Effect of anaesthetics and haemorrhage on the release of neurohypophysial antidiuretic hormone

Histochemical and ultrastructural study of renal cortical necrosis in rats treated with oestrone + vasopressin, and its prevention with a vasopressin antagonist

Renal cortical necrosis was induced by the administration of vasopressin to oestrogen-pretreated rats. Histochemical (succinic dehydrogenase, trichrome, perjod acid Schiff) and electronmicroscopic methods were applied to examine how the vasopressin antagonist d(CH2)5Tyr(Met)AVP influences the development of this renal cortical necrosis. The experiments revealed that vasopressin did not induce hypoxia or necrosis in the renal tubules if the antagonist was administered simultaneously, even after oestrogen pretreatment. The conclusion is drawn that this pressor antagonist may be of value for the prevention of renal cortical necrosis in rats or in human beings.

Suitability of urethane anesthesia for physiopharmacological investigations. Part 3: Other systems and conclusions

The suitability of urethane anesthesia for physiopharmacological experiments in various systems is briefly reviewed. Urethane anesthesia appears to be suitable for various types of studies on respiratory function and on reflex activation of motility of the urinary bladder and some sections of the intestinal tract. However, urethane produces a variety of potentially disturbing side-effects at endocrine and renal level.

Bromocriptine and apomorphine stimulation of cortisol secretion in conscious dogs; evidence for a stimulatory site located outside the blood brain barrier

IV injections of the dopamine receptor agonists bromocriptine (0.1 mg/kg) and apomorphine (0.05 mg/kg) induced rapid and long-lasting increases of cortisol levels as measured by RIA in peripheral venous plasma of conscious dogs. Pretreatment with dopamine receptor antagonists which do not readily penetrate the blood brain barrier (domperidone, halopemide, sulpiride) abolished the release responses induced by the dopamine agonists. These results suggest that the dopamine receptor agonists stimulate cortisol release at a site located outside the blood brain barrier. In addition, some dopamine receptor antagonists (haloperidol, chlorpromazine, milenperone) were shown to cause a rapid and long-lasting increase of cortisol levels.

Vasopressin: its role in antipyresis and febrile convulsion

When pyrogenic substances are injected intravenously into experimental animals, a sequence of events is set in motion which involves the hypothalamus and perhaps other portions of the diencephalon to produce a febrile response. We now present evidence that the brain produces its own endogenous antipyretic which may serve as a means of controlling the extent of the fever. When arginine vasopressin is perfused through the lateral septal area of the hypothalamus of the sheep, fever is suppressed. Vasopressin alone does not lower normal body temperature when perfused through this region of the brain. In addition, evidence is provided to indicate that vasopressin is released within the lateral septal area during the febrile response. It is concluded that, in fever, arginine vasopressin may be released in the lateral septal area of the brain and serve as an endogenous antipyretic. Results indicate that, following an initial application of vasopressin into the brain itself, a subsequent similar administration of vasopressin produces seizure-like activity. Therefore, it is suggested that this release of arginine vasopressin may contribute to the production of febrile convulsion.

Renal blood flow and its distribution following haemorrhage in the rat: the role of vasopressin

The effect of an acute arterial haemorrhage of 15 ml/kg body weight on renal and intrarenal blood flow (measured using radioactive microspheres) was determined in anaesthetized rats. In order to assess the role of vasopressin in mediating the observed changes, the response to haemorrhage of animals lacking the ability to synthesize vasopressin (Brattleboro rats) was compared with that of the parent strain (Long Evans rats). In addition, a group of Long Evans rats was treated with an antagonist of the vascular action of vasopressin before being bled. Thirty minutes after haemorrhage the mean arterial blood pressure of untreated Long Evans rats was significantly higher than that of Brattleboro or vasopressin antagonist-treated Long Evans rats. Following haemorrhage, total renal vascular resistance increased markedly in untreated Long Evans rats. In these animals there were substantial increases in calculated vascular resistance in both inner and outer halves of the renal cortex. In Brattleboro rats there was only a small increase in renal vascular resistance, confined to the outer cortex, whilst in vasopressin antagonist-treated Long Evans rats there was no significant change in the vascular resistance of either cortical region. It is concluded that much of the increase in renal vascular resistance which follows haemorrhage is due to vasopressin. This vasoconstrictor effect of the hormone, which contributes to its pressor action after haemorrhage, occurs in both the inner and outer regions of the renal cortex.

