Facilitatory role of the renin-angiotensin system in controlling adrenal catecholamine release in hemorrhaged dogs

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

The effects of candesartan and ramipril on adrenal catecholamine release in anaesthetized dogs

We have investigated the effects of the angiotensin II type 1 receptor antagonist candesartan, and the angiotensin II converting enzyme inhibitor ramipril, on catecholamine release from the anaesthetized dog's adrenal gland. These drugs were given systemically in low and high doses. The gland was stimulated electrically (0.5-12 Hz) and by angiotensin II infusion (40 ng/kg/min). Electrical stimulation resulted in frequency-dependent increases in catecholamine release. Candesartan (0.8, 4.0 mg/kg) and ramipril (0.125, 0.625 mg/kg) increased basal catecholamine release along with decreases in blood pressure. Both drugs diminished direct nerve stimulation-induced catecholamine release. When both drugs were combined, their inhibitory effect was slightly enhanced. Candesartan blocked catecholamine release induced by angiotensin II. Ramipril was not tested in this respect. The percentage of noradrenaline released during electrical stimulation of the gland remained constant and ranged from 14% to 22%. Both drugs appear to act by blocking local modulation of catecholamine release by the chromaffin cells.

Effect of angiotensin II and endothelin-1 receptor blockade on the haemodynamic and hormonal changes after acute blood loss and after retransfusion in conscious dogs

AIM

This study investigates angiotensin II and endothelin-1 mediated mechanisms involved in the haemodynamic, hormonal, and renal response towards acute hypotensive haemorrhage.

METHODS

Conscious dogs were pre-treated with angiotensin II type 1 (AT1) and/or endothelin-A (ETA) receptor blockers or not. Protocol 1: After a 60-min baseline period, 25% of the dog's blood was rapidly withdrawn. The blood was retransfused 60 min later and data recorded for another hour. Protocol 2: Likewise, but preceded by AT1 blockade with i.v. Losartan. Protocol 3: Likewise, but preceded by ETA blockade with i.v. ABT-627. Protocol 4: Likewise, but with combined AT1 plus ETA blockade.

RESULTS

In controls, haemorrhage decreased mean arterial pressure (MAP) by approximately 25%, cardiac output by approximately 40%, and urine volume by approximately 60%, increased angiotensin II (3.1-fold), endothelin-1 (1.13-fold), vasopressin (116-fold), and adrenaline concentrations (3.2-fold). Glomerular filtration rate and noradrenaline concentrations remained unchanged. During AT1 blockade, the MAP decrease was exaggerated (-40%) and glomerular filtration rate fell. During ETA blockade, noradrenaline increased after haemorrhage instead of adrenaline, and the MAP recovery after retransfusion was blunted. The decrease in cardiac output was similar in all protocols.

CONCLUSIONS

Angiotensin II is more important than endothelin-1 for the short-term regulation of MAP and glomerular filtration rate after haemorrhage, whereas endothelin-1 seems necessary for complete MAP recovery after retransfusion. After haemorrhage, endothelin-1 seems to facilitate adrenaline release and to blunt noradrenaline release. Haemorrhage-induced compensatory mechanisms maintain blood flow more effectively than blood pressure, as the decrease in cardiac output--but not MAP--was similar in all protocols.

Functional role of local angiotensin-converting enzyme (ACE) in adrenal catecholamine secretion in vivo

The aim of the present study was to investigate whether exogenous angiotensin I (AngI) is locally converted to angiotensin II (AngII), which in turn results in an increase in the adrenal catecholamine (CA) secretion in the adrenal gland in anesthetized dogs. Plasma CA concentrations in adrenal venous and aortic blood were determined by an HPLC-electrochemical method. Adrenal venous blood flow was measured by gravimetry. Local administration of AngI (0.0062 to 6.2 µg, 0.0096 to 9.6 µM) to the left adrenal gland resulted in significant increases in CA output in a dose-dependent manner. Following administration of 0.62 µg (0.96 µM) of AngI, adrenal epinephrine and norepinephrine outputs increased from 20.8 ± 13.6 to 250.9 ± 96.4 ng·min-1·g-1 (p < 0.05, n = 5) and from 2.8 ± 1.7 to 29.6 ± 11.1 ng·min-1·g-1 (p < 0.05, n = 5), respectively. From the same left adrenal gland, the output of AngII increased from -0.02 ± 0.04 to 26.39 ± 11.38 ng·min-1·g-1 (p < 0.05, n = 5), while plasma concentrations of AngII in aortic blood remained unchanged. In dogs receiving captopril (12.5 µg, 0.5 mM) 10 min prior to AngI, the net amounts of CA and AngII secreted during the first 3 min after AngI were diminished by about 80% (p < 0.05, n = 5) compared with those obtained from the control group. There was a close correlation (r2 = 0.91, n = 6) between the net increases in AngII and CA outputs induced by AngI. The results indicate that the local angiotensin converting enzyme is functionally involved in regional AngII formation in the canine adrenal gland in vivo. The study suggests that AngII thus generated may play a role in the local regulation of adrenal CA secretion.Key words: angiotensin I, angiotensin II, captopril, adrenal gland, anesthetized dog.

