Effect of pimozide on the cytology of the eel pituitary

Pimozide, a specific blocker of dopaminergic receptors, was injected for 4 to 9 days in freshwater (FW) eels or eels acclimated to sea water (SW), for 10 to 30 days. The daily dose was 100 or 200 microgram/100 g. In FW, pimozide induces a nuclear hypertrophy in the prolactin (PRL) cells of eels; these elongated cells increase in height. The amount of erythrosinophilic granules in the cytoplasm, initially reduced, increases. Plasma electrolyte values are not modified: only the plasma sodium level slightly rises with the higher dose. In SW, PRL cells appear less active. After 10 days, this hypoactivity is not yet fully evident; pimozide stimulates PRL cells without affecting electrolyte values. After 1 month in SW, PRL cells are stimulated with pimozide and a slight regranulation may occasionally occur. The response in SW is never as marked as it is in FW; a high dose is not more effective than a low one. The higher dose significantly raises Na+, Ca2+ and Cl- plasma levels. These data suggest that prolactin synthesis and release increase with pimozide. They corroborate the hypothesis of a hypothalamic inhibitory control on PRL secretion mediated through dopaminergic fibers in the eel, but other factors may also be involved in this regulation in addition to the effect of salinity.     

Response of prolactin cells to environmental calcium in the eel (Anguilla anguilla L.)

Prolactin (PRL) cell activity was investigated in eels kept in fresh water (FW), deionized water (DW) supplemented or not with Ca (2 mM), in Ca-enriched FW (10 mM), in normal (Ca 3.4 mM) or Ca-free 1/3 sea water (SW), and in SW (Ca 10.2 mM) or Ca-free SW (Ca 0.15 mM). Light-microscopic studies, including measurement of the nuclear area and cell height, showed that PRL cell activity, reduced in DW, is not affected by Ca supplementation. Activity is reduced in Ca-enriched FW, in 1/3 SW and in SW, conditions inducing an increase in the plasma sodium level. The lack of calcium in saline environments partly suppresses the nuclear atrophy occurring in SW. There is no significant correlation between external or total plasma calcium concentration and PRL cell activity. In artificial Ca-free SW, eels show a rapid increase in plasma osmolarity and sodium levels; there is a significant negative correlation between these two plasma values and the nuclear area or cell height of PRL cells. As in some other teleosts, plasma osmolarity and plasma sodium seem to play a more important role than external or internal calcium in controlling PRL secretion. This correlation is not apparent in eels kept in SW, having unstimulated PRL cells but active calcium-sensitive (Ca-s) cells in the pars intermedia.     

The Calcium-sensitive Cells of the Pars Intermedia in the Eel

In the pituitary, the PAS-positive calcium-sensitive (Ca-s) cells of the pars intermedia appear less active in seawater (SW)- than in freshwater (FW)-adapted eels. The kinetics of their response during adaptation to SW or readaptation to FW was investigated. Morphometric studies show that transfer to SW induces a rapid nuclear atrophy which accentuates in eels kept for several weeks in SW. Readaptation to FW stimulated the Ca-s cells after 2–10 days; after 1 or 2 months, the cells tend to be similar to those of eels kept in FW. Plasma calcium decreases slightly but significantly in SW eels. The response of the Ca-s cells is not modified by an ovine prolactin treatment inducing hypercalcemia, hypernatremia and stimulation of the corpuscles of Stannius. Minor changes occurring in the MSH cells remain difficult to interpret; the short stimulation during readaptation to FW may be related to a stress effect and/or to release of other peptides present in the MSH cells of fish.     