Plasma vasopressin concentration in the anaesthetized dog before, during and after atrial distension

Plasma vasopressin (AVP) concentration in dogs anaesthetized with chloralose was measured by radioimmunoassay and was within the range of 2-5 pg/ml. during control periods. Distension of the left atrium led within 2 min to a fall in plasma AVP concentration which reached a steady lower value within 4 min. After cessation of atrial distension the AVP concentration returned to pre-distention values within 4 min. Cooling the cervical vagosympathetic nerves to 8-10 degrees C led to a rise in plasma AVP concentration. Atrial distension during cooling of the vagi resulted in a further increase of plasma AVP concentration.

Evidence that endogenous vasopressin plays a protective role in circulatory shock. Role for reticuloendothelial system using Brattleboro rats

Experiments performed on male Wistar, Long Evans and Brattleboro rats indicate that the latter strain of animals, lacking vasopressin in their posterior pituitaries, are extremely sensitive to hemorrhagic and bowel ischemic shock. Mild forms of both hemorrhagic and bowel ischemic shock, as produced in Wistar or Long Evans rats, results in marked hypotension, hemoconcentration and blockade of the reticuloendothelial system (RES) in Brattleboro animals of similar sex, age and weight. These direct findings indicate that release of endogenous vasopressin in shock syndromes may be critical in maintenance of circulatory homeostasis and RES function.

The role of vasopressin in blood pressure regulation immediately following acute haemorrhage in the rat

1. The possible pressor effect of vasopressin immediately after acute haemorrhage has been studied using anaesthetized Brattleboro rats with diabetes insipidus and rats of the Long Evans parent strain.2. A blood loss of 0.5% of the body weight caused a significant decrease in mean arterial blood pressure, measured 10 min later, in Brattleboro rats, whereas this degree of haemorrhage was non-hypotensive in the control Long Evans rats. Following subsequent blood losses (each of 0.5% of the body weight), mean arterial blood pressure in Brattleboro rats was always significantly lower than in Long Evans rats.3. While no antidiuretic activity was at any time found in the plasma of Brattleboro rats, haemorrhages greater than 1% of the body weight were associated with marked increases in plasma arginine vasopressin (AVP) of Long Evans rats.4. When Brattleboro and Long Evans rats were subjected to a single haemorrhage of 2% of the body weight, the immediate decrease in arterial blood pressure was similar in the two groups. However, 5 and 10 min after the haemorrhage the arterial blood pressure was significantly higher in the Long Evans rats. When vasopressin was infused into Brattleboro rats so that plasma levels of the hormone approached those found in Long Evans rats, the mean arterial blood pressure 0, 5 and 10 min after haemorrhage was similar to that in the Long Evans animals.5. It is concluded that in the anaesthetized rat, vasopressin plays an important role in the regulation of arterial blood pressure during the period immediately following acute haemorrhage.

Effects of hormone-releasing stimuli on the area of the perivascular space in the neural lobe of the rat

Neural lobes from rats subjected to neurohypophysial hormone-releasing stimuli were examined electron microscopically following fixation in 4% tannic acid in 2.5% glutaraldehyde. This fixation allowed the delineation of the perivascular space in the neural lobe tissue. Measurement of the area of the perivascular space showed that it was significantly increased in the rats subjected to vagal stimulation and intraarterial calcium ions compared to the control rats. The rats which had been subjected to haemorrhage as a hormone-releasing stimulus did not show any significant change in the area of the perivascular space. The significance of these findings in relation to hormone release is discussed.

