Importance of the renin-angiotensin system in the regulation of arterial blood pressure in conscious mice and rats

AIM: The present experiments were designed to determine the mechanism(s) for increased sensitivity to blockade of the renin-angiotensin system in mice in comparison with rats. METHODS: Mice and rats, with indwelling femoral arterial and venous catheters, were chronically administered angiotensin II or pharmacological inhibitors of the renin-angiotensin system as sodium intake was altered. RESULTS: Increasing sodium intake led to suppression of circulating renin, angiotensin II, and aldosterone in rats and mice in the absence of alterations in arterial blood pressure. Additional experiments demonstrated that continuous intravenous infusion of angiotensin II (20 ng kg(-1) min(-1)) significantly increased arterial blood pressure by approximately 35 mmHg in conscious rats at all levels of sodium intake (n = 6). In contrast, arterial pressure was unaffected by angiotensin II infusion in conscious mice under conditions of low sodium intake, although arterial pressure was increased by 16 mmHg when mice were administered a high sodium intake while infused with angiotensin II (n = 6). In comparison, blockade of the endogenous renin-angiotensin system led to significantly greater effects on arterial pressure in mice than rats. Continuous infusion of captopril (30 microg kg(-1) min(-1)) or losartan (100 microg kg(-1) min(-1)) resulted in a 55-90% greater fall in blood pressure in conscious mice in comparison with conscious rats. CONCLUSION: The present studies indicate that arterial pressure in mice is more dependent upon the endogenous renin-angiotensin system than it is in rats, but mice are more resistant to the hypertensive effects of exogenous angiotensin II.     

Systemic effects of angiotensin III in conscious dogs during acute double blockade of the renin-angiotensin-aldosterone-system

Aims: The study was designed to determine (i) whether the effects of angiotensin III (AngIII) are similar to those of angiotensin II (AngII) at identical plasma concentrations and (ii) whether AngIII operates solely through AT₁‐ receptors. Methods: Angiotensin II (3 pmol kg^?1 min^?1–3.1 ng kg^?1 min^?1) or AngIII (15 pmol kg^?1 min^?1–14 ng kg^?1 min^?1) was infused i.v. during acute inhibition of angiotensin converting enzyme (enalaprilate; 2 mg kg^?1) and of aldosterone (canrenoate; 6 mg kg^?1 plus 1 mg kg^?1 h^?1). Arterial plasma concentrations of angiotensins were determined by radioimmunoassay using a cross‐reacting antibody to AngII. During ongoing peptide infusion, candesartan (2 mg kg^?1) was administered to block the AT₁‐receptors. Results: Angiotensin immunoactivity in plasma increased to 60 ± 10 pg mL^?1 during infusion of AngII or infusion of AngIII. AngII significantly increased mean arterial blood pressure (+14 ± 4 mmHg) and plasma aldosterone by 79% (+149 ± 17 pg mL^?1) and reduced plasma renin activity and sodium excretion (?41 ± 16 mIU L^?1 and ?46 ± 6 μmol min^?1 respectively). AngIII mimicked these effects and the magnitude of AngIII responses was statistically indistinguishable from those of AngII. All measured effects of both peptides were blocked by candesartan. Conclusion: At the present arterial plasma concentrations, AngIII is equipotent to AngII with regard to effects on blood pressure, aldosterone secretion and renal functions, and these AngIII effects are mediated through AT₁‐ receptors. The metabolic clearance rate of AngIII is five times that of AngII.     