Effects of Hypophysectomy and Prolactin Replacement on Eel Kidney Structure During Adaptation to Sea Water

After 20 to 50 days in sea water (SW), regressive changes of the kidney occur at the same rate in intact and hypophysectomized eels (Anguilla anguilla). In SW, ovine prolactin (oPRL) increases plasma electrolytes and restores a fresh water (FW) kidney structure; cell height and nuclear area increase in main segments of the nephron: first (P1) and second proximal (P2), distal (D) and initial collecting (C) tubules. A differentiation of new tubules also occurs. This effect is less intense after hypophysectomy and greater in FW. A dose of 10 μg/g body weight/day for 10 days in SW produces some harmful renal effects, barely detected at 6 μg/g/day. The kidney plays a minor role in osmotic adjustment in SW when PRL secretion is reduced (intact eels) or suppressed (hypophysectomized); PRL treatment reverses effects of SW adaptation. An inhibition of gill sodium extrusion may explain the high blood sodium level; however, kidney histological changes suggest a renal participation, perhaps through reduced water permeability as in Platichthys.     

Quantitative ultrastructural evidence of alterations in prolactin secretion related to external salinity in a teleost fish (Poecilia latipinna)

Summary Quantitative ultrastructural morphometric studies were made on the prolactin cells of Poecilia latipinna adapted to freshwater (FW), one-third seawater (1/3 SW) and full-strength seawater (SW), and at various times after transfers between 1/3 SW and FW.In fully-adapted fish the rates of prolactin (PRL) synthesis and PRL release are inversely related to environmental salinity. During adaptation to a new salinity the two rates are temporarily uncoordinated, with release increasing or decreasing more readily than synthesis. Synthesis appears to take 30 h or longer to come into balance with the increased release rate following transfer from 1/3 SW to FW, and 72 h or longer to adjust to the reduction in release rate that follows the reverse transfer. The excess PRL granules that accumulate in the latter situation appear to be removed by lysosomal digestion. As in other teleosts, in fish adapted to the external medium the size of the stored PRL granules is inversely related to external salinity, but this relationship breaks down during adaptation to a new salinity.The stellate cells which penetrate between the PRL cells are more prominent, more extensively ramified, and appear more metabolically active in FW-adapted fish than in the other groups. These cells seem to be closely related in function to the secretory activity of the PRL cells.We thank Mr. W.A. Thomson and Mr. D.I. Hollingworth for technical assistance and Dr. D.I.C. Pearson (Department of Physics, University of Nancy, Nancy, France) for advice on mathematical analysis and computer programs. The work was carried out during tenure of an S.R.C. Studentship by T.F.C. Batten     

Effect of environmental salinity change on osmotic permeability of the isolated gill of the eel, Anguilla anguilla L.

Net water fluxes in the isolated gills of Anguilla anguilla were studied during incubation in fresh water (FW) and in sea water (SW). When incubated in FW, water entry was greater in SW-adapted eels than in FW-adapted eels. In contrast, water loss in SW was less in SW-adapted eels than in FW-adapted eels. Rectification of osmotic water fluxes was observed for both FW and SW-adapted eels, net water fluxes in the mucosal-serosal (m-s) direction being greater than those in the opposite (s-m) direction. These results indicate that adaptation to a given external medium brings about a decrease in the osmotic permeability so that water gain in FW or water loss in SW is minimal.     

Feeding habitat and silvering stage affect lipid content and fatty acid composition of European eel Anguilla anguilla tissues

Lipids, particularly fatty acids (FAs), are major sources of energy and nutrients in aquatic ecosystems and play key roles during vertebrate development. The European eel Anguilla anguilla goes through major biochemical and physiological changes throughout its lifecycle as it inhabits sea- (SW), and/or brackish- (BW) and/or freshwater (FW) habitats. With the ultimate goal being to understand the reasons for eels adopting a certain life history strategy (FW or SW residency vs. ‘habitat shifting’), we explored differences in lipid content and FA composition of muscle, liver and eyes from eels collected across Norwegian SW, BW and FW habitats, and at different lifecycle stages (yellow to silver). FW and SW eels had a higher lipid content overall compared to BW eels, reflecting differences in food availability and life history strategies. SW eels had higher proportions of certain monounsaturated FAs (MUFAs; 18:1n-9, 20:1n-9), and of the essential polyunsaturated FAs 20:5n-3 (eicosapentaenoic acid, EPA) and 22:6n-3 (docosahexaenoic acid) than FW eels, reflecting a marine-based diet. In contrast, the muscle of FW eels had higher proportions of 18:3n-3, 18:2n-6 and 20:4n-6 (arachidonic acid), as is typical of FW organisms. MUFA proportions increased in later stage eels, consistent with the hypothesis that the eels accumulate energy stores prior to migration. In addition, the decrease of EPA with advancing stage may be associated with the critical role that this FA plays in eel sexual development. Lipid and FA information provided further understanding of the habitat use and overall ecology of this critically endangered species.     

