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

Neurocardiac regulation: from cardiac mechanisms to novel therapeutic approaches

Cardiac sympathetic overactivity is a well-established contributor to the progression of neurogenic hypertension and heart failure, yet the underlying pathophysiology remains unclear. Recent studies have highlighted the importance of acutely regulated cyclic nucleotides and their effectors in the control of intracellular calcium and exocytosis. Emerging evidence now suggests that a significant component of sympathetic overactivity and enhanced transmission may arise from impaired cyclic nucleotide signalling, resulting from compromised phosphodiesterase activity, as well as alterations in receptor-coupled G-protein activation. In this review, we address some of the key cellular and molecular pathways that contribute to sympathetic overactivity in hypertension and discuss their potential for therapeutic targeting.

Angiotensin-II type 1 receptor (AT1R) and alpha-1D adrenoceptor form a heterodimer during pregnancy-induced hypertension

Pregnancy courses with low response to angiotensin II and adrenergic agonists. In preeclampsia, both effects are reverted. It is known that angiotensin II regulates adrenergic system. It is not known, however, the interaction between both systems receptors. Our aim was to study if AT(1)R and alpha1D adrenoceptor heterodimerize in preeclampsia. We used subrenal aorctic coarctation in pregnant rats. Aortic tissues were prepared for confocal imaging and coimmunoprecipitated for alpha1D and AT(1) receptors. We found that AT(1)R and alpha1D adrenoceptor heterodimerize in both, healthy and preeclamptic groups. In healthy pregnant rats, heterodimer is barely detected. In preeclamptic rats however, we found higher heterodimerization. These results suggest that AT(1)R and alpha1D -adrenoceptor may form heterodimers, and may play a role in preeclampsia.

AT1-receptor blockade and sympathetic neurotransmission in cardiovascular disease

1. The present survey is dealing with the interactions between the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS) in various organs and tissues, with an emphasis on the angiotensin AT-receptors located at the sympathetic nerve endings. 2. Angiotensin II, the main effector of the RAAS is known to stimulate sympathetic nerve traffic and its sequelae in numerous organs and tissues, such as the central nervous system, the adrenal medulla, the sympathetic ganglia and the sympathetic nerve endings. These stimulatory effects are mediated by AT(1)-receptors and counteracted by AT(1)-receptor antagonists. 3. Sympatho-inhibition at the level of the sympathetic nerve ending appears to be a class effect of the AT(1)-receptor blockers, mediated by presynaptic AT(1)-receptors. With respect to the ratio pre-/postsynaptic AT(1)-receptor antagonism important quantitative differences between the various compounds were found. 4. Both the pre- and postjunctional receptors at the sympathetic nerve endings belong to the AT(1)-receptor population. However, the presynaptic receptors belong to the AT(1B)-subtype, whereas the postjunctional receptors probably belong to a different AT(1)-receptor subpopulation. 5. Sympatho-inhibition is a class effect of the AT(1)-receptor antagonists. In conditions in which the SNS plays a pathophysiological role, such as hypertension and congestive heart failure, this property may well be of therapeutic relevance.

Sympatho-inhibitory actions of irbesartan in pithed spontaneously hypertensive and Wistar-Kyoto rats

Angiotensin II (Ang II) can enhance sympathetic neurotransmission by acting on (AT1) receptors that are located on sympathetic nerve terminals. We investigated presynaptic blockade by the selective AT1-receptor antagonist irbesartan in pithed spontaneously hypertensive rats and normotensive Wistar-Kyoto rats (WKY). We compared the presynaptic inhibitory dose with that required for the blockade of AT1-receptors on vascular smooth muscle in both strains. To investigate blockade of presynaptic AT1-receptors, we studied the effect of irbesartan on the sequelae of electric stimulation of the thoraco-lumbar sympathetic outflow (0.25-8 Hz). To study the interaction between postsynaptic AT1-blockers and alpha-adrenoceptors, the effects of irbesartan on pressor responses to exogenous noradrenaline (NA) were established. Additionally, we studied the effect of irbesartan on dose-response curves for the vasoconstriction induced by exogenous Ang II. Pressor responses to electrical stimulation of thoracolumbar sympathetic neurones, to exogenous Ang II, as well as to (NA) were enhanced in spontaneously hypertensive rats (SHR) compared with WKY. The stimulation-induced rise in DBP could be dose-dependently reduced by irbesartan (0.3-10 mg/kg) in both SHR and WKY. The pIC50 values (doses which suppress the rise in DBP by 50% compared with control) were 5.60 +/- 0.09 and 5.72 +/- 0.08 for SHR and WKY, respectively (P > 0.05). In SHR, no effect of irbesartan (3 mg/kg) on pressor responses to exogenous NA was observed. In contrast, in WKY, irbesartan (3 mg/kg) caused a rightward shift of the dose-response curve to exogenous NA. Irbesartan (0.3-3 mg/kg) caused a depression of E(max) values and a rightward shift of the dose-response curves to exogenous Ang II in a similar fashion in both SHR and WKY. From these results we conclude that both in SHR and in WKY, Ang II exerts a facilitatory effect on sympathetic neurotransmission, which is mediated by prejunctional AT1-receptors in both strains. Irbesartan displays comparable sympatho-inhibitory potency in the normotensive and hypertensive pithed rat preparations. A facilitatory effect via postsynaptically located AT1-receptors on alpha-adrenoceptor-mediated responses exists in WKY, but not in SHR. In both strains the required dose to inhibit presynaptic effects is somewhat higher than the dose required to inhibit postsynaptic effects. No differences, therefore, seem to exist between the two strains regarding the affinity of irbesartan for pre- and postjunctional AT1-receptors, respectively.

