Osmoregulation In Seawater Research Articles

Landlocked populations have small but detectable differences in ionoregulatory physiology compared to anadromous sea lamprey, Petromyzon marinus

Life histories of anadromous and landlocked sea lamprey are similar, though landlocked populations lack seawater (SW) exposure, likely experiencing relaxed selection on SW survival traits. This study investigated SW osmoregulation in juvenile sea lamprey from one anadromous and three landlocked populations from the Great Lakes and Lake Champlain. Juveniles in all populations had strongly elevated gill Na+/K+-ATPase activity compared to larvae, indicating all populations underwent osmoregulatory changes associated with metamorphosis. Survival in SW was high in anadromous sea lamprey (90%) and highly variable among landlocked populations (40%–100%). Plasma ions’ levels were higher and hematocrit was lower after SW exposure in landlocked compared to anadromous sea lamprey. Freshwater (FW) gill ion transporter (H+-ATPase; Na+:Cl− cotransporter) mRNA levels were higher in FW and remained high after SW exposure in landlocked relative to anadromous juveniles. Landlocked lamprey had 24%–33% higher gill Na+:K+:2Cl− cotransporter abundance after SW exposure compared to anadromous lamprey. Our results indicate ionoregulatory differences that are consistent with relaxed selection on traits for SW entry and positive selection on FW traits in landlocked populations, suggestive of a recent Great Lakes invasion.

Osmoregulatory Performance among Prickly Sculpin (Cottus asper) Living in Contrasting Osmotic Habitats.

AbstractDuring the colonization of freshwater by marine fish, adaptation to hypoosmotic conditions may impact their ability to osmoregulate in seawater. The prickly sculpin (Cottus asper) is a euryhaline fish with marine ancestors that postglacially colonized many freshwater habitats. Previous work on C. asper suggested that isolation in freshwater habitats has resulted in putative adaptations that improve ion regulation in freshwater populations compared with populations with current access to estuaries. To determine whether long-term colonization of freshwater is associated with a reduced ability to ion regulate in seawater, we acclimated C. asper populations from three habitat types that vary in the extent to which they are isolated from marine habitats and compared their seawater osmoregulation. Seawater acclimation revealed that lake populations exhibited a reduced capacity to osmoregulate in seawater compared with coastal river populations with ongoing access to estuaries. In particular, when acclimated to seawater for several weeks, lake populations had lower gill Na+/K+-ATPase activity and lower intestinal H+-ATPase activity than coastal river populations. Lake populations also had a reduced ability to maintain plasma ion concentrations, and they produced lower quantities of intestinal carbonate precipitates in seawater than coastal river populations. Furthermore, there was a positive relationship between the anterior intestinal Na+/K+-ATPase activity and the amount of precipitate produced by the intestine, which suggests that the anterior intestine plays a role in seawater osmoregulation. Our results suggest that the extent of isolation from the sea could, in part, explain the reduced osmoregulation in seawater in postglacial freshwater populations of C. asper.

Continuous gastric saline perfusion elicits cardiovascular responses in freshwater rainbow trout (Oncorhynchus mykiss)

When in seawater, rainbow trout (Oncorhynchus mykiss) drink to avoid dehydration and display stroke volume (SV) mediated elevations in cardiac output (CO) and an increased proportion of CO is diverted to the gastrointestinal tract as compared to when in freshwater. These cardiovascular alterations are associated with distinct reductions in systemic and gastrointestinal vascular resistance (RSys and RGI, respectively). Although increased gastrointestinal blood flow (GBF) is likely essential for osmoregulation in seawater, the sensory functions and mechanisms driving the vascular resistance changes and other associated cardiovascular changes in euryhaline fishes remain poorly understood. Here, we examined whether internal gastrointestinal mechanisms responsive to osmotic changes mediate the cardiovascular changes typically observed in seawater, by comparing the cardiovascular responses of freshwater-acclimated rainbow trout receiving continuous (for 4 days) gastric perfusion with half-strength seawater (½ SW, ~ 17 ppt) to control fish (i.e., no perfusion). We show that perfusion with ½ SW causes significantly larger increases in CO, SV and GBF, as well as reductions in RSys and RGI, compared with the control, whilst there were no significant differences in blood composition between treatments. Taken together, our data suggest that increased gastrointestinal luminal osmolality is sensed directly in the gut, and at least partly, mediates cardiovascular responses previously observed in SW acclimated rainbow trout. Even though a potential role of mechano-receptor stimulation from gastrointestinal volume loading in eliciting these cardiovascular responses cannot be excluded, our study indicates the presence of internal gastrointestinal milieu-sensing mechanisms that affect cardiovascular responses when environmental salinity changes.

