Interference by inhibitors of the renin-angiotensin system with neurogenic vasoconstriction

Neurohumoral interactions contributing to renal vasoconstriction and decreased renal blood flow in heart failure

In heart failure (HF), increases in renal sympathetic nerve activity (RSNA), renal norepinephrine spillover, and renin release cause renal vasoconstriction, which may contribute to the cardiorenal syndrome. To increase our understanding of the mechanisms causing renal vasoconstriction in HF, we investigated the interactions between the increased activity of the renal nerves and the renal release of norepinephrine and renin in an ovine pacing-induced model of HF compared with healthy sheep. In addition, we determined the level of renal angiotensin type-1 receptors and the renal vascular responsiveness to stimulation of the renal nerves and α1-adrenoceptors. In conscious sheep with mild HF (ejection fraction 35%–40%), renal blood flow (276 ± 13 to 185 ± 18 mL/min) and renal vascular conductance (3.8 ± 0.2 to 3.1 ± 0.2 mL·min−1·mmHg−1) were decreased compared with healthy sheep. There were increases in the burst frequency of RSNA (27%), renal norepinephrine spillover (377%), and plasma renin activity (141%), whereas the density of renal medullary angiotensin type-1 receptors decreased. In anesthetized sheep with HF, the renal vasoconstrictor responses to electrical stimulation of the renal nerves or to phenylephrine were attenuated. Irbesartan improved the responses to nerve stimulation, but not to phenylephrine, in HF and reduced the responses in normal sheep. In summary, in HF, the increases in renal norepinephrine spillover and plasma renin activity are augmented compared with the increase in RSNA. The vasoconstrictor effect of the increased renal norepinephrine and angiotensin II is offset by reduced levels of renal angiotensin type-1 receptors and reduced renal vasoconstrictor responsiveness to α1-adrenoceptor stimulation.

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.

In vivo pharmacological characterization of UP 269-6, a novel nonpeptide angiotensin II receptor antagonist

UP 269-6, 5-methyl-7-propyl-8(-)[2'-(1H-tetrazol-5-yl)biphenyl-4- yl)methyl]-1,2,4-triazolo]1,5-c]pyrimidin-2(3H)-one is a novel nonpeptide angiotensin II receptor antagonist. In vivo studies were performed to evaluate UP 269-6 for its angiotensin II antagonistic action. In pithed rats, i.v. administration of UP 269-6 (0.03-1 mg/kg) shifted dose dependently to the right the dose-pressor response curve for angiotensin II and decreased the maximum response. The angiotensin II antagonistic effect of UP 269-6 was as potent as that of L-158,809 (5,7-dimethyl-2-ethyl-3(-)[[2'- (1H-tetrazol-5-yl)biphenyl-4-yl]methyl]-imidazo[4,5-b]pyridine) and 10 times more potent than that of losartan. UP 269-6 antagonized the angiotensin II sympathetic-mediated tachycardiac response. UP 269-6 inhibited dose dependently the pressor response to angiotensin II with an ID50 of 4.5 micrograms/kg, i.v. in conscious normotensive dogs. Oral administration of UP 269-6 (0.1 to 30 mg/kg) resulted in a dose-dependent and long-lasting inhibition of the angiotensin II-induced pressor response in conscious normotensive rats and dogs. Compared to losartan, UP 269-6 presented a more rapid onset of action. UP 269-6 caused similar angiotensin II antagonistic effects in rats and dogs but the duration of the effect was greater in dogs than in rats. UP 269-6 did not alter the tachycardiac response to isoproterenol and the pressor response to vasopressin. UP 269-6 was demonstrated to be devoid of agonistic properties in rats and dogs. Furthermore, UP 269-6 did not induce hypotension and did not cause alteration in heart rate and ECG waveforms in dogs even at a dose 1000 times higher than the angiotensin II antagonistic effective dose. These results demonstrate that UP 269-6 is a potent and specific angiotensin II receptor antagonist and dose not possess agonistic properties.

ACE-inhibitors in coronary artery disease?

