Plasma Arginine Vasotocin Concentrations 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.

Changes in plasma angiotensin II, aldosterone, arginine vasotocin, corticosterone, and electrolyte concentrations during acclimation to dry condition and seawater in the crab-eating frog

The crab-eating frog Fejervarya cancrivora inhabits mangrove swamps and marshes in Southeast Asia. In the present study, circulating angiotensin II (Ang II), aldosterone (Aldo), arginine vasotocin (AVT), and corticosterone (Cort) concentrations as well as various blood parameters were studied under osmotically stressful conditions. Following acclimation to hyperosmotic seawater and dry condition for 5days, body weight was significantly decreased. Under both conditions, plasma Na+, Cl−, and urea concentrations, hematocrit values (Ht; blood volume indicator), and osmolality were significantly increased. Dehydration associated with hypovolemic and hyperosmotic states of body fluids was induced during acclimation to hyperosmotic seawater and dry condition in the crab-eating frogs. Ang II, Aldo, AVT, and Cort were maintained within relatively narrow concentration ranges in the control frogs; however, in frogs under dry and hyperosmotic seawater conditions, large variations were observed among individuals in each group. Mean plasma Ang II and Aldo concentrations significantly increased in hyperosmotic seawater-acclimated and desiccated frogs. Although mean plasma AVT concentrations in dehydrated frogs of both the groups were approximately 2.0–3.5 times higher than those in the control frogs, the differences were not significant because of the variation. There was a significant correlation between plasma osmolality and AVT as well as Ang II but not Aldo. A significant correlation was also observed between Ht and AVT as well as Ang II. Plasma Ang II was significantly correlated with plasma Aldo. These results indicate that the crab-eating frogs may exhibit similar physiological responses to both seawater-acclimated and dry conditions. It appears that under dehydrated conditions, osmoregulatory mechanisms participate in stabilization of the situation. The renin–angiotensin system may have pivotal roles in body fluid regulation under volemic and osmotic stress in the Fejervarya species with unique osmoregulation.

Water Balance and Arginine Vasotocin in the Cocooning FrogCyclorana platycephala(Hylidae)

It is well established that forming a cocoon, for frog species capable of doing so, markedly reduces evaporative water loss; however, the capacity of cocooned frogs to maintain hydration during extended estivation is not well understood. The combined effects of long-term estivation and water loss were examined in the cocoon-forming species Cyclorana platycephala by assessing the hydration state of the frogs throughout a 15-mo estivation period. Frogs lost mass throughout the 15-mo period to a maximum of 36%+/-6.5% of their initial standard mass. Plasma osmolality reached maximal levels by the ninth month of estivation at 487 mOsm kg(-1) and then remained stable to the fifteenth month of estivation. Urine osmolality continued to increase to the fifteenth month of estivation, at which point plasma and urine concentrations were isosmotic. The use of bladder water to counter losses from circulation was indicated by the relatively slow rate of increase in plasma osmolality with mass loss and the progressive increase in urine osmolality. For estivating frogs, evidence was found for a possible threshold relationship between plasma osmolality and plasma arginine vasotocin (AVT) concentration. After estivation, plasma AVT concentrations decreased markedly after 15-mo estivators were placed in water for 2 h, suggesting that high levels of AVT may not be integral to rapid rehydration in this species.

Plasma aldosterone, angiotensin II, and arginine vasotocin concentrations in the toad, Bufo marinus, following osmotic treatments