Enhanced expression and release of C-type natriuretic peptide in freshwater eels

C-type natriuretic peptide (CNP) is recognized as a paracrine factor acting locally in the brain and periphery. To assess the role of CNP in teleost fish, a cDNA encoding a CNP precursor was initially cloned from the eel brain. CNP message subsequently detected by ribonuclease protection assay, using the cDNA as probe, was most abundant in the brain followed by liver, gut, gills, and heart. Expression was generally higher in freshwater (FW) than in seawater (SW) eels, but not in the brain. Plasma CNP concentration measured by a newly developed homologous radioimmunoassay for eel CNP was higher in FW than in SW eels. The CNP concentration was also higher in the heart of FW eels but not in the brain. These results show that CNP is abundantly synthesized in peripheral tissues of FW eels and secreted constitutively into the circulation. Therefore, CNP is a circulating hormone as well as a paracrine factor in eels. Together with our previous demonstration that CNP-specific receptor expression is enhanced in FW eels, it appears that CNP is a hormone important for FW adaptation. Because atrial NP (ANP) promotes SW adaptation in eels, CNP and ANP, despite high sequence identity, appear to have opposite effects on environmental adaptation of the euryhaline fish.     

Osmoregulation during the development of glass eels and elvers

Drinking rates in glass eels and elvers of the European eel increased with environmental acclimation salinity from 0·07±0·02 (FW) to 0·70±0·09 μl g-¹ h-¹ (SW) at month 1 and from 1·12±0·42 (FW) to 12·85±1·05 ± l g-¹ h-¹ (SW) at month 5. Drinking rates increased with time in both FW and SW groups. FW acclimated eels when challenged acutely with SW increased drinking rate rapidly immediately upon transfer (0–15 min) and the magnitude of this response increased with developmental time from month 1 to month 5.     

Ultrastructural changes in the calcium-sensitive (PAS-positive) cells of the pars intermedia of eels kept in deionized water and in normal and concentrated sea water

Summary The ultrastructure of the calcium-sensitive (Ca-s) (PAS-positive) cells of the pars intermedia was investigated in eels kept in hypo and hyperosmotic environments. Although the cells were moderately active in fresh water (FW), they were highly stimulated in deionized water (DW) and displayed an enlarged Golgi apparatus, a distinct rough endoplasmic reticulum, few secretory granules, some microtubules and an extended area of contact with the basal lamina that separates nervous and glandular tissues. Some mitosing cells were seen. A similar picture was observed in eels kept in sea water (SW) for 45 days, returned to FW and subsequently to DW for 21 days. In SW (30 and 33), and particularly in concentrated SW (50, 60 and 63), the Ca-s cells were inactive. Their granules were significantly smaller than in eels kept in FW, and the area of contact with the basal lamina was greatly reduced. However, signs of granule-release were seen in eels adapted to 50 and 60 SW. Nerve fibers rarely contacted the Ca-s cells and did not synapse with them. The ultrastructural data support the hypothesis that the Ca-s cells of Anguilla, like those of Carassius, are involved in ionic regulation. MSH cells were not greatly affected by the present experiments.     

