On the role of vasopressin and angiotensin in the development of irreversible haemorrhagic shock

Hypometabolism and hypothermia in the rat model of endotoxic shock: independence of circulatory hypoxia

We tested the hypothesis that development of hypothermia instead of fever in endotoxic shock is consequential to hypoxia. Endotoxic shock was induced by bacterial lipopolysaccharide (LPS, 500 μg kg(-1) i.v.) in rats at an ambient temperature of 22 °C. A β3-adrenergic agonist known to activate metabolic heat production, CL316,243, was employed to evaluate whether thermogenic capacity could be impaired by the fall in oxygen delivery (ḊO2) during endotoxic shock. This possibility was rejected as CL316,243 (0.15 mg kg(-1) i.v.) evoked similar rises in oxygen consumption (V̇O2) in the presence and absence of endotoxic shock. Next, to investigate whether a less severe form of circulatory hypoxia could be triggering hypothermia, the circulating volume of LPS-injected rats was expanded using 6% hetastarch with the intention of improving tissue perfusion and alleviating hypoxia. This intervention attenuated not only the fall in arterial pressure induced by LPS, but also the associated falls in V̇O2 and body temperature. These effects, however, occurred independently of hypoxia, as they were not accompanied by any detectable changes in NAD(+)/NADH ratios. Further experimentation revealed that even the earliest drops in cardiac output and ḊO2 during endotoxic shock did not precede the reduction in V̇O2 that brings about hypothermia. In fact, ḊO2 and V̇O2 fell in such a synchrony that the ḊO2/V̇O2 ratio remained unaffected. Only when hypothermia was prevented by exposure to a warm environment (30 °C) did an imbalance in the ḊO2/V̇O2 ratio become evident, and such an imbalance was associated with reductions in the renal and hypothalamic NAD(+)/NADH ratios. In conclusion, hypometabolism and hypothermia in endotoxic shock are not consequential to hypoxia but serve as a pre-emptive strategy to avoid hypoxia in this model.

Pressor and intra-renal effects of angiotensins I and II, and noradrenaline, in anaesthetized and conscious sheep

The pressor and intra-renal actions and effects of octa - and deca-peptides angiotensins II and I and of the catecholamine noradrenaline, in anaesthetized and conscious sheep, are considered. The halothane anaesthetic substantially lowers pressor sensitivity to both peptides but does not influence their ability to liberate K(+) ions into the circulating plasma. In comparison with angiotensin II, both angiotensin I and noradrenaline -- with direct presentation to the kidney -- are ineffective in decreasing intra-renal blood flow. However, with left ventricular injection, both pressor compounds immediately increase the blood pressure, as does angiotensin II. Combined doses of the decapeptide and catecholamine are thus highly effective in raising the blood pressure while having a minimal effect on blood flow through the kidney. This overall situation could provide a basis for treating clinical shock, especially regarding septicaemia and septic shock. The lowered hind-limb blood flow, with administration of the pressor compounds into the femoral artery, contrasts strongly with the raised flow resulting from intravenous injection. Experimental procedures to establish, or otherwise, relevant hypothetical situations are detailed.

Specific angiotensin II receptor blockage improves intestinal perfusion during graded hypovolemia in pigs

OBJECTIVE

To investigate the potential of specific angiotensin II subtype 1 (AT1) receptor blockade to modify the mesenteric hemodynamic response to acute hypovolemia and retransfusion.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University-affiliated animal research laboratory.

SUBJECTS

Fasted, anesthetized, ventilated, juvenile domestic pigs of both sexes.

INTERVENTIONS

Acute, graded hypovolemia by 20% and 40% of the total estimated blood volume followed by retransfusion in control animals (CTRL; n = 10) and animals pretreated with the AT1 receptor blocker candesartan (CAND; n = 10).