Osmoregulatory role of the intestine in the sea lamprey (Petromyzon marinus).

Lampreys are the most basal vertebrates with an osmoregulatory strategy. Previous research has established that the salinity tolerance of sea lamprey increases dramatically during metamorphosis, but underlying changes in the gut have not been examined. In the present work, we examined changes in intestinal function during metamorphosis and seawater exposure of sea lamprey (Petromyzon marinus). Fully metamorphosed juvenile sea lamprey had 100% survival after direct exposure to 35 parts per thousand seawater (SW) and only slight elevations in plasma chloride (Cl-) levels. Drinking rates of sea lamprey juveniles in seawater were 26-fold higher than juveniles in freshwater (FW). Na+-K+-ATPase (NKA) activity in the anterior and posterior intestine increased 12- and 3-fold, respectively, during metamorphosis, whereas esophageal NKA activity was lower than in the intestine and did not change with development. Acclimation to SW significantly enhanced NKA activity in the posterior intestine but did not significantly change NKA activity in the anterior intestine, which remained higher than that in the posterior intestine. Intestinal Cl- and water uptake, which were observed in ex vivo preparations of anterior and posterior intestine under both symmetric and asymmetric conditions, were higher in juveniles than in larvae and were similar in magnitude of those of teleost fish. Inhibition of NKA by ouabain in ex vivo preparations inhibited intestinal water absorption by 64%. Our results indicate drinking and intestinal ion and water absorption are important to osmoregulation in SW and that preparatory increases in intestinal NKA activity are important to the development of salinity tolerance that occurs during sea lamprey metamorphosis.

How Does Temperature Affect Primary Barrier Functions in Atlantic Salmon (Salmo Salar L.) Post‐Smolts?

Environmental temperature has a considerate impact over near all aspects of physiology in both wild and cultured salmonids. In addition to climate change, insight regarding the effects of temperature over physiology is crucial for the aquaculture sector, as temperature differences during the seaward transfer of fish from (semi‐) closed containment systems to open sea pens may be great and ultimately dependent on the prevailing season. For example, transferring of fish will likely occur towards higher temperatures during the summer months and lower temperatures during winter months. Further, open‐pen rearing of post‐smolt salmonids occur throughout the year at varying seasonal temperatures. Due to these temperature differences, it is therefore important to know how temperature influences tissue characteristics and modulates physiological functioning and adaptation, which processes maybe challenged during stress, thereby contributing to the stress response and overall robustness of the fish. Here we present how differences in temperature, ranging from 4°C to 18°C, affects Atlantic salmon (Salmo salar L.) post‐smolt primary barrier composition and functioning of osmoregulation in seawater. Our results suggest that primary barrier characteristics change with shifting temperatures, resulting in new physiological set points to match osmoregulatory demand in those conditions. Further, we elucidate how prior acclimation history affects the capacity of these barrier systems to respond when confronted by an additional acute stress. In this study, we investigated key changes in gill and skin phospholipid (PL) and fatty acid (FA) compositions utilizing electrospray ionization‐tandem mass spectrometry (ESI‐MS/MS) and gas chromatography (GC‐FID), in concert with qPCR expression profile changes of vital genes involved in osmoregulation, thereby aiming to understand how these correlate to changing patterns in primary barrier performance at different temperatures.Support or Funding InformationCenter for Research Based Innovation, “CtrlAqua” Project Number 237856/030This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cardiac remodeling and increased central venous pressure underlie elevated stroke volume and cardiac output of seawater-acclimated rainbow trout.

