Involvement of the AT(2)-receptor in angiotensin II-induced facilitation of sympathetic neurotransmission

Angiotensin II (Ang II) causes facilitation of sympathetic neurotransmission via prejunctionally-located AT(1)-receptors. The pithed rat is a suitable model to study the interactions between endogenously produced Ang II and the sympathetic nervous system at the peripheral level. Previously, we demonstrated that inhibition of the facilitatory actions of Ang II is a class effect of all AT(1)-receptor blockers (ARB). However, all ARBs caused less than maximal inhibition after the highest dose, thus causing a U-shaped dose-response curve with respect to sympatho-inhibition. In the present study, we investigated whether the AT(2)-receptor is involved in this upturn of the dose-response relationship. Accordingly, we studied the effect of the ARB, irbesartan (1 60 mg/kg), on the sequelae of electric stimulation of the thoraco-lumbar sympathetic outflow in the presence and absence of the AT(2)-blocker, PD 123319 (0.5 mg/kg +50 g/kg/min). Additionally, the effect of the combined (non- selective) AT(1)/AT(2)-receptor antagonist saralasin (0.001, 0.003, 0.01 or 0.03 mg/kg/min), on stimulation-induced responses was studied. In addition, we measured PRA-levels after administration of irbesartan, in this model. The stimulation-induced increase in diastolic blood pressure (DBP) could be dose-dependently reduced by irbesartan. Co-infusion with PD 123319 increased the sympatho-inhibitory potency of irbesartan, possibly through displacement of irbesartan from plasma protein binding sites. The U-shaped dose-response relationship observed with irbesartan, which is illustrative for other ARBs in this model, was not observed when PD 123319 was co-administered with irbesartan, nor with the non-selective AT(1)/AT(2)-blocker, saralasin. PRA-levels increased from 111.0+17.8 to 198.7+22.2 ng/ml/hour after administration of irbesartan. PRA-levels did not differ when measured after the three highest doses of irbesartan.

CONCLUSIONS

The present findings indicate a facilitatory role for the AT(2)-receptor, which is unmasked by the highest dose of irbesartan. Different plasma Ang II-levels are unlikely to have caused the less than maximal inhibition after the highest dose of irbesartan.

Sympatholytic properties of several AT1-receptor antagonists in the isolated rabbit thoracic aorta

OBJECTIVE

To evaluate the facilitating effect of angiotensin II on sympathetic neurotransmission to quantitatively compare the sympatho-inhibitory potencies of the selective AT1 -receptor antagonists losartan, irbesartan and telmisartan in the isolated rabbit thoracic aorta.

DESIGN

To investigate the influence of pharmacological compounds on pre-junctional sympathetic transmission, the quantification of sympathetic transmitter release is the most straightforward approach.

METHODS

To investigate the sympatholytic properties of AT1 -blockers, we studied their effects on the enhancement by angiotensin II of electrical field stimulation (EFS)-evoked (2 Hz) sympathetic transmission in a modified spillover model.

RESULTS

Angiotensin II (0.01 nmol/l-0.1 micromol/l) caused a concentration-dependent enhancement of EFS-evoked noradrenaline release (control versus concentrations 0.1 nmol/l-0.1 micromol/l, P<0.05). The maximal augmentation, by almost 100%, was observed at a concentration of 1 nmol/l (FR2/FR1, 2.03 +/- 0.11 versus control, 0.99 +/- 0.03). Higher concentrations (up to 0.1 micromol/l) produced less than maximal facilitation. The AT1 -receptor antagonists losartan (0.1 nmol/l-0.1 micromol/l), telmisartan (0.01-10 nmol/l) and irbesartan (0.1 nmol/l-0.1 micromol/l) concentration dependently attenuated the angiotensin II-mediated (1 nmol/l) enhancement of EFS-evoked sympathetic outflow. The concentrations that reduced the enhancement by 50% (IC50 values, expressed as -log mol/l +/- SEM) were 9.05 +/- 0.16 losartan, 10.28 +/- 0.20 telmisartan and 9.20 +/- 0.23 irbesartan. Accordingly, the order of potency with respect to sympatho-inhibition proved telmisartan> irbesartan = losartan (where > signifies P<0.05).

CONCLUSIONS

The facilitating effect of angiotensin II on the sequelae of neuronal stimulation appears to be mediated by pre-synaptically located AT1 -receptors. Facilitation can be concentration dependently attenuated by AT1 -blockade. The order of potency with respect to sympatho-inhibition is telmisartan irbesartan = losartan. These differences may be explained by differences in affinity for the pre-synaptic AT1 -receptor.

A mitogen-activated protein kinase is involved in the inotropic but not chronotropic actions of adrenoceptor agonists and endothelin-1

The activation of mitogen-activated protein kinase (MAPK) pathways in the heart, for instance by alpha(1)-adrenoceptor agonists and endothelin-1, has primarily been associated with cellular growth regulation. Here we have investigated a possible role of MAPK pathways in the inotropic and chronotropic effects of adrenoceptor agonists and endothelin-1 in isolated rat left and right atria. Inotropic and chronotropic responses of the isolated atria to methoxamine, isoprenaline and endothelin-1 were measured in the absence and presence of inhibitors of MAPK pathways. The MAPK kinase (MKK(mek)) inhibitors PD98059 (100 microM) and U0126 (10 microM) significantly inhibited the inotropic responses to the alpha(1)-adrenoceptor agonist methoxamine (300 microM) and endothelin-1 (50 nM), but not the chronotropic responses to these agonists. U0126 but not PD98059 inhibited the inotropic response to 3 microM isoprenaline. None of the aforementioned inotropic and chronotropic effects were inhibited by the MAPKP(p38) inhibitor SB203580 (2 microM). We conclude that activation of the PD98059/U0126-sensitive MAPK pathway is essential for the inotropic but not chronotropic actions of adrenoceptor agonists and endothelin-1.

