The cardiovascular effects of vasopressin after haemorrhage in anaesthetized rats

NMDA and non-NMDA glutamate receptors in the paraventricular nucleus of the hypothalamus modulate different stages of hemorrhage-evoked cardiovascular responses in rats

Here we report the involvement of N-Methyl-d-Aspartate (NMDA) and non-NMDA glutamate receptors from the paraventricular nucleus of the hypothalamus (PVN) in the mediation of cardiovascular changes observed during hemorrhage and post-bleeding periods. In addition, the present study provides further evidence of the involvement of circulating vasopressin and cardiac sympathetic activity in cardiovascular responses to hemorrhage. Systemic treatment with the V1-vasopressin receptor antagonist dTyr(CH2)5(Me)AVP (50 μg/kg, i.v.) increased the latency to the onset of hypotension during hemorrhage and slowed post-bleeding recovery of blood pressure. Systemic treatment with the β1-adrenergic receptor antagonist atenolol (1 mg/kg, i.v.) also increased the latency to the onset of hypotension during hemorrhage. Moreover, atenolol reversed the hemorrhage-induced tachycardia into bradycardia. Bilateral microinjection of the selective NMDA glutamate receptor antagonist LY235959 (2 nmol/100 nL) into the PVN blocked the hypotensive response to hemorrhage and reduced the tachycardia during the post-hemorrhage period. Systemic treatment with dTyr(CH2)5(Me)AVP inhibited the effect of LY235959 on hemorrhage-induced hypotension, without affecting the post-bleeding tachycardia. PVN treatment with the selective non-NMDA receptor antagonist NBQX (2 nmol/100 nL) reduced the recovery of blood pressure to normal levels in the post-bleeding phase and reduced hemorrhage-induced tachycardia. Combined blockade of both NMDA and non-NMDA glutamate receptors in the PVN completely abolished the hypotensive response in the hemorrhage period and reduced the tachycardiac response in the post-hemorrhage period. These results indicate that local PVN glutamate neurotransmission is involved in the neural pathway mediating cardiovascular responses to hemorrhage, via an integrated control involving autonomic nervous system activity and vasopressin release into the circulation.

V1a vasopressin receptors maintain normal blood pressure by regulating circulating blood volume and baroreflex sensitivity

Arginine-vasopressin (AVP) is a hormone that is essential for both osmotic and cardiovascular homeostasis, and exerts important physiological regulation through three distinct receptors, V1a, V1b, and V2. Although AVP is used clinically as a potent vasoconstrictor (V1a receptor-mediated) in patients with circulatory shock, the physiological role of vasopressin V1a receptors in blood pressure (BP) homeostasis is ill-defined. In this study, we investigated the functional roles of the V1a receptor in cardiovascular homeostasis using gene targeting. The basal BP of conscious mutant mice lacking the V1a receptor gene (V1a-/-) was significantly (P < 0.001) lower compared to the wild-type mice (V1a+/+) without a notable change in heart rate. There was no significant alteration in cardiac functions as assessed by echocardiogram in the mutant mice. AVP-induced vasopressor responses were abolished in the mutant mice; rather, AVP caused a decrease in BP, which occurred in part through V2 receptor-mediated release of nitric oxide from the vascular endothelium. Arterial baroreceptor reflexes were markedly impaired in mutant mice, consistent with a loss of V1a receptors in the central area of baroreflex control. Notably, mutant mice showed a significant 9% reduction in circulating blood volume. Furthermore, mutant mice had normal plasma AVP levels and a normal AVP secretory response, but had significantly lower adrenocortical responsiveness to adrenocorticotropic hormone. Taken together, these results indicate that the V1a receptor plays an important role in normal resting arterial BP regulation mainly by its regulation of circulating blood volume and baroreflex sensitivity.

Losartan increases renal blood flow during isoflurane anesthesia in sheep

BACKGROUND

Inhaled anesthetics cause a transient reversible depression of renal function by direct renal effects or indirectly by changes in neurohumoral systems or cardiovascular performance. When the sympathetic nervous activity is decreased during anesthesia, other vasoactive systems like vasopressin (AVP) and particularly the renin angiotensin system (RAS) are of importance for blood pressure maintenance. Little is known about how the renal circulation is affected by angiotensin receptor blockade during isoflurane anesthesia.

METHODS

The study was performed on isoflurane anesthetized sheep equipped with flow probes (placed around a femoral and a renal artery) and a pulmonary artery catheter. During stable conditions the sheep were given one or more of the following substances: isotonic saline (NaCl); losartan (LOS) 10 mg x kg(-1); prazosin (PRAZ) 0.2 mg x kg(-1) and a vasopressin V1-receptor antagonist (AVP-a) 10 microg x kg(-1).