MEASUREMENTS AND MAIN RESULTS

Invasive monitoring of arterial and central venous blood pressures, cardiac output, portal venous blood flow, and jejunal mucosal blood flow. Blood gases were repeatedly analyzed to calculate oxygen delivery and consumption. Thirty minutes after each level of hypovolemia at 20% and 40%, cardiac output was decreased in CTRL animals from a baseline of 2.9 +/- 0.1 to 1.8 +/- 0.2 and 1.1 +/- 0.2 L/min, with no differences compared with CAND animals. Cardiac output was restored to 3.0 +/- 0.3 L/min 30 mins after retransfusion in CTRL animals, with no significant intergroup differences. Baseline portal venous blood flow (Q(MES)) and jejunal mucosal perfusion (PU(JEJ)) were greater in CAND animals compared with CTRL animals. During graded hypovolemia, CAND animals maintained Q(MES) and PU(JEJ) at significantly higher levels compared with CTRL animals, particularly after 40% hemorrhage (+221% and + 244%, respectively, relative to the mean values in CTRL animals). The same pattern was observed after retransfusion. Moreover, the calculated mesenteric critical oxygen delivery was significantly greater in CTRL animals (74 mL/min) compared with CAND animals (34 mL/min). No animals died in the CAND group, whereas four animals died during 40% hypovolemia or retransfusion in the CTRL group.

CONCLUSIONS

Specific AT1 blockade before acute hypovolemia significantly ameliorated mesenteric and, in particular, jejunal mucosal hypoperfusion. In addition, cardiovascular stability was improved, and mortality in conjunction with acute hypovolemia and retransfusion could be completely avoided. These findings support a fundamental role of the renin-angiotensin system in the mesenteric response to acute hypovolemia and indicate a substantial interventional potential for candesartan in conjunction with circulatory stress.

Catecholamine and renin-angiotensin response during controlled hypotension induced by prostaglandin E1 combined with hemodilution during isoflurane anesthesia

STUDY OBJECTIVE

To evaluate the catecholamine and renin-angiotensin response during controlled hypotension combined with hemodilution in the clinical setting.

DESIGN

Randomized, prospective study.

SETTING

Inpatient surgery at Nagasaki Rosai Hospital.

PATIENTS

30 ASA physical status I and II female patients scheduled for total hip arthroplasty.

INTERVENTIONS

Patients were randomly divided into three groups. Group A (N = 10) received hemodilution alone. Group B (N = 10) received controlled hypotension alone. Group C (N = 10) received hemodilution plus controlled hypotension. Hemodilution was carried out after induction of anesthesia, in which drawn blood was replaced with 6% hydroxyethyl starch, and the final hematocrit value was approximately 22%. Controlled hypotension was induced with prostaglandin E1 (PGE1) to maintain mean arterial blood pressure at 55 mmHg for 80 minutes.

MEASUREMENTS AND MAIN RESULTS

Measurements included plasma renin activity (RA) and plasma concentrations of angiotensin-II (AG-II), aldosterone (AS), norepinephrine (NE), and epinephrine (EP). These indices were measured before hemodilution, after hemodilution, 80 minutes after starting hypotension, and 60 minutes after recovery from hypotension. Hemodilution alone caused no significant change in the values throughout the time course. Controlled hypotension alone caused significant increases in plasma concentrations of NE (+295%) and EP (+203%) at 80 minutes after starting hypotension, whereas it caused no change in plasma RA and plasma concentrations of AG-II and AS. Hemodilution plus controlled hypotension caused significant increases in plasma RA (+271%) and plasma concentrations of AG-II (+188%), AS (+199%), NE (+279%), and EP (+184%) at 80 minutes after starting hypotension.

CONCLUSION

The combination of hemodilution and PGE1 induced controlled hypotension causes significant responses, especially in a renin-angiotensin-aldosterone system under isoflurane anesthesia.

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.