Prejunctional and postjunctional inhibitory actions of eprosartan and candesartan in the isolated rabbit mesenteric artery

Effects of angiotensin II type 1 (AT1) receptor antagonists eprosartan and candesartan and AT2 receptor antagonist PD123319 on Ang II-induced facilitation of noradrenergic neurotransmission were investigated in isolated rabbit mesenteric artery under isometric conditions. Sympathoinhibitory potency of AT1 blockers was compared with their potency concerning inhibition of direct vasoconstrictor effect of Ang II. To investigate blockade of presynaptic AT1 and AT2 receptors, effects of Ang II on electrical field stimulation (EFS)-induced contractions in presence or absence of eprosartan, candesartan, or PD123319 were studied. To investigate blockade of postsynaptic AT1 receptors, effects of either eprosartan or candesartan on concentration-response curves of Ang II were studied. In addition, effect of Ang II on postsynaptic alpha-adrenoceptor-mediated responses was studied using noradrenaline. EFS (1, 2, and 4 Hz) caused an increase of contractile force. At stimulation frequencies of 1, 2, and 4 Hz, a subpressor concentration of Ang II (0.5 nM) increased stimulation-induced vasoconstrictor responses by 2.8 +/- 0.5, 2.4 +/- 0.4, and 1.6 +/- 0.1 of control values, respectively (p < 0.05 compared with control for all frequencies). The enhancement could be antagonized by eprosartan (1 nM-0.1 microM) and candesartan (1 nM-0.1 microM). The AT2 antagonist PD123319 (10 nM) did not influence Ang II-induced facilitation of stimulation-induced contractions. Contractile responses to exogenous noradrenaline were unaltered in presence of Ang II 0.5 nM. Ang II (1 nM-0.3 microM) caused a concentration-dependent increase in contractile force, which could be antagonized by eprosartan (pD2; 8.8 +/- 0.19) and candesartan (pD2; 11.3 +/- 0.23). Thus, the facilitating effect of Ang II on noradrenergic neurotransmission is mediated by presynaptically located AT1 receptors and not by AT2 receptors. For eprosartan, sympathoinhibition was achieved at concentrations that also block AT1 receptors on vascular smooth muscle. In contrast, for candesartan, presynaptic inhibitory concentrations were considerably higher than those required for postsynaptic inhibition.

Sympatho-inhibitory properties of various AT1 receptor antagonists

It is well known that angiotensin II (Ang II) can facilitate the effects of sympathetic neurotransmission. In the present study, using various experimental models, we investigated the inhibitory effects of several Ang II subtype 1 receptor (AT1) antagonists on this Ang II-induced facilitation. We compared the sympatho-inhibitory potencies of the AT1 blockers with their respective potencies regarding inhibition of the direct vasoconstrictor effects of Ang II. In the isolated mesenteric artery, we investigated the effects of Ang II in the presence and absence of losartan, irbesartan and telmisartan on stimulation-induced vasoconstrictor responses. In the pithed rat, we studied the effect of AT1 blockade on the sequelae of electrical stimulation of the thoracolumbar sympathetic outflow (presynaptic AT1 blockade) as well as on dose-response curves elicited by exogenous Ang II (postsynaptic AT1 blockade). Additionally, we compared the sympatho-inhibitory of irbesartan in spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. In the isolated mesenteric artery, Ang II (10 nM) significantly enhanced stimulation-induced vasoconstrictor responses. The enhancement could be antagonized in a concentration-dependent manner by losartan (1 nmol/l to 1 mumol/l), irbesartan (0.1 nmol/l to 0.1 mumol/l) and telmisartan (0.01 nmol/l to 0.01 mumol/l). The sympatho-inhibitory potency was telmisartan > irbesartan > losartan. In the pithed normotensive rat, the stimulation-induced increase in diastolic blood pressure (DBP) as well as the Ang II-elicited DBP response were dose-dependently reduced by all the AT1 receptor blockers investigated. The order of potency with respect to sympatho-inhibition was eprosartan > valsartan = candesartan = embusartan = telmisartan > losartan > irbesartan (comparison of doses which at 2 Hz reduced delta DBP by 20 mmHg, differences significant at P < 0.05). The order of potency regarding inhibition of the Ang II-induced DBP increase was candesartan > embusartan = valsartan = eprosartan = telmisartan > irbesartan > losartan (comparison of the antagonist concentration, in the presence of which twice the agonist concentration, in the presence of which twice the agonist concentration is needed to cause the same effect [pA2 values], differences significant at P < 0.05). In the pithed SHR and the normotensive WKY rat the sympatho-inhibitory potency of irbesartan did not differ significantly between both strains. It can be concluded that all AT1 receptor antagonists appear to possess sympatho-inhibitory properties, which may be of potential interest in the treatment of hypertension and heart failure. Our findings suggest differences in pre- and postsynaptic inhibition between the various compounds, since for eprosartan and losartan the sympatholytic doses and postsynaptic inhibitory doses differed far less than for the other AT1 receptor antagonists.

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.