Saline‐induced natriuresis and renal blood flow in conscious dogs: effects of sodium infusion rate and concentration

Aim: This study focused on static and dynamic changes in total renal blood flow (RBF) during volume expansion and tested whether a change in RBF characteristics is a necessary effector mechanism in saline‐induced natriuresis. Methods: The aortic flow subtraction technique was used to measure RBF continuously. Identical amounts of NaCl (2.4 mmol kg^?1) were given as slow isotonic (Iso, 120 min), slow hypertonic (Hyper, 120 min), and rapid isotonic loads (IsoRapid, 30 min). Results: During Iso and IsoRapid, arterial blood pressure increased slightly (6–7 mmHg), and during Hyper it remained unchanged. Iso and Hyper increased sodium excretion (4 ± 1 to 57 ± 27 and 10 ± 4 to 79 ± 28 μmol min^?1, respectively) and decreased plasma renin activity (by 38% and 29%), angiotensin II (by 56% and 58%) and aldosterone (by 47% and 65%), while RBF remained unchanged. IsoRapid caused a similar increase in sodium excretion (to 72 ± 19 μmol min^?1), a similar decrease in renin system activity, but a 15% elevation of RBF (282 ± 22 to 324 ± 35 mL min^?1). Selected frequency domain parameters of RBF autoregulation did not change in response to any load. Conclusions: In response to slow saline loading simulating daily sodium intake, the rate of sodium excretion may increase 10–20‐fold without any change in mean arterial blood pressure or in RBF. Regulatory responses to changes in total body NaCl levels appears, therefore, to be mediated primarily by neurohumoral mechanisms and may occur independent of changes in arterial pressure or RBF.     

Upregulation of the brain renin-angiotensin system in rats with chronic renal failure

Aim: We investigated how the brain renin–angiotensin system is involved in regulation of the sympathetic activity and arterial pressure in rats with chronic renal failure. Methods: Systolic arterial pressure, heart rate and diurnal urinary noradrenaline excretion were measured for 12 weeks in spontaneously hypertensive rats (SHR) with or without subtotal nephrectomy. Expression of mRNAs related to the brain renin–angiotensin system was measured using polymerase chain reaction. Effects of a 6‐day intracerebroventricular infusion of a type 1 angiotensin II receptor antagonist (candesartan) or bilateral dorsal rhizotomy on these variables were also investigated. Results: Systolic arterial pressure and urinary excretion of noradrenaline were consistently higher in subtotally nephrectomized SHR than in sham‐operated SHR (262 ± 5 vs. 220 ± 3 mmHg, P < 0.001; 2.71 ± 0.22 vs. 1.69 ±0.19 ng g^?1 body weight day^?1, P < 0.001). Expression of renin, angiotensin‐converting enzyme and type 1 angiotensin II receptor mRNAs in the hypothalamus and lower brainstem was greater in subtotally nephrectomized SHR than in sham‐operated SHR. Continuous intracerebroventricular infusion of candesartan attenuated hypertension and the increase in urinary noradrenaline excretion in subtotally nephrectomized SHR. Dorsal rhizotomy decreased arterial pressure, urinary excretion of noradrenaline and expression of renin–angiotensin system‐related mRNAs in brains of subtotally nephrectomized SHR. Conclusion: The brain renin–angiotensin system in subtotally nephrectomized SHR appears to be activated via afferent nerves from the remnant kidney, resulting in sympathetic overactivity and hypertension in this chronic renal failure model.     

Calciuresis elicited by spontaneous rehydration in dehydrated sheep

Cardiovascular and renal responses to a step-up infusion of endothelin-1 (ET-1) (1, 5, and 15 ng kg^-1 min^-1) were investigated in conscious dogs. In addition, the disappearance of ET-l in arterial and central venous plasma after an infusion of 10 ng kg^-1 min^-1 was quantified, and the effects of vasopressin (AVP, 10 ng kg^-1 min^-1) and angiotensin II (AII, 2, 5, and 10 ng kg^-1 min^-1) on plasma ET-1 were investigated. The step-up infusion of ET-1 increased the plasma level from 3.6 ± 0.3 to 243 ± 23 pg ml^-1. Concomitantly, arterial blood pressure increased and heart rate (HR) decreased dose-dependently. Diuresis, sodium, and potassium excretion did not change significantly. However, free water clearance increased during the infusion. Clearance of creatinine and excretion of urea decreased (39 ± 4 to 29 ± 3 ml min^-1 and 87 ± 16 to 71 ± 14 μmol min^-1, respectively). Decay curves for ET-1 in venous and arterial plasma were identical, and initial t_½ was 1.1 ± 0.1 min. Vasopressin increased arterial blood pressure (107 ± 4 to 136 ± 3 mmHg) beyond the infusion period and increased plasma ET-1 (85%). An equipressor dose of AII tended to decrease plasma ET-1. It is concluded that the lung is apparently not important in the removal of ET-1, that the disappearance of ET-1 follows a complex pattern, and vasopressin – in contrast to angiotensin II – is able to increase the plasma concentration of ET-1. The latter may suggest that ET-1 is involved in the prolonged pressor action of AVP observed.     