Effects of vasopressin on smooth muscle cells of guinea-pig mesenteric vessels

1 The effects of vasopressin on the membrane and contractile properties of smooth muscle cells of guinea-pig mesenteric arteries, and mesenteric and portal veins were investigated in various ionic environments by means of a micro-electrode technique and an isometric tension recording method. The results were compared with those obtained with oxytocin and noradrenaline (NA).2 In the mesenteric jejunal artery, the mean membrane potential was -56.6 +/- 2.3 mV, s.d, and the membrane was electrically quiescent. Application of outward current pulses generated small graded responses, and the current voltage relationship was linear with application of an inward current pulse.3 Vasopressin and NA depolarized the membrane and increased the membrane resistance. Vasopressin was a 1000 times more potent than oxytocin in depolarizing the membrane. In high concentrations, vasopressin (1 x 10(-3) or 1 x 10(-2) iu/ml) or NA (5.9 x 10(-5) M) generated slow oscillatory membrane potential changes (slow waves) and spikes during the depolarization. The excitatory actions of vasopressin and NA were not suppressed by tetrodotoxin (3.1 x 10(-7) M) or ouabain (1.3 x 10(-6) M) and the actions of vasopressin were not suppressed by adrenoceptor blocking agents (3.9 x 10(-7) M phentolamine or 3.6 x 10(-7) M propranolol).4 The depolarization induced by vasopressin or NA is mainly due to a decrease in the K-permeability of the membrane. However, the contribution of other ionic species to the depolarization induced by vasopressin or NA differed, e.g. in low concentrations of [Na](o), the NA-induced depolarization was suppressed to a greater extent than that due to vasopressin. In low concentrations of [Ca](o), the vasopressin-induced depolarization was suppressed to a greater extent than with NA.5 In low concentrations of [Ca](o) and in the presence of vasopressin or NA, spike generation was inhibited but slow waves were not. In low concentrations of [Na](o), the vasopressin-induced slow waves and spikes were for the great part preserved, but with a high concentration of [Ca](o), vasopressing-induced slow waves were suppressed.6 Both vasopressin and NA produced contractions in the jejunal mesenteric artery. However, the maximum contraction in response to vasopressin was larger than that to NA, although both induced similar membrane depolarization. In a low concentration of [Na](o), vasopressin but not NA produced a contraction.7 In the cranial mesenteric artery, NA (5.9 x 10(-5) M) depolarized the membrane and produced a contraction, while vasopressin (1 x 10(-1) iu/ml) and oxytocin (1 x 10(-1) iu/ml) neither depolarized the member nor produced a contraction. In the mesenteric vein, NA (5.9 x 10(-5) M) slightly depolarized the membrane and produced a small contraction. On the other hand, in the portal vein, NA (5.9 x 10(-7) M) produced a marked depolarization and a contraction. Vasopressin (1 x 10(-1) iu/ml) and oxytocin (1 x 10(-1) iu/ml) produced neither excitatory nor inhibitory actions in these veins.8 It is concluded that vasopressin acts on only small muscular arteries, while NA acts on all mesenteric vessels, particularly the portal vein. Therefore, the hepatic portal vascular resistance may be increased by NA and reduced by vasopressin.

Influence of carotid and aortic baroreceptors on neurosecretory neurons in supraoptic nuclei

(1) The effects of arterial baroreceptor stimulation on the activity of 'identified' neurosecretory cells in supraoptic nuclei (SON) of the hypothalamus were investigated in anesthetized cats. (2) Stimulation of baroreceptors by distension of an 'isolated' carotid sinus greatly inhibited SON activity. A linear relationship was found to exist between stimulus intensity (an increase in the sinus pressure) and the degree of inhibition of SON neuron activity. This inhibitory effect was abolished by section of the sinus nerve. (3) The inhibitory effect of baroreceptors on SON neuron activity was most pronounced in the first 5 sec during stimulation, and the effect became less when the stimulus lasted for a long period. At the end of stimulation there was a transient reversal of the response. (4) The excitation of aortic baroreceptors by occluding descending aorta strongly inhibited SON neuron activity. (5) Occlusion of carotid arteries augmented the SON neuron activity. Electrical stimulation of the sinus nerve caused an excitation or an inhibition of SON neurons, depending on the stimulus intensities. Apparently these stimuli produced different degrees of excitation in baro- and chemoreceptors. (6) Combination of chemo- and baroreceptor stimulations revealed that the excitatory effect exerted by chemoreceptor stimulation on SON neurons was reversed or blocked by baroreceptor activation, indicating the powerful inhibitory influence of baroreceptors.