RESULTS

LOS and AVP-a did not affect mean arterial pressure (MAP), whereas PRAZ lowered MAP significantly (from 98+/-12 to 65+/-7 mmHg). Renal blood flow (RBF) increased after LOS treatment (148+/-34 to 222+/-33 ml x min(-1)). The other substances were without effect on RBF. Femoral blood flow remained unchanged after all treatments.

CONCLUSION

We conclude that the sympathoadrenal system is still the major determinant for blood pressure maintenance during isoflurane anesthesia in sheep. The apparently increased activity of the renin angiotensin system in this situation causes a reduction in renal blood flow, which is counteracted by angiotensin II AT1-receptor blockade.

Desamino-8D-arginine vasopressin treatment of Brattleboro rats: effect on sensitivity to pressor hormones

The cardiovascular effects of intravenous injections of vasopressin, angiotensin II and noradrenaline were studied in anaesthetized adult male Brattleboro rats with hereditary diabetes insipidus on lifelong treatment with the vasopressin V2 receptor agonist desamino-8D-arginine vasopressin in the drinking fluid, which restored fluid input and output to normal rat values. The pressor response to 20 ng.kg-1 vasopressin was significantly greater (P < 0.005) in the vasopressin V2 receptor agonist-treated rats than in the control animals, but the responses to all higher doses of the peptide were comparable. Doses of noradrenaline from 40 to 160 ng.kg-1 had similar pressor effects in the treated and control rats, while the pressor response to the highest dose of noradrenaline (320 ng.kg-1) was significantly lower (P < 0.01) in the vasopressin V2 receptor agonist-treated rats. Furthermore the pressor responses to all three doses of angiotensin II (40, 80 and 160 ng.kg-1) were significantly attenuated in the treated rats compared to the control group (P < 0.001, P < 0.05 and P < 0.0005 respectively), as were the decreases in heart rate (P < 0.005 at 40 ng.kg-1, P < 0.01 at 80 ng.kg-1). The hypovolaemic stimulus induced by a blood loss of 20 ml.kg-1 resulted in a lower mean arterial blood pressure initially in the treated Brattleboro rats, but subsequent recovery was similar in both treated and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The abnormal quinine drinking aversion in the Brattleboro rat with diabetes insipidus is reversed by the vasopressin agonist DDAVP: a possible role for vasopressin in the motivation to drink

The Brattleboro rat with hypothalamic diabetes insipidus (BDI) has an abnormal aversion to drinking quinine-adulterated water compared with normal rats of the parent Long Evans (LE) strain. This BDI animal tolerates marked hypovolemia and decreased body weight in preference to drinking the quinine-adulterated fluid, indicative of a reduced motivation to drink. Acute or chronic treatment of BDI rats with desamino-8D arginine vasopressin (DDAVP) restored to normal their drinking response to quinine solution. Partial restoration of fluid turnover in BDI rats with hydrochlorothiazide, which has an antidiuretic effect in diabetes insipidus (when vasopressin is absent), failed to abolish the abnormal drinking response to quinine-adulterated solution in 8 out of 12 animals. In contrast, induction of diabetes mellitus in LE rats, which resulted in a marked polydipsia and polyuria even though vasopressin was still present, did not impair the drinking response to quinine solutions. These results suggest that the abnormal drinking response to quinine-adulterated fluid in BDI rats is reversed by treatment with the vasopressin V2-receptor agonist DDAVP but is unlikely to be a consequence of the restoration of fluid turnover to normal levels by a renal action. A possible central action involving vasopressin and the motivation to drink is discussed.

Tolerance to haemorrhage during vasopressin antagonism and/or captopril treatment in conscious sheep

The effect of separate and combined blockade of vasopressin (AVP) V1-receptors and angiotensin II formation on resistance to a slow venous haemorrhage (0.7 ml kg-1 min-1) was studied in six conscious adult sheep by bleeding to the point of an abrupt fall in the mean systemic arterial pressure (MSAP). Intravenous administration of the V1-receptor antagonist [d(CH2)5Tyr(Me)AVP] (10 micrograms kg-1) and/or the angiotensin I converting enzyme inhibitor captopril (20 mg + 1 mg h-1) did not cause any significant haemodynamic changes in the normovolaemic animal. The volume of haemorrhage necessary to induce acute hypotension (MSAP < 50 mmHg) was significantly smaller after AVP blockade alone (13.8 +/- 0.7 ml kg-1; P < 0.01) but not after captopril treatment (14.7 +/- 1.6 ml kg-1; n.s.) compared to control animals receiving no drug treatment (16.8 +/- 0.6 ml kg-1). The combined treatment with the AVP antagonist and captopril caused a further decrease in tolerance to haemorrhage (9.4 +/- 1.2 ml kg-1; P < 0.001). Blockade of AVP V1-receptors was associated with an attenuated increase in systemic vascular resistance immediately after the end of haemorrhage, concomitant with an accentuated lowering of the central venous pressure. In contrast, captopril treatment decreased the degree of vasoconstriction mainly during the second half of the posthaemorrhage observation period of 1 hour. It is concluded that both AVP and angiotensin II contribute to the maintenance of the MSAP during haemorrhage in conscious sheep.(ABSTRACT TRUNCATED AT 250 WORDS)

