Effect of captopril on cardiovascular, sympathetic and vasopressin responses to hemorrhage

Angiotensin II during Experimentally Simulated Central Hypovolemia

Central hypovolemia, defined as diminished blood volume in the heart and pulmonary vascular bed, is still an unresolved problem from a therapeutic point of view. The development of pharmaceutical agents targeted at specific angiotensin II receptors, such as the non-peptidergic AT₂-receptor agonist compound 21, is yielding many opportunities to uncover more knowledge about angiotensin II receptor profiles and possible therapeutic use. Cardiovascular, anti-inflammatory, and neuroprotective therapeutic use of compound 21 have been suggested. However, there has not yet been a focus on the use of these agents in a hypovolemic setting. We argue that the latest debates on the effect of angiotensin II during hypovolemia might guide for future studies, investigating the effect of such agents during experimentally simulated central hypovolemia. The purpose of this review is to examine the role of angiotensin II during episodes of central hypovolemia. To examine this, we reviewed results from studies with three experimental models of simulated hypovolemia: head up tilt table test, lower body negative pressure, and hemorrhage of animals. A systemic literature search was made with the use of PubMed/MEDLINE for studies that measured variables of the renin–angiotensin system or its effect during simulated hypovolemia. Twelve articles, using one of the three models, were included and showed a possible organ-protective effect and an effect on the sympathetic system of angiotensin II during hypovolemia. The results support the possible organ-protective vasodilatory role for the AT₂-receptor during hypovolemia on both the kidney and the splanchnic tissue.

Endogenous and exogenous vasopressin in shock

PURPOSE OF REVIEW

Vasopressin is critical for blood pressure regulation when cardiovascular homeostasis is threatened and some patients with shock have inappropriately low levels of hormone in plasma. The present review focuses on recent work that addresses the role of endogenous vasopressin in the pathogenesis of shock and the potential therapeutic indications and secondary effects of exogenous hormone in patients with shock.

RECENT FINDINGS

Examples of types of shock resistant to catecholamine pressors in which exogenous vasopressin was effective in restoring arterial pressure continued to accumulate. Widespread determinations of plasma vasopressin in patients with shock suggest that endogenous vasopressin deficiency may be more frequent than previously thought. The generation of mice with deletion of vasopressin's V1a receptor highlighted the important role of the hormone on cardiovascular homeostasis.

SUMMARY

Vasopressin administration is very effective in restoring arterial pressure in many forms of shock and this appears to be due, at least in part, to deficiency of endogenous hormone. Generation of mice lacking vasopressin V1a receptor open new and exciting avenues of inquiry to clarify the role of the hormone in cardiovascular homeostasis.

The role of brain angiotensin in thirst and AVP release induced by hemorrhage

This study investigated the role of brain angiotensin (Ang II) in thirst induced by hemorrhage. Hemorrhage by blood withdrawal from the femoral artery to 33% and 44% blood volume loss produces a dose response increase in plasma Ang II. In the brainstem there was no Ang II response to hemorrhage. In the hypothalamus, Brain Ang II was maximally elevated to 33% hemorrhage. Thus, plasma Ang II and brain Ang II had an independent response to hemorrhage. To further test the role of central versus peripheral Ang II, we tested the effect of central (50 mg) and peripheral (50 mg/kg) administration of captopril or central injection of 1 mg losartan or 3 mg CGP 42112A prior to a 33% hemorrhage in unanesthetized male Sprague-Dawley rats (250 g). Drinking was measured and AVP blood samples were taken before and after hemorrhage. The results show that central (i.v.t.) administration of captopril and losartan inhibited drinking compared to controls (0.33 +/- 0.3 ml vs. 2.3 +/- 0.8 ml: P < 0.05 and 0.20 +/- 0.09 ml vs.3.05 +/- 0.81 ml; P < 0.01, respectively) while peripheral (i.p.) captopril alone increased drinking in response to hemorrhage (5.81 +/- 0.81 ml vs. 2.3 +/- 0.8 ml; P < 0.05). AVP levels were elevated at 5 and 15 min, but neither injections of losartan or CGP 42112A i.v.t. affected this response to hemorrhage. We conclude that increased hypothalamic brain Ang II after hypovolemic hemorrhage stimulates thirst and blood pressure restoration and acts through AT1 receptors. The release of AVP in hemorrhage, however, does not rely exclusively on the angiotensinergic pathway in the brain.