The effects of hypochlorite-induced oxidative stress on presynaptic M2-receptors at sympathetic nerve endings in the rat tail artery

1 It was shown recently that stimulation of cardiac muscarinic M2-receptors revealed an enhanced negative inotropic response in isolated rat left atria after exposure to hypochlorite-induced oxidative stress. This phenomenon was not observed after stimulation of the cardiac A1-receptor, which like the M2-receptor is coupled to Gi-proteins. Since even the contractile response to M3-receptor stimulation was not amplified in the rat portal vein, we hypothesized a M2-receptor specificity of this hypochlorite-induced enhancement. 2 The present study was performed in order to investigate whether the sympathoinhibitory response to presynaptically located M2-receptor stimulation would also be modified after exposure to hypochlorite in the rat tail artery. We applied electrical field stimulation (EFS) in order to mimic sympathetic neurotransmission. 3 EFS increased the vascular tone frequency-dependently (0.3-4 Hz). EFS-induced vasoconstriction could be attenuated by acetylcholine (30 nM-1 microM) in a concentration-dependent manner. Hypochlorite (10 and 100 microM) did not affect the sympathoinhibitory effect of acetylcholine (100 nM). 4 In conclusion, in contrast to cardiac M2-receptors, hypochlorite did not amplify the sympathoinhibitory effects of presynaptic M2-receptors. The different responsiveness between neuronal and cardiac M2-receptors to hypochlorite may be explained by the different G-protein subunits involved in the activation of the underlying signalling cascade.

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.

Vasomotor effects of arg-gly-asp (RGD) peptides are limited and not related to endothelium-derived hyperpolarizing factor-mediated relaxation in rat mesenteric arteries

1. In the present study we tested the effect of arg-gly-asp (RGD) peptides on vasomotor responses in rat isolated mesenteric arteries. More specifically, the hypothesis was tested that RGD interaction with integrins mediates relaxation attributed to endothelium-derived hyperpolarizing factor (EDHF). 2. The presence of the beta3 integrin subunit was shown by western blot analysis. To study its functional role, arteries (355 +/- 11 microm; n = 50) were mounted in a wire myograph set-up to measure isometric force generation. After blockade of nitric oxide synthesis with N(G)-nitro-L-arginine (0.1 mmol/L) and prostaglandin synthesis with indomethacin (10 micromol/L), methacholine (10 micromol/L) induced a transient relaxation within 1 min of 72 +/- 4.0% (as percentage of precontraction with phenylephrine; n = 27). 3. These responses were inhibited by a 60 mmol/L potassium buffer (18 +/- 6.0%; n = 6) or endothelium denudation (12 +/- 3.2%; n = 7), consistent with EDHF. 4. A function-blocking monoclonal antibody against the integrin beta3 chain did not affect relaxation. 5. The RGD peptides gly-arg-gly-asp-thr-pro (GRGDTP), gly-arg-gly-asp-ser (GRGDS) and cyclic RGD, ligands for the RGD binding site of integrins, also did not affect relaxation induced by methacholine. 6. Cyclic RGD increased contraction from 91 +/- 3 to 98 +/- 3% (as percentage of 120 mmol/L potassium). 7. In conclusion, these data show that vasomotor responses related to integrins are small and not involved in hyperpolarization attributed to EDHF in rat mesenteric artery.

Characterisation of a modified approach to the study of sympathetic neurotransmission and its presynaptic modulation in the isolated rabbit thoracic aorta

INTRODUCTION

The quantification of [(3)H]noradrenaline spillover from electrically stimulated, sympathetic nerves is a widely used method to study presynaptic effects of hormones, transmitters and drugs. Although a straightforward approach, the execution of the experiments is not trivial. This holds true mainly for a reliable control of the experimental conditions, a major pitfall of the commonly used superfusion setup, and problems concerning the sampling of the tritium containing probes.

METHODS

These difficulties prompted us to develop a variant of this method, which is based on a classical organ bath setup. Rabbit thoracic aortic rings were incubated with [(3)H]-labeled noradrenaline. Instead of being constantly washed away by superfusion, the [(3)H]noradrenaline is allowed to accumulate in the medium.

RESULTS

Electrical field stimulation (EFS) (2 Hz, 3 ms, 150 mA) caused a significant increase of [(3)H]noradrenaline outflow by approximately a factor 4.2 (P<.05). The fractional release of noradrenaline during consecutive periods of stimulation remained unaltered (FR(2)/FR(1) 0.99+/-0.03). The EFS-evoked release could be nearly abolished by the selective sodium channel blocker tetrodotoxin (1 microM) (FR(2)/FR(1) 0.06+/-0.03, P<.05). The N-type calcium antagonist omega-conotoxin GVIA (0.3 microM) abolished the stimulation-evoked outflow (FR(2)/FR(1) 0.01+/-0.06, P<.05), whereas the antisympathotonic agent guanethidine (10 microM) attenuated the EFS-evoked noradrenaline outflow by approximately a factor 2 (FR(2)/FR(1) 0.46+/-0.07, P<.05). Angiotensin II (0.1 and 1 nM) enhanced the EFS-evoked [(3)H]noradrenaline outflow by nearly a factor 1.5 and 2, respectively (FR(2)/FR(1) of 1.43+/-0.11 (0.1 nM) and 2.03+/-0.11 (1 nM); n=6-8, P<.05). All agents failed to influence basal outflow.

DISCUSSION

Our modified experimental approach appears to be suitable to study presynaptic influences on sympathetic transmission in the rabbit thoracic aorta. In addition to optimal control of the experimental conditions, the method offers the advantage of a safe and reliable sampling.