Vasopressin-induced presynaptic facilitation of sympathetic neurotransmission in the pithed rat

OBJECTIVE

Several studies have shown that arginine vasopressin (AVP) potentiates the sympathetic nervous transmission in isolated vessels. The present study investigates such a potentiation in the pithed rat model.

METHODS

Male Wistar rats weighing 270-310 g were used. Spinal-cord stimulation was applied, with frequencies of 0.25-4 Hz, in the presence or absence of a subpressor dose of intravenous (i.v.) AVP (1 pmol/kg per min). In addition, the effect of AVP on postsynaptic alpha-adrenoceptor-mediated responses was studied using exogenously administered noradrenaline (NA). For this purpose dose-response curves (DRCs) for NA (i.v.) were constructed.

RESULTS

In the pithed rat model endogenously generated angiotensin II facilitates neurally mediated increments in vascular resistance. Without the administration of the angiotensin II type 1 (AT1) antagonist, irbesartan, the facilitating effect of AVP was not visible. However, after the administration of the AT1 antagonist, irbesartan, the facilitating effect of AVP became apparent. The stimulation-induced rise in diastolic blood pressure (DBP) was enhanced in the presence of AVP from 63.7 +/- 4.5 to 78.6 +/- 4.2 mmHg, at a stimulation frequency of 4 Hz. The vasopressin receptor V1 antagonist, SR-49059, completely inhibited this AVP-induced facilitation, whereas the V2 antagonist, SR-121463B, or the V2 agonist, desmopressin, did not. The DRC of exogenously administered NA was not influenced by AVP.

CONCLUSION

The stimulating effect of AVP on sympathetic neurotransmission is completely dependent on the stimulation of presynaptically located V1 receptors. The facilitating effect of angiotensin II on the sympathetic nervous system (SNS) in the pithed rat model masks the facilitating effect of AVP in this preparation.

Inhibition of facilitation of sympathetic neurotransmission and angiotensin II-induced pressor effects in the pithed rat: comparison between valsartan, candesartan, eprosartan and embusartan

BACKGROUND

In the pithed rat model, endogenously generated angiotensin (Ang) II can enhance sympathetic neurotransmission by acting on Ang II type 1 (AT1) receptors that are located on sympathetic nerve terminals.

OBJECTIVE

To compare the inhibitory potency of candesartan, valsartan, eprosartan and embusartan in blocking presynaptically and postsynaptically located AT1 receptors.

DESIGN

To investigate blockade of presynaptic AT1 receptors, we studied the effect of AT1 receptor blockade on the sequelae of electrical stimulation of the thoracolumbar sympathetic outflow (0.25-8 Hz). To investigate the interaction between postsynaptic AT1 blockers and alpha-adrenoceptors, the effects of these compounds on pressor responses to exogenous noradrenaline were determined. To investigate blockade of postsynaptic AT1 receptors, we studied the effect of the AT1 antagonists on dose-response curves elicited by exogenous Ang II.