Angiotensin-converting enzyme inhibitors in cardiovascular anesthesia

Besides the long-term regulation of extracellular fluid volume, the RAS plays an important physiologic role in maintaining venous return and blood pressure during acute hemodynamic stresses. ACE inhibitors may therefore alter venous return and cardiac output regulation during anesthesia and surgery. This may be regarded as a drawback of ACE inhibition when other factors interfere with cardiovascular homeostasis; deleterious hemodynamic events may therefore occur when blood volume is decreased, which may be frequent during cardiovascular anesthesia and surgery. However, the alternative solution should not be to stop ACE inhibitors preoperatively. This would allow recovery of RAS control of blood pressure, but at the expense of some regional circulations. From this point of view, preliminary results from early studies during cardiovascular anesthesia and surgery showing redistribution of regional blood flow with inhibition of ACE are encouraging; whether postoperative outcome can be improved deserves further studies. At this time, the evidence is that ACE inhibition does not allow the anesthesiologist to be tolerant of hypovolemia.

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)

Effect of lignocaine on arginine-vasopressin plasma levels: baseline or induced by frusemide

1. To assess whether or not lignocaine influences baseline and frusemide-induced (5 mg kg-1) plasma concentrations of arginine-vasopressin (AVP), 2 groups of rabbits received an infusion of lignocaine (130 micrograms min-1 kg-1) for 6 h. Lignocaine-induced changes in AVP plasma concentrations were substantiated by measurement of diuresis and natriuresis and hepatic plasma flow, by means of an infusion of indocyanine green (ICG) (249 micrograms min-1 kg-1). 2. Baseline plasma AVP levels were 4.9 +/- 0.9 pg ml-1 (+/- s.e.), and following lignocaine, these values were reduced to 0.7 +/- 0.1 pg ml-1 (P less than 0.01). Frusemide increased AVP levels to 134.1 +/- 73.6 pg ml-1 (P less than 0.05) and lignocaine totally prevented this increase, e.g. mean AVP levels of 2.7 pg ml-1. 3. Lignocaine enhanced baseline diuresis secondary to an increase in free water clearance; none of the experimental conditions affected the diuresis and natriuresis induced by frusemide. 4. Frusemide reduced the hepatic plasma flow and this decrease was not reversed by the infusion of lignocaine. 5. It is concluded that in healthy rabbits lignocaine reduces baseline secretion of AVP and its antidiuretic effect; in addition, lignocaine prevents the rise in AVP induced by frusemide.

[Anesthetic consequences of hemodynamic effects of angiotensin converting enzyme inhibitors]

Angiotensin converting enzyme inhibitors (ACEI) are used increasingly to treat cardiovascular diseases, and so, therefore, the number of patients scheduled for surgery and treated preoperatively with these drugs. Haemodynamic instability has sometimes been observed during anaesthesia in these patients, leading some authors to discontinue ACEI administration before anaesthesia. However, recent physiological data concerning the renin angiotensin system (RAS) and ACEI pharmacological data may increase our understanding of the mechanisms of cardiovascular interaction between ACEI and anaesthesia. The RAS is involved in blood pressure regulation when extracellular fluid volume is decreased and in case of hypovolaemia, by inducing vasoconstriction and longterm volume regulation. Arterial vasoconstriction is the target for ACEI. However, venoconstriction may maintain venous return and cardiac output in spite of reduced blood volume. On the other hand, ACEI treatment impedes cardiac adaptation to acute changes in extracellular fluid volume. This effect may be increased by underlying pathology (especially in hypertension) as well as by anaesthesia. A combination of an increased sensitivity to acute changes in ventricular load due to treatment with ACEI and anaesthesia in hypertensive patients or in patients with cardiac failure may carry a high risk of hypotension. Specific studies on haemodynamic tolerance of anaesthesia in patients chronically treated with ACEI are required to assess the prevalence of this risk and how to manage it.

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.