Oxidative stress impairs the haemodynamic activity of cardiovascular agents with antioxidant properties

Reactive oxygen species (ROS) are known to be involved in the pathogenesis and progression of various cardiovascular diseases. For some therapeutics like carvedilol and captopril used in the treatment of such diseases antioxidant properties have been proposed to play a role in addition to their haemodynamic activities. It was the aim of the present study to assess whether ROS may affect the molecular integrity and the primary pharmacological actions of compounds with additional antioxidant properties. Accordingly, well-known drugs as mentioned were exposed to ROS, generated by electrolysis and analyzed by means of functional and chemical investigations. For this purpose rat thoracic aortic rings were incubated with either the beta1,2/alpha1-adrenoceptor antagonist carvedilol (100 nM), the alpha1-adrenoceptor antagonist prazosin (5 nM), the thiol-containing ACE-inhibitor captopril (3 microM) or lisinopril (300 nM), an ACE-inhibitor without a thiol moiety. Furthermore, isolated rat left atria were incubated with either carvedilol (14 nM) or with the beta1,2-adrenoceptor antagonist timolol (50 nM). After an incubation period of 15 min, electrolysis was applied to the buffer medium in order to generate ROS. After an additional 15 min, concentration-response curves were constructed for angiotensin I and phenylephrine in thoracic aortic rings incubated with the ACE-inhibitors and the alpha1-adrenoceptor antagonists, respectively. In addition, concentration-response curves were constructed for isoprenaline in presence of the beta1,2-adrenoceptor antagonists in isolated left atria. After exposure to oxidative stress the alpha1- and beta-adrenoceptor blocking activity of carvedilol was significantly impaired, when compared to control conditions. In contrast, the pharmacological effects of prazosin and timolol remained unaffected. The ACE-inhibition by captopril was completely abolished after electrolysis, while the pharmacological action of lisinopril was only slightly reduced. In addition, a complete oxidative degradation of captopril and carvedilol could be demonstrated by using UV/Vis spectroscopy and HPLC/fluorospectroscopy, respectively. From these results we conclude that the haemodynamic therapeutics with additional radical scavenging properties may undergo a chemical modification due to ROS-exposure which results in a loss of pharmacological activity.

Reactive oxygen species potentiate the negative inotropic effect of cardiac M2-muscarinic receptor stimulation

The aim of the present study was to investigate the influence of reactive oxygen species (ROS) on the contractile responses of rat isolated left atria to muscarinic receptor stimulation. ROS were generated by means of electrolysis (30 mA, 75 s) of the organ bath fluid. Twenty minutes after the electrolysis period, the electrically paced atria (3 Hz) were stimulated with the adenylyl cyclase activator forskolin (1 microM). Subsequently, cumulative acetylcholine concentration-response curves were constructed (0.01 nM-10 microM). In addition, phosphoinositide turnover and adenylyl cyclase activity under basal and stimulated conditions were measured. For these biochemical experiments we used the stable acetylcholine analogue carbachol. The atria exposed to reactive oxygen species were influenced more potently (pD2 control: 6.2 vs. 7.1 for electrolysis-treated atria, P<0.05) and more effectively (Emax control: 40% vs. 90% reduction of the initial amplitude, P<0.05) by acetylcholine. In contrast, ROS exposure did not alter the responses to adenosine, whose receptor is also coupled via a Gi-protein to adenylyl cyclase. The basal (40% vs. control, P<0.05) as well as the carbachol-stimulated (-85% vs. control, P<0.05) inositol-phosphate formation was reduced in atria exposed to ROS. The forskolin-stimulated adenylyl cyclase activity was identical in both groups but carbachol stimulation induced a more pronounced reduction in adenylyl cyclase activity in the electrolysis-treated atria. Accordingly we may conclude that ROS enhance the negative inotropic response of isolated rat atria to acetylcholine by both a reduction of the positive (inositide turnover) and increase of the negative (adenylyl cyclase inhibition) inotropic components of cardiac muscarinic receptor stimulation. This phenomenon is most likely M2-receptor specific, since the negative inotropic response to adenosine is unaltered by ROS exposure.

Effect of the AT1-receptor antagonists losartan, irbesartan, and telmisartan on angiotensin II-induced facilitation of sympathetic neurotransmission in the rat mesenteric artery

SUMMARY

The effect of the AT1-receptor antagonists losartan, irbesartan, and telmisartan on angiotensin II (Ang II)-induced facilitation of noradrenergic neurotransmission was investigated in the isolated rat mesenteric artery under isometric conditions. Electrical field stimulation (2, 4, and 8 Hz) caused a frequency-dependent increase of contractile force. At stimulation frequencies of 2, 4, and 8 Hz, Ang 11 (10 nM) increased the stimulation-induced vasoconstrictor responses by a factor 4.8 +/- 0.9, 2.9 +/- 0.7, and 1.3 +/- 0.1, respectively (p < 0.05 compared with control for all frequencies). The enhancement could be concentration-dependently antagonized by losartan (1 nM-1 microM), irbesartan (0.1 nM-0.1 microM), and telmisartan (0.01 nM-0.01 microM). At a stimulation frequency of 2 Hz, the relation between stimulation-induced vasoconstrictor responses (in presence of Ang II 10 nM) and the concentration of the AT1-antagonists used could be described by linear regression. The order of potency concerning sympathoinhibition was telmisartan > irbesartan > losartan (p < 0.05 between linear regression lines). Contractile responses to exogenous noradrenaline were unaltered in the presence of Ang II 10 nM. We conclude that the facilitating effect of Ang II on noradrenergic neurotransmission is mediated by presynaptically located AT1-receptors. Conversely, this facilitating effect can be dose-dependently counteracted by blockade of these receptors. Sympathoinhibitory properties are likely to contribute to the therapeutic effect of AT1-blockers, in particular in conditions in which the sympathetic nervous system is activated, such as congestive heart failure and hypertension.