To clarify the physiological roles of the renin–angiotensin–aldosterone system (RAAS) and arginine vasotocin (AVT) on body fluid regulation in amphibians, we measured plasma concentrations of aldosterone (ALDO), angiotensin II (ANG II), and AVT after various osmotic challenges in the marine toad, Bufo marinus (Bufonidae). Hematocrit value (Ht) as an indicator of plasma volume, plasma osmolality and concentrations of plasma components (Na +, Cl −, K +, and urea) were also measured. The toads were maintained under various osmotic treatments for 7 days. In dehydrated toads, plasma concentrations of ALDO, ANG II, AVT, and all plasma components measured were increased. In toads maintained in 300 mosmol/kg H 2O NaCl solution, plasma osmolality, Na +, Cl −, urea, and plasma AVT concentrations were significantly increased, and Ht and plasma concentrations of ALDO and ANG II were significantly decreased. In toads maintained in tap water, plasma osmolality, and concentrations of Na + and ALDO were significantly decreased. We also estimated total body water (TBW), plasma volume (PV) using Evans Blue dye and Ht in the toads under various osmotic treatments. In dehydrated toads, TBW and PV were significantly decreased and Ht was significantly increased in comparison with those of control. In toads maintained in 300 mosmol/kg H 2O NaCl solution, TBW and PV were significantly increased and Ht was significantly decreased in comparison with those of control. There was a significant negative correlation between Ht and PV or TBW. These results show that dehydration, which induces hypovolemic and hyperosmotic conditions, stimulates increases of plasma ALDO, ANG II, and AVT concentrations, while hypervolemic treatment induces decreases of plasma ALDO and ANG II concentrations. There were significant correlations between plasma osmolality and AVT concentration, between Ht and concentrations of RAAS hormones, and between plasma concentrations of ALDO and ANG II. These results suggest that volumetric and osmometric systems regulated by RAAS hormones and AVT are present in B. marinus.

Dietary intake effects on arginine vasotocin and aldosterone in cloacal fluid of whitebellied sunbirds ( Nectarinia talatala)

The main objective of this study was to determine whether or not the renal outputs of the osmoregulatory hormones arginine vasotocin (AVT) and aldosterone (ALDO) reflect the osmotic status of whitebellied sunbirds ( Nectarinia talatala). The birds were fed a range of sucrose concentrations (from 0.07 to 2.5 mol/l, with osmolalities of 70 to ∼5,800 mosM/kg), and adjusted their intakes so that they drank large volumes of dilute diets and small volumes of concentrated diets. Renal fluid outputs were appropriately regulated so that large volumes of cloacal fluid (CF) were voided on the dilute diets and small volumes on the concentrated diets. Accordingly, plasma AVT concentrations increased with increasing sugar concentration; however, AVT outputs in CF did not change in a similar manner, rather they decreased as dietary concentration increased. It was not possible to measure plasma ALDO concentrations in the small sunbirds because of insufficient blood samples available; however, ALDO outputs in CF did vary with the sucrose diets and renal function, being highest on the most concentrated diet. In addition ALDO output in CF fell markedly when sodium was added to the 0.5 mol/l sucrose diet. We conclude that in sunbirds fed increasingly concentrated sucrose solutions, changes in CF outputs of ALDO, but not AVT, appear to reflect the water flux and hydration state of these birds.

Changes in expression of AVT and AVT receptor (VT1) gene in hypothalamus and shell gland in relation to egg laying in white leghorn hen

Oviposition is a complex phenomenon involving various regulatory mechanisms at the neuroendocrine levels. Present study was designed to access the changes in arginine vasotocin (AVT) and its receptor (VT1) gene expression in relation to the time of egg laying of white leghorn hen. The expression of AVT gene (Northern blot analysis and in situ hybridization) in the hypothalamus and localization of ir-AVT in the magnocellular neurons of paraventricular nuclei was studied 2 h before (−2 h), immediately after (0 h) and 2 h after (+2 h) egg laying. Simultaneous changes in the AVT and VT1 receptor gene in the shell gland, which finally responds to AVT for smooth muscle contraction and expulsion of egg, were also determined by semi-quantitative reverse transcriptase-polymerase chain reaction. The findings indicated increased hypothalamic AVT gene expression immediately after egg laying (0 h) when compared to 2 h before and 2 h after egg laying. AVT receptor gene expression in the shell gland also followed the same pattern. However, AVT gene expression in the shell gland, unlike that of hypothalamus was higher at −2 h compared to 0 and +2 h of oviposition. While highly significant increase was noted in plasma AVT concentration at the time of egg laying, other parameters such as plasma osmolality and ionic concentration (Na+, K+, Ca2+, and Cl−) did not show any change. It is suggested that in addition to increased hypothalamic AVT transcript and peripheral release, local synthesis of AVT in the shell gland (paracrine release) may contribute to the contraction of shell gland smooth muscles during egg laying. Moreover, these findings clearly indicate temporal correlation of AVT and its receptor gene expression in different tissues during oviposition.