Lability of renal papillary tissue composition in the rat

1. The acute effects of (a) a minor operative procedure using ether as the anaesthetic, and (b) the administration of 0.9% saline as a single I.V. injection in the conscious rat, on renal tissue composition were studied in hydropenic and normally hydrated rats. 2. The operative procedure and anaesthesia induced a rapid and transient decrease in papillary osmolality in both hydropenic and normally hydrated animals, the important contributing factor being a significant decrease in urea content. 3. Administration of a small volume of saline caused a rapid decrease in urea content, and an increase in water content. 4. It is concluded that papillary composition is extremely labile, large changes being produced by relatively minor experimental procedures.

The effects of hypotension and hypovolaemia on the liberation of vasopressin during haemorrhage in the unanaesthetized monkey (Macaca mulatta)

Using unanaesthetized monkeys, experiments were performed to examine the effects of haemorrhage on the liberation of arginine vasopressin (AVP). Haemorrhages of 10%, 15% or 20% total blood volume were performed via a catheter with its tip in the abdominal vena cava. A catheter in the left internal jugular vein was used for blood sampling. Arterial blood pressure was monitored via a catheter whose tip resected in an iliac artery. The monkeys showed no signs of discomfort from this catheterisation. Blood samples for AVP assay were taken at different times from 0-90 min after the end of the haemorrhage. At the end of the experiment, blood removed was reinfused. Results show that haemorrhage resulted in liberation of AVP, but only if there was a fall in arterial blood pressure. AVP release occurred more readily as the total volume of blood withdrawn increased, but the absolute rise in hormone concentration was not related to the total volume of blood withdrawn. However, comparing the area under the curve of mean arterial blood pressure with that for AVP concentration showed the two to have a significant exponential relationship. It is concluded that, as in other species, haemorrhage is a potent stimulus for AVP liberation in the monkey. However, in contrast to some other species, the fall in arterial pressure seems to be the prime stimulus rather than hypovolemia per se.

A sensitive method for the assay of oxytocin in the circulating blood

1 The sensitivity of a wide range of isolated tissues to oxytocin was investigated. The longitudinal muscle strip of duck pulmonary vein proved the most suitable tissue for use with the superfusion technique, contracting to concentrations to oxytocin as low as 10 muu/ml.2 The duck pulmonary vein superfused with Krebs solution was contracted by oxytocin, the vasopressins, adrenaline, noradrenaline, 5-hydroxytryptamine, histamine and angiotensin II. Pre-treatment of the preparation with phenoxybenzamine (1-2 mug/ml) abolished the contractions to catecholamines and reduced the effects of histamine and 5-hydroxytryptamine without affecting the sensitivity to oxytocin.3 The pulmonary vein contracted when superfused with blood from an anaesthetized dog. This contraction was accompanied by a non-specific loss of responsiveness. When the pulmonary vein was superfused with Krebs solution that had been dialysed against blood the initial contraction was greatly reduced or abolished as was the loss in responsiveness.4 Oxytocin was stable in circulating dog's blood but approximately 50% was bound to plasma proteins. Oxytocin was not destroyed in the pulmonary circulation or the hind limbs. In the doses used oxytocin had a half-life of 60-90 s in the circulation of the dog under steady-state conditions. Disappearance occurred chiefly in the visceral vascular beds.

Electrophysiological evidence for the activation of supraoptic neurones during the release of oxytocin

