Plasma Arginine Vasotocin Levels Research Articles

Metabolic status and ghrelin regulate plasma levels and release of ovarian hormones in layer chicks.

The aim of the present study was to examine the role of nutritional status, the metabolic hormone ghrelin and their interrelationships in the control of chicken hormones involved in the regulation of reproduction. For this purpose, we identified the effect of food deprivation, administration of ghrelin 1-18 and their combination on plasma levels of testosterone (T), estradiol (E), arginine-vasotocin (AVT) and growth hormone (GH) as well as the release of these hormones by isolated and cultured ovarian fragments. It was observed that food deprivation reduces plasma T and E and increases plasma AVT and GH levels. Food restriction also reduced the amount of E produced by isolated ovaries, but it did not affect the ovarian secretion of T and AVT. No ovarian GH secretion was detected. Ghrelin administered to ad libitum fed chickens did not affect plasma T and E levels, but it did increase plasma GH and AVT concentrations. Moreover, it partially prevented the effect of food deprivation on plasma E and AVT levels, but not on T or GH levels. Ghrelin administration to control birds promoted ovarian T, but not E or AVT release and reduced T and no other hormonal outputs in birds subjected to food restriction. Our results (1) confirmed the ovarian origin of the main plasma T and E and the extra-ovarian origin of the main blood AVT and GH; (2) showed that food deprivation-induced suppression of reproduction may be caused by suppression of T and E and the promotion of AVT and GH release; (3) suggest the involvement of ghrelin in control chicken E, AVT and GH output; and (4) indicates that ghrelin can either mimic or modify the effect of the intake of low calories on chicken plasma and ovarian hormones, i.e. it can mediate the effect of metabolic state on hormones involved in the control of reproduction.

Osmoregulatory role of vasotocinergic and isotocinergic systems in the gilthead sea bream (Sparus aurata L)

Gilthead sea bream, Sparus aurata L., is an important fish species for the Mediterranean aquaculture and is considered a good model for studying the osmoregulatory process, due to its capacity to cope with great changes in environmental salinity (5–60‰). Our group studied the osmoregulatory role of different endocrine systems in this species, focusing on the vasotocinergic and isotocinergic systems over several years. For this purpose, the cDNAs coding for pro-vasotocin (pro-vt), pro-isotocin (pro-it), two arginine vasotocin (AVT) receptors (avtr v1a2- and v2-types) and one IT receptor (itr) were cloned. Acclimation to different environmental salinities induced a direct lineal relationship between plasma AVT levels and salinity, with no changes in plasma IT values. In addition, higher values in vasotocinergic, isotocinergic and stress pathways (pro-vt and pro-it gene expression, AVT and IT storage and plasma cortisol levels) in both hypo- and/or hyper-osmotic transfers, suggest an interaction between cortisol and AVT/IT pathways. Moreover, gene expression of specific receptors, as well as the use of different in vitro techniques, demonstrated an important osmoregulatory orchestration in different organs. In addition, individuals intraperitoneally injected with AVT and transferred to different environmental salinities enhanced plasma cortisol levels and/or gill Na+, K+-ATPase activity. These effects could be related to the energy repartitioning process occurring during osmotic adaptation of S. aurata to extreme environmental salinities, which could be mediated not only by plasma cortisol but also by AVT. Finally, our results indicated a very important role of the vasotocinergic and/or isotocinergic systems in both osmoregulatory and non-osmoregulatory organs.