Substantial increases in cardiac output (CO), stroke volume (SV), and gastrointestinal blood flow are essential for euryhaline rainbow trout (Oncorhyncus mykiss) osmoregulation in seawater. However, the underlying hemodynamic mechanisms responsible for these changes are unknown. By examining a range of circulatory and cardiac morphological variables of seawater- and freshwater-acclimated rainbow trout, the present study revealed a significantly higher central venous pressure (CVP) in seawater-acclimated trout (~0.09 vs. -0.02 kPa). This serves to increase cardiac end-diastolic volume in seawater and explains the elevations in SV (~0.41 vs. 0.27 ml/kg) and CO (~21.5 vs. 14.2 ml·min-1·kg-1) when compared with trout in freshwater. Furthermore, these hemodynamic modifications coincided with a significant increase in the proportion of compact myocardium, which may be necessary to compensate for the increased wall tension associated with a larger stroke volume. Following a temperature increase from 10 to 16.5°C, both acclimation groups exhibited similar increases in heart rate (Q10 of ~2), but SV tended to decrease in seawater-acclimated trout despite the fact that CVP was maintained in both groups. This resulted in CO of seawater- and freshwater-acclimated trout stabilizing at a similar level after warming (~26 ml·min-1·kg-1). The consistently higher CVP of seawater-acclimated trout suggests that factors other than compromised cardiac filling constrained the SV and CO of these individuals at high temperatures. The present study highlights, for the first time, the complex interacting effects of temperature and water salinity on cardiovascular responses in a euryhaline fish species.

Relaxed selection causes microevolution of seawater osmoregulation and gene expression in landlocked Alewives.

Ecological transitions from marine to freshwater environments have been important in the creation of diversity among fishes. Evolutionary changes associated with these transitions likely involve modifications of osmoregulatory function. In particular, relaxed selection on hypo-osmoregulation should strongly affect animals that transition into novel freshwater environments. We used populations of the Alewife (Alosa pseudoharengus) to study evolutionary shifts in hypo-osmoregulatory capacity and ion regulation associated with freshwater transitions. Alewives are ancestrally anadromous, but multiple populations in Connecticut have been independently restricted to freshwater lakes; these landlocked populations complete their entire life cycle in freshwater. Juvenile landlocked and anadromous Alewives were exposed to three salinities (1, 20 and 30 ppt) in small enclosures within the lake. We detected strong differentiation between life history forms: landlocked Alewives exhibited reduced seawater tolerance and hypo-osmoregulatory performance compared to anadromous Alewives. Furthermore, gill Na(+)/K(+)-ATPase activity and transcription of genes for seawater osmoregulation (NKCC-Na(+)/K(+)/2Cl(-) cotransporter and CFTR-cystic fibrosis transmembrane conductance regulator) exhibited reduced responsiveness to seawater challenge. Our study demonstrates that adaptations of marine-derived species to completely freshwater life cycles involve partial loss of seawater osmoregulatory performance mediated through changes to ion regulation in the gill.

Impact of ultraviolet-B radiation on planktonic fish larvae: Alteration of the osmoregulatory function

Coastal marine ecosystems are submitted to variations of several abiotic and biotic parameters, some of them related to global change. Among them the ultraviolet-B (UV-B) radiation (UVBR: 280–320nm) may strongly impact planktonic fish larvae. The consequences of an increase of UVBR on the osmoregulatory function of Dicentrarchus labrax larvae have been investigated in this study. In young larvae of D. labrax, as in other teleosts, osmoregulation depends on tegumentary ion transporting cells, or ionocytes, mainly located on the skin of the trunk and of the yolk sac. As early D. labrax larvae passively drift in the top water column, ionocytes are exposed to solar radiation. The effect of UVBR on larval osmoregulation in seawater was evaluated through nanoosmometric measurements of the blood osmolality after exposure to different UV-B treatments. A loss of osmoregulatory capability occured in larvae after 2 days of low (50μWcm−2: 4h L/20h D) and medium (80μWcm−2: 4h L/20h D) UVBR exposure. Compared to control larvae kept in the darkness, a significant increase in blood osmolality, abnormal behavior and high mortalities were detected in larvae exposed to UVBR from 2 days on. At the cellular level, an important decrease in abundance of tegumentary ionocytes and mucous cells was observed after 2 days of exposure to UVBR. In the ionocytes, two major osmoeffectors were immunolocalized, the Na+/K+-ATPase and the Na+/K+/2Cl− cotransporter. Compared to controls, the fluorescent immunostaining was lower in UVBR-exposed larvae. We hypothesize that the impaired osmoregulation in UVBR-exposed larvae originates from the lower number of tegumentary ionocytes and mucous cells. This alteration of the osmoregulatory function could negatively impact the survival of young larvae at the surface water exposed to UVBR.

