Acute Stress in Lesser-Spotted Catshark (Scyliorhinus canicula Linnaeus, 1758) Promotes Amino Acid Catabolism and Osmoregulatory Imbalances

Simple Summary

In catsharks (Scyliorhinus canicula), air exposure induces amino acid catabolism altogether with osmoregulatory imbalances. This study describes a novel NHE isoform being expressed in gills that may be involved in ammonium excretion.

Abstract

Acute-stress situations in vertebrates induce a series of physiological responses to cope with the event. While common secondary stress responses include increased catabolism and osmoregulatory imbalances, specific processes depend on the taxa. In this sense, these processes are still largely unknown in ancient vertebrates such as marine elasmobranchs. Thus, we challenged the lesser spotted catshark (Scyliorhinus canicula) to 18 min of air exposure, and monitored their recovery after 0, 5, and 24 h. This study describes amino acid turnover in the liver, white muscle, gills, and rectal gland, and plasma parameters related to energy metabolism and osmoregulatory imbalances. Catsharks rely on white muscle amino acid catabolism to face the energy demand imposed by the stressor, producing NH₄⁺. While some plasma ions (K⁺, Cl⁻ and Ca²⁺) increased in concentration after 18 min of air exposure, returning to basal values after 5 h of recovery, Na⁺ increased after just 5 h of recovery, coinciding with a decrease in plasma NH₄⁺. These changes were accompanied by increased activity of a branchial amiloride-sensitive ATPase. Therefore, we hypothesize that this enzyme may be a Na⁺/H⁺ exchanger (NHE) related to NH₄⁺ excretion. The action of an omeprazole-sensitive ATPase, putatively associated to a H⁺/K⁺-ATPase (HKA), is also affected by these allostatic processes. Some complementary experiments were carried out to delve a little deeper into the possible branchial enzymes sensitive to amiloride, including in vivo and ex vivo approaches, and partial sequencing of a nhe1 in the gills. This study describes the possible presence of an HKA enzyme in the rectal gland, as well as a NHE in the gills, highlighting the importance of understanding the relationship between acute stress and osmoregulation in elasmobranchs.

Molecular and biochemical characterization of the bicarbonate-sensing soluble adenylyl cyclase from a bony fish, the rainbow trout Oncorhynchus mykiss

Soluble adenylyl cyclase (sAC) is a HC O 3   - -stimulated enzyme that produces the ubiquitous signalling molecule cAMP, and deemed an evolutionarily conserved acid-base sensor. However, its presence is not yet confirmed in bony fishes, the most abundant and diverse of vertebrates. Here, we identified sAC genes in various cartilaginous, ray-finned and lobe-finned fish species. Next, we focused on rainbow trout sAC (rtsAC) and identified 20 potential alternative spliced mRNAs coding for protein isoforms ranging in size from 28 to 186 kDa. Biochemical and kinetic analyses on purified recombinant rtsAC protein determined stimulation by HC O 3   - at physiologically relevant levels for fish internal fluids (EC₅₀ ∼ 7 mM). rtsAC activity was sensitive to KH7, LRE1, and DIDS (established inhibitors of sAC from other organisms), and insensitive to forskolin and 2,5-dideoxyadenosine (modulators of transmembrane adenylyl cyclases). Western blot and immunocytochemistry revealed high rtsAC expression in gill ion-transporting cells, hepatocytes, red blood cells, myocytes and cardiomyocytes. Analyses in the cell line RTgill-W1 suggested that some of the longer rtsAC isoforms may be preferentially localized in the nucleus, the Golgi apparatus and podosomes. These results indicate that sAC is poised to mediate multiple acid-base homeostatic responses in bony fishes, and provide cues about potential novel functions in mammals.

A review of reductionist methods in fish gastrointestinal tract physiology

A holistic understanding of a physiological system can be accomplished through the use of multiple methods. Our current understanding of the fish gastrointestinal tract (GIT) and its role in both nutrient handling and osmoregulation is the result of the examination of the GIT using multiple reductionist methods. This review summarizes the following methods: in vivo mass balance studies, and in vitro gut sac preparations, intestinal perfusions, and Ussing chambers. From Homer Smith's initial findings of marine fish intestinal osmoregulation in the 1930s through to today's research, we discuss the methods, their advantages and pitfalls, and ultimately how they have each contributed to our understanding of fish GIT physiology. Although in vivo studies provide substantial information on the intact animal, segment specific functions of the GIT cannot be easily elucidated. Instead, in vitro gut sac preparations, intestinal perfusions, or Ussing chamber experiments can provide considerable information on the function of a specific tissue and permit the delineation of specific transport pathways through the use of pharmacological agents; however, integrative inputs (e.g. hormonal and neuronal) are removed and only a fraction of the organ system can be studied. We conclude with two case studies, i) divalent cation transport in teleosts and ii) nitrogen handling in the elasmobranch GIT, to highlight how the use of multiple reductionist methods contributes to a greater understanding of the organ system as a whole.

