Christa Simon-Oppermann

Radioimmunoassay for arginine-vasotocin (AVT) in serum of Pekin ducks: AVT concentrations after adaptation to fresh water and salt water

A radioimmunoassay for arginine-vasotocin (AVT), the antidiuretic principle in birds, was developed using the high cross-reactivity of AVT with an AVP antiserum raised in rabbits. This assay is specific for the measurement of AVT in serum of birds. The sensitivity and precision is such that serum AVT concentrations above 0.5 fmol/ml can be measured quantitatively.A serum AVT concentration of 5.1±1.4 fmol/ml was found in normally hydrated, fresh water adapted ducks with a serum osmolality of 293.7±2.2 mosmol/kg. When the same animals were acutely hydrated, no or<0.5 fmol/ml AVT was present at an osmolality of 289.9±2.4 mosmol/kg. In salt water adapted ducks with a serum osmolality of 333.0±7.6 mosmol/kg, a serum AVT of 22.7±3.0 fmol/ml was found. When fresh water adapted ducks were hydrated and infused with different doses of AVT, a linear relationship between serum AVT and the amount infused was obtained. During AVT infusions, serum osmolality and urine flow rate were negatively correlated and urine osmolality was positively correlated with serum AVT.

Serum arginine-vasotocin (AVT) and afferent and central control of osmoregulation in conscious Pekin ducks

In conscious Pekin ducks adapted either to fresh water or to hypertonic saline (1.9%) as drinking fluid, urinary excretion, salt gland secretion and the serum concentration of radioimmunoassayable arginine-vasotocin (AVT) were examined with regard to their afferent and central control. The experiments were carried out under conditions of water diuresis, osmotic diuresis or supraorbital salt gland secretion, which were induced by continuous infusions of appropriate solutions.Temporary bilateral vagus blockade caused rises in AVT serum concentration accompanied by antidiuresis in hydrated ducks and by inhibition of salt gland secretion in salt-stressed ducks.Rostral brainstem cooling caused decreases of AVT serum concentration and water diuresis in ducks under osmotic diuresis and reduction of AVT serum concentration and inhibition of salt gland secretion in saltstressed ducks.Cerebral osmotic stimulation in hydrated ducks by intracarotid injection or by intracerebroventricular microinfusion of hypertonic NaCl solutions caused antidiuretic reactions associated with rises of AVT serum concentration.

Osmolality and volume factors in salt gland control of pekin ducks after adaptation to chronic salt loading

SummaryPekin ducks were adapted to permanent osmotic stress by rearing them on a NaCl solution of increasing concentration up to 2% as drinking water. Their salt and water balance was compared with that of non-adapted ducks maintained on tap water. Amounts and osmolalities of salt gland secretion and cloacal discharges, plasma osmolality and electrolytes were measured during stepwise osmotic loading by intravenous infusion of NaCl solution of about 740 mosm·kg−1, at rates of 0.25, 0.45 and 0.65 ml·min−1. Before loading, the plasma osmolality of the adapted ducks was about 22 mosm·kg−1 higher than in non-adapted animals. The initial step of loading induced salt gland secretion in the adapted ducks after an average rise of plasma osmolality of 3.6 mosm·kg−1 and in the non-adapted animals after a rise of 7.8 mosm·kg−1. The method of osmotic loading enabled both groups of animals to balance their water input and output. However, only the adapted ducks were able to balance NaCl input and output, predominantly by salt gland secretion, thus maintaining a stable plasma osmolality. The nonadapted ducks retained 42% of the salt load which resulted in a rise of plasma osmolality of 49 mosm·kg−1, more salt being excreted by the kidneys than by the salt glands.In the salt-adapted ducks, salt gland activity, plasma osmolality and Na+ concentration did not correlate during balanced states of salt input and output. The involvement of tonicity receptors in salt gland control was confirmed by the stimulating effects of various hypertonic solutions. On the other hand, continuous loading by a constant infusion of NaCl solution of 1,300 mosm·kg−1 induced a steady salt gland secretion at a rising plasma osmolality and thus suggested that a volume factor is involved in salt gland control. Inhibition of salt gland activity by withdrawing blood and activation by blood infusion confirmed this assumption. While a direct cause and effect relationship between volume changes and salt gland secretion cannot be demonstrated, the results indicate that volume changes in one or more extracellular compartments do affect salt gland secretion.

Blood volume changes and arginine vasotocin (AVT) blood concentration in conscious fresh water and salt water adapted ducks.