The genetic basis of salinity tolerance traits in Arctic charr (Salvelinus alpinus)

BackgroundThe capacity to maintain internal ion homeostasis amidst changing conditions is particularly important for teleost fishes whose reproductive cycle is dependent upon movement from freshwater to seawater. Although the physiology of seawater osmoregulation in mitochondria-rich cells of fish gill epithelium is well understood, less is known about the underlying causes of inter- and intraspecific variation in salinity tolerance. We used a genome-scan approach in Arctic charr (Salvelinus alpinus) to map quantitative trait loci (QTL) correlated with variation in four salinity tolerance performance traits and six body size traits. Comparative genomics approaches allowed us to infer whether allelic variation at candidate gene loci (e.g., ATP1α1b, NKCC1, CFTR, and cldn10e) could have underlain observed variation.ResultsCombined parental analyses yielded genome-wide significant QTL on linkage groups 8, 14 and 20 for salinity tolerance performance traits, and on 1, 19, 20 and 28 for body size traits. Several QTL exhibited chromosome-wide significance. Among the salinity tolerance performance QTL, trait co-localizations occurred on chromosomes 1, 4, 7, 18 and 20, while the greatest experimental variation was explained by QTL on chromosomes 20 (19.9%), 19 (14.2%), 4 (14.1%) and 12 (13.1%). Several QTL localized to linkage groups exhibiting homeologous affinities, and multiple QTL mapped to regions homologous with the positions of candidate gene loci in other teleosts. There was no gene × environment interaction among body size QTL and ambient salinity.ConclusionsVariation in salinity tolerance capacity can be mapped to a subset of Arctic charr genomic regions that significantly influence performance in a seawater environment. The detection of QTL on linkage group 12 was consistent with the hypothesis that variation in salinity tolerance may be affected by allelic variation at the ATP1α1b locus. IGF2 may also affect salinity tolerance capacity as suggested by a genome-wide QTL on linkage group 19. The detection of salinity tolerance QTL in homeologous regions suggests that candidate loci duplicated from the salmonid-specific whole-genome duplication may have retained their function on both sets of homeologous chromosomes. Homologous affinities suggest that loci affecting salinity tolerance in Arctic charr may coincide with QTL for smoltification and salinity tolerance traits in rainbow trout. The effects of body size QTL appear to be independent of changes in ambient salinity.

Cytosolic carbonic anhydrase in the Gulf toadfish is important for tolerance to hypersalinity

Carbonic anhydrase (CA) is a ubiquitous enzyme involved in acid–base regulation and osmoregulation. Many studies have demonstrated a role for this enzyme in fish osmoregulation in seawater as well as freshwater. However, to date CA responses of marine fish exposed to salinities exceeding seawater (∼ 35 ppt) have not been examined. Consequently, the aim of the present study was to examine CA expression and activity in osmoregulatory tissues of the Gulf Toadfish, Opsanus beta, following transfer to 60 ppt. A gene coding, for CAc of 1827 bp with an open reading frame of 260 amino acids was cloned and showed high expression in all intestinal segments and gills. CAc showed higher expression in posterior intestine and rectum than in anterior and mid intestine and in gills of fish exposed to 60 ppt for up to 4 days. The enzymatic activity, in contrast, was higher in all examined tissues two weeks following transfer to 60 ppt. Comparing early expression and later activity levels of acclimated fish reveals a very different response to hypersalinity among tissues. Results highlight a key role of CAc in osmoregulation especially in distal regions of the intestine; moreover, CAc play a role in the gill in hypersaline environments possibly supporting elevated branchial acid extrusion seen under such conditions.