Repeated hypotension induced by nitroprusside and haemorrhage in sheep: effects on vasopressin release and recovery of arterial blood pressure

The arginine vasopressin (AVP) release in response to repeated hypotension caused by intravenous (i.v.) infusion of sodium nitroprusside (SNP) or haemorrhage was studied in conscious euhydrated sheep. Parallel determinations of renal excretion and plasma concentration of AVP were made in experiments involving two consecutive 10-min i.v. infusions of SNP (about 35 micrograms kg-1 min-1) with a 3-h interval between and repeated the next day. The AVP response to the second SNP administration was significantly reduced, but partial recovery was observed in response to the initial infusion the next day. Maximal fall in mean arterial blood pressure (MABP) and its recovery pattern did not differ in response to any of the four SNP infusions. In contrast, impaired recovery of the MABP together with markedly reduced AVP response was seen as a consequence of a hypotensive haemorrhage repeated after 3 h, but not when the interval between haemorrhages was extended to 24 h. The haemorrhage-induced increase in plasma renin activity was not affected by variations in the interval between experiments. It is concluded that the massive AVP liberation normally seen as an effect of acute isovolaemic hypotension becomes markedly reduced upon a renewed fall in the MABP occurring within 3 h. An iteration of hypotensive haemorrhage accentuates this fatigue of the hormonal response, which may contribute to the impaired recovery of the MABP.

Naloxone reversal of hemorrhagic hypotension in the conscious guinea-pig is impeded by inhibition of the renin-angiotensin II system

Naloxone reverses hemorrhagic hypotension in the conscious guinea-pig. Captopril and saralasin impede this naloxone effect, suggesting that angiotensin II is involved in naloxone action. This is compatible with previous work which has shown that B-endorphin inhibits the centrally mediated pressor action of angiotensin II, and that naloxone blocks this effect. Naloxone may be interacting with the postulated brain renin-angiotension II system or may be blocking the action of shock-induced circulating angiotensin II on a centrally located area such as the hypothalamus.

Effect of bilateral nephrectomy on the recovery of blood pressure after acute hemorrhage in rats: role of renin-angiotensin system

The effect of bilateral nephrectomy, and administration of an inhibitor of angiotensin converting enzyme, on the recovery of arterial blood pressure after hemorrhage (loss of 1% of b.wt), was studied in male Sprague-Dawley rats. Neither manoeuver significantly affected the recovery of blood pressure within the first 10 min after hemorrhage. Thereafter, the recovery of the blood pressure was markedly suppressed. The study suggests that the initial recovery of blood pressure is unrelated to the kidneys, but the later one requires their presence and depends on the activity of the renin-angiotensin system.

Effects of captopril on arterial blood pressure, plasma renin activity and vasopressin concentration in sodium-repleted and sodium-deficient goats. A serial study during pregnancy, lactation and anestrus

The effects of captopril (intravenous loading dose of 20 mg, 1 h later followed by 10 mg infused during the next h) on arterial blood pressure and plasma renin activity were followed in 4 goats during the last months of pregnancy, during lactation and during anestrus. Experiments were made both when the animals were sodium-repleted and sodium-deficient. Furosemide and dietary restriction were used to deprive the animals of sodium. In sodium-replete animals, captopril caused a more pronounced fall in mean arterial blood pressure and a larger increase in plasma renin activity (PRA) when the animals were pregnant than when they were lactating or in anestrus. During sodium-deficient conditions the response was similar as during sodium repletion in pregnant goats. In anestral goats, PRA rose in response to captopril, but the blood pressure fall was similar as when the goats were sodium-replete. In lactating sodium-deficient goats, captopril caused a marked fall in mean arterial blood pressure concomitant with a 2-3 times higher rise in PRA than during corresponding sodium-repletion experiments. The respiratory rate was elevated in pregnant animals and increased further during captopril. The plasma vasopressin concentration did not change during captopril-induced hypotension. If the blood pressure fell greater than or equal to 10 mmHg the animals became very quiet and occasionally they fell asleep. All goats delivered healthy kids. The fact that the blood pressure fall was marked and consistent in all animals during pregnancy, but less and more variable during anestrus indicates that the activity of the renin-angiotensin system is of greater importance for blood pressure maintenance during pregnancy than during anestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral pressor effects of sympathetic stimulation, noradrenaline, angiotensin II and vasopressin in Brattleboro rats

The following study was designed to test the hypothesis that peripheral blood vessels of vasopressin deficient (Di/Di) rats are less responsive to pressor substances than normal rats. To address this question, pithed Di/Di and normal Long-Evans rats (LE) were exposed to intravenous injections of arginine-vasopressin, angiotensin II and noradrenaline. In addition, blood pressure increments and noradrenaline release in response to spinal cord stimulation in pithed Di/Di and LE rats were studied. The results show no abnormalities in peripheral vascular sensitivity to any of the pressor substances administered, nor was there any change in blood pressure and sympatho-adrenomedullary response to graded stimulation of the sympathetic outflow from the spinal cord. This study suggests that the failure of vasopressin deficient rats to recover from acute hemorrhage is not due to hyporesponsiveness of the peripheral vasculature to pressor agents but, rather, to the deficiency in the direct pressor effect of vasopressin.