Relative antidipsogenic potencies of six homologous natriuretic peptides in eels

Atrial natriuretic peptide (ANP) exhibits a potent antidipsogenic effect in seawater (SW) eels to limit excess Na(+) uptake, thereby effectively promoting SW adaptation. Recently, cardiac ANP, BNP and VNP and brain CNP1, 3 and 4, have been identified in eels. We examined the antidipsogenic effect of all homologous NPs using conscious, cannulated eels in both FW and SW together with parameters that affect drinking. A dose-response study (0.01-1 nmol/kg) in SW eels showed the relative potency of the antidipsogenic effect was in the order ANP ≥ VNP > BNP = CNP3 > CNP1 ≥ CNP4, while the order was ANP = VNP = BNP > CNP3 = CNP1 = CNP4 for the vasodepressor effect. The minimum effective dose of ANP for the antidipsogenic effect is much lower than that in mammals. ANP, BNP and VNP at 0.3 nmol/kg decreased drinking, plasma Na(+) concentration and aortic pressure and increased hematocrit in SW eels. The cardiac NPs induced similar changes in drinking, aortic pressure and hematocrit in FW eels, but aside from BNP no change in plasma Na(+) concentration. CNPs had no effect on drinking, plasma Na(+) concentration and hematocrit but induced mild hypotension in both FW and SW eels, except for CNP3 that inhibited drinking in SW eels. These results show that ANP, BNP and VNP are potent antidipsogenic hormones in eels in spite of other regulatory factors working to induce drinking, and that CNPs are without effects on drinking except for the ancestor of the cardiac NPs, CNP3.     

Effect of transfer to sea water and back to fresh water on the histological structure of the eel kidney

Summary A cytometrical study of the various segments of kidney tubules was performed on silver or silvering male eels during acclimation to sea water (SW) for 2 to 160 days and their return to fresh water (FW). Tubular epithelial cell height and nuclear area are markedly reduced during the first two days, and further reduced in the following period to reach a stable level around the 20th day. The glomerular size is slightly decreased. Phospholipids are less abundant and the brush borders become thinner. In a single eel kept for 9 months in pure SW slowly concentrated by evaporation (NaCl 45 g/l), cell and nuclear values were similar to those obtained in full strength SW after 160 days. Distal and collecting tubules exhibit maximal reactivity, that of the first proximal tubule being minimal.When eels kept for 20 days in SW are returned to FW, there is a rapid increase (48 h), sometimes with an overshoot, of all measured parameters, followed by a rapid decrease (5 days) and a normalization of the values in the distal and collecting tubules.The rapid response of eel kidney to salinity changes does not appear to be linked to cellular hydration as the various segments of the kidney tubules react differently. The data are discussed with respect to osmotic acclimation and in relation to variations of prolactin secretion. Prolactin (PRL) release during transfer to FW stimulates the renal tubules less strongly than mammalian PRL: while mitotic activity and differentiation of new nephrons are induced by ovine PRL treatment in intact or hypophysectomized FW eels (Olivereau and Lemoine, 1969b), no such events were detected during acclimation to this hypotonic medium, at least during the first ten days. This rather reflects insufficient PRL release than a different biological activity, despite immunological differences between fish and ovine prolactin.