Changes in expression of AVT and AVT receptor (VT1) gene in hypothalamus and shell gland in relation to egg laying in white leghorn hen

Oviposition is a complex phenomenon involving various regulatory mechanisms at the neuroendocrine levels. Present study was designed to access the changes in arginine vasotocin (AVT) and its receptor (VT1) gene expression in relation to the time of egg laying of white leghorn hen. The expression of AVT gene (Northern blot analysis and in situ hybridization) in the hypothalamus and localization of ir-AVT in the magnocellular neurons of paraventricular nuclei was studied 2 h before (−2 h), immediately after (0 h) and 2 h after (+2 h) egg laying. Simultaneous changes in the AVT and VT1 receptor gene in the shell gland, which finally responds to AVT for smooth muscle contraction and expulsion of egg, were also determined by semi-quantitative reverse transcriptase-polymerase chain reaction. The findings indicated increased hypothalamic AVT gene expression immediately after egg laying (0 h) when compared to 2 h before and 2 h after egg laying. AVT receptor gene expression in the shell gland also followed the same pattern. However, AVT gene expression in the shell gland, unlike that of hypothalamus was higher at −2 h compared to 0 and +2 h of oviposition. While highly significant increase was noted in plasma AVT concentration at the time of egg laying, other parameters such as plasma osmolality and ionic concentration (Na +, K +, Ca 2+, and Cl −) did not show any change. It is suggested that in addition to increased hypothalamic AVT transcript and peripheral release, local synthesis of AVT in the shell gland (paracrine release) may contribute to the contraction of shell gland smooth muscles during egg laying. Moreover, these findings clearly indicate temporal correlation of AVT and its receptor gene expression in different tissues during oviposition.

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.

Day–night variations in plasma melatonin and arginine vasotocin concentrations in chronically cannulated flounder ( Platichthys flesus)

Chronically catheterised, free swimming flounder ( Platichthys flesus) have been used in experiments examining the day–night variations in circulating levels of melatonin (Mel) and arginine vasotocin (AVT). Under normal photoperiod (16 h light/8 h dark) serial blood samples taken from individual fish demonstrated a Mel rhythm with daytime levels at 09.00 and 15.00 h (238±14 and 179±12 fmol·ml −1, respectively) lower than those at 23.00 h (1920±128 fmol·ml −1). Maintenance of fish in 24-h light abolished the light/dark Mel rhythm and circulating levels were comparable to those measured during the day in fish under normal photoperiod illumination. In fish maintained under 24 h dark, although a daily rhythm was still apparent, at the time when it would be normally dark, plasma Mel concentration was reduced and at times when it would be normally light, levels were higher than in fish maintained under normal light/dark illumination. Plasma AVT concentrations were higher in fish during the day (4.4±0.8 fmol·ml −1) than those at night (1.5±0.4 fmol·ml −1), the opposite to that seen with Mel. During acute study infusion of AVT resulted in reduced levels of plasma Mel, although this did not achieve statistical significance. Infusion of Mel did not alter circulating AVT concentration.

Modulation of plasma antidiuretic hormone levels does not change the magnitude of the LPS-induced febrile response in Pekin ducks.

The objective of this study was to determine the effect of modulating the plasma concentrations of the avian antidiuretic hormone, arginine vasotocin (AVT), upon the febrile response to lipopolysaccharide (LPS) in Pekin ducks. LPS, intravenously administered into conscious control birds at a dose of 1 microg x kg(-1), caused a monophasic increase in body temperature of 0.85 +/- 0.12 degrees C associated with a Thermal Response Index of 2.5 +/- 0.6 C degrees h. Plasma AVT concentrations in the control birds also increased with the progression of the fever response, more than doubling from their basal values. Ducks in which the circulating level of AVT had either been elevated by the intravenous infusion of the peptide or dehydration, or reduced by the administration of a specific AVT antibody prior to LPS administration, produced body temperature profiles and Thermal Response Index values that did not differ significantly from those of the control birds. The lack of any direct effect of variations in plasma AVT concentrations upon the magnitude of the fever response indicates that the LPS-induced elevation in plasma AVT is not associated with modulating the rise in body temperature obtained in avian fever.

Antidiuretic hormone and angiotensin II plasma concentrations in febrile Pekin ducks.