1. Antidromically identified supraoptic (SO) units were recorded in lactating rats anaesthetized with urethane (1.1 g/kg), and their activity was studied during milk ejection evoked by the suckling of the young. Fifty-eight SO units were recorded through 174 milk ejections. Each milk ejection was the result of a neurohypophysial release of an oxytocin pulse of 0.5-1.5 m-u.2. Fifty-five of the SO units displayed background activity and three were silent. The firing rates ranged from 0 to 15.4 spikes/sec, the distribution was exponential with 26% of the units firing at < 1 spike/sec. Sixteen (28%) of the SO units displayed a phasic pattern of activity characterized by periods of activity (6-132 sec) and silence (4-71 sec).3. Twenty-five of the non-phasic units displayed a large and stereotyped acceleration in spike activity some 10.5-17.4 sec before the rise in intramammary pressure at milk ejection. Units accelerated to rates between 9-66 spikes/sec, an increase of about thirtyfold (median) on background activity. The response was brief (0.9-4.7 sec) and was followed by a period of after-inhibition.4. It was concluded from studies of double recordings and observations of multi-unit activity that all the responsive units were synchronously activated. The mean latency of 13.3 sec between the onset of the neurosecretory response and milk ejection was similar to that observed following the experimental release of endogenous oxytocin by electrical stimulation of the neurohypophysis (50 pulses/sec for 2-4 sec).5. Four of the phasically active units were correlated with the oxytocin release for milk ejection. Three of these units displayed a burst of activity superimposed on the terminal portion of an active phase, some 10.2-14.7 sec before milk ejection. The fourth unit, recorded in conjunction with a responsive non-phasic unit, consistently switched from silence to activity coincident with the onset of the SO activation.6. The remaining SO units and a further ten units that were not antidromically activated by neurohypophysial stimulation displayed no change in activity during either the period of neurosecretory activation or the period of after-inhibition.7. This activation of the SO neurones, in the formulation of oxytocin release and milk ejection, is the same as that we have previously observed in recordings from the paraventricular (PV) region, and the proportion of neurones displaying the response is similar: 48% in the SO nuclei, 58% in the PV nuclei. We conclude, since the SO nuclei contain 80% of the neurosecretory cells that project to the neurohypophysis, that the SO nuclei are as important, if not more so, than the PV nuclei in the control of oxytocin release.

The influence of anaesthetics on the increase in the water permeability of the toad bladder induced by vasopressin

1 Single lobes of the bladder of Bufo marinus were isolated and filled with, and suspended in, oxygenated Ringer solution. The fluid in contact with the outside (serosa) of the lobes had a total osmolarity of 225 m-osmol/litre, and that bathing the inside (mucosa) of 45 m-osmol/litre.2 Osmotic water flow from mucosa to serosa was measured by weighing the lobes every 30 minutes. It was negligible unless vasopressin was added to the serosal bath. Standard concentrations of either 1.25 or 6.25 mu/ml were used to render the bladder lobes permeable to water.3 The presence in the serosal medium of pentobarbitone or thiopentone in concentrations ranging from 0.25 to 2.5 mM, or of chloralose in concentrations ranging from 0.65 to 6.5 mM, diminished the increase in water permeability induced by vasopressin.4 The three anaesthetics exerted similar inhibitory effects on the action of vasopressin from the serosal and from the mucosal surface of the bladder.5 In the presence of a constant high concentration of anaesthetic, increasing the concentration of vasopressin over three orders of magnitude led to stepwise increases of osmotic water flow out of the lobes, although at every dose level the effect of vasopressin was depressed by the anaesthetic. However, it was not completely abolished even if the concentration of vasopressin was close to threshold.6 The increase in water permeability of the bladder induced by 3',5'-adenosine monophosphate (cyclic AMP) was also depressed by the three anaesthetics.7 Possible explanations of the findings are discussed.

The role of blood osmolality and volume in regulating vasopressin secretion in the rat