Effects of bradykinin and papaverine on renal autoregulation and renin release in the anaesthetized dog

The present study on six anaesthetized dogs investigates the influences of two different vasodilators, bradykinin and papaverine, on the relationship between autoregulation of renal blood flow and glomerular filtration rate, sodium excretion and renin release. At control conditions renal blood flow and glomerular filtration rate was autoregulated to the same levels of renal arterial pressure, 55 ± 3 and 58 ± 3 mmHg, respectively. Renin release increased from 0.3±0.1 to 22±4 μg AI min^-1, and sodium excretion decreased from 99 +29 to 4.6 ± 3.3 μmol min^-1 when renal arterial pressure was reduced from 122±6 to 44±2 mmHg. Infusion of bradykinin (50 ng kg^-1 min^-1) increased renal blood flow by 50% at control blood pressure without changing glomerular filtration rate, and both renal blood flow and glomerular filtration rate autoregulated to the same pressure levels as during control. Sodium excretion increased threefold at control renal arterial pressure, but was unchanged at low renal arterial pressure. Bradykinin did not change renin release neither at control nor low renal arterial pressure. Papaverine infusion at a rate of 4 mg min^-1 increased renal blood flow 50% without changing glomerular filtration rate. The lower pressure limits of renal blood flow and glomerular filtration rate autoregulation were increased to 94±6 and 93±6 mmHg, respectively. Sodium excretion increased sixfold at control renal arterial pressure and was still as high as the initial control values at low renal arterial pressure (97±27 μmol min^-1) accompanied by only a small increase in renin release (1.4±0.3 to 6±2 μg AI min^-1). We conclude that bradykinin does not influence autoregulatory pressure limits of renal blood flow and glomerular filtration rate nor the accompanying increase in renin release during reductions in renal arterial pressure. Papaverine on the other hand maintains high sodium chloride delivery to macula densa at low renal arterial ressure, suppressing renin release and impairing autoregulation through effects on the tubulo-glomerular feedback mechanism.     

Inhibitory effect of endothelin on renin release in dog kidneys

To investigate the effect of endothelin on renin release, experiments were performed in barbiturate-anaesthetized dogs with denervated kidneys. Intrarenal infusion of endothelin (1 ng min^-1kg^-1body wt) reduced renal blood flow (RBF) from 145 ± 10 ml min^-1to 98 ± 9 ml min^-1without altering renin release (1 ± 1 μg angiotensin I (AI) min^-1). Renin release was then increased either by renal arterial constriction or ureteral occlusion. When renal arterial pressure was reduced to 50 mmHg, renin release averaged 79 ± 20 μg AI min^-1in six dogs and fell significantly to 24 ± 6 μg AI min^-1during endothelin infusion. During ureteral occlusion the inhibitory effect of endothelin on renin release either during inhibition of β-adrenergic activity with propranolol or after inhibiting prostaglandin synthesis by indomethacin during intrarenal infusion of isoproterenol was examined. After propranolol administration ureteral occlusion increased renin release from 5 ± 2 μg AI min^-1to 38 ± 12 μg AI min^-1in six dogs. Subsequent intrarenal endothelin infusion (1 ng min^-1kg^-1body wt) during maintained ureteral occlusion reduced renin release to 10 ± 3 μg AI min^-1. In six other dogs prostaglandin synthesis was inhibited by indomethacin. Subsequent infusion of isoproterenol (0.2 μg min^-1kg^-1body wt) to stimulate β-adrenoceptor activity increased renin release from 13 ± 4 μg AI min^-1to 68 ± 8 μg AI min^-1during ureteral occlusion. Intrarenal endothelin infusion (1 ng min^-1kg^-1body wt) reduced renin release to 22 ± 3 μg AI min^-1during continuous isoproterenol infusion and ureteral occlusion. Hence endothelin inhibits renin release induced by renal arterial constriction or ureteral occlusion. Similar inhibitory effects whether renin release was raised by increasing prostaglandin synthesis or by stimulating β-adrenergic activity suggest a direct effect of endothelin on the juxtaglomerular cells.     