1. The objective of this study was to determine the changes in plasma concentrations of the hormones arginine vasotocin (AVT) and angiotensin II (AII) associated with lipopolysaccharide (LPS)-induced fever in Pekin ducks. 2. LPS, intravenously administered into conscious birds at doses of 1, 10 and 100 microgram kg-1, caused dose-dependent and monophasic increases in body temperature, with fever index values of 3.5, 7.0 and 10.6, respectively. 3. Plasma AVT concentrations also increased with the progression of the fever, with the largest elevation (from 8.4 +/- 1. 6 to 25.2 +/- 3.2 pg ml-1; means +/- s.e.m., n = 7) being caused by the highest dose of LPS. 4. Plasma AII concentrations did not significantly change from basal values (mean of 45.5 +/- 6.3 pg ml-1 for all groups) during the acute phase of the fever response. 5. The osmotic status of the birds, as indicated by plasma osmolality and electrolyte values, did not significantly change in any of the experimental animals. 6. The elevation of AVT in avian fever leads to speculation about a possible antipyretic action of this hormone, which would have particular relevance to understanding the evolution of fever.

Changes in Plasma Arginine Vasotocin (AVT) Concentration and Dorsal Aortic Blood Pressure Following AVT Injection in the TeleostPlatichthys flesus

Dorsal aortic blood pressure and plasma arginine vasotocin (AVT) concentrations have been assessed in free swimming, chronically cannulated flounder following AVT injection. Intraarterial AVT at doses of 4.76 × 10−12mol · kg−1and greater caused a biphasic change in blood pressure (an initial fall followed by a sustained pressor response). Doses above 4.76 × 10−12mol · kg−1were associated with plasma AVT concentrations (20 min after injection) 2–3 orders of magnitude greater than the physiological range and must be considered pharmacological. Injection of the lowest pressor AVT dose (4.76×10−12mol·kg−1), 20 min after injection, increased plasma AVT concentrations to 23.4 ± 6.1 fmol · ml−1. This increase is close to plasma AVT concentrations recently reported in untreated fish; however, in the initial period after injection plasma levels were calculated to be considerably higher than the physiological range. These results confirm that AVT is pressor in flounder but suggest that the pressor response may occur only at circulating AVT levels above the normal physiological range. The biphasic response to AVT and the differing responses to mammalian V1and V2type receptor agonists in the current work suggests that AVT may contribute to regional blood flow distribution in teleosts rather than blood pressure regulation.

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.

Effect of acute manipulation of blood volume and osmolality on plasma [AVT] in seawater flounder

Chronically cannulated seawater (SW)-adapted flounder (Platichthys flesus) were used unanesthetized and unrestrained in an experimental series that acutely manipulated blood volume and plasma osmolality to determine their influence on plasma arginine vasotocin (AVT) concentrations. Immunoreactive AVT was measured using a radioimmunoassay validated for flounder and other teleosts. After hemorrhage-induced hypovolemia or hypervolemia produced by saline infusion, no major changes in plasma AVT concentrations were detected. Raising plasma osmolality by intraperitoneal injection of 1 M NaCl compared with control 150 mM NaCl-injected fish (329.4 +/- 3.4 vs. 320.4 +/- 3.0 mosmol/kgH2O, P < 0.05) produced an increase in plasma AVT concentration (6.7 +/- 1.2 vs. 4.2 +/- 0.2 fmol/ml, P < 0.05). In a separate study, plasma composition in a large number of uncannulated SW-adapted flounder was determined. This demonstrated a positive linear relationship between the natural variation in plasma AVT concentrations and plasma osmolality and Na+ and Cl- concentrations observed between fish. These data indicate that AVT secretion in SW-adapted flounder is closely related and perhaps directly sensitive to changes in plasma tonicity.

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.

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.

Pituitary and plasma arginine vasotocin levels in the lamprey, Lampetra japonica

Pituitary and plasma arginine vasotocin (AVT) concentrations were examined in correlation with plasma mineral concentrations in the lamprey acclimated to freshwater (FW) and diluted seawater (SW). There were significant differences in plasma osmolality and plasma concentrations of sodium, chloride, calcium and magnesium between lampreys acclimated to FW and those acclimated to diluted SW. Although the plasma AVT concentration in the FW group was not significantly different from that in the diluted SW group, the pituitary AVT content in the FW group was significantly higher than that in the SW group. These results seem to show that synthesis of AVT is increased in FW lampreys.