A sensitive and specific radioimmunoassay for plasma arginine vasopressin (AVP) has been used to study the effects of blood osmolality and volume in regulating AVP secretion in unanesthetized rats. Under basal conditions, plasma AVP and osmolality were relatively constant, averaging 2.3+/-0.9 (SD) pg/ml and 294+/-1.4 mosmol/kg, respectively. Fluid restriction, which increased osmolality and decreased volume, resulted in a progressive rise in plasma AVP to about 10 times basal levels after 96 h. A 2-3-fold increase in plasma AVP occurred as early as 12 h, when osmolality and volume had each changed by less than 2%. Intraperitoneal injections of hypertonic saline, which had no effect on blood volume, also produced a rise in plasma AVP that was linearly correlated with the rise in osmolality (r > 0.9) and quantitatively similar to that found during fluid restriction (plasma AVP increased 2-4-fold with each 1% increase in osmolality). Intraperitoneal injection of polyethylene glycol, which decreased blood volume without altering osmolality, also increased plasma AVP but this response followed an exponential pattern and did not become significant until volume had decreased by 8% or more. At these levels of hypovolemia, the osmoregulatory system continued to function but showed a lower threshold and increase sensitivity to osmotic stimulation. We conclude that AVP secretion is regulated principally by blood osmolality but that the responsiveness of this mechanism may be significantly altered by modest changes in blood volume.

Vasopressin clearance and secretion during haemorrhage in normal dogs and in dogs with experimental diabetes insipidus

1. The secretion of vasopressin in response to haemorrhagic shock has been investigated in anaesthetized dogs.2. The changes in the plasma concentrations of vasopressin were followed over a period of 5 hr, during which the arterial blood pressure was kept constant at 40 mm Hg. It was found that vasopressin concentration in plasma rose to a high peak shortly after the onset of shock and gradually declined thereafter. Five hours later, it was still 3.5 times higher than control. Re-transfusion of blood was followed by a return to control levels.3. The clearance of vasopressin was calculated before and during shock in normal dogs and in dogs with experimental diabetes insipidus. Soon after the onset of shock, the clearance rate dropped to one quarter of its normal level but slowly recovered, returning to near control values at the fifth hour of shock. Clearance rates did not vary as a function of infusion rates, suggesting that there is no maximal transport rate for the removal of the hormone over the entire secretory range found in normal and hypotensive dogs.4. From the clearance rates and from the plasma concentrations of endogenously secreted vasopressin it has been possible to calculate the approximate secretory rates of the hormone in response to shock. Secretion rose to a very high level, some 40 times greater than control, at the onset of shock. This was followed by a fairly constant secretory plateau. At the fifth hour of shock secretion was 3.5 times higher than control.5. The half-life of vasopressin was measured in normal and hypotensive dogs. Control measurements confirm the generally accepted value of approximately 5 min. The half-life was significantly higher in the early stage of shock, but returned to control values in the later stage.6. Haemorrhage experiments performed in normal and diabetic dogs suggest that vasopressin may play a part in the development of irreversible haemorrhagic shock: all normal animals died within a few hours of retransfusion, whereas four out of eight diabetic dogs similarly treated survived a 24 hr observation period. In a separate set of experiments, eight diabetic dogs were subjected to the haemorrhage procedure while receiving a constant infusion of vasopressin: only two of these survived. Surviving dogs showed none of the characteristic lesions of irreversible haemorrhagic shock.

Release of neurohypophysial hormones in vitro

1. The rate of release of neurohypophysial hormones in vitro, using isolated, halved neural lobes of the rat in an incubation medium containing excess K(+) and Ca(2+), was measured. The highest average rate of release was observed between 10 and 20 min after commencement of incubation.2. Incubation of isolated, halved rat neural lobes in the presence of acetylcholine, with or without eserine, did not stimulate hormone release. When complete isolated hypothalamo-neurohypophysial systems were incubated in a suspension medium containing 10(-7) mg/ml. acetylcholine a significant increase in the release of oxytocin occurred (P < 0.01); the increase in vasopressin release was less pronounced (P < 0.05).3. Uptake of O(2) by the isolated, halved neural lobes and the hypothalamo-neurohypophysial systems continued for 2-3 hr, i.e. in excess of the experimental incubation time.4. During the first 40 min of incubation the control halved neural lobes increased in weight; the neural lobes incubated in buffer containing high potassium and calcium showed no increase in weight.5. Neural lobes incubated in buffer containing excess K(+) and Ca(2+) contained about 3 times as much potassium as controls. The sodium content was not affected significantly.6. Factors involved in the process of neurohypophysial hormone release are discussed.