Effect of acute restraint on hypothalamic pro-vasotocin mRNA expression in flounder, Platichthys flesus

Arginine vasotocin (AVT) stimulates release of adenocorticotrophin hormone (ACTH) in trout. However, AVT’s role in fish hypothalamic–pituitary–interrenal-axis (HPIA) is not fully understood. Here, we examined distribution of AVT and glucocorticoid receptor (GR) in the magnocellular preoptic nucleus (PM) and the AVT/cortisol response to acute restraint in flounder. The GR/AVT distribution in the PM was determined using double immunohistochemistry (IHC). Flounder were confined in nets, immersed in water for 30 m, with plasma and tissue samples taken prior to, 3, 24 and 48 h post-confinement. Plasma osmolality, Na +, Cl − and cortisol were taken as indicators of HPIA activation. Plasma AVT was measured proVT mRNA expression in the PM was detected using in situ hybridisation (ISH) with a S35 labelled oligoprobe for homologous flounder proVT. Double IHC showed the presence of GR in AVT synthesising neurones of the PM. Plasma Na +, Cl −, osmolality and cortisol (1.0 ± 0.9 to 183.6 ± 3.1 mM; p < 0.001) increased significantly 3 h post-restraint: recovering to control levels after 48 h. Plasma AVT levels did not change. However, a concomitant increase in proVT mRNA expression in the magnocellular (PMm) and gigantocellular (PMg) neurones of the PM was observed (11.1 ± 1.8 to 55.2 ± 9.1% 24 h post-restraint; p < 0.001) and levels still remained significantly elevated at 48 h ( p < 0.01). This suggests that PMm and PMg AVT neurones are associated with HPIA activation following acute restraint, including potential cortisol negative feedback. The extended elevation of hypothalamus proAVT mRNA expression following a single acute stressor affords a possible mechanism to moderate sensitivity of the HPIA to subsequent challenges.

The effects of freshwater to seawater transfer on circulating levels of angiotensin II, C-type natriuretic peptide and arginine vasotocin in the euryhaline elasmobranch, Carcharhinus leucas

This study examined the effect of transfer to increased environmental salinity on the circulating levels of angiotensin II (ANG II), C-type natriuretic peptide (CNP), and arginine vasotocin (AVT) in the euryhaline elasmobranch, Carcharhinus leucas. Plasma levels of ANG II and CNP were significantly increased in C. leucas chronically acclimated to seawater (SW) in comparison to freshwater (FW) acclimated fish. There was no difference in plasma AVT levels. Acute transfer of FW fish to 75% SW induced an increase in plasma ANG II levels within 12 h, and subsequent transfer from 75 to 100% SW further increased plasma ANG II levels at both 24 and 72 h. No change in plasma CNP was observed during acute transfer to increased salinity. However, a significant increase in plasma AVT levels was observed following 96 h in 75% SW and 24 h in 100% SW. In chronically SW acclimated C. leucas plasma osmolality, sodium, chloride, and urea were all significantly higher than FW acclimated fish but there was no difference in haematocrit. Acute transfer of C. leucas to 75% SW induced a significant increase in plasma osmolality, sodium and urea concentrations within 96 h of transfer. Subsequent transfer from 75 to 100% SW induced a further increase in these variables within 24 h in addition to a significant increase in plasma chloride above control levels. Haematocrit did not differ between the experimental and control groups throughout the acute study. Circulating levels of ANG II were significantly correlated to plasma, sodium, chloride, and urea concentrations during acclimation to SW. Conversely, circulating levels of CNP and AVT did not correlate to plasma osmolytes, however, CNP was significantly correlated to haematocrit during acclimation to seawater.

Plasma Concentrations of Arginine Vasotocin and Urotensin II Are Reduced Following Transfer of the Euryhaline Flounder (Platichthys flesus) from Seawater to Fresh Water

