Jippe C. Balt

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 μmol/l) caused a concentration-dependent enhancement of EFS-evoked noradrenaline release (control versus concentrations 0.1 nmol/l−0.1 μmol/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 μmol/l) produced less than maximal facilitation. The AT1-receptor antagonists losartan (0.1 nmol/l−0.1 μmol/l), telmisartan (0.01–10 nmol/l) and irbesartan (0.1 nmol/l−0.1 μmol/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.

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 &agr;-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 n M) 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 n M–0.1 &mgr;M) and candesartan (1 n M–0.1 &mgr;M). The AT2 antagonist PD123319 (10 n M) 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 n M. Ang II (1 n M–0.3 &mgr;M) 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 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.

Different AT1 receptor subtypes at pre- and postjunctional sites: AT1A versus AT1B receptors.

Angiotensin (AT) II is known to enhance responses to electrical field stimulation (EFS) via AT1 receptors located on sympathetic nerve terminals. Differences in potency exist between AT1 receptor antagonists regarding the inhibition of the prejunctional and postjunctional AT1 receptors. It is hypothesized that prejunctional AT1 receptors might belong to the AT1B receptor subtype. Accordingly, the authors investigated whether AT1B receptor inhibition by high concentrations of PD123319 could suppress ATII-augmented noradrenergic transmission (prejunctional) in the rabbit thoracic aorta by means of a noradrenaline spillover model. Additionally, the influence of PD123319 on ATII-enhanced constrictor responses to electrical field stimulation was investigated in the isolated rabbit mesenteric artery. Furthermore, the authors investigated whether PD123319 could influence the constrictor responses (postjunctional) to ATII in both preparations. In the thoracic aorta, ATII (10 n M) caused a significant enhancement of EFS-evoked [3H]-noradrenaline release by a factor of 2.0 ± 0.1. This reinforcement could be inhibited by PD123319 (0.1, 1, and 10 &mgr;M). The constrictor response to ATII was unaffected by PD123319. In the mesenteric artery, ATII (0.5 n M) caused a significant enhancement of constrictor responses to EFS by factors of 2.9 ± 0.3, 2.3 ± 0.3, and 1.6 ± 0.1 at 1, 2, and 4 Hz, respectively. This enhancement could be attenuated by PD123319 (1 and 10 &mgr;M). The constrictor response to ATII was unaffected by PD123319. It is concluded that the prejunctional AT1 receptors belong to the AT1B subtype whereas postjunctional AT1 receptors do not.

Decreased facilitation by angiotensin II of noradrenergic neurotransmission in isolated mesenteric artery of rabbits with chronic heart failure.

Both in human and in experimental heart failure (HF), the renin-angiotensin system and the sympathetic nervous system are activated. In a previous study a facilitatory action of angiotensin II (Ang II) was shown in the rabbit mesenteric artery, which was mediated via prejunctionally located Ang II type 1 (AT1) receptors. Very little is known about the effects of Ang II on sympathetic neurotransmission at the peripheral level in congestive heart failure (CFH). Accordingly, in the isolated mesenteric arteries obtained from rabbits with experimentally induced CHF, as well as in age-matched control rabbits, the effect of Ang II on contractions provoked by electrical field stimulation was investigated in the presence and absence of the AT1 receptor antagonist eprosartan. Additionally, to investigate a possible postjunctional facilitation, the effects of Ang II on &agr;-adrenoceptor-mediated responses were studied using noradrenaline (NA). Lastly, the vasoconstrictor effects of Ang II were compared between HF rabbits and controls, by constructing concentration-response curves to Ang II. In control rabbits, Ang II 0.5 n M caused an enhancement of stimulation-induced responses by a factor 3.2 ± 0.5, 2.4 ± 0.3, and 1.5 ± 0.08, at 1, 2, and 4 Hz, respectively (P < 0.05 at all frequencies compared with vehicle). In rabbits with HF, the enhancement by Ang II (0.5 n M) amounted to a factor 2.1 ± 0.2, 1.7 ± 0.1, and 1.2 ± 0.04, at 1, 2, and 4 Hz, respectively (P < 0.05 compared with vehicle at all frequencies). Accordingly, the enhancing effect of Ang II was more pronounced in the control group compared with rabbits with HF (P < 0.05 at each frequency). Eprosartan (1 n M–0.1 &mgr;M) could inhibit the facilitatory effects of Ang II in arteries from HF as well as from control rabbits. Contractile responses to exogenous NA (3 n M–0.1 m M) were the same in HF rabbits and controls, and they were unaltered in the presence of Ang II 0.5 n M. Ang II (0.1 n M–1 &mgr;M) caused a concentration-dependent increase in contractile force, which was the same in HF rabbits and controls. From these findings it can be concluded that in rabbits with CHF as well as in control animals, Ang II facilitates the stimulation-induced vasoconstrictor responses via prejunctionally located AT1 receptors. The facilitating effect was decreased in vessels obtained from rabbits with CHF, whereas responses to exogenous Ang II were unchanged. These findings may be explained by downregulation or uncoupling of the prejunctional AT1 receptor.