Angiotensin converting enzyme (ACE)-inhibitors are established in the treatment of arterial hypertension and heart failure. In recent years ACE-inhibitors have also been used in the treatment of patients with coronary artery disease (CAD), since from experimental data an antiischemic action of these agents is suggested. Antiischemic effects of ACE-inhibitors may be exerted through a reduction of myocardial oxygen demand, by a reduction of angiotensin-mediated coronary vasoconstriction, by an interaction with bradykinin and the prostaglandin system, by a modulation of endothelial control of vascular tone, and by an interaction with the sympathetic nervous system. However, clinical findings on potential beneficial effects of ACE-inhibitors in patients with CAD are inconsistent and controversial. While in hypertensive patients with CAD ACE-inhibitors generally seem to attenuate myocardial ischemia at rest and during exercise, a significant fraction of about 30% of normotensive patients with CAD does not benefit or even deteriorates. Lowering of coronary perfusion pressure and alteration of transmural blood flow distribution may be responsible for this. In patients with left ventricular dysfunction (SOLVD) or congestive heart failure (CONSENSUS, SOLVD) ACE-inhibitors have been proven to prevent progressive deterioration in left ventricular function and to reduce mortality. In patients with asymptomatic left ventricular dysfunction after myocardial infarction (SAVE), long-term administration of captropril was associated with an improvement in survival and reduced morbidity and mortality due to major cardiovascular events. Therefore, from a prognostic viewpoint patients with CAD and left ventricular dysfunction or congestive heart failure should be treated with ACE-inhibitors, although the clinical use of ACE-inhibitors in patients with ongoing angina pectoris may be limited by an aggravation of angina, presumably due to critically lowering coronary perfusion pressure. Finally, ACE-inhibitors failed to prevent restenosis after successful PTCA. In conclusion, from a prognostic viewpoint patients with CAD and congestive heart failure or left ventricular dysfunction should be treated with ACE-inhibitors. In hypertensive patients ACE-inhibitors generally seem to attenuate myocardial ischemia. In normotensive patients with CAD and angina pectoris but without left ventricular dysfunction ACE-inhibitors cannot generally be recommended at present, unless the patients, which may have benefit from ACE-inhibitor treatment can be better defined.

Interactions between angiotensin II and alpha-adrenoceptor agonists mediating pressor responses in the pithed rat

1. The aim of the study was to investigate the interactions between angiotensin II (AII) and adrenoceptor-mediated pressor responses in the pithed rat. Emphasis was placed on the effects of AII on blood pressure per se and the possibility of differential effects on alpha 1- and alpha 2-adrenoceptor-mediated pressor responses. 2. A low concentration of the angiotensin converting enzyme (ACE) inhibitor, teprotide (1 mg kg-1) lowered the resting diastolic blood pressure (BP) and attenuated only the second phase components of pressor responses to both alpha 1- and alpha 2-adrenoceptor agonists. Infusion of AII (50 ng kg-1 min-1) did not reverse the attenuating effect of teprotide and did not reliably restore the basal diastolic BP. 3. Although teprotide (10 mg kg-1) did not produce a greater fall in diastolic BP than did the low dose (1 mg kg-1), it attenuated the peak and second phase pressor responses to alpha 1- and alpha 2-adrenoceptor agonists but had no effect on pressor responses to AII or 5-hydroxytryptamine (5-HT). Infusion of AII reversed the effects of teprotide (10 mg kg-1) provided that rats were pretreated with flurbiprofen (5 mg kg-1), confirming that the depressor effects of the higher dose of teprotide are AII-dependent but that demonstration of this was complicated by products of cyclo-oxygenase. 4. The AII-receptor antagonist, saralasin (4 micrograms kg-1 min-1) attenuated alpha 1- and alpha 2-adrenoceptor-mediated pressor responses in a manner similar to that of teprotide (10 mg kg-1), suggesting that in this pithed rat model the alpha-adrenoceptor-mediated responses were selectively facilitated by endogenous AII. 5. Infusion of AII (50 ng kg-1 min-1) over a 60 min period did not produce a pressor response in the absence of other drugs but did facilitate pressor responses to alpha-adrenoceptor agonists. This confirms that AII can modulate alpha-adrenoceptor-mediated responses independently of basal blood pressure. 6. Overall the results indicate a facilitatory role for endogenous AII on alpha-adrenoceptor-mediated pressor responses. This is discussed in relation to the failure to demonstrate this convincingly under similar conditions on sympathetic nerve-mediated pressor responses.

Effects of captopril on sympathetic control of the heart and vasculature in dogs

Experiments were conducted in pentobarbital anesthetized dogs to investigate the effects of captopril on sympathetic neuronal control of the heart and hindlimb vasculature. Captopril, 3.1 mg/kg, i.v. produced marked reductions in blood pressure and hindlimb perfusion pressure, an observation consistent with the high plasma renin activity in the test animals. Increments in hindlimb perfusion pressure elicited by electrical stimulation of the lumbar sympathetic chain were also significantly reduced following captopril administration (p less than .002). The subsequent administration of a ten fold higher dose of captopril, 31.0 mg/kg, produced no further attenuation of the neurally mediated responses. In contrast to the decreased vascular responses to nerve stimulation after captopril, the tachycardia produced by stimulation of pre- or post-ganglionic neurons to the stellate ganglion were not altered. The results of the present study suggest that captopril acts by inhibiting vascular sympathetic neuronal function when the activity of the renin-angiotensin system is elevated. The attenuation of neurally mediated vasoconstriction may be due to the interruption of angiotensin II formation, thereby, preventing the facilitatory effects of angiotensin on sympathetic neurons.