Blood volume changes consisting in the removal and reinfusion respectively, of 10% of the estimated blood volume (23.2 ml on average) were induced to determine their effects on the blood concentration of arginine vasotocin (AVT), the antidiuretic hormone (ADH) of birds, in fresh water adapted ducks (water ducks) with blood osmolalities and ADH concentrations similar to those of normally hydrated mammals, and in salt water adapted ducks (salt ducks) with chronically elevated blood osmolalities and ADH concentrations. The investigations were carried out in steady state conditions, when infusion of 1 ml·min−1 of isotonic saline was matched by the excretion in water ducks and when infusion of 0.4 ml·min−1 of 1,000 mosmolal saline was matched by the salt gland excretion in the salt ducks.After blood removal, AVT blood concentration (mean ±SE) increased from 6.5±0.4 to 8.4±0.6 pg·ml−1 in water ducks and from 18.1±1.6 to 22.6±1.9 pg·ml−1 in salt ducks. The respective blood osmolalities of 297.4±1.4 and 318.6±3.3 mOsm·kg−1 did not change. Reinfusion of the blood after steady-state conditions had been reattained decreased blood AVT from 7.9±0.7 to 6.7±0.5 pg·ml−1 in water ducks. In the salt ducks AVT concentration had already returned to the control level before blood reinfusion which induced no further reduction. The blood osmolalities remained unchanged in both groups.During blood removal and reinfusion, the excretion rate of the kidneys in water ducks and the salt glands in salt ducks were temporarily reduced and enhanced respectively, the effect being not symmetrical for salt gland secretion.For water ducks, the volume sensitivity of AVT release was found comparable to that of mammals, when related to the induced blood volume changes, the responses to blood removal and reinfusion being approximately equal in absolute terms. In the salt ducks, the volume sensitivity of AVT release was clearly expressed during blood removal but insignificant during blood reinfusion.

Osmotic and volume control of diuresis in conscious ducks (Anas platyrhynchos)

SummaryIn conscious Pekin ducks made diuretic either by infusing hyposmotic glucose solution or isosmotic saline, osmotic and volume effects on renal water excretion were investigated. As in mammals, antidiuresis mediated by enhanced release of antidiuretic hormone was induced by increasing carotid blood osmolality while a decrease augmented diuresis, indicating cerebral osmotic control of renal water excretion in birds.In contrast to the situation in mammals, a sensitive diuretic response to isosmotic volume expansion, corresponding to 1% of the extracellular volume, can be demonstrated, with intracarotid and intravenous application of the isosmotic saline infusion having identical effects.Volume loading with isosmotic saline produced a greater diuretic response than loading with the same amount of autologous blood, thereby indicating a major contribution of volume changes in the interstitial compartment to the control of renal water excretion. This corresponds to the importance of the interstitial fluid compartment for the control of salt gland activity in this species.

Central and systemic antidiuretic hormone and angiotensin II in salt and fluid balance of birds as compared to mammals

1. Tonicity dominates the release of ADH with similar sensitivities (0.2-1 pg/ml per mOsm/kg) for both birds and mammals. 2. There is an inverse relationship between the volume of the extracellular fluid compartments and the plasma level of ADH. 3. Angiotensin II formation is governed by volume factors. 4. In birds the factors reducing the delivery of Na+ to the nephron distal tubules stimulate ANGII formation. 5. Mammals have a high vascular constrictor sensitivity to ADH and ANGII; there is little or no vascular sensitivity to these in birds. 6. In birds and mammals the subfornical organ and other circumventricular organs have receptors that specifically bind ANGII. 7. Dog and duck CSF levels of ADH and AII indicate their function as specific mediators of intrinsic neuronal systems controlling salt and fluid balance.

Afferent connections of physiologically identified neuronal complexes in the paraventricular nucleus of conscious Pekin ducks involved in regulation of salt- and water-balance