Different types of antagonism by losartan and irbesartan on the effects of angiotensin II and its degradation products in rabbit arteries

A previous study by our group has demonstrated that the selective AT1-receptor antagonist losartan behaves as a noncompetitive antagonist in rabbit isolated renal artery (RA). In the present investigation, the influence of losartan and irbesartan on the contractile effects of angiotensin II (AII) and its degradation products angiotensin III (AIII) and angiotensin IV (AIV) was determined in the rabbit isolated RA and femoral artery (FA). The arteries were set up in organ chambers and changes in isometric force were recorded. In both rabbit isolated RA and FA preparations, AII, AIII and AIV elicited significant contractile responses with a similar efficacy. These effects were impaired by the presence of functional endothelium in RA preparations but not in FA preparations. In both preparations studied, the effects of AII, AIII and AIV were influenced neither by the aminopeptidase-A and -M inhibitor amastatin (10 microM), nor by the aminopeptidase-B and -M inhibitor bestatin (10 microM). In endothelium-denuded FA preparations, preincubation with losartan (3-300 nM) antagonized AII-, AIII- and AIV-induced contractions in a competitive manner. However, in endothelium-denuded RA preparations, losartan depressed the maximal contractile responses induced by AII but not those induced by AIII and AIV. In the same preparations, preincubation of another selective AT1-receptor antagonist irbesartan (3-30 nM) concentration-dependently shifted AII and AIII curves to the right in an insurmountable manner. The reduction of the maximal response of AII is more potent when compared to that of AIII (47.7 +/- 1.51% vs. 66.7 +/- 1.88%, percentage of the initial maximal response; P < 0.05; n=5). The selective AT2-receptor antagonist PD123177 (1 microM) did not influence the responses to all three peptides in both RA and FA preparations. These heterogeneous antagonistic effects of the two AT1-receptor antagonists studied with respect to the contractile actions of AII, AIII and AIV suggest the possible existence of multiple, functionally relevant AT1-receptor subtypes in rabbit RA preparations.

Influence of losartan and nicardipine on the contractile responses of human subcutaneous arteries and veins to angiotensin II

In the human forearm vascular bed, the arterial constrictor effects of angiotensin II were found to be caused by an AT1-receptor mediated calcium influx, while the venous constrictor effects appeared to be independent of L-type calcium channels. In this study, we investigated the influences of the AT1-receptor antagonist losartan and the calcium channel blocker nicardipine on the angiotensin II-induced constriction of small isolated subcutaneous arteries and veins obtained from human mammary tissue. Subcutaneous arteries and veins were isolated from mammary tissue from 9 healthy women who underwent breast reduction surgery. Effects of angiotensin II (0.3 nM to 1 mM), losartan (0.1 mM) and nicardipine (0.1 mM) were investigated in a myograph set up. Identification of arteries and veins was confirmed histologically after the experiments. Drug effects were expressed relatively to the potassium-induced contraction. Angiotensin II concentration-dependently contracted arteries and veins by maximally 1.66 +/- 0.31 N/m and 0.43 +/- 0.08 N/m, respectively (P < 0.05). In arteries the angiotensin II were subject to a mild degree of tachyphylaxis: the Emax of the repetitive concentration-response curve (CRC) was reduced from 105 +/- 4% of the potassium-induced contraction to 84 +/- 6% (P < 0.05); the EC50 value was unchanged (P > 0.05). In veins no tachyphylaxis was observed. Losartan caused a rightward shift of the CRC of angiotensin II in arteries and veins (P < 0.05), and reduced the Emax in arteries from 105 +/- 4 to 85 +/- 9% (P < 0.05), but did not change the Emax in veins. Nicardipine significantly decreased the Emax in arteries and veins (to residual values of 10 +/- 2 and 20 +/- 4%, respectively, of the control values). In conclusion, the angiotensin II-induced constriction of human arteries and veins isolated from mammary tissue are AT1-receptor mediated and inhibited by losartan. The nearly complete inhibition by nicardipine indicates that the constrictor effects in both types of vessels are dependent on L-type calcium channels.

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.

The influence of chronic inhibition of nitric oxide synthesis on contractile and relaxant properties of rat carotid and mesenteric arteries

Balloon denudation of the rat carotid artery leads to an immediate decrease in beta-adrenoceptor-medi-ated vasodilator response. However, this arterial function becomes significantly enhanced during subsequent formation of neointima with the endothelial cell lining still being absent. It was therefore hypothesized that chronic suppression of endothelium-dependent nitric oxide (NO) synthesis may eventually upregulate the beta-adrenoceptor system on vascular smooth muscle. To investigate this hypothesis, male Wistar rats were treated chronically with the L-arginine analogue NG-nitro-L-arginine methyl ester (L-NAME) to inhibit the synthesis of NO (i.e. 15 mg/kg per day or 0.06 mmol/kg per day for 6 weeks p.o.). Prior to the experiments the mean arterial blood pressure (MAP) was significantly elevated in the L-NAME-treated rats (i.e. 128.4+/-3.4 mmHg vs. 100.0+/-2.9 mmHg, L-NAME vs. control, n=4, P<0.05). The functional properties of the isolated vessel preparations were established by isometric force measurement in a myograph setup, in the absence of L-NAME. The maximal contractile responses to high potassium chloride solution (100 mM), to the alpha1-adrenoceptor agonist phenylephrine, and to the thromboxane A2-agonist U46619, were not influenced by chronic L-NAME-treatment in the carotid and mesenteric artery preparations. The vasodilator responses to the cholinergic agonist methacholine were significantly impaired in the carotid arteries of L-NAME-treated animals: 30.9+/-7.9% vs. 64.6+/-2.0%, P<0.05, L-NAME vs. control, n=10. However, these responses appeared not to be modulated in the mesenteric artery preparations after chronic L-NAME treatment. Separate experiments showed that these responses could be blocked both in the rat carotid and mesenteric artery with L-NAME (10 mM) in vitro. Addition of the NO synthase substrate L-arginine could partially but significantly reverse this blockade. Chronic inhibition of NO synthesis caused significant deterioration of beta-adrenoceptor-mediated vasodilator responses. For carotid arteries: Emax=36.1+/-9.4% vs. 65.9+/-6.0%, P<0.05, L-NAME vs. control, n=5; and pD2=6.7+/-0.2 and 7.4+/-0.1, respectively, P<0.05, n=5. For mesenteric arteries: pD2=7.7+/-0.0 and 8.0+/-0.0, respectively, P<0.05, n=5. From these data, it may be concluded that chronic L-NAME treatment results in a stable impairment of the endothelium-dependent NO system in the rat carotid but not mesenteric arteries. The stated hypothesis fails as the beta-adrenoceptor-induced vasorelaxation of carotid and mesenteric arteries became significantly impaired, rather than enhanced. Taken together, the beta-adrenoceptor function in the rat carotid artery is apparently more dependent on endothelial NO synthesis than that in the rat mesenteric artery.