The effect of angiotensin AT1 receptor blockade in the brain on the maintenance of blood pressure during haemorrhage in sheep

The effect of systemic or intracerebroventricular (ICV) infusion of the angiotensin AT₁ receptor antagonist losartan on blood pressure during hypotensive haemorrhage was investigated in five conscious sheep. Mean arterial pressure (MAP) was measured during haemorrhage (15 mL kg^?1 body wt). Losartan (1 or 0.33 mg h^?1) was given to sheep by ICV, intravenous or intracarotid administration, beginning 60 min before and continuing during the haemorrhage. During control infusion of ICV artificial cerebrospinal fluid, MAP was maintained until 13.16 ± 0.84 mL kg^?1 blood loss, when a rapid reduction of at least 15 mmHg in arterial pressure occurred (the decompensation phase). ICV infusion of losartan at 1 mg h^?1 caused an early onset of the decompensation phase after only 9.8 ± 0.8 mL kg^{? 1} of blood loss compared with control. Intravenous infusion of losartan (1 mg h^?1) also caused an early onset (P < 0.05) of the decompensation phase at 10.2 ± 1.0 mL kg^?1 blood loss. This dose of losartan inhibited the pressor response to ICV angiotensin II, but not to intravenously administered angiotensin II, indicating that only central AT₁ receptors were blocked. Bilateral carotid arterial administration of losartan at 0.33 mg h^?1 caused an early onset of the decompensation phase during haemorrhage at 11.06 ± 0.91 mL kg^?1 blood loss (P < 0.05), which did not occur when infused by intravenous or ICV routes. The results indicate that an angiotensin AT₁-receptor-mediated mechanism is involved in the maintenance of MAP during haemorrhage in sheep. The locus of this mechanism appears to be the brain.     

Inefficiency of intracerebroventricular ANP to alter haemodynamic,plasma vasopressin and renin responses to haemorrhage in sheep

Whether intracerebroventricular (i.e.v.) infusion of atrial natriuretic peptide (human-ANP, 1–28) 25 pmol min^-1 influences the tolerance to blood loss and haemorrhage induced cardiovascular, vasopressin and renin responses were studied in five conscious sheep. The i.e. v. infusion was started 60 min prior to a slow (0.7 ml kg^-1 min^-1) venous haemorrhage, was run concurrently with bleeding, and for 90 min thereafter. Venous blood was removed until the mean systemic arterial pressure suddenly fell to about 50 mmHg. There were no statistically significant differences in either the bleeding volume necessary to induce the sudden decrease in blood pressure, or in cardiovascular parameters measured by venous heart thermodilution catheterization, compared with control experiments with i.e.v. infusion of artificial CSF. The plasma protein and vasopressin concentrations and renin activity were unaffected by the i.c.v. infusion of ANP as were the changes in these parameters occurring during the subsequent haemorrhage. The same negative findings were obtained with a three times higher dose of ANP(l-28) (75 pmol min^-1), tested in three of the animals. Thus the i.c.v. infusion of ANP(l-28), in amounts expected to elevate the CSF concentration far above basal levels does apparently not influence normal blood pressure regulation or alter haemodynamic, vasopressin and renin responses to haemorrhage in conscious sheep.     

