Involvement of endothelial NO in the dilator effect of VIP on rat isolated pulmonary artery
Introduction
There is growing evidence that pulmonary hypertension and associated chronic heart failure [1] involve dysregulation of pulmonary endothelial function and vascular tone. Specifically, the production of vasodilator mediators such as prostacyclin and nitric oxide (NO) is decreased and the formation of vasoconstrictor mediators such as endothelin is enhanced, and this change in the function of the pulmonary endothelium ultimately results in pulmonary vascular remodelling [2], [3], [4]. Together with endothelial injury, the prevailing release of vasoconstrictor mediators leads to a pro-coagulant state and vascular obstruction. Endothelial function in pulmonary arteries is also impaired in patients with chronic obstructive pulmonary disease [5], [6] as well as in rats with chronic hypoxia [7], [8]. The changes in pulmonary vascular function, most of which favour abnormal vasoconstriction, occur in both the endothelium and vascular smooth muscle [9] but are largely independent of changes in blood vessel structure [10].
The pulmonary vascular tone is regulated by many vasoactive mediators either released locally from vascular endothelium, airway epithelium, vascular smooth muscle or nerve fibres, or delivered systemically via the blood [11]. The neural regulation of the pulmonary circulation involves α-adrenoceptor-mediated sympathetic vasoconstriction and a biphasic vascular response to acetylcholine (ACh) released from parasympathetic nerve endings. The biphasic response to ACh depends on the vascular tone [12], given that ACh induces constriction of the vascular bed under resting tone, and dilatation following pre-contraction [13], [14]. Relaxation of the pulmonary artery in response to ACh is due to activation of muscarinic receptors on the endothelium, while the ACh-evoked contraction at resting tone arises from activation of muscarinic receptors on the smooth muscle [15].
Neuropeptides constitute a third class of neurotransmitters released from autonomic and sensory nerve fibres supplying the vasculature [16]. These peptide transmitters comprise vasoactive intestinal polypeptide (VIP), peptide histidine methionine (PHM), peptide histidine valine (PHV), pituitary adenylate cyclase activating peptide (PACAP-27 and PACAP-38) [17] and calcitonin gene-related peptide [18], [19].
VIP is a 28-amino acid neuropeptide [20] which exerts various biological actions including modulation of neurotransmission, immune regulation and vascular relaxation [21]. The mode of action of VIP on blood vessels has remained controversial, since responses to this peptide either depend on the presence of endothelium [22], [23], [24] or are endothelium-independent [25], [26]. The same is true for the effect of VIP to cause vasodilatation in the isolated rat lung. On the one hand, it has been reported that NO and/or cyclooxygenase metabolites participate in VIP-induced pulmonary vasodilatation [27]. On the other hand, Chen et al. [28] found that the relaxant effect of VIP on pulmonary arteries of chronically hypoxic rats was endothelium-independent. Similarly, Sata et al. [29] hold that the VIP-induced relaxation of pulmonary artery strips isolated from the guinea-pig, rat and rabbit is endothelium-independent.
In an attempt to clarify these observations, the aim of the present study was to analyse the role of the endothelium and NO in the relaxant action of VIP on the rat isolated pulmonary artery and to compare the action of VIP with that of ACh.
Section snippets
Isolated preparations
Seventy-two adult male Sprague–Dawley rats (Division of Laboratory Animal Science and Genetics, Department of Biomedical Research, Medical University of Vienna, Himberg, Austria) weighing 250–280 g were used in this study. All investigations were according to national law governing the care and use of experimental animals. The animals were anaesthetized with diethyl ether (Merck, Darmstadt, Germany). The heart and lungs were excised en bloc and fixed on dental wax surface in a petri dish
Histology
As shown in Fig. 1A and B, mechanical rubbing of the arterial preparations caused severe damage to the endothelium.
Effects of ACh and VIP on endothelium-native and endothelium-denuded rat isolated pulmonary artery
ACh elicited a concentration-dependent relaxation of the rat isolated pulmonary artery with native endothelium (Fig. 2, Fig. 4), pre-contracted with Phe. Maximum relaxation was obtained at a concentration of 10 μM. Higher concentrations of ACh failed in most experiments to produce greater responses (n = 6). In endothelium-damaged pulmonary arteries ACh had almost no effect (n = 6). The
Discussion
The present results show that ACh as well as VIP significantly relax the endothelium-native rat isolated pulmonary artery pre-contracted by phenylephrine. The ACh-induced vasodilatation was abolished and the response to VIP significantly attenuated by the removal of the vascular endothelium. Extrapolation of EC50 values revealed that VIP was 9–10 times more potent than ACh in dilating the pulmonary artery with native endothelium. In a further analysis of the mechanism of action of ACh it was
Acknowledgements
This work was co-financed by the European commission (grant No. QLK3-CT-2002-02031) and the Austrian Nano-Initiative as part of the Nano-Health project (no. 0200), the sub-project NANO-BREATH being financed by the Austrian FWF (Fonds zur Förderung der Wissenschaftlichen Forschung; Project no. 206). We are grateful for support by the Austrian National Bank through grant no. 9673.
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