Plasma concentrations and stored levels of the neuroendocrine peptides arginine vasotocin (AVT) and urotensin II (UII) were measured in the euryhaline flounder (Platichthys flesus) following the acute hypo-osmotic challenge of direct seawater (SW) to fresh water (FW) transfer. Hormone measures, plasma osmolality, and ion concentrations and tissue water content were determined 1, 4, 8, 24, 72, and 144 h after transfer. Plasma AVT concentration fell initially following FW transfer but then returned toward pretransfer levels by day 6. Plasma UII concentration decreased while urophysial UII content was increased following hypo-osmotic challenge relative to SW time-matched controls, suggesting down regulation of the UII system during the initial stages after FW transfer. These changes in neuroendocrine activity were associated with a significant fall in plasma osmolality and major plasma ions. Positive correlations were observed between plasma AVT and osmolality and Cl− and Mg2+ concentrations, suggesting functional association of these plasma parameters with AVT action and/or control of AVT secretion. The initial response to hypotonic challenge involves reduced plasma AVT and UII levels consistent with the proposed role for these hormones, supporting flounder osmoregulation in hypertonic media.

WATER DEPRIVATION AND CIRCADIAN CHANGES IN PLASMA ARGININE VASOTOCIN AND MESOTOCIN IN THE DOMESTIC HEN (GALLUS DOMESTICUS)

Possible circadian variations in plasma levels of arginine vasotocin (AVT) and mesotocin (MT) were assessed in domestic hens (Gallus domesticus) under a 12h:12h light-dark (LD) schedule. Blood samples were taken at 4h intervals, and neurohypophyseal hormone levels were determined by radioimmunoassay. Marked circadian changes in both AVT and MT were observed in hens provided free access to water. Minimal and maximal AVT levels occurred at 08:00 and 20:00, respectively. Minimal MT levels occurred at 20:00, whereas maximal MT levels occurred over a broad time period of 04:00 to 12:00. In water-deprived hens, plasma AVT levels were elevated at each time point, and the circadian variations in plasma AVT and MT levels were attenuated. These results demonstrate that rhythmicity in neurohypophyseal function in a lower vertebrate species, like that in mammals, is disrupted by osmotic stress. (Chronobiology International, 18(6), 947–956, 2001)

Effect of AVT antisense oligodeoxynucleotides on AVT release induced by hypertonic stimulation in chicks

In birds, arginine vasotocin (AVT) and mesotocin (MT) are the neurohypophyseal hormones. AVT is known to be an avian antidiuretic hormone and is released from the neurohypophysis by dehydration or hyperosmotic stimulation. The purpose of this study was to determine whether the mechanism of AVT synthesis is related to the mechanism of hormone release from the neurohypophysis. Four-day-old chicks received an AVT antisense oligodeoxynucleotide (ODN) injection into the cerebral ventricle (icv). Following antisense administration, the chicks received hypertonic saline stimulation. Plasma levels of AVT and MT were measured by radioimmunoassays. In control birds, a hypertonic saline injection resulted in the increase of plasma AVT level. The administration of a high dose (50 μg) of antisense ODN inhibited the increase of plasma AVT level induced by the hypertonic saline stimulation. Plasma levels of MT did not change with the administration of hypertonic saline or antisense ODN. These results suggest that the mechanisms that regulate the secretion of AVT from the neurohypophysis may be coupled to the mechanisms that regulate the synthesis of AVT.

Regulation of opioid peptides on the release of arginine vasotocin in the hen

Arginine vasotocin (AVT), an avian neurohypophysial hormone, is released during osmotic stimulation and oviposition. In the present study, the role of opioid peptides on AVT release was studied by examining the effects of an opioid agonist and antagonist on osmotic- and oviposition-induced secretion of AVT. The administration of hypertonic saline (1.5 M NaCl) induced an increase in the plasma levels of AVT. The simultaneous administration of morphine, an opioid receptor agonist, inhibited the osmotically induced increase in plasma levels of AVT in a dose-dependent manner. On the other hand, the co-administration of morphine with naloxone, an opioid receptor antagonist, attenuated the inhibitory effect of morphine. Moreover, injection of naloxone alone enhanced the osmotically induced increase in plasma levels of AVT. However, the administration of morphine did not inhibit the oviposition-induced increase in plasma levels of AVT. These results suggest that osmotic-induced release of AVT may be under opioid regulation, while oviposition-induced release of AVT may be controlled by a different mechanism. J. Exp. Zool. 286:481-486, 2000.