The vasodepressor action of prostacyclin (PGI2) and its effect on plasma angiotensin II and vasopressin in unanaesthetized normotensive and hypertensive rats

The action of prostacyclin (PGI2) on arterial pressure, heart rate, plasma concentration of angiotensin II and vasopressin was studied in groups of normal, renal hypertensive and DOC hypertensive rats. PGI2 was given by continuous iv. infusion at successive doses of 0.25, 0.5 and 1.0 microgram/kg x min, each rate for one hour. Arterial pressure was reduced to normal or below normal in the hypertensive rats, though the fall of blood pressure was greatest in the DOC hypertensive animals. Mean arterial pressure at the end of infusion was 89 mm Hg for normal rats, 87 mm Hg for renal hypertensive rats and 69 mm Hg for DOC hypertensive rats. Diastolic pressure fell more than systolic pressure suggesting a vasodilator mechanism. Heart rate was reduced significantly at the end of the infusion in the three groups of rats. Prostacyclin was also infused for 3 hours at a constant rate of 0.5 microgram/kg x min. The arterial pressure lowering effect was maintained throughout the infusion period.

Neurohypophyseal vasopressor principle: vasopressor hormone as well as antidiuretic hormone?

Studies on the vasopressor role of the antidiuretic hormone arginine-vasopressin (AVP) in DOC hypertension, in two-kidney Goldblatt hypertension, and in spontaneous hypertension of rats, and during acute blood pressure elevation after intracerebroventricular injection of angiotensin II and in glycerol-induced acute renal failure of rats are reviewed. For the measurement of plasma AVP a radioimmunoassay has been developed. For this assay, a series of criteria has been met which allows the conclusion that, in plasma of rats, the antibody measures AVP only. For the blockade of vasopressor effects of AVP a specific antiserum has been used. On the basis of a series of control studies it has been concluded, but not proven that the antiserum lowers blood pressure exclusively by blockade of AVP. It could be shown that in the various animal models of hypertension and of acute blood pressure elevation AVP exerts systemic vasoconstriction when its plasma concentrations are elevated. In those models where the renin-angiotensin system played no role in blood pressure control, the height of blood pressure was closely related to the plasma AVP concentrations. When this relationship was compared with that obtained after the i.v. infusion or injection of AVP, a marked shift to the left became apparent. Hence, sensitization to the vasopressor effect of AVP had occurred, the factor of sensitization amounting to more than 1,000. It is concluded that AVP is not only an antidiuretic hormone but also a vasopressor hormone, and that any systemic vasopressor effect of AVP requires a mechanism of sensitization.

Effect of the inhibitor of angiotensin I converting enzyme on endocrine function and renal perfusion in haemorrhagic shock

The present study was undertaken to investigate the effect of inhibition of angiotensin I converting enzyme on endocrine responses and renal perfusion in haemorrhagic shock. Plasma levels of vasopressin (VP), aldosterone, and plasma renin activity (PRA), and renal cortical tissue blood flow were measured both in control dogs and in those treated with an inhibitor of angiotensin I converting enzyme (SQ20881). The drug was administered with an intention of preventing vasoconstriction induced by angiotensin II and to improve tissue perfusion. No significant differences were found in plasma levels of vasopressin, aldosterone and plasma renin activity between control and SQ20881-treated groups. Secondary elevation of blood pressure following haemorrhage was significantly delayed and reduced by the administration of the inhibitor. Decrease in renal cortical tissue blood flow was observed in the SQ20881-treated group. It is suggested that angiotension II appears to play a role in spontaneous recovery of arterial blood pressure following haemorrhage. Furthermore, angiotensin II does not seem to play an important role in the stimulation of secretion of vasopressin in response to haemorrhage. Our data failed to demonstrate any favourable effects of inhibition of angiotension I converting enzyme on renal perfusion during haemorrhagic shock in dogs.