The differential time courses of the vasodilator effects of various 1,4-dihydropyridines in isolated human small arteries are correlated to their lipophilicity

OBJECTIVES

To investigate a possible relationship between the time courses of action of various calcium antagonists and their lipophilicity, characterized as log P-values.

METHODS

The functional experiments were performed in vitro in human small subcutaneous arteries (internal diameter 591 +/- 51 microm, n = 7 for each concentration), obtained from cosmetic surgery (mamma reduction and abdominoplasty). The vessels were investigated in an isometric wire myograph. The vasodilator effect of the calcium antagonists was quantified by means of log IC50-values, and the onset of the vasodilator effect for each concentration studied was expressed as time to Eeq90-values (time to reach 90% of the maximal effect).

RESULTS

Log IC50-values were -8.46 +/- 0.09, -8.33 +/- 0.25 and -8.72 +/- 0.16 for nifedipine, felodipine and (S)-lercanidipine, respectively (not significant). On average, nifedipine reached time to Eeq90 in 11 +/- 1 min. For felodipine and (S)-lercanidipine the corresponding values were 60 +/- 11 min and 99 +/- 9 min, respectively. The differences between these values were statistically significant (P< 0.01). In spite of these differences in the in-vitro human vascular model, the three calcium antagonists are equipotent with regard to their vasodilator effects. Linear regression analysis of the correlation between the logarithm of the membrane partition coefficient (log P-values) of the calcium antagonists tested [2.50, 4.46 and 6.88 for nifedipine, felodipine and (S)-lercanidipine, respectively] and their respective values found for time to Eeq90 was highly significant.

CONCLUSIONS

It appears that a higher log P-value is correlated with a slower onset of action.

Beta-adrenergic responses are significantly enhanced in rat carotid artery with intimal hyperplasia

The objective of the present study was to investigate the influence of balloon injury and subsequent neointima formation in the rat carotid artery on the beta-adrenoceptor function. Rat left common carotid artery was subjected to balloon injury with an arterial embolectomy catheter; the contralateral artery was sham-operated. Immediately, and at 2, 8 and 16 weeks post-injury, both the injured and the sham-operated carotid arteries were isolated and mounted in an isometric wire-myograph set-up. Subsequently, concentration-response curves (CRCs) were constructed for the beta-adrenoceptor agonist isoprenaline after precontraction with the thromboxane A2 (TP)-receptor agonist U46619 (30 nM) of the injured and sham-operated artery preparations. To evaluate the involvement of the beta1- and the beta2-adrenoceptor subtypes, CRCs were constructed in the presence of CGP 20712A (0.1 nM, a beta1-adrenoceptor-selective antagonist) and ICI 118,551 (10 nM, a beta2-adrenoceptor-selective antagonist). L-NAME (100 microM) and indomethacine (10 microM) were used to evaluate the influence of nitric oxide (NO) or prostanoids, respectively. Immediately post-injury, isoprenaline-induced vasorelaxation was impaired in the injured carotid artery preparations: Emax=19.6 +/- 2.2% vs. 64.0 +/- 4.6%, injured vs. sham, n=8, P<0.05. However, from 2 weeks post-injury onwards, this response appeared enhanced in the injured preparations: Emax, 2 weeks= 86.4 +/- 2.2% vs. 49.7 +/- 5.7%, injured vs. sham, n=5, P<0.05. In addition, the sensitivity for isoprenaline was increased in these preparations: pD2, 2 weeks=7.48 +/- 0.08 vs. 6.88 +/- 0.10, injured vs. sham, n=5, P<0.05. The beta-adrenoceptor population in both types of preparations consisted mainly of the beta2-adrenoceptor subtype, although at 8 and 16 weeks post-injury, the beta1-adrenoceptor subtype appeared to be present as well in the injured artery preparations. Inhibition of NO synthesis led to significant decreases of beta-adrenoceptor-mediated vasorelaxation both in injured and in sham-operated artery preparations for all time points, except at 16 weeks. Cyclo-oxygenase inhibition had no influence on isoprenaline-induced vasorelaxation in injured and sham-operated preparations. From this, it is concluded that beta-adrenoceptor-mediated vasorelaxation in rat carotid artery is partially NO-dependent and occurs mainly via activation of the beta2-adrenoceptor subtype. Balloon injury and subsequent neointima formation in the rat carotid artery lead initially to an impairment, but subsequently to an enhancement of the beta-adrenoceptor-mediated vasorelaxation. The impairment is attributable to the removal of endothelium, whereas the enhanced beta-adrenoceptor-mediated function may be related to the occurrence of an NO system in the neointimal smooth muscle cells.