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Résumé L'étude cytométrique (hauteur épithéliale et aire nucléaire) des divers segments du tube rénal, réalisée chez l'Anguille mâle argentée ou en cours d'argenture lors de l'adaptation à l'eau de mer (SW) montre une réduction marquée dès les premières 48 h en SW qui s'accuse légèrement pour atteindre un niveau stable vers le 20^e jour; la taille des glomérules est réduite. Les phospholipides sont moins abondants, la bordure en brosse est plus mince. Chez une Anguille conservée 9 mois en SW initialement normale et lentement concentrée par évaporation (NaCl 45 g/l), les valeurs obtenues sont peu différentes de celles après 160 jours en SW pure. La réactivité maximale s'observe au niveau du tube distal et du collecteur, celle du tube proximal l étant minimale.Le retour en eau douce (FW) provoque une rapide augmentation (2 jours), parfois excessive, de ces paramètres, suivie d'une baisse rapide (5 jours) et de la normalisation des valeurs du tube distal et collecteur (10 jours).Les réponses du rein aux changements de salinité, très rapides, ne paraissent pas liées à des processus de déshydratation ou hydratation cellulaire car elles affectent inégalement les divers segments du néphron. Elles sont discutées en fonction des variations des électrolytes plasmatiques et de la sécrétion hypophysaire de prolactine (PRL): sa décharge qui accompagne le retour en FW stimule moins intensément le tube rénal que l'injection de PRL ovine, elle ne provoque pas une intense activité mitotique ni une différenciation de nouveaux néphrons, au moins pendant les 10 premiers jours. Ce fait représente probablement plus une sécrétion insuffisante de PRL lors du retour en milieu hypotonique qu'une différence d'activité biologique, malgré des différences immunologiques entre prolactines ovine et de Poisson.

     

How does salinity influence habitat selection and growth in juvenile American eels Anguilla rostrata?

The influence of salinity on habitat selection and growth in juvenile American eels Anguilla rostrata captured in four rivers across eastern Canada was assessed in controlled experiments in 2011 and 2012. Glass eels were first categorized according to their salinity preferences towards fresh (FW), salt (SW) or brackish water (BW) and the growth rate of each group of elvers was subsequently monitored in controlled FW and BW environments for 7 months. Most glass eels (78–89%) did not make a choice, i.e. they remained in BW. Salinity preferences were not influenced by body condition, although a possible role of pigmentation could not be ruled out. Glass eels that did make a choice displayed a similar preference for FW (60–75%) regardless of their geographic origin but glass eels from the St Lawrence Estuary displayed a significantly higher locomotor activity than those from other regions. Neither the salinity preferences showed by glass eels in the first experiment nor the rearing salinities appeared to have much influence on growth during the experiments. Elvers from Nova Scotia, however, reached a significantly higher mass than those from the St Lawrence Estuary thus supporting the hypothesis of genetically (or epigenetically) based differences for growth between A. rostrata from different origins. These results provide important ecological knowledge for the sustained exploitation and conservation of this threatened species.     

The natriuretic peptide system in eels: a key endocrine system for euryhalinity?

The natriuretic peptide system of a euryhaline teleost, the Japanese eel (Anguilla japonica), consists of three types of hormones [atrial natriuretic peptide (ANP), ventricular natriuretic peptide (VNP), and C-type natriuretic peptide (CNP)] and four types of receptors [natriuretic peptide receptors (NPR)-A, -B, -C, and -D]. Although ANP is recognized as a volume-regulating hormone that extrudes both Na(+) and water in mammals, ANP more specifically extrudes Na(+) in eels. Accumulating evidence shows that ANP is secreted in response to hypernatremia and acts to inhibit the uptake and to stimulate the excretion of Na(+) but not water, thereby promoting seawater (SW) adaptation. In fact, ANP is secreted immediately after transfer of eels to SW and ameliorates sudden increases in plasma Na(+) concentration through inhibition of drinking and intestinal absorption of NaCl. ANP also stimulates the secretion of cortisol, a long-acting hormone for SW adaptation, whereas ANP itself disappears quickly from the circulation. Thus ANP is a primary hormone responsible for the initial phase of SW adaptation. By contrast, CNP appears to be a hormone involved in freshwater (FW) adaptation. Recent data show that the gene expression of CNP and its specific receptor, NPR-B, is much enhanced in FW eels. In fact, CNP infusion increases (22)Na uptake from the environment in FW eels. These results show that ANP and CNP, despite high sequence identity, have opposite effects on salinity adaptation in eels. This difference apparently originates from the difference in their specific receptors, ANP for NPR-A and CNP for NPR-B. VNP may compensate the effects of ANP and CNP for adaptation to respective media, because it has high affinity to both receptors. On the basis of these data, the authors suggest that the natriuretic peptide system is a key endocrine system that allows this euryhaline fish to adapt to diverse osmotic environments, particularly in the initial phase of adaptation.     