Effect of angiotensin-converting enzyme inhibitor (SQ 20881) on the plasma concentration of angiotensin I, angiotensin II, and arginine vasopressin in the dog during hemorrhagic shock

The effect of an angiotensin-converting enzyme inhibitor on the circulating levels of angiotensin I, angiotensin II, and arginine vasopressin was studied in dogs subjected to hypotensive hemorrhagic shock. In dogs subjected to hemorrhage but not given the inhibitor, angiotensin II rose 20-fold (from 69 to 1,343 pg/ml of plasma), whereas in dogs subjected to hemorrhage but pretreated with the inhibitor, angiotensin II rose only 2-fold (from 92 to 171 pg/ml of plasma). In the pretreated dogs angiotensin I rose 30-fold (from 108 to 3,232 pg/ml of plasma). There was no statistically significant difference between the vasopressin levels found in the untreated dogs and the levels found in dogs given the inhibitor (1,016 and 1,095 pg/ml of plasma). Of the 15 dogs in the untreated group, five died before retransfusion was completed (four of cardiac failure and one of cardiac arrhythmia); none of the 10 dogs in the inhibitor-treated group died. These observations suggest that the very high levels of angiotensin II observed following severe hemorrhage do not contribute significantly to the increased secretion of vasopressin and that the inhibitor protects against death, possibly by suppressing the very high blood levels of angiotensin II observed following this type of experimental hemorrhagic shock.

Heart failure in experimental refractory shock

Severe haemorrhagic shock was studied in a series of animal experiments. Reduction in myocardial oxygen and substrate utilisation in association with cardiac failure was demonstrated when the duration of shock was prolonged. The probable aetiological factors included ischaemia and the presence in the blood of certain vasoactive and metabolic depressant substances. It was concluded that the process leading to irreversible shock was multi-factorial in origin but failure appeared to play an important role.

The involvement of noradrenergic nerves in the cardiovascular reflex responses to lower body negative pressure in the anaesthetised rabbit

1. Resting cardiovascular status and the reflex responses to lower body negative pressure (LBNP) were examined in control rabbits and in rabbits sympathectomised with 6-hydroxydopamine (6-OHDA). 2. Resting systemic arterial pressure and total peripheral resistance were significantly higher in control than in 6-OHDA treated animals. 3. Control animals responded to LBNP at -25 mm Hg with a peripheral vasoconstriction and tachycardia. These responses were absent in 6-OHDA-treated animals. 4. Control animals responded to LBNP at -70 mm Hg with a peripheral vasoconstriction and a bradycardia followed by a tachycardia. There was no significant change in heart rate in response to this stimulus in 6-OHDA-treated animals, although a slight vasoconstriction was seen. 5. Bethanidine blocked the tachycardia and vasoconstriction seen in response to LBNP in control and 6-OHDA-treated rabbits. Under these conditions there was no significant difference between the total peripheral resistance of the control and treated animals. 6. The results indicated that neither the maintenance of the resting systemic arterial pressure nor the reflexes elicited by LBNP in control or in chronically sympathectomised rabbits involved hormonal mechanisms, but LBNP was maintained for 1 min only. These findings are discussed in the context of previous observations.

Conversion of angiotensin I to angiotensin II

The angiotensin I converting enzyme has two important functions: it inactivates bradykinin and converts angiotensin I to angiotensin II. Inhibition of the enzyme blocks the renin-angiotensin system and decreases systemic blood pressure if the pressure is maintained or increased by renin. The enzyme occurs in a variety of tissues and cell forms. The vascular endothelial cells of the lung and of peripheral blood vessels, and the epithelial cells of the kidney tubules are major sources of the enzyme. In addition to inactivating hypotensive peptides and activating a hypertensive one in the systemic circulation, the enzyme may affect organ functions by hydrolyzing peptides that are formed and released locally.