Functional properties of the saphenous vein harvested by minimally invasive techniques

BACKGROUND

Since surgical techniques affect the functional properties of the vessel wall, the present study investigated the influence of minimally invasive harvesting techniques on the vascular reactivity of the saphenous vein.

METHODS

Saphenous vein remnants were obtained after aortocoronary bypass operation from patients subjected to conventional (n = 6), mediastinoscope-assisted (n = 4), or endoscope-assisted venectomy (n = 5). After preservation in University of Wisconsin solution (UW), ring preparations were mounted in a standard organ bath setup and concentration-response curves were constructed for phenylephrine, sodium nitroprusside, and acetylcholine.

RESULTS

Saphenous vein reactivity was not altered after preservation in UW. For the vein preparations harvested by means of the three venectomy methods, no differences were demonstrated for responses to KCl, phenylephrine, or sodium nitroprusside. The maximal endothelium-dependent acetylcholine-induced dilation of precontracted vein rings varied between 5% and 12%, independent of the surgical technique applied.

CONCLUSIONS

It was demonstrated that minimally invasive surgical techniques for harvesting the saphenous vein, which are developed to reduce postoperative complications at the site of explantation, did not affect the vascular reactivity in a different manner than the conventional method.

Evidence for a sympatholytic effect of mibefradil in the pithed rat preparation

OBJECTIVE

The T-type prevalent calcium channel blocker mibefradil (MIB) was shown to possess N-type calcium channel blocking properties. As this particular type of calcium channel is known to be crucially involved in the neuronal release of noradrenaline, we have investigated whether MIB could be a sympatholytic drug.

METHODS

To evaluate the sympathoinhibitory action, the effects of 3 and 10 micromol/kg MIB on the tachycardic effect of electrical stimulation of the preganglionic cardioaccelerator nerves in the pithed rat were investigated. The effect of MIB on the dose-response curve of externally applied noradrenaline was also studied. To compare the results with a classic L-type calcium channel blocker, the experiments were repeated with 3 and 10 micromol/kg verapamil (VER).

RESULTS

The maximal increase in heart rate in response to electrical nerve stimulation was 96 +/- 7 bpm (control, n = 6), 70 +/- 6 bpm (3 micromol/kg MIB, n = 8), 57 +/- 6 bpm (10 micromol/kg MIB, n = 5), 93 +/- 5 bpm (3 micromol/kg VER, n = 6) and 46 +/- 7 bpm (10 micromol/kg VER, n = 5). The tachycardic response to electrical stimulation at 1, 5 and 10 Hz was completely blocked by 5 mg/kg intravenous guanethidine. The maximal increase in heart rate in response to noradrenaline was 96 +/- 4 bpm (control, n = 6), 103 +/- 6 (3 micromol/kg MIB, n = 6), 42 +/- 9 bpm (10 micromol/kg MIB, n = 5), 73 +/- 5 bpm (3 micromol/kg VER, n = 5) and 40 +/- 7 bpm (10 micromol/kg VER, n = 6). Under control conditions and in the presence of 3 micromol/kg MIB and VER the maximal effect of noradrenaline was reached at 0.1 micromol/kg whereas in the presence of 10 micromol/kg MIB and VER it was reached at a dose of 1 micromol/kg. MIB at a dose of 3 micromol/kg was significantly more effective in reducing the chronotropic response to electrical stimulation compared with externally applied noradrenaline. For VER the opposite holds true. These differences were not observed with doses of 10 micromol/kg MIB and VER.

CONCLUSION

Mibefradil, besides its direct effect on cardiac T- and L-type calcium channels, reduces the release of noradrenaline from sympathetic nerve endings, most probably by inhibition of presynaptic N-type calcium channels. In the model used this effect is only observable at relatively low concentrations, most probably because of the direct cardiodepressant action of MIB provoked by L-type channel blockade.

Probing the selectivity of allosteric modulators of muscarinic receptors at other G-protein-coupled receptors

1. The aim of the present investigation was to analyse whether three prototype allosteric modulators of ligand binding to muscarinic receptors, i.e. alcuronium, gallamine, and the alkane-bis-ammonium compound W84 (hexane-1,6-bis[dimethyl-3'-phthalimidopropylammonium bromide]), may have allosteric effects on radioligand-binding characteristics at other G-protein-coupled receptors, such as cerebral A1 adenosine receptors (Gi-coupled), cardiac left ventricular alpha1-adrenoceptors (Gq), and beta-adrenoceptors (Gs). 2. The modulators were applied at concentrations known to be high with regard to the allosteric delay of the dissociation of the antagonist [3H]-N-methylscopolamine (NMS) from muscarinic M2-receptors: 30 micromol l(-1) W84, 30 micromol l(-1) alcuronium, 1000 micromol l(-1) gallamine. As radioligands, we used the adenosine A1-receptor ligand [3H]-cyclopentyl-dipropylxanthine (CPX), the alpha1-adrenoceptor ligand [3H]-prazosin (PRAZ), and the beta-adrenoceptor ligand (-)-[125I]-iodocyanopindolol (ICYP). Allosteric actions on ligand dissociation and the equilibrium binding were measured in the membrane fractions of rat whole forebrain (CPX) and of rat cardiac left ventricle (PRAZ, ICYP, NMS), respectively. 3. CPX and PRAZ showed a monophasic dissociation with half-lives of 5.88+/-0.15 and 12.27+/-0.46 min, respectively. In the case of CPX, neither the binding at equilibrium nor the dissociation characteristics were influenced by the allosteric agents. With PRAZ, the binding at equilibrium remained almost unaltered in the presence of W84, whereas it was reduced to 36+/-2% of the control value with alcuronium and to 42+/-2% with gallamine. The dissociation of PRAZ was not affected by W84, whereas it was moderately accelerated by alcuronium and gallamine. In the case of ICYP, the binding at equilibrium was not affected by the allosteric modulators. The dissociation of ICYP was slow, and after 3 h, more than 50% of the radioligand was still bound, so that a reliable half-life could not be calculated. ICYP dissociation was not affected by W84. In the presence of alcuronium and gallamine, the dissociation curve of ICYP revealed an initial drop from the starting level, followed by the major phase of dissociation being parallel to the control curve. 4. In summary, the allosteric action of the applied agents is not a common feature of G-protein-coupled receptors and appears to be specific for muscarinic receptors.