Regulation of ion transport in eel intestine by the homologous guanylin family of peptides

Since the gene expression of guanylin peptides and their receptors, guanylyl cyclase Cs, is enhanced in the intestine of seawater (SW)-adapted eels compared with fresh water (FW)-adapted fish, the guanylin family may play an important role in SW adaptation in eels. The present study analyzed the effect of three homologous guanylin peptides, guanylin, uroguanylin and renoguanylin, on ion movement through the eel intestine, and examined the target of guanylin action using Ussing chambers. The middle and posterior parts of the intestine, where water and ion absorption occurs actively in SW eels, exhibited serosa-negative transepithelial potential, while the anterior intestine was serosa-positive. Mucosal application of each guanylin in the middle or posterior intestine reduced the short-circuit current (Isc) dose dependently and reversed it at high doses, and reduced electric tissue resistance. The effects were greater in the middle intestine than in the posterior intestine. All three guanylins showed similar potency in the middle segment, but guanylin was more potent in the posterior segment. 8-bromo cGMP mimicked the effect of guanylins. The intestinal response to guanylin was smaller in FW eels. The mucosal presence of NPPB utilized as a CFTR blocker, but not of other inhibitors of the channels/transporters localized on the luminal surface in SW fish intestine, inhibited the guanylin-induced decrease in Isc. In eels, therefore, the guanylin family may be involved in osmoregulation by the intestine by binding to the receptors and activating CFTR-like channels on the mucosal side through cGMP production, perhaps resulting in Cl(-) and HCO3(-) secretion into the lumen.     

Effects of environmental salinity and 17alpha-methyltestosterone on growth and oxygen consumption in the tilapia, Oreochromis mossambicus

Effects of environmental salinity and 17α-methyltestosterone (MT) on growth and oxygen consumption were examined in the tilapia, Oreochromis mossambicus. Yolk-sac fry were collected from brood stock in fresh water (FW). After yolk-sac absorption, they were assigned randomly to one of four groups: FW, MT treatment in FW, seawater (SW) and MT treatment in SW. All treatment groups were fed to satiation three times daily. The fish reared in SW (both control and MT-treated groups) grew significantly larger than either group in FW from day 43 throughout the experiment (195 days). The fish fed with MT added to their feed grew significantly larger than their respective controls from day 85 in FW and in SW until the end of the experiment. The routine metabolic rate (RMR) was determined monthly from month 2 (day 62) to month 5 (day 155). A significant negative correlation was seen between RMR and body mass in all treatment groups. Among fish of the same age, the SW-reared tilapia had significantly lower RMRs than the FW-reared fish. The MT-treated fish in SW showed significantly lower RMRs than the SW control group at months 3–5, whereas MT treatment in FW significantly increased the RMR at month 3. Comparison of regression lines between RMR and body mass indicates that MT treatment in FW caused a significant increase in oxygen consumption at a given mass of the fish, whereas MT treatment was without effect on RMR in SW-reared fish. These results clearly indicate that SW-rearing and MT treatment accelerate growth of tilapia, and that RMR decreases as fish size increased. It is also likely that the increased RMR and growth in MT-treated tilapia in FW may be due to the metabolic actions of MT, although the reason for the absence of MT treatment in SW is unclear.     

Lipid and fatty acid variations in muscle tissues of the ‘yellow’ stage of the European eel (Anguilla anguilla) during short-term adaptation to freshwater and seawater under food deprivation

Changes in body mass and water content (WC), lipids, and fatty acid (FA) composition of muscle were determined in European eels (Anguilla anguilla) in the ‘yellow’ stage (sexually immature). The animals were held in either freshwater (FW) or seawater (SW) and deprived of food for four weeks. During this period, body mass decreased steadily. This decrease was accompanied by reduced muscle WC and total lipid in both SW and FW. ‘Yellow’ eels appeared to use lipids in muscle to provide metabolic and osmoregulatory energy. However, they maintained relative constancy in the FA profiles, so that total FA, saturated, monounsaturated, and polyunsaturated FA, were similar between salinities during short-term starvation.     