Atrial Natriuretic Peptide Suppresses Osmotically Stimulated Plasma Arginine Vasotocin Levels in the Pekin Duck

Plasma arginine vasotocin (AVT) concentrations were measured at 15-min intervals in conscious Pekin ducks intravenously infused with 1 M saline at a rate of 0.5 ml/min for 1 hr. The responses were compared in eight birds, each given in random order a polyclonal antiserum against atrial natriuretic peptide (anti-ANP) or normal rabbit serum (control). The elevations in plasma osmolalities and sodium concentrations and the reductions in hematocrit values caused by the hypertonic saline were similar in both conditions. Plasma ANP in the control animals was elevated threefold during the saline infusion (from 66.8 ± 8.8 pg/ml to a peak of 193.3 pg/ml), whereas in the same birds treated with specific antiserum, circulating ANP concentrations fell to less than 10 pg/ml and remained at this level for the duration of the experiment. Plasma AVT concentrations rose steadily and significantly from basal levels of 5.8 ± 0.5 and 5.8 ± 0.8 pg/ml in the control and anti-ANP groups, respectively; however, the concentrations measured in the birds with low plasma ANP were consistently higher ( P < 0.02) than those under the control condition. Furthermore, the sensitivity of AVT release for the control birds (0.35 pg/ml per mOsm/kg) was significantly lower ( P < 0.001) than that in the anti-ANP animals (0.86 pg/ml per mOsm/kg). Elevated plasma ANP may suppress osmotically stimulated AVT release during states of vascular volume expansion.

Arginine vasotocin and fish osmoregulation

Pituitary arginine vasotocin (AVT) secretion is sensitive to the osmotic challenge associated with transfer of euryhaline teleosts between sea water (SW) and fresh water (FW). Pituitary AVT content in FW-adapted flounders greatly exceeds that in SW-adapted fish. Plasma AVT concentrations are in the range 10(-12)-10(-11) M (1-100 pg/ml). In euryhaline species, like the eel, flounder and trout, there were no consistent, marked differences in plasma AVT concentrations between FW- and SW-adapted fish. In SW- but not FW-adapted flounders plasma AVT and sodium concentrations are correlated. During the initial period of acclimation from FW to SW eels, show a transitory rise in plasma AVT concentration which is associated with a transitory increase in plasma Angiotensin II. In view of the range of plasma AVT concentration observed in SW- and FW-adapted fish, it is evident that of the described dose-dependent effects of AVT on urine production, only the antidiuretic responses are likely to be of physiological significance. In addition to the presence of a V1-type vascular receptor for AVT, the nephron also possesses a V2-type receptor, coupled to adenylate cyclase. In the gill tissue AVT receptors are also present, but in this tissue receptor occupancy leads to inhibition of cAMP production rather than the stimulation observed in renal tissue. The functional significance of the gill AVT receptor remains to be established.

Arginine vasotocin gene expression and hormone synthesis during ontogeny of the chicken embryo and the newborn chick.

Chicken embryos at different developmental stages (embryonal day (E) 6 to 21) and chicks at posthatch day 1 (D1) were monitored for the development of their hypothalamo-neurohypophysial system as indicated by the kinetics of arginine vasotocin (AVT) gene expression via mRNA concentration and brain AVT content. Our data concerning the onset of gene expression support previous results from our laboratory and others about an early activation of the AVT gene transcriptional and translational activity around E6. We could detect measurable amounts of AVT in chicken embryo brains at E6 and an exponential increase during further development until D1. Dot blots of hypothalamic RNA extracts indicated that AVT gene transcript concentrations rose between E12 and E17 and slightly dropped thereafter. Northern hybridization showed that this drop was caused by a decrease of full length message and an increase of smaller transcripts during late embryonal and D1 stages, probably an AVT mRNA specific degradation phenomenon. The dissociation between the increase of AVT concentration and AVT mRNA concentration visible at the D1 stage might be due to accumulation and storage of AVT in the magnocellular neurons, preferentially in their axon terminals in the neurohypophysis. Blood samples taken from E14 onwards revealed a constant increase in plasma osmolality and plasma AVT concentration. Our data suggest that, in the chicken, AVT seems to be required early during embryonal development, either for osmoregulatory or further unknown functions.