Energetics of osmoregulation: II. water flux and osmoregulatory work in the euryhaline fish, Fundulus heteroclitus

Teleost fish experience passive osmotic water influx in fresh water (FW) and water outflux in salt water, which is normally compensated by water flow driven by active ion transport mechanisms. Euryhaline fish may also minimize osmotic energy demand by "behavioral osmoregulation", seeking a medium isotonic with their body fluids. Our goal was to evaluate the energy requirement for osmoregulation by the euryhaline fish Fundulus heteroclitus, to determine whether it is of sufficient magnitude to favor behavioral osmoregulation. We have developed a method of weighing small fish repetitively for long periods without apparent damage, which was used to assess changes in water content following changes in external salinity. We found that cold (4 degrees C) inhibits osmoregulatory active transport mechanisms in fish acclimated to warmer temperatures, leading to a net passive water flux which is reversed by rewarming the fish. A sudden change of salinity at room temperature triggers a transient change in water content and the initial slope can be used to measure the minimum passive flux at that temperature. With some reasonable assumptions as to the stoichiometry of the ion transport and ATP-generating processes, we can calculate the amount of respiration required for ion transport and compare it to the oxygen uptake measured previously under the same conditions. We conclude that osmoregulation in sea water requires from 6% to 10% of the total energy budget in sea water, with smaller percentages in FW, and that this fraction is probably sufficient to be a significant selective driving force favoring behavioral osmoregulation under some circumstances.

Cortisol receptor blockade and seawater adaptation in the euryhaline teleost Fundulus heteroclitus

To examine the role of cortisol in seawater osmoregulation in a euryhaline teleost, adult killifish were acclimated to brackish water (10 per thousand) and RU486 or vehicle was administered orally in peanut oil daily for five days at low (40 mg.kg(-1)) or high dose (200 mg.kg(-1)). Fish were transferred to 1.5 x seawater (45 per thousand) or to brackish water (control) and sampled at 24 h and 48 h after transfer, when Cl- secretion is upregulated. At 24 h, opercular membrane Cl- secretion rate, as Isc, was increased only in the high dose RU486 group. Stimulation of membranes by 3-isobutyl-1-methylxanthine and cAMP increased Isc in vehicle treated controls but those from RU486-treated animals were unchanged and membranes from brackish water animals showed a decrease in Isc. At 48 h, Isc increased and transepithelial resistance decreased in vehicle and RU486 groups, compared to brackish water controls. Plasma cortisol increased in all groups transferred to high salinity, compared to brackish water controls. RU486 treated animals had higher cortisol levels compared to vehicle controls. Vehicle treated controls had lower cortisol levels than untreated or RU486 treated animals, higher stimulation of Isc, and lower hematocrit at 24 h, beneficial effects attributed to increased caloric intake from the peanut oil vehicle. Chloride cell density was significantly increased in the high dose RU486 group at 48 hours, yet Isc was unchanged, suggesting a decrease in Cl- secretion per cell. Thus cortisol enhances NaCl secretion capacity in chloride cells, likely via glucocorticoid type receptors.

Changes in the apical surface of chloride cells following acclimation of lampreys to seawater.

Scanning electron microscopy (SEM) was used to study the changes that occur in the morphological relationships between chloride and pavement cells in the gills during acclimation of young adult lampreys to seawater. Because chloride cells are located predominantly between lamellae and are thus obscured from view, the lamellae were removed with the use of a micromanipulator installed in a SEM. In gills of animals maintained in river water, chloride cells could then be seen to be dislike and typically to form single rows between successive lamellae. After acclimation to seawater, the apical surfaces of chloride cells lose their microvilli and change in shape from small circles to rectangles that extend the full width between successive lamellae. These changes result in an increase in the length of the paracellular pathway between chloride cells. Previous work has shown that the number of strands of the zonulae occludentes sealing this pathway declines under these conditions. This presumably leads to an increase in paracellular permeability of the gill epithelium, thereby providing the low-resistance paracellular shunt required for the passive movement of sodium into the environment during osmoregulation in seawater. The above changes are reversed by transfer of lampreys downward to 10% seawater.

Cortisol Mediates the Increase in Intestinal Fluid Absorption in Atlantic Salmon during Parr-Smolt Transformation

