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

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 other's 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], FR2/FR12.03±0.11 and CHF-preparations [n=7], FR2/FR11.71±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 pIC508.81±0.31; CHF 8.65±0.42).

Ang II (1 nM—0.3 µM) concentration-dependently increased the contractile force in control preparations (Emax21.64±3.86 mN, pD27.63±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 pD26.93±0.05, Emax131.0±2.7; CHF pD27.00±0.05, Emax136.7±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.

Analysis of Vasoreactivity of Isolated Human Radial Artery

Radial artery (RA) is increasingly used as graft for coronary artery bypass grafting due to its good long-term patency. However, the mechanism of peri- and post-operative spasm is still unclear. Because of that, the aim of our study is to analyze the contractility of RA and to determine whether the presence of functional endothelium alters its contractile properties. Contractions of isolated RA rings were provoked by exogenously applied vasoconstrictors or by electrical field stimulation (EFS, 20 Hz). The order of vasoconstrictors potency based on their EC50 values was as follows: angiotensin II > phenylephrine > 5-hydroxytriptamine. Presence of endothelium increased both EC50 and maximal contraction to phenylephrine and angiotensin II, but inhibited reactivity of RA to 5-hydroxytriptamine. Spontaneous rhythmic contractions (SRC, <4 mHz) and EFS-induced contractions of RA are endothelium-independent and weaker than contractions induced by exogenously applied vasoconstrictors. Our study concludes that RA shows marked sensitivity and reactivity to angiotensin II, phenylephrine, and 5-hydroxytriptamine. Further investigations are necessary to answer why angiotensin II and phenylepehrine induce stronger contractions in the presence of endothelium. In addition, SRC as well as contractions of neurogenic origin may take part in developing vascular spasm of RA.

The evaluation of the N-type channel blocking properties of cilnidipine and other voltage-dependent calcium antagonists

Sympathetic neurotransmission in tissues with intact sympathetic nerve arborization is extensively dependent on calcium influx via N-type calcium-channels. It was the objective of the present study to assess and compare the claimed sympatholytic effect of the 1,4-dihydropyridine compound cilnidipine with other voltage-dependent calcium-channel (VDCC) antagonists. We studied these compounds by means of three different models. In the rabbit isolated thoracic aorta, the alleged sympatholytic properties displayed by these compounds were evaluated in the noradrenaline spillover model. Additionally, the influence of cilnidipine on stimulation-induced constrictor responses was studied in the rat isolated tail artery (male Wistar rats, 250-300 g) in addition to its effect on noradrenaline-induced contractions. Finally, we studied the influence of cilnidipine and other calcium-channel blockers on stimulation-induced chronotropic responses, in order to address N- or L-type selectivity, in the pithed rat model (male Wistar rats, 260-320 g). Furthermore, we evaluated their effect on noradrenaline-induced tachycardia. In the isolated rabbit thoracic aorta preparation omega-conotoxin GVIA (0.1 microM) nearly abolished the sympathetic outflow caused by stimulation, whereas nifedipine (0.1 microM) and amlodipine (1 microM) did not influence the evoked noradrenaline release. Cilnidipine (1 microM) significantly attenuated the response by nearly 18% and mibefradil (1 microM) by c. 42%. The stimulation-induced constrictor response (prejunctional effect) in the rat isolated tail artery could be blocked by omega-conotoxin GVIA (0.5 and 1 microM). Cilnidipine (10 nm and 0.1 microM) significantly attenuated responses to stimulation by maximally 20%, whereas it did not influence the constrictor response to noradrenaline (postjunctional effect). The mean heart rate in the pithed rat model amounted to 309.3 +/- 3.6 beats/min (bpm). Electrical stimulation of the cardio-accelerator nerves (C7-Th1) resulted in an increase by 106.7 +/- 2.2 bpm. All antagonists studied, except for nifedipine, attenuated the chronotropic response to stimulation (P < 0.05). The rank order of sympatholytic efficacy was: omega-conotoxin GVIA (84.8%), mibefradil (75.1%), cilnidipine (43.0%) and amlodipine (34.8%). Noradrenaline (10 nmol/kg) increased the heart rate by 117.8 +/- 2.7 bpm. This chronotropic response was influenced equally well by the calcium-channels blockers as observed in the stimulation (prejunctional) experiment. In conclusion, the N-type channel blocking properties and thus sympatholytic effect of cilnidipine could be demonstrated in some (vascular) but not all (cardiac) models studied. At the level of the vasculature cilnidipine reduced the neurotransmitter release to electrical stimulation in both the noradrenaline spillover model and in the model of the rat isolated tail artery, respectively. For amlodipine and nifedipine no sympatholytic activity could be demonstrated. In the pithed rat model, we were unable to demonstrate a selective N-type blocking effect for the VDCC-antagonists.