Antidiuretic Effect of Eel ANP Infused at Physiological Doses in Conscious, Seawater-Adapted Eels, Anguilla japonica

Atrial natriuretic peptide (ANP) is known as a potent natriuretic/diuretic hormone in vertebrates. However, eel ANP infused at doses that did not alter arterial blood pressure (0.3-3.0 pmol/kg/min) decreased urine volume and increased urinary Na concentration in seawater (SW)-adapted eels but not in freshwater (FW)-adapted eels. The renal effects were dose-dependent and disappeared after infusate was switched back to a vehicle (0.9% NaCl). Urinary Na excretion (volume x Na concentration) did not change during ANP infusion. ANP infusion increased plasma ANP concentration, but the increase at the highest dose was still within those observed endogenously after injection of hypertonic saline. Urinary Mg and Ca concentrations increased during ANP infusion in SW eels, but urinary Ca excretion decreased in FW eels. Plasma Na concentration profoundly decreased during ANP infusion only in SW eels, suggesting that ANP stimulates Na extrusion via non-renal routes. These results indicate that ANP is a hormone which specifically extrudes Na ions and thereby promotes SW adaptation in the eel. This is in sharp contrast with mammals where ANP is a volume regulating hormone that extrudes both Na and water.     

The ultrastructure of prolactin cells in the annual cyprinodont Cynolebias whitei during its life cycle

Summary Prolactin (PRL) cells were studied electron-microscopically and morphometrically in the annual cyprinodont fish, Cynolebias whitei during its life cycle. In prehatching larvae, PRL cells possessed small secretory granules, giant mitochondria and a well-developed Golgi apparatus. During hatching, no changes were observed in the volume density of the secretory granules, indicating that no increased release of PRL occurs at hatching. A significant change in the composition of PRL cells, i.e., the volume densities per cytoplasm volume of the different organelles, occurred between one day and one week of age. Thereafter, only minor differences were observed between age groups, indicating that no major changes occur in PRL cell activity during the lifespan of C. whitei. However, the volume density per cell volume of the nucleus decreased steadily with age during the lifespan. A comparison of the PRL cells in young and adult fish reared in fresh water (FW) with siblings reared from hatching in diluted sea water (1/3 SW) did not reveal any differences with respect to the volume densities of the organelles, including the secretory granules. However, significant differences were observed with respect to the diameter, electron-dense content and affinity to anti- PRL serum of the secretory granules. These differences indicate that, despite the similar volumetric composition of the PRL cells, their secretory granules contain a substantially higher concentration of PRL in FW-reared fish than in 1/3 SW-reared fish.     

Calcium-sensitive cells of the pars intermedia and osmotic balance in the eel

Summary Freshwater eels were adapted to calcium-free sea water (SW) or 1/3 Ca-free SW. Survival was generally poor in Ca-free SW, although three eels were still in good condition after 19–30 days; survival in 1/3 Ca-free SW was excellent. Osmotic disturbances (increase of plasma osmolarity and sodium levels), which initially occur in Ca-free SW, were no longer detectable after 19–30 days, or in eels in 1/3 Ca-free SW after one month. Plasma calcium sharply decreases initially; it is less depressed after 19–30 days and in 1/3 Ca-free SW. Alterations in the mucus production may be involved in the osmotic changes. Under these conditions no clear stimulation of the calcium-sensitive (Ca-s) cells of the pars intermedia was registered, but in Ca-free SW (1/3 or full strength) the inhibitory effect normally observed in SW does not occur. In a hyperosmotic environment, other ion(s), possibly magnesium, may reduce the response of the Ca-s cells to a lack of environmental calcium.