A role for vasopressin in the stress-induced inhibition of gonadotrophin secretion: studies in the brattleboro rat

Abstract The effects of stress on the secretion of adrenocorticotrophin, corticosterone and luteinizing hormone (LH) in rats congenially lacking hypothalamic vasopressin (Brattleboro rats) and in normal controls of the parent strain (Long Evans) have been compared in an attempt to examine the role of vasopressin in the stress-induced depression of gonadotrophin secretion. In the Long Evans rats, stress (0.6 mg/100g histamine, ip) initiated, within 5 and 20 min respectively, significant (P <0.01, Student's t-test) increases in the plasma adrenocorticotrophin and corticosterone concentrations. It also caused a reduction in the serum LH concentration which was maximal at 5 min. By contrast, in the vasopressin deficient Brattleboro rats, stress had no effect on the serum LH concentration and produced only modest increases in pituitary adrenocortical activity compared with those in Long Evans controls. Pretreatment of both Long Evans and Brattleboro rats with dexamethasone (20mug/100 g ip, daily for 3 days) effectively abolished the pituitary-adrenal response to stress. The steroid treatment also prevented the stress-induced suppression of LH in the Long Evans rats; indeed, these animals, unlike the vehicle-treated controls, exhibited a rise in serum LH concentration within 5 min of exposure to stress. Stress did not affect the serum LH concentrations in steroid-treated Brattleboro rats. The results confirm previous reports that vasopressin is required for the full expression of the pituitary-adrenocortical response stress. They also provide novel evidence which suggests that vasopressin released in stress contributes to the impairment of gonadotrophin secretion.

The renal vascular response to mild and severe haemorrhage in the anaesthetized rat

1. In order to document the characteristics of the renal vascular response to blood losses of varying severity, Inactin-anaesthetized rats were subjected to a haemorrhage of 5, 10, 15 or 20 ml (kg body weight)-1, while a fifth group (control rats) remained unbled. Radioactive microspheres (diameter 10 microns) were used to determine cardiac output, total renal blood flow and the distribution of blood flow within the kidneys; measurements were made before and 5-120 min after haemorrhage. 2. In control animals none of the variables studied changed significantly during the experimental period. 3. Immediately after haemorrhage there were reductions in arterial pressure and cardiac output which were roughly proportional to the severity of haemorrhage. Arterial pressure recovered to some extent during the next 30 min, then stabilized; cardiac output recovered only slightly. 4. Total renal blood flow fell to an extent dependent on the degree of haemorrhage, with no evidence of subsequent recovery. The approximate reductions in renal blood flow were 2% (n.s.), 15%, 30% and 50% after bleeds of 5, 10, 15 and 20 ml kg-1, respectively. Renal vascular resistance increased consistently only in the groups bled 15 and 20 ml kg-1. When renal blood flow was expressed as a fraction of cardiac output, it increased during the period immediately after haemorrhage, indicating some degree of 'protection' of the renal circulation in the face of hypotension. 5. Measurements of intrarenal blood flow indicated a significantly reduced flow to the superficial cortex after every degree of haemorrhage. Inner cortical flow was less affected and fell significantly only in the groups bled 15 and 20 ml kg-1; blood flow to the mid-cortex was intermediate.

Cardiovascular and hormonal changes following haemorrhage in the anaesthetized Brattleboro rat with an extracorporeal circulation

An extracorporeal circulation technique was developed for use in rats to provide equilibrated blood samples for multiple hormone assays. The inclusion of the extracorporeal circulation did not significantly alter arterial blood pressure, cardiac output, heart rate or central venous pressure in either Brattleboro rats with hereditary diabetes insipidus (BDI) or normal rats of the parent Long Evans (LE) strain. Plasma adrenaline and noradrenaline levels did not alter in either BDI or LE rats following inclusion of the extracorporeal circulation but the vasopressin concentration rose significantly in the LE rats. The impaired recovery of the mean arterial blood pressure following haemorrhage in the BDI rats compared with normal LE animals was not further influenced by the inclusion of the extracorporeal circulation. Plasma vasopressin and adrenaline (but not nor-adrenaline) levels were significantly raised during, and after, haemorrhage in the LE rats while in the BDI rats only plasma adrenaline levels were significantly increased. These results show that the insertion of an extracorporeal circulation into an anaesthetized BDI or LE rat does not adversely affect the cardiovascular system despite the increase in baseline plasma vasopressin concentration in normal rats, and its subsequent removal provides an additional equilibrated blood sample for multiple hormone assay within the same animal. The increased release of both adrenaline and vasopressin (but not noradrenaline) after haemorrhage in the same animal is detected using this technique, and the importance of vasopressin to the normal recovery process confirmed.