A Sexual Dimorphism in Hypothalamic Arginine Vasotocin (AVT) Gene Expression and AVT Plasma Levels in the Japanese Quail (Coturnix coturnix japonica) in Response to Water Deprivation

To examine a possible sexual dimorphism in the osmotic control of arginine vasotocin (AVT) release in birds, age-matched male and female Japanese quail were subjected to water deprivation. The observed increased plasma osmolalities were accompanied by increased plasma AVT levels. Plasma mesotocin levels did not change with water deprivation. The sensitivity of the osmotic control of AVT release as determined by the slope of the relationship between plasma AVT levels and plasma osmolalities was significantly (P < 0.05) higher in males than in females. By Northern blot analysis, levels of hypothalamic AVT gene transcripts were increased 2.3 ± 0.14- and 3.5 ± 0.13-fold in water-deprived male and female Japanese quail, respectively, compared to normally hydrated birds. Our data suggest gender-related differences in the osmotic control of AVT release and in hypothalamic AVT gene expression in the Japanese quail.

Effects of Naloxone on Neurohypophyseal Peptide Release by Hypertonic Stimulation in Chicks

The effects of opioid peptides on the osmotic release of neurohypophyseal hormones, arginine vasotocin (AVT) and mesotocin (MT), were determined in 2-day-old chicks. Experiment 1 examined the effect of a variety of doses of naloxone, an opioid antagonist, on chicks administered isotonic or hypertonic solution. Plasma osmolality in chicks administered hypertonic solution was significantly higher than that in groups administered isotonic solution. None of the doses of naloxone affected plasma osmolality in response to isotonic and hypertonic solution. Plasma levels of AVT increased in hypertonic solution and this response was further enhanced by naloxone injection as the doses increased. The hypertonic solution alone did not affect plasma levels of MT, but additional treatment with naloxone slightly increased plasma levels of MT. Experiment 2 examined the effect of DAMGO ([d-Ala2,N-Me-Phe4,Gly-ol]-enkepha lin), a specific μ receptor agonist. Relatively high plasma osmolality caused by hypertonic solution was not affected by additional treatment with DAMGO. Plasma levels of AVT in response to hypertonic solution and to additional treatment with naloxone were reduced by higher doses of DAMGO. Experiment 3 examined the effect of naloxone on chicks administered different concentrations of NaCl. Administration of hypertonic solution resulted in an increase in plasma osmolality and plasma levels of AVT. Naloxone administration enhanced the increase in plasma AVT levels in response to hypertonic solution. Experiment 4 examined the effect of naloxone on different kinds of hypertonic solution, 0.15 M NaCl, 1.5 M NaCl, 2.55 M urea, and 1.95 M sucrose. The increases in plasma osmolality resulting from the administration of the urea and sucrose solutions were the same as those in the chicks injected with 1.5 M NaCl. In sucrose-treated chicks, plasma levels of AVT increased in chicks administered naloxone but not in chicks injected with normal saline. In contrast, no significant changes in plasma levels of AVT were observed in urea treatment with or without naloxone. In Experiments 3 and 4, plasma levels of MT after administration of hypertonic solutions did not change. However, naloxone administration enhanced plasma levels of MT in osmotically stimulated chicks. The results of the present study suggest that opioid peptides attenuate the increase in plasma AVT and MT in hypertonic states.