Reactive oxygen species-induced aortic vasoconstriction and deterioration of functional integrity

Oxygen derived free radicals and other reactive oxygen species (ROS) are involved in a variety of disease states, which can have cardiac and vascular implications. The present study was performed to investigate the mechanism of ROS-induced vasoconstriction and the influence of ROS on the functional integrity of isolated rat thoracic aorta. ROS were generated by means of electrolysis (30 mA, during 0.5, 1, 2 or 3 min) of the organ bath fluid. ROS induced a transient (approximately 60 min) vasoconstriction and the maximally induced contraction was dependent on the duration of electrolysis. Dimethyl sulfoxide (DMSO) diminished the ROS-induced vasoconstriction almost completely, indicating a major influence of hydroxyl radicals on contractility. The dual cyclooxygenase/lipoxygenase inhibitor, meclofenamate, completely prevented the ROS-induced vasoconstriction. The phospholipase A2 (PLA2) inhibitor, oleyloxyethyl phosphorylcholine, was able to reduce the vasoconstriction elicited by ROS by approximately 70%. Conversely, the specific cytoplasmic PLA2 inhibitor arachidonyl trifluoromethylketone proved ineffective in this respect. By using the specific mitogen-activated protein kinase (MAPkinase) kinase inhibitor PD98059, it was shown that the activation of extracellular-regulated kinase (ERK) MAPkinase contributes to the ROS-induced vasoconstriction. The effects of ROS on the functional integrity of the aortae were investigated, in particular with respect to receptor (alpha1-adrenoceptor) and non-receptor-mediated contractile responses (high potassium solution). In addition, both the endothelium dependent (methacholine) and endothelium independent (sodium nitroprusside) vasorelaxation were investigated before and after ROS exposure. Electrolysis periods of 0.5 and 1 min induced a modest leftward shift of the concentration response curves for the alpha1-adrenoceptor agonist methoxamine. Longer electrolysis periods of 2 and 3 min additionally decreased the maximal response to (alpha1-adrenoceptor stimulation. Methacholine-induced vasorelaxation proved diminished in aortae subjected to electrolysis (0.5, 1, 2 and 3 min), whereas relaxation to sodium nitroprusside was nearly complete in all groups. KCl-induced contractions proved attenuated only after longer electrolysis periods of 2 and 3 min. This ROS-induced deterioration of functional integrity was almost completely prevented by 0.6% DMSO. From these results we may conclude that ROS induce an eicosanoid and ERK MAPkinase-mediated vasoconstriction in isolated rat thoracic aorta. In addition, exposure to ROS leads to a deterioration of functional integrity characterized by endothelial dysfunction and decreased contractile function.

Inhibitory effect of mibefradil on contractions induced by sympathetic neurotransmitter release in the rat tail artery

This study tested whether mibefradil exerts a stronger inhibitory effect than verapamil on sympathetic neurotransmitter release provoked by electrical field stimulation. Tail arteries (diameter 620+/-9 microm) were obtained from male Wistar rats. Ring segments of 2 mm length were mounted in an isometric wire myograph. After an appropriate period of equilibration and a priming procedure the vessels were either subjected to electrical field stimulation (EFS; frequency 0.25-4 Hz for 30 s) or a concentration-response curve was generated with either noradrenaline (concentration range 0.03-3 microM) or ATP (concentration 0.3 mM) which served as baseline parameters. EFS-induced contractions were stable and reproducible and were blocked by tetrodotoxin (1 microM), guanethidine (3 microM), and the combination of suramin (0.5 mM) and prazosin (3 microM). EFS-induced contractions (1 Hz) were almost completely inhibited by 10 microM mibefradil (97%) but only partly by 10 microM verapamil (73%). Log IC50 values were -5.6 for mibefradil and -6.6 for verapamil. Calcium antagonists were equipotent in inhibiting noradrenaline (maximum inhibition by mibefradil and verapamil by 70% and 75%, respectively; log IC50: -6.5 and -6.7, respectively) and ATP-mediated contractions (maximum inhibition by mibefradil and verapamil by 92% and 97%, respectively; log IC50: -6.5 and -7.0, respectively). Consequently mibefradil displays an additional effect on contractions provoked by EFS-induced sympathetic noradrenaline release which cannot be explained by L-type calcium channel blockade. Probably this effect of mibefradil is mediated by the blockade of prejunctional N-type calcium channels, thereby inhibiting sympathetic noradrenaline release. Since activation of the sympathetic nervous system in hypertension is both common and undesirable, a calcium antagonist displaying both L- and N-type calcium channel blocking activities, would have major advantages over calcium antagonists lacking N-type calcium channel blocking activities.