Effects of chronic central administration of losartan on the cardiovascular and hormonal responses to hemorrhage in conscious rats

Our objective was to assess the effects of chronic central angiotensin II (Ang II) blockade on the basal regulation of blood pressure, heart rate (HR), arginine vasopressin (AVP), renin, epinephrine (EPI), norepinephrine (NE) and on cardiovascular and hormonal responses to hemorrhage in conscious rats. Losartan (4 micrograms/h), or artificial cerebrospinal fluid (aCSF), was chronically infused into a lateral ventricle by using an osmotic minipump for 6 days at a rate of 1 microliter/h. Compared with aCSF controls, chronic losartan treatment significantly decreased the basal level of blood pressure (from 117 +/- 2.3 to 106 +/- 2.2 mmHg, P < 0.01) and increased the HR (from 357 +/- 3.7 to 410 +/- 6.6 beats/min, P < 0.01). Plasma renin concentration increased 3-fold (from 6.1 +/- 0.6 to 19.2 +/- 1.6 ng.ml(-1).h(-1), P < 0.01). Basal levels of AVP, EPI and NE were not different between two groups. Blood pressure immediately after hemorrhage and its compensatory recovery following hemorrhage was not different in both groups. Immediately after hemorrhage, however, in the losartan-treated rats, the HR was distinctly lower than that of aCSF controls, even at 10 min after hemorrhage. Hemorrhage produced a significant increase in the plasma concentrations of AVP, renin, EPI and NE. Chronic losartan treatment markedly augmented the AVP, renin and EPI responses to hemorrhage. These results strongly suggest that Ang II acting through AT1 receptors in the brain plays a significant physiological role in the regulation of basal blood pressure, HR and renin release. In addition, centrally acting Ang II may be one of the important mediators for cardiovascular regulations and hormone releases in response to hemorrhage.

The effect of global ischemia and reperfusion on the plasma levels of vasoactive peptides. The neuroendocrine response to cardiac arrest and resuscitation

OBJECTIVE

Return of spontaneous circulation with CPR is a function of coronary perfusion pressure, which is determined by vasomotor tone and the force of compression. Vasomotor tone is affected by the relative stimulation of arterial vasoconstricting and vasorelaxing receptors by vasoactive substances. We measured the plasma levels of the endogenous vasoactive peptides arginine vasopressin (AVP) angiotensin II (ANG-II) and atrial natriuretic peptide (ANP) during cardiac arrest and resuscitation.

DESIGN

A fibrillatory canine model of canine arrest was used. 'Down time' was greater than 10 min, during which no therapy, including BLS, was given. Standard ACLS was initiated at the end of the down time with manual external chest compression standardized to an esophageal pulse pressure of 50 mmHg. Blood samples were collected through an aortic catheter during spontaneous circulation and 3 min after initiation of ACLS. Peptide levels were measured using standard RIA techniques. Results are reported as the mean +/- S.D. in pg/ml.

RESULTS

AVP levels increased from a baseline of 1.7 +/- 1.0 pg/ml during spontaneous circulation to 29.9 +/- 33.3 during cardiac arrest and CPR (P = 0.01). There was a moderate positive correlation between aortic pressure and circulating AVP levels after the first dose of epinephrine (R = 0.5). There was a trend towards higher AVP levels in animals with return of spontaneous circulation (P = 0.12). ANG-II levels increased from a baseline of 14.7 +/- 12.9 pg/ml during spontaneous circulation to 151 +/- 105 during cardiac arrest and CPR (P < 0.05). ANP levels increased from a baseline of 55 +/- 46 pg/ml during spontaneous circulation to 293 +/- 73 during cardiac arrest and CPR (P < 0.01).

CONCLUSION

There were significant increases in the levels of these endogenous vasoactive peptides. This reflects the neuroendocrine response to global ischemia and CPR reperfusion. Plasma levels of these peptides may effect the vital organ perfusion pressures, response to exogenous vasopressors, and outcome of resuscitative efforts. Future therapies may be directed at enhancing or blocking the effect of these peptides so as to optimize perfusion pressure which is one of the principle determinants of outcome during CPR.