SummaryThe afferent connections of the paraventricular nucleus (PVN) of the domestic mallard (Pekin duck), Anas platyrhynchos, were demonstrated by means of microiontophoretic injection of horseradish peroxidase (HRP). To place the HRP injection exactly into the PVN, its location was identified prior to the injection by observing antidiuretic reactions to electrostimulations within the rostral hypothalamus of conscious, hydrated animals. Antidiuresis was induced only when electrostimulation was applied to a distinct hypothalamic area. Two different patterns of antidiuresis were observed: (i) an immediate reduction in rate of production of urine, and (ii) antidiuresis preceded by a period of increase in production of urine. Repeated stimulation of the same site with the same parameters resulted in decreasing antidiuretic effects. At the site where stimulation had elicited the most pronounced antidiuresis of either response type, HRP was injected microiontophoretically.Histological examination after 3–8 days of survival revealed delicate injection sites located exclusively in the periventricular portion of the PVN. Adjacent to the dorsal portion of the PVN retrogradely labeled tanycytes and intraependymal neurons were scattered in the ventricular wall. As demonstrated in neurohistological and electron-microscopic investigations, this ependymal region exhibits a particular arrangement of tanycytes and small neurons (10–15 μm in diameter), some of which belong to the neurosecretory type.Additional HRP-labeled neuronal perikarya afferent to the PVN were demonstrated in the contralateral PVN, and on the ipsilateral side in the lateral septum, lateral hypothalamic area and locus coeruleus. Within the nuclei of the solitary tract, stained nerve cells were found ipsilateral as well as contralateral to the injection site.Several of the neurons demonstrated may be considered as candidates for the transmission of signals originating from various receptive structures relevant for the control of avian salt- and water-balance. The physiological results conform to the concept that neurons of the PVN influence urine formation by controlling the release of arginine-vasotocin (AVT). Evidence that suggests additional modes of control exerted by these neurons in salt- and water-balance is presented.

Hypothalamic temperature and osmoregulation in the Pekin duck

The temperature of the anterior and middle hypothalamus of conscious Pekin ducks was altered with chronically implanted thermodes. Both urine formation and salt secretion by the supraorbital glands were influenced by hypothalamic cooling. When osmotic diuresis was induced by continuous intravenous infusion of 1.2 ml·min−1 of 293 mosm·kg−1 mannitol in H2O solution, hypothalamic cooling increased urine flow rate at reduced urine osmolality and unchanged osmolal excretion rate. The degree of this cold induced diuresis increased with cooling intensity. Additional ADH administration by continuous infusion at a supramaximal dose abolished the diuretic effect of hypothalamic cooling. When water diuresis was induced by intragastric continuous infusion of 1.2 ml·min−1 of distilled water, hypothalamic cooling enhanced the diuresis, but hypothalamic warming had equivocal effects. The diuretic effects of hypothalamic cooling suggest an inhibition of endogeneous ADH release by lowering hypothalamic temperature. When the salt glands of salt adapted ducks were stimulated by continuous intravenous infusion of 0.2 ml·min−1 of 800 mosm·kg−1 NaCl in H2O solution, hypothalamic cooling reduced the salt gland secretion rate to an extent depending on cooling intensity. It is concluded that the activities of those integrative and/or efferent hypothalamic neurons, which mediate the hormonal control of renal water absorption and the nervous control of salt secretion by the supraorbital gland, depend on their own temperature.

Blood pressure and arginine vasotocin in normonatremic and hypernatremic ducks

SummaryAs a model to study effects of chronic, excessive salt loading on circulation, Pekin ducks were adapted to 2% saline solution as their sole water supply, while fresh-water-adapted animals were used as controls. Due to the development of salt-climinating glands, salt-adapted ducks are able to cope indefinitely with this salt stress which means a daily ingestion of 5–6 g NaCl per kg body weight per day, associated with a chronic elevation of plasma osmolality and plasma sodium by 5–8% above normal and an up to 3-fold increase of antidiuretic hormone concentration in comparison to animals maintained on fresh water. Salt loading for up to 14 months did neither increase arterial mean, nor diastolic, nor pulse pressure. On the contrary, arterial mean and diastolic pressure were slightly lower in the salt-adapted than in the fresh-water-adapted animals, while pulse pressure and heart rate did not differ. Circulatory adaptation to removal and reinfusion of 10% of the estimated blood volume was identical in salt-water and fresh-water-adapted ducks. It is concluded that even excessive chronic salt loading resulting in chronic hyperosmolality with high plasma levels of sodium and antidiuretic hormone does not alter hemodynamic adaptation, provided that efficient compensating mechanisms are at the animals disposal.

Neuropeptides and temperature regulation

Abstract 1. 1.|A method is described, which makes it possible to analyse, both qualitatively and quantitatively, the effect of neuropeptides on individual thermoregulatory outputs. Use of the method also facilitates speculation on neuronal sites in the anterior hypothalamus, which might be specifically affected. 2. 2.|Data show that neurotensin shifts the threshold T core to higher temperatures whilst bombesin shifts the threshold T core downwards and reduces the efficiency of individual thermoregulatory outputs. 3. 3.|Data suggest that peptidergic receptor sites in the brain are specifically involved in thermoregulation and confirm that the effect of neuropeptides on resting nerve terminals consists in inhibiting spontaneous transmitter release.