RESULTS

The stimulation-induced increase in diastolic blood pressure (DBP) and the Ang II-elicited DBP response were dose-dependently reduced by all AT1 receptor blockers. Interestingly, the greatest doses of the AT1 antagonists caused less than maximal reduction in the stimulation-induced increase in DBP, resulting in a U-shaped dose-response relationship. To compare sympathoinhibitory potencies, the doses that, at 2 Hz, reduced the change in DBP by 20 mmHg (ED20 values, expressed as -log mol/kg) were calculated; they were 5.50 +/- 0.12, 5.77 +/- 0.10, 6.32 +/- 0.12 and 5.62 +/- 0.13 for valsartan, candesartan, eprosartan and embusartan, respectively. The order of potency, therefore, was eprosartan> valsartan = candesartan = embusartan (where > signifies P < 0.05). To compare the order of potency for inhibition of the Ang II-induced increase in DBP, we calculated pA2 values (the X intercept in Schild regression). They were 7.20 +/- 0.17, 8.01 +/- 0.01, 7.20 +/- 0.03 and 7.25 +/- 0.16, for valsartan, candesartan, eprosartan and embusartan, respectively. Accordingly, the order of potency for inhibition of the direct pressor effects of Ang II was candesartan> valsartan = eprosartan = embusartan (where > signifies P < 0.05).

CONCLUSION

In the pithed rat, the effects on DBP of stimulation of the thoracolumbar spinal cord are partly dependent on endogenously formed Ang II. These effects can be counteracted by blockade of presynaptically located AT1 receptors. No interaction was found between postsynaptically located AT1 receptors and alpha-adrenoceptors. The order of potency of the agents tested for sympathoinhibition clearly differed from that for inhibition of the direct pressor effects of Ang II. These findings suggest considerable differences in affinity of the various AT1 blockers for pre- and postsynaptic AT1 receptors.

Inhibition of angiotensin II-induced facilitation of sympathetic neurotransmission in the pithed rat: a comparison between losartan, irbesartan, telmisartan, and captopril

OBJECTIVE

Numerous studies have shown that angiotensin II enhances sympathetic nervous transmission. The objective of the present study was to quantify the inhibitory effect of the angiotensin II type 1 (AT1) receptor blockers losartan, irbesartan and telmisartan and the angiotensin converting enzyme (ACE) inhibitor captopril on sympathetic neurotransmission and to compare the potency of these agents both at the presynaptic and the postsynaptic levels.

DESIGN

In the male, normotensive pithed rat model, we studied the effect of losartan (1, 3, 10 and 30 mg/kg), irbesartan (3, 10, 30 and 60 mg/kg), telmisartan (0.3, 1, 3 and 10 mg/kg) and captopril (1.5, 5, 15 and 45 mg/kg) on electrical stimulation of the thoraco-lumbar spinal cord. To investigate the interaction between postsynaptic AT1-receptors and alpha-adrenoceptors, the effects of these compounds on pressor responses to exogenous noradrenaline were studied.

RESULTS

Stimulation of the thoracolumbar spinal cord caused a stimulation-frequency dependent rise in diastolic blood pressure (DBP) that could be dose-dependently reduced by both AT1 receptor blockade and ACE inhibition. Interestingly, the highest doses of the AT1 antagonists caused less than maximal reduction in the rise in DBP. This phenomenon was not observed after ACE inhibition by captopril. In experiments with exogenous noradrenaline, no effect of AT1 blockade or ACE inhibition on alpha-adrenoceptor-mediated blood pressure responses was seen.

CONCLUSION

We conclude that, in the pithed rat model, the effects of stimulation of the thoraco-lumbar spinal cord on DBP are counteracted by blockade of presynaptically located AT1 receptors. The order of potency concerning sympatico-inhibition is telmisartan > losartan > irbesartan. Regarding the inhibition of angiotensin II-induced facilitation of sympathetic neurotransmission, marked differences were observed between selective AT1 blockade and ACE inhibition. The finding that all three AT1 blockers cause less than maximal inhibition in their highest doses, as opposed to captopril, suggests that this is a class effect of the AT1 antagonists.

Disparate effects of thrombin receptor activating peptide on platelets and peripheral vasculature in rats

The hemodynamic and platelet effects of the thrombin receptor activating peptide SFLLRN (TRAP) were evaluated in rats. TRAP failed to aggregate rat platelets in vitro (platelet rich plasma) or in vivo in the pulmonary microcirculation. In contrast, TRAP aggregated washed human platelets. Intravenous injection of TRAP (1 mg/kg) in inactin-anesthetized rats produced a biphasic response in blood pressure characterized by an initial depressor response (-25 +/- 3 mmHg for 15-30 s) followed by a pronounced pressor response (50 +/- 7 mmHg for 2-3 min). This increase in blood pressure can be attributed to increases in total peripheral resistance since cardiac output remained unchanged. Further, only the pressor responses were observed in pithed rats suggesting a direct effect of TRAP in causing smooth muscle contraction. Consequently, rat platelets differ from human platelets in that they are resistant to TRAP whereas rat vasculature is highly sensitive to TRAP. These observations suggest that while the thrombin receptors on rat vasculature may be similar to those on human platelets, the receptors and/or the coupling mechanisms in rat platelets appear different from human platelets.