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 nM) 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 microM). The constrictor response to ATII was unaffected by PD123319. In the mesenteric artery, ATII (0.5 nM) 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 microM). 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.

No involvement of the AT2-receptor in angiotensin II-enhanced sympathetic transmission in vitro

Angiotensin II (Ang II) enhances sympathetic neurotransmission via AT(1)-receptors located on sympathetic nerve terminals. We recently demonstrated that inhibition of Ang II-mediated facilitation in the pithed rat by irbesartan resulted in a U-shaped dose response curve, which was not observed when PD 123319, at a concentration that selectively blocks the AT(2)-receptor, was co-administered. Hence, the irbesartan-mediated upstroke might be explained by the involvement of the AT(2)-receptor after AT(1) blockade with high-dose irbesartan. In the present study, we further investigated the possible role of the AT(2)-receptor in Ang II-mediated facilitation in vitro. We studied the effect of the AT(2)-receptor antagonist PD 123319 (10 nM) on Ang II-enhanced sympathetic outflow evoked by electrical field stimulation (EFS) in the rat isolated inferior vena cava. Additionally, we investigated the effect of the AT(1)-receptor blocker irbesartan (0.1 nM 1 M) on the sequelae of Ang II-enhanced, EFS-evoked sympathetic nerve traffic in the presence or absence of PD 123319 (10 nM). PD 123319 did not influence Ang II-enhanced sympathetic outflow. Irbesartan dose-dependently attenuate Ang II-augmented transmitter release (pIC50 7.99+0.03), whereas no U-shaped concentration-response relationship for irbesartan was observed. Co-administration of PD 123319 with irbesartan proved unable to influence Ang II-mediated facilitation differently compared with irbesartan alone. The experimental observations indicate that the AT(2)-receptor is not involved in Ang II-mediated enhancement of sympathetic nerve traffic in the present in vitro study.

Pre- and postsynaptic inhibitory potencies of the angiotensin AT1 receptor antagonists eprosartan and candesartan

The aim of the present study was to determine the inhibitory potency of two selective angiotensin AT(1) receptor antagonists, eprosartan and candesartan, at the level of the sympathetic nerve terminal and the vascular smooth muscle. Male New Zealand White rabbits, weighing 2100-2550 g, were used. To study eprosartan and candesartan at the neuronal angiotensin AT(1) receptor, we investigated their influence on the angiotensin II-enhanced, electrical field stimulation-evoked sympathetic transmission in the rabbit isolated thoracic aorta in a noradrenaline spillover model. To study both antagonists at the vascular angiotensin AT(1) receptor, concentration-response curves for angiotensin II were constructed in the presence or absence of the two angiotensin AT(1) receptor antagonists. Angiotensin II (10 nM) caused a significant increase by 107+/-11.1% of the stimulation-evoked sympathetic outflow, which was concentration-dependently inhibited by both eprosartan (pIC(50) 7.91+/-0.12) and candesartan (pIC(50) 10.76+/-0.13). Angiotensin II (1 nM-0.3 microM) caused a concentration-dependent increase in contractile force (E(max) 20.62+/-2.24 mN, pD(2) 8.16+/-0.04). Both eprosartan (pA(2) 8.90+/-0.11, pIC(50) 8.87+/-0.12 (10 nM angiotensin II)) and candesartan (pD(2)' 10.80+/-0.13) counteracted the contractions evoked by cumulative concentrations of angiotensin II. Candesartan proved a more potent antagonist than eprosartan at both the pre- and postjunctional angiotensin AT(1) receptor. For eprosartan, vascular inhibitory concentrations were 10-fold lower than sympatho-inhibitory concentrations, whereas for candesartan, inhibitory concentrations at both sites were similar. The results may be explained by differences between the pre- and postjunctional angiotensin AT(1) receptor subtype.