Effects of angiotensin II on arterial pressure,renin and aldosterone during exercise

Summary To evaluate the effect of isotonic exercise on the response to angiotensin II, angiotensin II in saline solution was infused intravenously (7.5 ng · kg⁻¹ · min⁻¹) in seven normal sodium replete male volunteers before, during and after a graded uninterrupted exercise test on the bicycle ergometer until exhaustion. The subjects performed a similar exercise test on another day under randomized conditions when saline solution only was infused. At rest in recumbency angiotensin II infusion increased plasma angiotensin II from 17 to 162 pg · ml⁻¹ (P<0.001). When the tests with and without angiotensin II are compared, the difference in plasma angiotensin II throughout the experiment ranged from 86 to 145 pg · ml⁻¹. The difference in mean intra-arterial pressure averaged 17 mmHg at recumbent rest, 12 mmHg in the sitting position, 9 mmHg at 10% of peak work rate and declined progressively throughout the exercise test to become non-significant at the higher levels of activity. Plasma renin activity rose with increasing levels of activity but angiotensin II significantly reduced the increase. Plasma aldosterone, only measured at rest and at peak exercise, was higher during angiotensin II infusion; the difference in plasma aldosterone was significant at rest, but not at peak exercise. In conclusion, the exercise-induced elevation of angiotensin II does not appear to be an important factor in the increase of blood pressure. It is suggested that the vasodilating mechanisms in the working muscles and the vasoconstricting mechanisms in the non-working vascular beds are powerful and dominant during isotonic exercise and attenuate the opposing or additive vasoconstrictor effects of angiotensin II. The negative feedback effect of angiotensin II on renal renin secretion, however, is not inhibited by exercise.     

Influence of elevated renin substrate on angiotensin II and arterial blood pressure in conscious mice

The present experiments were performed to determine the influence of intravenous administration of renin substrate on plasma angiotensin II levels and mean arterial blood pressure in conscious C57BL/6J mice. Mice with chronic indwelling femoral arterial and venous catheters were acutely or chronically administered intravenous doses of a synthetic peptide corresponding to the 14 amino acids on the N-terminal of angiotensinogen. A dose-dependent increase in arterial blood pressure was observed as the intravenous bolus dose of the renin substrate was increased from 0.18 to 180 nmol kg(-1) with a maximal increase in pressure of 40 +/- 3 mmHg achieved following administration of the 18 nmol kg(-1) bolus (n = 11). Additional experiments demonstrated that a sustained intravenous infusion of the renin substrate led to a long-term increase in arterial blood pressure. The continuous infusion of renin substrate at 0.05 nmol kg(-1) min(-1) for 3 days did not alter arterial blood pressure from the control level of 119 +/- 5 mmHg (n = 5); however, arterial blood pressure significantly increased to 129 +/- 6 mmHg with an infusion rate of 0.5 nmol kg(-1) min(-1) and further increased to 141 +/- 3 mmHg when the renin substrate infusion was increased to 5.0 nmol kg(-1) min(-1). Finally, the infusion of renin substrate at 5.0 nmol kg(-1) min(-1) resulted in a significant increase in plasma angiotensin II concentration from 34 +/- 6 pg ml(-1) in vehicle-infused mice to 288 +/- 109 pg ml(-1). These results demonstrate that modulation of the circulating level of angiotensinogen can alter the plasma angiotensin II level and arterial blood pressure in normal animals.     