Pressor responses to the alpha 1-adrenoceptor agonist cirazoline: effects of captopril, phenoxybenzamine and nifedipine

We have examined the effects of captopril on pressor responses to the selective alpha 1-adrenoceptor agonist cirazoline in the pithed rat preparation following treatment with phenoxybenzamine and/or nifedipine. Pretreatment with captopril reduced the pressor responses to cirazoline and displaced the dose-response curve for this agonist to the right, significantly increasing the ED50 without altering the maximum response. Pretreatment with phenoxybenzamine accentuated the inhibitory actions of captopril and a combination of phenoxybenzamine and captopril significantly increased the ED50 without altering the maximum response. Administration of nifedipine in animals, which had already received phenoxybenzamine and captopril, led to a further displacement to the right of the cirazoline dose-response curve. The ED50 was found to be significantly increased and the maximum response was now significantly depressed. Captopril produced further additive inhibition with nifedipine and phenoxybenzamine of the vasoconstrictor effects of cirazoline. These data indicate, perhaps not surprisingly, that the cellular basis for the inhibitory effects of captopril is different from that of nifedipine and phenoxybenzamine, however, more importantly, that captopril may directly, or indirectly, inhibit receptor-operated cation channel mediated pressor responses.

Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.

Reduction in postsynaptic alpha 2-adrenoceptor activity by endogenous angiotensin III in the nucleus reticularis gigantocellularis of the rat

We investigated in adult, male Sprague-Dawley rats anesthetized with pentobarbital sodium the synaptic location of the interaction between endogenous angiotensin III (AIII) and the alpha 2-adrenoceptors in the medulla oblongata that are involved in cardiovascular regulation. The circulatory suppressant efficacy of a centrally acting alpha 2-adrenoceptor agonist, guanabenz, was used as the experimental index. Direct bilateral microinjection of AIII (40 pmol) into the nucleus reticularis gigantocellularis (NRGC), a medullary site believed to be intimately related to the cardiovascular inhibitory actions of guanabenz, attenuated, whereas the selective AIII receptor antagonist, Ile7-AIII (20 nmol), potentiated, the circulatory suppressant effects of guanabenz (100 micrograms/kg, i.v.). These two respective actions were essentially unaffected by immunocytochemically verified depletion of noradrenergic nerve terminals in the NRGC, elicited by a selective noradrenergic neurotoxin, DSP4. These data suggest that endogenous AIII may exert a tonic inhibitory action on the alpha 2-adrenoceptors located postsynaptically on neurons in the NRGC that are involved in central cardiovascular regulation.

Postjunctional alpha-adrenoceptors in the rabbit isolated distal saphenous artery: indirect sensitivity to prazosin of responses to noradrenaline mediated via postjunctional alpha 2-adrenoceptors

1. Under normal experimental conditions, the rabbit isolated distal saphenous artery appears to contain a homogeneous population of postjunctional alpha 1-adrenoceptors. Prazosin competitively antagonized responses to noradrenaline (NA) with a pA2 value of 8.6, while a relatively high concentration of rauwolscine (1 microM), produced only a 2 fold rightward displacement of the NA cumulative concentration-response curve (CCRC). 2. Despite the fact that angiotensin II (AII) was without effect on responses to NA or phenylephrine in this preparation, this peptide made responses to NA less susceptible to the antagonistic action of prazosin. This was particularly evident on the lower portion of the CCRC for NA. These results suggest that in the presence of AII, NA produces contractile responses by an action mediated through a prazosin-resistant adrenoceptor. 3. An attempt was made to isolate a homogeneous population of postjunctional alpha 2-adrenoceptors by use of a receptor protection procedure involving the combination of rauwolscine and phenoxybenzamine. After the protection protocol no responses were observed to the alpha-adrenoceptor agonists NA, phenylephrine or UK-14304. In the presence of angiotensin II however, concentration-dependent contractions were observed to each of these agonists. Under these conditions the rank order of potency, UK-14304 greater than NA greater than phenylephrine, is consistent with that of an effect mediated through postjunctional alpha 2-adrenoceptors. 4. The responses to NA, after the protection protocol, in the presence of AII, were susceptible to the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM), but resistant to the selective alpha 2-adrenoceptor antagonist prazosin (0.1 microM). Furthermore, the combination of rauwolscine (1 microM) and prazosin (0.1 I microM) was no more effective in blocking responses to NA than was rauwolscine (1 microM) alone. These results are consistent with the presence of a homogeneous population of postjunctional alpha 2-adrenoceptors. 5. Inducing a small degree of tone with a low concentration of the selective alpha 1-adrenoceptor agonist, phenylephrine, markedly increased the threshold sensitivity to the selective alpha 2-adrenoceptor agonist UK- 14304, in a manner analogous to that seen with All. 6. The results in the present study indicate that responses mediated via postjunctional alpha 2-adrenoceptors in the rabbit isolated distal saphenous artery are dependent upon a degree of vascular smooth muscle stimulation by some other receptor system. It is hypothesized that under normal experimental conditions, this function is fulfilled by stimulation of alpha l-adrenoceptors, while after alpha 1-adrenoceptor blockade the necessary positive influence can be provided by stimulation of All receptors. The implications for such an interaction between postjunctional alpha-adrenoceptor subtypes in demonstrating prazosin-resistant, rauwolscine- or yohimbine-sensitive responses in isolated blood vessels is discussed.

