines in the regulation of fish metabolism: An overview

The physiological role of the catecholamines (CA), adrenaline and noradrenaline in fish has been frequently reviewed, but the metabolic consequences of these hormones have received less attention. The purpose of this review is to examine the recent literature dealing with CA actions on whole fish and tissue metabolism. The CA increase glucose production both in vivo and in vitro, at least in isolated hepatocytes. Although the data are less clear, lipid mobilization is also a consequence of elevated circulating CA. The difficulty with using the whole fish for such studies is that CA may alter other circulating hormone levels, CA turnover in the circulation quickly, and it is difficult to define precisely the tissue being affected. Much of our understanding is derived, therefore, from the study of isolated tissues, and especially the hepatocyte. Catecholamines stimulate both glycogenolysis and gluconeogenesis in hepatocytes isolated from a large number of fish species. This review examines the steps involved in the signal transduction system, from the binding of CA to alpha- and beta-adrenoceptors to the ultimate effects of specific enzyme phosphorylation. Recent literature demonstrates that the complexity of the adrenoceptor system noted for mammals, also is expressed in fish. Adrenoceptor subtypes are specific to species, to tissues and to function of the tissues, and these issues are discussed especially as they are related to external and to internal stressors. Future research will pursue better definitions of the adrenoceptor systems, molecular biology of the components of these receptor systems and development of alternative cell models. There still remains a poor explanation of the reason for the diversity of adrenoceptor systems, and there are a number of fish systems that may provide unique opportunities to understand this question.

Corticotropin-releasing factor and neuropeptide Y mRNA levels are elevated in the preoptic area of socially subordinate rainbow trout

The objectives of this study were to characterize rainbow trout (Oncorhynchus mykiss) corticotropin-releasing factor (CRF) and neuropeptide Y (NPY) cDNAs and to determine their mRNA levels in response to social stress. Standard cloning techniques were used to obtain cDNAs, sequences for trout NPY and two CRF isoforms. At the predicted amino acid level, our NPY sequence differs from the trout amino acid sequence reported by. A phylogenetic analysis suggests that the two CRF isoforms result from a gene duplication that occurred in a common ancestor of salmonids. A tissue distribution demonstrated that the mRNAs of both CRF isoforms are predominantly present in the preoptic area of the trout brain, whereas NPY mRNA is more abundant in the telencephalon. Pairs of sized-matched juvenile female trout were allowed to interact for 72 h and social ranks were assigned on the basis of behavioural observations. Mean plasma cortisol levels were 13-fold higher in subordinate than in dominant trout. As measured by ribonuclease protection assay, CRF1 and NPY mRNA levels were respectively 51 and 32% higher in the preoptic area of subordinate trout; in addition, CRF1 and NPY mRNA levels were positively correlated (R2=0.44). These results suggest that subordinate rainbow trout chronically maintain high levels of CRF mRNA during social stress and that NPY may be involved in the control of the stress axis in trout.

Starvation and the activities of glycolytic and gluconeogenic enzymes in skeletal muscles and liver of the plaice,Pleuronectes platessa

SummaryChanges in body index parameters and liver, red muscle and white muscle enzyme profiles have been determined in fed and four month starved plaice,Pleuronectes platessa. The results are compared to other vertebrates to estimate specific tissue metabolic patterns and changes in these patterns with starvation.1.Liver demonstrates the lowest glycolytic but highest gluconeogenic capacity of the three tissues. Red muscle has little, if any, gluconeogenic potential, based upon low activities of phosphoenol pyruvate carboxykinase and glucose-6-phosphatase and no detectable activities of pyruvate carboxylase. Plaice white skeletal muscle has the highest glycolytic potential of the tissues studied.2.Plaice starved for four months demonstrate significant reductions in liver-somatic index and red muscle-somatic index, and increases in tissue water contents (Table 1). Enzyme activities generally decline in both muscle types, but are maintained in the liver (Table 2). Activities of liver soluble phosphoenol pyruvate carboxykinase increase by approximately 8-fold, suggesting that the enzymic response to starvation in plaice is similar to that of mammals.3.These results suggest that starvation in plaice is associated with both a decrease in spontaneous activity and metabolic capacity of skeletal muscles, and an enhanced potential for liver gluconeogenesis. Also, it is possible that the precursors for liver gluconeogenesis do not form pyruvate as an intermediate step.

Phase I and II enzymes and antioxidant responses in different tissues of brown bullheads from relatively polluted and non-polluted systems.

Brown bullheads (Ameiurus nebulosus) were collected from the St. Lawrence River and compared in their detoxication capacities to bullheads from a relatively non-polluted aquatic system, Lac La Pêche. The content of polychlorinated biphenyls (PCBs) in white muscle was significantly higher (22-fold) in bullheads from the St. Lawrence River compared with those from Lac La Pêche. Activities of liver ethoxyresorufin O-deethylase (EROD), a common Phase I enzyme, were 2.8-fold higher in St. Lawrence River bullheads than in fish from La Pêche. Conjugation activity by hepatic glutathione Stransferase (GST) was 3-fold higher in the St. Lawrence River fish, and significantly higher activities were also detected in kidney and white muscle in these fish as compared with the Lac La Pêche group. UDP-Glucuronosyltransferase (UDPGT) activities in liver and kidney did not differ between the two groups. Activities of cytosolic superoxide dismutase (SOD) were significantly higher, while lower activities of catalase (CAT) in kidney and glutathione peroxidase (GPX) in red and white muscles were noted in the St. Lawrence River bullheads. Concentrations of total glutathione (TGSH) in the different tissues revealed significantly lower levels in liver, kidney and white muscle of bullheads from the St. Lawrence River. The changes in Phases I and II enzymes and TGSH levels in the various tissues relate to higher PCB concentrations of muscle tissue and suggest activation of detoxication capacities, but weakened antioxidant status in the bullheads from the polluted area. The results implicate the involvement of white muscle along with liver and kidney in the whole fish detoxication process.

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors

Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).