Effects of short-term dehydration on plasma osmolality, levels of arginine vasotocin and its hypothalamic gene expression in the laying hen

The neurohypophysial hormone, arginine vasotocin (AVT), plays an important role in the osmoregulation of birds. After a prolonged period of water deprivation, plasma osmolality and plasma concentration of AVT are elevated. In this study, the effects of short term dehydration were examined in laying hens by measuring plasma osmolality, plasma levels of potassium, sodium and AVT and hypothalamic concentrations of mRNA encoding AVT during 8 h of water deprivation. Plasma osmolality increased significantly after at 6 h of water deprivation. Plasma sodium levels, however, did not change. Plasma potassium concentrations gradually decreased during dehydration. Plasma AVT levels and hypothalamic AVT mRNA levels increased significantly after 8 h. The results of this study demonstrate that depriving chickens of water results first in an increase in plasma osmolality followed by increases in AVT levels in plasma and AVT mRNA levels in the hypothalamus. The data indicate that the synthesis of AVT in the magnocellular neurons in the hypothalamus is activated soon after the animals are deprived of water. This indicates that both de novo synthesized AVT as well as AVT stored in the neurohypophysis are available to meet the increasing demands for the hormone during osmotic stress.

Renal function and plasma levels of arginine vasotocin during free flight in pigeons.

We examined urinary water loss and plasma levels of arginine vasotocin (AVT) in free-flying, tippler pigeons trained to fly continuously for up to 5 h. First, we used [3H]polyethyleneglycol ([3H]PEG) as a glomerular filtration marker by implanting an osmotic minipump into each bird. In two flights (10 birds in winter at an ambient temperature of 13-15 degrees C and seven in summer at 23 degrees C), we measured pre-flight (hydrated, resting control birds) and post-flight [3H]PEG activity and osmolality in blood and ureteral urine. For comparison, we measured these variables in 10 birds in winter before and after controlled dehydration (24 h at 25 or 30 degrees C). Second, we measured plasma levels of AVT in 6-8 birds before and immediately after each of three different summer flights. Urine osmolality increased significantly by up to three times the control level in both post-flight and dehydrated pigeons; urine:plasma osmolality ratios did not exceed 2. Compared with controls, glomerular filtration rate (GFR) was significantly lower after flight in summer, but did not change in either post-flight or dehydrated winter pigeons. In winter, mean post-flight urine flow rate (UFR) decreased significantly to less than half the control level, while in summer, post-flight UFR did not differ from control levels. In general, mean filtered water reabsorption (FrH2O) increased from 95 % in controls to 98 % in post-flight and dehydrated birds. Plasma levels of AVT increased after flight to between three and eight times the preflight levels. The data from this first study of kidney function during flight are consistent with previous studies of dehydration in birds and exercise in mammals in which both increased FrH2O and decreased GFR contribute to renal conservation of water.

Response of circulating arginine vasotocin and isotocin to rapid osmotic challenge in rainbow trout

The plasma levels of arginine vasotocin (AVT) and isotocin (IT) in rainbow trout ( Oncorhynchus mykiss) were studied to demonstrate changes in neurohypophysial nonapeptides following rapid osmotic challenge. Freshwater-adapted fish were transferred to brackish Baltic water (BW) and brackishwater-adapted fish were transferred to fresh water (FW). In both experiments blood samples for AVT, IT and osmolality were taken 30 min, 1, 2, 24 hr, and 10 days after transfer. Hormones were determined by gradient high-performance liquid chromatography preceded by solid-phase extraction. The pattern of AVT and IT response to external salinity changes was similar during the first hours and showed transient increase or decrease after transfer to brackish or fresh water, respectively. The AVT plasma concentration after 10 days in FW was significantly higher than that in BW, whereas the IT plasma concentrations in both salinities did not differ. These data suggest that the synthesis and or release of AVT and IT are controlled independently, and the roles of hormones in long-term osmotic adaptation are different.

Arginine vasotocin gene expression in hypothalamic neurons is up-regulated in chickens drinking hypertonic saline: an in situ hybridization study.