An in vitro analysis of intestinal ammonia transport in fasted and fed freshwater rainbow trout: roles of NKCC, K+ channels, and Na+, K+ ATPase

We examined mechanisms of ammonia handling in the anterior, mid, and posterior intestine of unfed and fed freshwater rainbow trout (Oncorhynchus mykiss), with a focus on the Na⁺:K⁺:2Cl^- co-transporter (NKCC), Na⁺:K ⁺-ATPase (NKA), and K⁺ channels. NKCC was localized by immunohistochemistry to the mucosal (apical) surface of enterocytes, and NKCC mRNA was upregulated after feeding in the anterior and posterior segments. NH₄⁺ was equally potent to K⁺ in supporting NKA activity in all intestinal sections. In vitro gut sac preparations were employed to examine mucosal ammonia flux rates (Jm_amm, disappearance from the mucosal saline), serosal ammonia flux rates (Js_amm, appearance in the serosal saline), and total tissue ammonia production rates (Jt_amm = Js_amm - Jm_amm). Bumetanide (10^-4 mol L^-1), a blocker of NKCC, inhibited Js_amm in most preparations, but this was largely due to reduction of Jt_amm; Jm_amm was significantly inhibited only in the anterior intestine of fed animals. Ouabain (10^-4 mol L^-1), a blocker of NKA, generally reduced both Jm_amm and Js_amm without effects on Jt_amm in most preparations, though the anterior intestine was resistant after feeding. Barium (10^-2 mol L^-1), a blocker of K⁺ channels, inhibited Jm_amm in most preparations, and Js_amm in some, without effects on Jt_amm. These pharmacological results, together with responses to manipulations of serosal and mucosal Na⁺ and K⁺ concentrations, suggest that NKCC is not as important in ammonia absorption as previously believed. NH₄⁺ appears to be taken up through barium-sensitive K⁺ channels on the mucosal surface. Mucosal NH₄⁺ uptake via both NKCC and K⁺ channels is energized by basolateral NKA, which plays an additional role in scavenging NH₄⁺ on the serosal surface to possibly minimize blood toxicity or enhance ion uptake and amino acid synthesis following feeding. Together with recent findings from other studies, we have provided an updated model to describe the current understanding of intestinal ammonia transport in teleost fish.

Amphibious fishes: evolution and phenotypic plasticity

Amphibious fishes spend part of their life in terrestrial habitats. The ability to tolerate life on land has evolved independently many times, with more than 200 extant species of amphibious fishes spanning 17 orders now reported. Many adaptations for life out of water have been described in the literature, and adaptive phenotypic plasticity may play an equally important role in promoting favourable matches between the terrestrial habitat and behavioural, physiological, biochemical and morphological characteristics. Amphibious fishes living at the interface of two very different environments must respond to issues relating to buoyancy/gravity, hydration/desiccation, low/high O2 availability, low/high CO2 accumulation and high/low NH3 solubility each time they traverse the air-water interface. Here, we review the literature for examples of plastic traits associated with the response to each of these challenges. Because there is evidence that phenotypic plasticity can facilitate the evolution of fixed traits in general, we summarize the types of investigations needed to more fully determine whether plasticity in extant amphibious fishes can provide indications of the strategies used during the evolution of terrestriality in tetrapods.

Mechanism of ammonia excretion in the freshwater leech Nephelopsis obscura: characterization of a primitive Rh protein and effects of high environmental ammonia

Remarkably little is known about nitrogenous excretion in freshwater invertebrates. In the current study, the nitrogen excretion mechanism in the carnivorous ribbon leech, Nephelopsis obscura, was investigated. Excretion experiments showed that the ribbon leech is ammonotelic, excreting 166.0 ± 8.6 nmol·grams fresh weight (gFW)(-1)·h(-1) ammonia and 14.7 ± 1.9 nmol·gFW(-1)·h(-1) urea. Exposure to high and low pH hampered and enhanced, respectively, ammonia excretion rates, indicating an acid-linked ammonia trapping mechanism across the skin epithelia. Accordingly, compared with body tissues, the skin exhibited elevated mRNA expression levels of a newly identified Rhesus protein and at least in tendency the Na(+)/K(+)-ATPase. Pharmacological experiments and enzyme assays suggested an ammonia excretion mechanism that involves the V-ATPase, Na(+)/K(+)-ATPase, and carbonic anhydrase, but not necessarily a functional microtubule system. Most importantly, functional expression studies of the identified Rh protein cloned from leech skin tissue revealed an ammonia transport capability of this protein when expressed in yeast. The leech Rh-ammonia transporter (NoRhp) is a member of the primitive Rh protein family, which is a sister group to the common ancestor of vertebrate ammonia-transporting Rh proteins. Exposure to high environmental ammonia (HEA) caused a new adjustment of body ammonia, accompanied with a decrease in NoRhp and Na(+)/K(+)-ATPase mRNA levels, but unaltered ammonia excretion rates. To our knowledge, this is only the second comprehensive study regarding the ammonia excretion mechanisms in a freshwater invertebrate, but our results show that basic processes of ammonia excretion appear to also be comparable to those found in freshwater fish, suggesting an early evolution of ionoregulatory mechanisms in freshwater organisms.