Effect on renin release of inhibiting renal nitric oxide synthesis in anaesthetized dogs

Nitric oxide plays an important role in the regulation of basal renal blood flow. This study was performed to examine whether selective inhibiti± of renal nitric oxide synthesis affects renin release in vivo. Accordingly, in six barbiturate-anaesthetized dogs renin release was examined before and after intrarenal infusion of the selective inhibitor of nitric oxide synthesis, N^G-nitro-l -arginine (NOARG). NOARG was infused into the renal artery to yield a renal arterial blood concentration of 0.4 μmol ml^-1. NOARG did not change systemic arterial blood pressure and glomerular filtration rate, but reduced basal renal blood flow by 26 ± 2%. Urine flow, sodium and potassium excretion were reduced after inhibition of renal nitric oxide synthesis. Basal renin release (3 ± 2 μg AI min^-1) was not altered by NOARG infusion (1 ± 1 μg AI min^-1). To stimulate renin release the renal artery was constricted to a renal perfusion pressure of 50 mmHg. At this perfusion pressure infusion of NOARG reduced renin release significantly from 48 ± 11 μg AI min^-1to 14 ± 4 μg AI min^-1. In conclusion, inhibition of renal nitric oxide synthesis reduces basal renal blood flow and reduces renin release stimulated by renal arterial constriction. These findings indicate that renal nitric oxide modulates both renal blood flow and renin release in vivo.     

Plasma Renin Activity Following Central Infusion of Angiotensin II and Altered CSF Sodium Concentration in the Conscious Goat

To study central influences on the renal release of renin, angiotensin II was infused into the lateral cerebral ventricle of conscious hydrated goats. CSF sodium concentration was increased or lowered by similar infusions of hypertonic NaCl or of isotonic fructose solution. Infusion of anglotensin II in doses from 0.5 to 1 μg caused a drop in plasma renin activity (PRA) and elicited a rise in blood pressure, antidiuresis, natriuresis, and thirst. Intraventricular infusion of hypertonic NaCl also suppressed PRA, induced anti-diuresis, natriuresis, and an inconsistent rise in blood pressure. Lowering of CSF [Na⁺] by infusion of isotonic fructose caused a rise in PRA and was followed by a water diuresis in the non-hydrated animal. The fructose infusions caused some decrease in renal K⁺ excretion but no consistent change in renal Na⁺ excretion. The results indicate that angiotensin II and changes in sodium balance modulate renal renin release also via the central nervous system.     

The progressive pressor response to angiotensin in the rabbit

1. The threshold for any detectable rise of systemic arterial pressure during the prolonged intravenous administration of angiotensin to conscious rabbits was observed to be an infusion rate of 0·003-0·006 μg.kg^-1.min^-1.

2. At infusion rates between threshold and 0·04 μg.kg^-1.min^-1 the systemic arterial pressure rose progressively over a 3- to 7-day period to a plateau.

3. On stopping the angiotensin infusion the blood pressure fell rapidly back to its base line much faster than it rose during the infusion. The time taken to reach control values was approximately related to the duration of the infusion.

4. At infusion rates of about 0·05 μg.kg^-1.min^-1 the full rise of blood pressure developed within a few minutes, and could be sustained without change for many days. At higher rates the blood pressure diminished with time.

5. Diurnal fluctuations of blood pressure were often seen during prolonged infusions of angiotensin at low rates; and more rapid fluctuations of blood pressure over an hour or two were frequently encountered immediately after an infusion was turned off.

6. The possible role of angiotensin in producing chronic renal hypertension is discussed in the light of these observations.

     

Angiotensin II infusion during β-adrenergic stimulation by isoproterenol. Effects on hepatic,splenic and cardiac blood volumes and on the magnitude and distribution of cardiac output in the dog

The cardiac and peripheral vascular adjustments to angiotensin II (0.1–0.2 μg kg^-1 min^-1 i.v.) during high β-adrenergic activity by a continuous isoproterenol infusion (0.2–0.3 μg kg^-1 min^-1 i.v.) were examined in anaesthetized, atropinized dogs. Hepatic, splenic and left ventricular (LV) volume changes were estimated by an ultrasonic-technique, and the blood flow distribution was measured by injecting radioactive microspheres and by electromagnetic flowmetry on the caval veins, the hepatic artery and the portal vein. During isoproterenol infusion, angiotensin II increased the systolic LV pressure by 45 ± 3 mmHg and the stroke volume by 17 ± 6 %. Concomitantly, the hepatic and splenic blood volumes declined by 29 ± 4 and 14 ± 6 ml, respectively, and the LV end-diastolic segment length increased by 3 ± 1 %. The flow through the inferior caval vein increased by 39 ± 9%, whereas the superior vena caval flow remained unchanged. The hepatic arterial flow more than doubled. Thus, at high inotropy by isoproterenol infusion, angiotensin II relocates blood from the liver and the spleen towards the heart. By activating the Frank-Starling mechanism, cardiac output is increased and conducted through the lower body, especially through the hepatic artery, because of the poor autoregulation of flow through this vessel.,     