Osmotic stress stimulates the release of the avian hypothalamic neuropeptide arginine vasotocin (AVT) into the peripheral circulation. We conducted the present study to investigate the effects of salt-loading on AVT secretion and AVT gene expression in specific hypothalamic nuclei in chickens. White Leghorn chickens were provided food ad lib and either water or 2% NaCl to drink. Both plasma osmolality and plasma AVT levels were significantly increased in chickens that drank 2% NaCl for either two or four days compared to that in chickens that drank water. Results from in situ hybridization analysis demonstrated an increase in the number of neurons expressing AVT mRNA in the supraoptic (SON) and paraventricular nuclei (PVN) in chickens provided 2% NaCl to drink compared to chickens that were provided water to drink. The number of grains per neuron increased in the PVN, but not in the SON of osmotically stimulated birds. Thus, increased osmolality resulting from ingestion of hypertonic saline is an effective stimulus to increase hypothalamic AVT mRNA content in chickens.

Diurnal Changes in Plasma Arginine Vasotocin and Isotocin in Rainbow Trout Adapted to Fresh Water and Brackish Baltic Water

The plasma levels of arginine vasotocin (AVT) and isotocin (IT) in rainbow trout (Oncorhynchus mykiss) were studied to assess possible diurnal variations in neurohypophysial nonapeptides. Fish were kept under natural photoperiod and adapted to fresh and brackish Baltic water. Blood was sampled at 5:00, 11:00, 16:00, 22:30, and again at 5:00. Hormones were determined by gradient high-performance liquid chromatography preceded by solid-phase extraction. The marked diurnal changes of AVT were detected in plasma of fish adapted to both fresh and brackish waters. AVT levels (fmol/ml) were maximal at 16:00 and minimal at 5:00 (253.4±35.7 and 45.5±17.3, respectively). Unlike AVT, isotocin levels displayed no diurnal changes. AVT concentrations at 11:00, 16:00, and 22:30 were significantly higher than IT values measured throughout the day. Plasma AVT concentrations determined in brackish water-adapted fish at 16:00 were significantly lower than those of freshwater-adapted fish at the same time. These data suggest that synthesis and/or release of AVT and IT are controlled independently, so that these nonapeptides have different physiological roles in teleost fish. AVT might participate in circadian time-keeping system in fish.

Urinary Clearance of Arginine Vasotocin in Pekin Ducks

To quantify the renal excretion of arginine vasotocin (AVT) in birds, synthetic AVT was infused into conscious Pekin ducks at rates of 0.03, 0.10, and 0.30 ng/kg/min for 30 min and the relationships between plasma and urine AVT concentrations were monitored by radioimmunoassay. As plasma AVT concentrations increased (from a basal value of 7.8 ± 1.1 pg/ml to levels of 11.5 ± 1.9, 17.6 ± 2.2, and 24.2 ± 2.5 pg/ml, respectively) the urinary concentrations and excretion rates increased linearly from basal levels of 26.0 ± 3.1 pg/ml and 14.9 ± 1.8 pg/min to 215.6 ± 36.6 pg/ml and 39.2 ± 3.7 pg/min, respectively. The metabolic clearance rate of AVT (approximately 23 ml/min/kg) and urinary clearance (approximately 0.91 ml/min/kg) remained constant at each infusion dose and were therefore independent of plasma AVT levels. Over the range of plasma AVT concentrations tested, a constant fraction of about 5% of the AVT cleared from the blood was excreted intact in the urine.

Saline-Infusion-Induced Increases in Plasma Osmolality Do Not Stimulate Nasal Gland Secretion in the Ostrich (Struthio camelus)

Nasal gland activity was monitored in five freshwater and five saltwater-acclimated adult ostriches given an intravenous infusion of 1,500 mosm/kg sodium chloride for 90 min at 5 mL/min. The hyperosmotic infusion increased the plasma osmolality from 303.4 ± 6.9 to 353.3 ± 13.1 mosm/kg in the freshwater birds and from 299.4 ± 16.2 to 328.8 ± 15.4 mosm/kg in the saltwater-acclimated animals. None of the freshwater ostriches produced any observable nasal secretion. Two of the saltwater birds did show signs of glandular activity, but this was of short duration (5-10 min) and yielded insignificant volumes of fluid. The nasal glands of ostriches do not appear to play a quantitatively important role in the elimination of the administered saline load. Changes in plasma concentrations of arginine vasotocin (AVT) in response to the hyperosmotic infusion were also determined. Plasma AVT levels increased in parallel with the elevations in plasma osmolality (from 5.4 ± 0.9 to 19.4 ± 8.6 pg/mL in the freshwater ostrich...