We have previously shown in Atlantic salmon that the rate of fluid absorption by the posterior intestine (Jv) is elevated during the smolt stage in spring as a preadaptive development for osmoregulation in seawater. In the present study, we examined developmental differences in the responsiveness of Jv to cortisol and the corticosteroid antagonist, RU 486, through the parr-smolt transformation. Freshwater, juvenile salmon were administered slow-release implants of cortisol (50 μg/g body wt), RU 486 (1 mg/g body wt), or the implant without steroid (controls) at seven times from November 1992 through June 1993. Seven and 8 days after implantation, plasma cortisol concentrations and in vitroJv were measured. In control salmon, both plasma cortisol and Jv peaked in April and were positively correlated over time. Cortisol implants stimulated Jv of salmon only during the parr and postsmolt stages, when Jv of controls was low. The exogenous cortisol was sufficient to stimulate Jv to a rate comparable to that measured for control salmon in the smolt stage. Conversely, RU 486 implants inhibited Jv only during the peak smolt period, when Jv of controls was elevated. Taken together, these results demonstrate that cortisol is a necessary and sufficient endocrine signal mediating this developmental change in intestinal function during parr-smolt transformation. This is the first report of plasma cortisol levels measured in fish treated with RU 486. Plasma cortisol was elevated by RU 496, suggesting that this corticosteroid antagonist blocked feedback inhibition on the pituitary-interrenal axis.

Effects of Environmental Salinity on Pituitary Growth Hormone Content and Cell Activity in the Euryhaline Tilapia, Oreochromis mossambicus

Studies were undertaken to determine whether several indicators of growth hormone (GH) cell activity, namely GH content, fine structure, and volume of the GH region, differ in the pituitaries of freshwater (FW) and seawater (SW) tilapia, Oreochromis mossambicus. Tilapia raised from the stage of yolk-sac absorption for 7 months in SW contain significantly more GH in their pituitaries than in those of fish reared in FW. Pituitary growth hormone content in tilapia raised in FW for 7 months and transferred to SW for 49 days is greater than that in sibling tilapia retained in FW. Conversely, GH content is significantly lower in the pituitaries of SW-reared tilapia transferred to FW for 49 days than that in the pituitaries from fish retained in SW. Likewise, the volume of the GH region and activity of the GH cells are enhanced in pituitaries from SW-reared tilapia over that seen in pituitaries from FW fish. Taken together, all data indicate heightened GH cell activity in SW-raised tilapia and suggest that GH may play a causal role in the greater growth rates observed in SW tilapia compared to FW fish and/or that GH may be involved in SW osmoregulation. The latter suggestion is supported, in part, by our observation that in vivo oGH treatment (2 μg/g body wt) stimulated gill Na +,K +-ATPase activity.

Changes in intestinal fluid transport in Atlantic salmon (Salmo salar L) during parr-smolt transformation

We examined changes in fluid transport by the intestine of Atlantic salmon (Salmo salar L) undergoing parrsmolt transformation during springtime. In vitro measurements of fluid transport rate (Jv) across non-everted middle and posterior intestinal sac preparations were made in late April and early June 1990 and from February through June 1991 for juvenile smolting fish. Intestinal Jv was also compared between parr- and smolt-stage salmon in both years. To evaluate the osmoregulatory role of the intestine, Jv was measured for smolts adapted to seawater and their cohorts remaining in fresh water. The middle intestine of smolting fish underwent a significant decrease in fluid transport during the springtime, while posterior intestinal Jv significantly increased. Parr-stage fish decreased Jv in the middle intestine during springtime similar to smolts. However, the posterior intestinal Jv of smolts showed a significant increase over the parr around the peak smolt period in both years. Seawater-adapted smolts generally exhibited posterior intestinal Jv approximately double that of freshwater cohorts. A decrease over time shown for the middle intestine, together with the increased Jv in the posterior intestine preceding and after seawater entry, suggests the development of a functional regionalization during parr-smolt transformation, with the posterior intestine taking on increased importance in osmoregulation in seawater.