Sexual dimorphism in vasopressin and cardiovascular response to hemorrhage in the rat

There is evidence for sex-related differences in the cardiovascular actions of vasopressin. Furthermore, receptors for the gonadal steroid hormones are located in centers in the brain involved in the control of vasopressin release and in cardiovascular regulation. We have, therefore, examined the effects of hemorrhage on mean arterial blood pressure, the plasma vasopressin concentration, and plasma renin concentration in conscious male and female rats. In preliminary experiments, no differences were found in blood and plasma volumes with respect to either sex or phase of the estrous cycle. In separate experiments, rats were subjected to two hemorrhages of 10% of blood volume, separated by an interval of 15 minutes. There were no substantial gender- or cycle-related differences in the ability of hemorrhaged rats to maintain mean arterial blood pressure or increase plasma renin concentration. The increase in plasma vasopressin concentration was greater in proestrous females than in males after the first hemorrhage and in diestrous, proestrous, and metestrous females than in males after the second hemorrhage. Pretreatment with a V1-receptor antagonist was without statistically significant effect on the mean arterial blood pressure responses in males, but it impaired blood pressure compensation in females. There are, then, gender- and cycle-related differences in vasopressin responses to hemorrhage, and vasopressin appears particularly important for blood pressure compensation to hemorrhage in female rats.

Cardiovascular and steroid responses to graded haemorrhage in rats with adrenal regeneration hypertension

Abstract Blood pressure (BP) and plasma steroid responses to haemorrhage (2 ml + 1 ml + 1 ml at 20-min intervals) were assessed in sham-operated (SO) rats and in rats with adrenal regeneration hypertension (ARH). Experiments were carried out between 0700 and 1000 h (a.m.) and between 1400 and 1700 h (p.m.), because rats with ARH have BPs that are higher a.m. than p.m. There were no differences in the BP responses following haemorrhage in SO or ARH rats either a.m. or p.m., although ARH rats were unable to increase their plasma steroid levels. Pretreatment with captopril alone, d(CH(2))(5) DAVP alone, or captopril and d(CH(2))(5)DAVP augmented the early hypotensive responses to haemorrhage but did not influence the later compensated levels of BP in either group of rats. There were no clear-cut a.m. to p.m. differences in the changes in BP in any of the drug-treated groups of SO or ARH rats. Under all conditions studied, the compensated level of systolic BP in ARH rats, 20 min after the final bleed, remained higher a.m. than p.m., indicating that this difference was not dependent on the renin-angiotensin system and vasopressin and suggesting that the sympathetic nervous system and/or other factors might be involved.

Cardiovascular interactions between vasopressin, angiotensin and noradrenaline in the Brattleboro rat

1. The cardiovascular effects of bolus intravenous injections of vasopressin, angiotensin II and noradrenaline were studied in 6-hydroxydopamine pretreated, anaesthetized Brattleboro rats with hereditary diabetes insipidus and normal rats of the parent Long Evans strain. 2. Pretreatment with 6-hydroxydopamine did not significantly affect control values for mean arterial blood pressure, cardiac output or total peripheral resistance in either Brattleboro or Long Evans rats but the pressor response to haemorrhage was reduced in both strains compared to the control animals. 3. The pressor responses of the untreated Brattleboro rats to 250 mu kg-1 vasopressin were significantly greater and more prolonged than in control rats of the Long Evans strain. 4. Pretreatment with 6-hydroxydopamine significantly enhanced the peak pressor response to vasopressin, but not to angiotensin II (1 microgram kg-1), in Brattleboro and Long Evans rats. 5. Pretreatment with 6-hydroxydopamine resulted in an enhanced pressor response to 1 microgram kg-1 noradrenaline in both Brattleboro and Long Evans rats, but the effect was significantly greater in the vasopressin-deficient animals. 6. These results indicate differences in the pressor responsiveness of Brattleboro rats to vasopressin and noradrenaline, but not to angiotensin II, compared with control Long Evans rats and provide evidence for important interactions between the sympathetic nervous system and these pressor hormones.