A Radioimmunoassay for the Determination of Arginine Vasotocin (AVT): Plasma and Pituitary Concentrations in Fresh- and Seawater Fish

A specific radioimmunoassay was developed and characterized for the measurement of arginine vasotocin (AVT) in teleost fish. Specificity of the antibody for AVT was demonstrated by parallelism of a series of AVT standards with serially diluted pituitary and plasma extracts. Crossreactivity of the antibody with the other teleost neurohypophysial peptide, isotocin, was less than 1% and the sensitivity of the assay was 0.24 fmol/assay tube. AVT was extracted from plasma by reverse-phase liquid chromatography [efficiency of 87.6 ± 9.3% ( n = 5)] and demonstrated as an effective procedure for plasma volumes ranging from 0.4 to 1.2 ml. Plasma AVT concentrations measured in a range of euryhaline and stenohaline teleost fish were between 10 -12 and 2 × 10 -11 M (1-20 pg/ml). There were no consistent differences between plasma AVT levels in euryhaline fish (flounder, trout, and eel) adapted to fresh water (FW) and sea water (SW). In flounder, pituitary AVT levels in FW- and SW-adapted fish were also similar.

Role of endogenous atrial natriuretic peptide in volume expansion diuresis and natriuresis of the Pekin duck

Polyclonal antibodies raised in a rabbit against avian atrial natriuretic peptide (ANP) were shown to reduce circulating endogenous ANP levels in Pekin ducks by more than 90%, and were subsequently used to investigate the role of this peptide in volume expansion diuresis and natriuresis. Conscious birds, undergoing a steady-state diuresis and natriuresis maintained by an i.v. infusion of hypotonic saline at a rate of 0.7 ml/min, responded to ANP antiserum (anti-ANP) with an immediate 30% reduction in urine flow rate and sodium excretion which lasted for about 30 min. Plasma arginine vasotocin levels were not changed by anti-ANP whereas circulating angiotensin II concentrations increased immediately following the administration of anti-ANP. Serum from non-immunized normal rabbits produced no changes in the renal and plasma parameters monitored. The results show that the high circulating levels of endogenous ANP associated with volume expansion promote renal salt and fluid excretion and thus have a major physiological role in avian volume homeostasis.

Pituitary and plasma arginine vasotocin levels in teleost fish

The role of arginine vasotocin (AVT) in teleostean osmoregulation is unclear. Its administration may produce dose-dependent effects on renal fluid loss though the physiological significance of such effects is unknown due to the paucity of data on the normal range in pituitary and plasma AVT levels. The present study addresses these questions by examining the pituitary AVT activity in the flounder and plasma AVT concentrations in the flounder, trout, and carp following alterations in environmental tonicity. Flounder pituitary AVT content, measured by bioassay, was elevated in freshwater (FW)-acclimated compared to seawater (SW)-acclimated fish and rapidly fell or rose, respectively, following transfer from FW to SW or from SW to FW to attain a new steady state. Plasma levels of AVT were higher in FW than in SW in both flounder and trout and it would, therefore, appear that the synthesis, storage, and secretion of AVT may be enhanced in FW. In the stenohaline carp plasma levels of AVT were unchanged on acclimation from FW to 40% SW. In SW flounder there was a significant positive correlation between plasma osmolality and plasma AVT concentration. Such a relationship was not apparent in FW fish, which may reflect differing roles for the hormone in these two extremes of osmotic environment.