Developmental aspects of the renal responses to hemorrhage during converting-enzyme inhibition in fetal lambs

The role of the renin-angiotensin system in modulating the renal hemodynamic and functional responses to reductions of fetoplacental blood volume (8.8-35.5%) was studied in two groups of fetal lambs (less than 120 days and greater than 130 days gestation; term 145 days) during infusion of either captopril (experimental fetuses) or dextrose 5% in water (control fetuses). At high hemorrhage levels (level III), renal blood flow decreased and renal vascular resistance increased significantly in both groups of fetuses (less than 120 days and greater than 130 days), either treated or not treated with captopril. However, at low hemorrhage levels (levels I and II), and contrary to what was observed in young fetuses (less than 120 days), near-term fetuses (greater than 130 days) receiving captopril showed neither significant decreases in renal blood flow nor increases in renal vascular resistance, whereas untreated fetuses of the same gestational ages demonstrated significant decreases in renal blood flow and increases in renal vascular resistance. It was found in both less than 120 day and greater than 130 day fetuses that hemorrhage is associated with a decrease in urinary flow rate and free water clearance accompanied by an increase in urine osmolality and sodium reabsorption. It was shown that captopril does not modify this response. The present study also demonstrated that the blood pressure response to hemorrhage was characterized by a similar decrease in less than 120 day fetuses, whether treated or untreated with captopril. On the other hand, blood pressure did not change in control fetuses greater than 130 days, but decreased slightly in captopril-treated fetuses during hemorrhage. Taken together, the present results tend to suggest that the renin-angiotensin system may be an important modulator of the renal hemodynamic response to low level hemorrhage as fetuses approach term, and may be more important in controlling blood pressure in near-term than in young fetuses.

Circulatory effects of renin-angiotensin system antagonists during halothane anaesthesia in hypertensive rats

The circulatory effects of captopril, an angiotensin I-converting enzyme inhibitor, and saralasin, a competitive angiotensin II antagonist, were studied during halothane anaesthesia in spontaneously hypertensive (SH) rats. Captopril decreased blood pressure significantly in unanaesthetized rats. Pretreatment with indomethacin, a prostaglandin synthesis inhibitor, did not modify the antihypertensive action of captopril. During 1 MAC halothane anaesthesia, the mean arterial pressure (MAP) in unmedicated SH control rats was maintained at a relatively high level (16.2 +/- 0.7 kPa, mean +/- s.e. mean), while in captopril-treated rats MAP decreased to 8.8 +/- 1.1 kPa. Indomethacin somewhat inhibited MAP decrease in the captopril-medicated group. Saralasin infusion in halothane-anaesthetized rats decreased MAP in the same way as captopril alone. The tolerance to haemorrhagic shock was markedly impaired in rats receiving captopril or saralasin, compared to untreated controls. During halothane anaesthesia, the plasma renin activities in the captopril, captopril + indomethacin, and saralasin groups were significantly higher than in untreated animals. Plasma kininogen was unaffected by any of the medications. The results suggest that the renin-angiotensin system is important in maintaining blood pressure in halothane anaesthesia, and that the tolerance to haemorrhagic shock is particularly impaired by drugs inhibiting the renin-angiotensin system.

Vasopressin: an essential pressor factor for blood pressure recovery following hemorrhage

Two experimental approaches were used to evaluate the importance of the pressor effects of vasopressin in blood pressure recovery following hypotensive hemorrhage. Experiments using homozygous Brattleboro rats demonstrated that the hemodynamic recovery of these animals was subnormal, even though the activation and efficacy of the sympathetic nervous and renin-angiotensin systems were intact. Experiments using an antipressor vasopressin analogue in normal rats during hypotensive hemorrhage demonstrated significantly blunted blood pressure recovery in the presence of the analogue. Thus, both experiments indicate that the pressor effects of circulating vasopressin play an essential role in blood pressure recovery following hypovolemic hypotension induced by hemorrhage.