Evidence for prazosin-resistant, rauwolscine-sensitive alpha-adrenoceptors mediating contractions in the isolated vascular bed of the rat tail

1. The postjunctional alpha-adrenoceptors mediating contractions in the isolated vascular bed of the perfused rat tail have been investigated, in the presence and absence of an increase in perfusion pressure by arginine vasopressin (AVP). 2. In the absence of AVP, bolus doses of noradrenaline (NA) and phenylephrine produced pressor responses of similar time course, while UK-14,304 was practically inactive. Responses to noradrenaline were inhibited more by 0.05 microM prazosin than by 1 microM rauwolscine, suggesting the presence of alpha1-adrenoceptors. 3. Following a sustained elevation in perfusion pressure by AVP, both UK-14,304 and NA (the latter in the presence of 0.05 microM prazosin to inhibit alpha 1-adrenoceptors) elicited dose-dependent pressor responses. The maximum response to UK-14,304 under these conditions was approximately 30% of the maximum response to NA in the absence of prazosin and AVP. Responses to phenylephrine were not affected by the AVP-induced increase in vascular tone. 4. In the presence of AVP, pressor responses to UK-14,304 were resistant to 0.05 microM prazosin and susceptible to antagonism by 1 microM rauwolscine (-log Kb 7.65 +/- 0.15). Similarly, responses to NA in the presence of 0.05 microM prazosin and AVP were inhibited by 1 microM rauwolscine. This represents the first demonstration of prazosin-resistant, rauwolscine-sensitive alpha 2-adrenoceptor-mediated responses in the vasculature of the rat tail. 5. These results suggest that in isolated vascular preparations, functional populations of postjunctional alpha 2-adrenoceptors may be 'uncovered' by the presence of AVP.

Interactions between angiotensin II, sympathetic nerve-mediated pressor response and cyclo-oxygenase products in the pithed rat

1. The influence of angiotensin II (AII) on resting blood pressure and on sympathetic nerve-mediated pressor responses in the pithed rat was investigated either by inhibiting the renin-angiotensin system or by infusing AII. 2. Plasma AII levels in the pithed rat were approximately 20 fold higher than in normotensive rats. 3. Infusion of a subpressor dose of AII (50 ng kg-1 min-1) had no effect on sympathetic nerve mediated pressor responses but a pressor dose of AII, (200 ng kg-1 min-1) facilitated nerve-mediated pressor responses. 4. The angiotensin converting enzyme inhibitor, teprotide, and the AII-receptor antagonist, saralasin, lowered the diastolic blood pressure and attenuated sympathetic nerve-mediated pressor responses. There was no difference in the effects of teprotide at 1 mg kg-1 and 10 mg kg-1. Infusion of sodium nitroprusside at concentrations producing a fall in diastolic blood pressure of similar magnitude to that produced by teprotide and saralasin significantly attenuated nerve-mediated pressor responses. 5. After teprotide, AII 50 mg kg-1 min-1 increased diastolic blood pressure. The inhibitory effect of teprotide on nerve-mediated pressor responses was antagonized by this infusion of AII only if the rats were pretreated with the cyclo-oxygenase inhibitor, flurbiprofen. 6. It is concluded that AII is a major determinant of vascular tone in the pithed rat and that inhibition of the renin-angiotensin system attenuates sympathetic nerve-mediated pressor responses at least in part through the fall in blood pressure per se. The demonstration of this is complicated by an excessive release of vasodilator prostaglandins possibly due to the infused AII. Since plasma All levels are high, the effects of blockade of the renin-angiotensin system will be exaggerated and so the importance of All as a modulator of sympathetic responses will be overestimated in this model.