Central AT1 and AT2 receptors mediate chronic intracerebroventricular angiotensin II‐induced drinking in rats fed high sodium chloride diet from weaning

Intracerebroventricular (ICV) angiotensin (AIl) administration stimulates central AII receptors to induce water consumption in rats. The aim of this study was to determine the role of brain AT₁ and AT₂ receptors in mediating chronic ICV AII‐induced drinking in rats raised on normal or high sodium chloride diets from weaning. Rats were weaned at 21 days of age and placed on normal or high sodium chloride diet for 10–12 weeks. At adulthood, the animals were instrumented with brain lateral ventricular cannulas and femoral arterial catheters. Low dose chronic central AII infusion (20 ng min^?1) significantly (P < 0.05) increased water intake in both groups of rats when compared with their respective controls of 24 h artificial cerebrospinal fluid infusions. In a separate group of high sodium fed rats, coinfusion of AII with the AT₁ receptor antagonist, losartan (0.25 μg min^?1) or the AT₂ receptor blocker, PD 123319 (0.50 μg min^?1) blocked chronic ICV AII‐induced drinking. Upon reinfusion of AII water intake increased above control. Following the cessation of AII infusions, water intake returned to values not significantly different from control (P > 0.05). In contrast, in the normal sodium fed rats losartan, but not PD 123319, blocked the AII‐mediated water intake. The data demonstrate that in high sodium chloride fed rats AII stimulates both central AT₁ and AT₂ receptors to induce drinking, while in the normal sodium chloride fed rats the peptide activates the drinking response primarily by stimulation of central AT₁ receptors.     

Effects of angiotensin-converting enzyme inhibition on arterial,venous and capillary functions in cat skeletal muscle in vivo

The aim of the present study was to analyse quantitatively, on a cat gastrocnemius muscle preparation in vivo, the effects of local angiotensin-converting enzyme (ACE) inhibition by enalaprilat on total regional vascular resistance (tone) and its distribution to the large-bore arterial resistance vessels (>25 μm), the small arterioles (<25 μm) and the veins. Associated effects on capillary pressure and fluid exchange were also studied. Close-arterial infusion of enalaprilat (0.05–0.20 mg kg muscle tissue min^-1) elicited a moderate dilator response in all three consecutive sections of the muscle vascular bed, an increase in capillary pressure and transcapillary fluid filtration. This dilation could be abolished by the selective bradykinin B₂-receptor antagonist Hoe 140 (2 mg kg^-1 min^-1, i.a.), indicating that the dilator mechanism of ACE inhibition was an increased local concentration of bradykinin, and hardly at all a decreased concentration of angiotensin (AT) II. The generalized dilator response to ACE inhibition along the vascular bed suggested a relatively uniform distribution of ACE from artery to vein and this was further supported by the finding that a close-arterial infusion of AT I (0.04–0.32 μg kg^-1 min^-1), which was vasoactive only after conversion to AT II by local ACE, elicited a generalized constrictor response in all three vascular sections. In contrast, infused AT II (0.01–0.16 μg kg^-1 min^-1) constricted almost selectively the large-bore arterial vessels. The specific angiotensin AT₁-receptor antagonist losartan (2 mg kg^-1 min^-1, i.a.) abolished the constrictor response to AT II but did not affect vascular tone under control conditions, indicating that AT II is not involved in the initiation of basal vascular tone in muscle. These results, taken together, indicate that under basal conditions vascular ACE contributes to the local control of vascular tone in skeletal muscle by degrading the endogenous dilator bradykinin, and not by converting AT I into vasoconstrictor AT II.     