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).

Impaired neuronal and vascular responses to angiotensin II in a rabbit congestive heart failure model

Congestive heart failure (CHF) is characterised by activation of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS). Both systems are known to interact and to potentiate each others activities. We recently demonstrated that angiotensin II (Ang II) enhances sympathetic nerve traffic via prejunctionally-located AT1-receptors. At present, little is known about the effects of Ang II at the level of the sympathetic neurones in CHF. Accordingly, we investigated the effect of Ang II in the presence and absence of the AT1-receptor antagonist, eprosartan, on stimulation-induced nerve traffic in isolated thoracic aorta preparations obtained from rabbits suffering from experimentally-induced CHF. Control-preparations were obtained from age-matched animals. Sympathetic activity was assessed by a [3H]noradrenaline spill-over model. Additionally, Ang II constrictor responses were compared between CHF and control vessels in the presence and absence of eprosartan. Additionally, to study postjunctional facilitation, the effects of Ang II on postsynaptic α-adrenoceptor-mediated responses were studied using noradrenaline. Stimulation-evoked SNS-neurotransmission was similar in both groups (CHF versus control). Ang II (0.1 nM—0.1 µM) caused a concentration-dependent increase of the stimulation-evoked sympathetic outflow in both groups, with a maximum at 10 nM (control [n=7], FR 2/FR1 2.03±0.11 and CHF-preparations [n=7], FR2/FR 1 1.71±0.07). The enhancement by Ang II was decreased in CHF-preparations compared with controls (p<0.05). Eprosartan concentration-dependently attenuated the Ang II-enhanced (10 nM) sympathetic outflow in both CHF- and control preparations. The sympathoinhibitory potency of eprosartan was similar in both groups (control pIC50 8.81±0.31; CHF 8.65±0.42). Ang II (1 nM—0.3 µM) concentration-dependently increased the contractile force in control preparations (Emax 21.64±3.86 mN, pD2 7.63±0.02, n=7). Eprosartan (1 nM—0.1 µM) influenced the Ang IIcontractions via a mixed form of antagonism. In CHF-preparations, Ang II caused impaired vascular contraction. The KCl-induced contraction was decreased in the CHF- compared with control preparations (13.02±0.64 mN versus 30.40±0.89 mN). The relative Ang II contraction (% of KCl) was also decreased (2.3% vs. 58.0%). Concentration-response curves to noradrenaline (%KCl) were similar (control pD2 6.93±0.05, Emax 131.0±2.7; CHF pD2 7.00±0.05, Emax 136.7±2.6) (p>0.05) and were not affected by Ang II. We conclude that Ang II-enhanced sympathetic neurotransmission is mediated by the prejunctional AT1 -receptor in both control and CHF-preparations. The decreased facilitation of SNS effects by Ang II may be explained by down-regulation or desensitisation of the neuronal AT1-receptor. Additionally, the aortic contractile capacity in heart failure rabbits appears to be decreased, probably as a result of heart failure-associated neuroendocrine and functional changes.

Vasopressin facilitates presynaptic sympathetic nerve activity in humans.

Objective It was the objective of this study to investigate whether a facilitatory role of vasopressin (AVP) on sympathetic nerve activity can be demonstrated in humans at the peripheral level. Methods Eight subjects (32 ± 2.3 years) participated in this study. Forearm blood flow (FABF) was measured using the venous occlusion plethysmography model. Each session was performed in the presence of a continuous infusion (into the brachial artery) of AVP in sub-pressor dosage of 0.008 ng/kg per min, or NaCl 0.9%. Using lower-body negative pressure (LBNP) (−10, −20 and −30 mmHg) the combined pre- and postsynaptic action of AVP on the sympathetic nervous system was investigated. This was followed by a second protocol in which the possible postsynaptic effects of AVP were evaluated with intra-arterial infused norepinephrine (NE). Results The baseline FABF was 5.2 ± 0.6 ml/100 ml per min. After infusion of AVP (0.008 ng/kg per min), the FABF remained unchanged at a flow of 5.5 ± 0.6 ml/100 ml per min (P = 0.26). LBNP caused a pressure-dependent decrease in FABF (25.6 ± 4.4, 29.0 ± 6.1 and 38.6 ± 6.9%, for −10, −20 and −30 mmHg, respectively). AVP significantly enhanced the FABF responses to lower-body negative pressures (38.0 ± 8.6, 49.3 ± 5.1 and 58.9 ± 6.3%, respectively (P = 0.014). NE caused a dose-dependent vasoconstriction by 3.1 ± 4.6, 17.0 ± 4.3 and 23.2 ± 4.9%, at dosages of 10, 20 and 40 pg/min, respectively, unaffected by AVP (P = 0.91). Conclusions We conclude that AVP can facilitate vasoconstriction mediated by the peripheral sympathetic nervous system at the presynaptic level in humans.