An Emerging Role for a Cardiac Peptide Hormone in Fish Osmoregulation

It is clear from the extant literature that various fish groups face chronic osmoregulatory problems that depend on the surrounding salinity. Their physiologic and hormonal responses are largely those seen in the mammals, but their terrestrial descendants have lost osmoregulatory structures such as gills and rectal glands and depend primarily on renal function. A data base is now emerging that strongly suggests that a putative atriopeptin plays a role in osmoregulation in fishes. This conclusion is supported by the fact that heterologous AP produces relevant physiologic responses (e.g. natriuresis, vasodilation, stimulation, or inhibition of Na+ secretion by intestine, gills, and rectal gland) in both teleosts and elasmobranchs. Moreover, cardiac and brain extracts from fish can produce similar effects in both fishes and mammals, and these tissues from various fish groups contain immunoreactive AP, as does plasma. Both physiologic and immunologic evidence suggests that the ventricle may be a significant source of AP in fishes, contrary to the situation in mammals. Finally, osmotic perturbations result in a change in plasma and tissue APir levels. The finding that plasma APir levels increase in sea water, and that heterologous AP stimulates salt secretion by the teleost gill and shark rectal gland, and inhibits salt uptake by the teleost intestine, suggests that AP may primarily play a role in salt, rather than fluid, secretion in fishes. The fact that in mammals AP inhibits prolactin secretion, but is itself stimulated by cortisol, supports this conclusion, since prolactin is generally considered to be the dominant osmoregulatory hormone in freshwater fishes, and cortisol serves this function in marine fishes. In addition, if AP inhibits brain AVT release in fishes, as it apparently inhibits vasopressin release in mammals, this also would be adaptive in marine fishes since AVT in fishes is diuretic, rather than antidiuretic. Interactions between AP and these hormones (prolactin, cortisol, and ATV) have not been studied in fishes to date, but these theoretical interactions do lend support to the hypothesis that AP may function primarily in salt homeostasis in fishes. At least one potential hormonal interaction counters this argument, however. Atriopeptin is known to inhibit the production and effects of angiotensin II in mammals, and since this hormone is apparently dipsogenic in fishes, it may play a critical role in osmoregulation in sea water. Finally, it is of some historical interest that in Keys' (67) original description of the eel heart-gill perfusion system in 1931 he commented that gill resistance remained constant for hours only if the heart itself was perfused.(ABSTRACT TRUNCATED AT 400 WORDS)

Urea efflux from the Squalus acanthias pup: the effect of stress.

ABSTRACT The extremely high concentration of urea found in elasmobranch blood (approx. 400 mmol 1–1) is secondary to nearly complete reabsorption (90–95%) of this solute in the renal tubules (Forster, 1967), and a branchial urea permeability which is approximately 1% that of the toad bladder (<10 × 10–8cm s–1, Payan, Goldstein & Forster, 1973). Evans, Oikari, Kormanik & Mansberger (1982) have recently shown that the premature spiny dogfish (Squalus acanthias) pup is capable of osmoregulation in sea water, despite an initial reduction in plasma urea concentration (13% in 3 days). This fall could have been secondary to a reduced synthetic rate, or caused by an increased efflux across surfaces such as the gills, renal and/or rectal gland epithelia, or yolk sac. The present study was therefore undertaken to measure the rate of urea efflux from intact spiny dogfish pups, and to determine the effect of stress on that efflux. Stress was induced by the experimentally common procedure of handling, MS-222 anaesthesia, and weighing.

Degeneration of the alimentary tract in sexually maturing European Anguilla anguilla (L.) and American eels Anguilla rostrata (LeSueur)

Gut morphology was examined in untreated and artifically matured European and American eels. In both species there was a decrease in gut size and gut muscle thickness with increasing sexual maturity. Hormone-treated, fully mature eels displayed advanced degeneration of the muscle layers. Total muscle thicknesses decreased from as high as 740 μm in sexually immature eels to less than 100 μm in sexually mature eels. Numbers of villi also decreased with maturity, with mature fish having relative lumenal circumferences that were less than half of those of immature fish. Epithelial structure changed with maturation and the number of mucous cells decreased. It is suggested that the degenerative changes are caused by elevated cortisol levels. Possible effects of gut changes on osmoregulation in seawater are discussed.

Stimulation of sodium transport across the teleost urinary bladder by urotensin II

Active transport of sodium (measured as short-circuit current) from the lumen to the serosal side of the seawater-acclimated Gillichthys mirabilis urinary bladder (columnar cell region) is stimulated in vitro by urotensin II. The response is dose related over a calculated physiologically meaningful range of concentrations. There is a concomitant reduction in transepithelial resistance. Arginine vasotocin, another urophysial peptide, is without effect on short-circuit current and does not reduce resistance. India ink injection via the caudal vein demonstrates a direct vascular path from the urophysis to the urinary bladder, supporting the notion of the urinary bladder as a target for urotensin II. The effect of urotensin II on ion transport across the teleost urinary bladder is consistent with a possible role in seawater osmoregulation.