Transmural distribution of biochemical markers of total protein and collagen synthesis,myocardial contraction speed and capillary density in the rat left ventricle in angiotensin ll-induced hypertension

The effect of angiotensin II-induced hypertension on selected biochemical parameters was studied in Sprague-Dawley rats. Angiotensin II infusion at rates of 41.7 μg h^-1 kg^-1 and 12.5 μg h^-1 kg^-1 for 2, 5, 10 and 15 days elevated the systolic blood pressure from 143 ± 7 mmHg to 215–230 mmHg (P < 0.001) and 185–195 mmHg (P < 0.001), respectively. The left ventricular weight/body weight ratio increased 10–14% (P < 0.05) and 23–32% (P < 0.001) after 2–15 days in rats treated at the lower and higher infusion rates, respectively. Prolyl 4-hydroxylase (PH) activity, a marker of collagen synthesis, was evenly distributed in the left ventricle. PH activity increased by about 100% in both subendocardial and subepicardial layers of the left ventricular wall after angiotensin II infusion for 10 days at 41.7 γ h^-1 kg^-1, but remained unaltered at 12.5 μg h^-1 kg^-1. No change was observed in hydroxyproline concentration. Myosin isoenzymes (V₁-V₃), which reflect myocardial contractility, were unevenly distributed in the left ventricular wall: the proportion of the fast-turnover isoenzyme (V₁) was smaller in the subendocardial layer than in the subepicardial layer. The proportion of V_l decreased after treatment in both layers. Alkaline phosphatase activity, a marker of capillary density, was evenly distributed transmurally in the left ventricular wall. Angiotensin II caused a slight decrease in this activity in both myocardial layers. The results suggest that the elevation of blood pressure leads to transmurally evenly distributed changes in biochemical parameters reflecting collagen synthesis, capillary density and contractile properties of the myocardium.     

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 μg 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 post-haemorrhage 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. During the spontaneous recovery after hypotensive blood loss, a vasoconstrictor effect of AVP is evident mainly during the initial 15 min, whereas at later stages angiotensin II appears to be of relatively greater importance.     

Long-lasting coronary vasoconstrictor effects and myocardial uptake of endothelin-1 in humans

The effect of intravenous administration of the endothelium-derived vasoconstrictor peptide endothelin-1 (ET-1 0.2, 1 and 8 pmol kg^?1 min^?1) on coronary blood flow in relation to plasma ET-1 as well as blood lactate and glucose levels were investigated in six healthy volunteers. Coronary sinus blood flow was measured by thermodilution. Administration of ET-1 elevated arterial plasma ET 35-fold, dose-dependently increased mean arterial blood pressure from 95±5 mmHg to 110±6 mmHg (P<0.01) and reduced heart rate from 64±4 beats min^?1 to 58±4 beats min^?1 (P<0.05) at 8 pmol kg^?1 min^?1. Coronary sinus blood flow was reduced maximally by 23±4% (P<0.01) and coronary vascular resistance increased by 48±11% (P<0.01). Coronary sinus oxygen saturation decreased from 35±1% to 22±2% at 2 min after the infusion (P<0.01). A coronary constrictor response was observed at a 4-fold elevation in plasma ET. The reduction in coronary sinus blood flow lasted 20 min and coronary sinus oxygen saturation was still reduced 60 min after the infusion. Myocardial oxygen uptake or arterial oxygen saturation were not affected by ET-1. Myocardial lactate net uptake decreased by 40% whereas glucose uptake was unaffected. At the highest infusion rate there was a net removal of plasma ET by 24±3% over the myocardium (P<0.05). The results show that ET-1 induces long-lasting reduction in coronary sinus blood flow via a direct coronary vasoconstrictor effect in healthy humans observable at a 4-fold elevation in plasma ET-1. Furthermore, there is a net removal of circulating ET-1 by the myocardium.