Our observation of LPS induced increase in
Our observation of LPS-induced increase in the sensitivity of cultured endothelium-intact coronary AP1903 receptor towards stimulation with selective ETB receptor agonist (S6c) could be explained by activation of TLRs receptors located on the endothelium of rat coronary arteries. However, the possible recruitment of vascular endothelial TLRs by LPS, and the specific pathways by which LPS could interact in coronary arteries are still unsolved and this field requires much attention and elucidation in the future. The ETB receptor-mediated contraction may be enhanced by pro-inflammatory cytokines and/or through an enhancement of intracellular signalling mechanisms (Leseth et al., 1999, White et al., 1999), but future studies will have to reveal whether LPS from P.g. interacts with these two major intracellular pathways.
In conclusion, the present findings demonstrate ETB receptor upregulation on vascular smooth muscle cells in organ culture, and that LPS from P.g. is able to further sensitize the coronary artery contractile response towards the selective ETB receptor agonist S6c only in endothelium intact coronary arteries. The selective competitive ETB receptor antagonist, BQ 788 induced a concentration-dependent parallel rightward shift in the log S6c-response curve in the isolated coronary arteries subjected to organ culture with and without LPS, supporting the receptor subtype characterization in both vessel groups. Immunohistochemical studies revealed an increased ETB receptor protein level in coronary arteries subjected to organ cultured, but LPS was not able to further increase the ETB receptor expression in the smooth muscle layer. No difference was observed regarding the contractile response induced by ET-1 or the ETA protein level when comparing the response of fresh coronary arteries with that of incubated vessels with and without LPS.
Conflict of interest
Acknowledgements This work was supported by The Simon Spies Foundation.
Introduction Endothelium-derived vasoactive substances have been shown to be of importance both under normal and various pathological conditions (Bassenge, 1996). One of these mediators, endothelin-1, with potent vasoconstrictive properties has been suggested to be involved in several diseases such as heart failure (Kiowski et al., 1995), ischaemia/reperfusion injury (Pernow and Wang, 1997), subarachnoid haemorrhage (Ehrenreich and Schilling, 1995), primary pulmonary hypertension (Goldie et al., 1996) and sepsis (Battistini et al., 1996). Apart from vasoconstriction, endothelin-1 also may mediate mitogenic stimulation of smooth muscle cells (Piero, 1995), neutrophil activation (Caramelo et al., 1997) and lymph vessel constriction (Reeder and Ferguson, 1996). The effects of this 21 amino acid peptide, converted from its pro-form, big endothelin-1, by an isoenzyme family named endothelin converting enzyme, are mediated by at least two receptors, the endothelin ETA and endothelin ETB receptors. These two receptors are found in most mammals and through the development of different receptor antagonists and agonists, their functions have been shown to be more complicated than first thought. Both endothelin ETA and endothelin ETB (ETB2) receptors located on smooth muscle cells mediate contraction whereas stimulation of a subgroup of endothelin ETB receptors (ETB1), located on the endothelium, cause vasodilation through release of prostacyclin and nitric oxide (NO) (De Nucci et al., 1988). Furthermore, the endothelin ETB1 receptors could be further subdivided depending on differences in sensitivity to various types of agonists and antagonists MacLean et al., 1998, Miasiro et al., 1998. In addition, the endothelial endothelin ETB1 receptor seems to participate in the clearance of circulating endothelin-1 (Dupuis et al., 1996), a process that has been suggested to be mediated by a splice variant of the endothelin ETB receptor (Elshourbagy et al., 1996). Other mechanisms proposed to be mediated by the endothelin ETB receptor includes positive inotropy (Beyer et al., 1995) and macrophage induced inflammatory response (Sakurai-Yamashita et al., 1997). Complicating the issue even more is the suggested cross-talk between the endothelin ETA and endothelin ETB receptors, leading to increased endothelin ETB receptor sensitivity during endothelin ETA receptor antagonism (Ozaki et al., 1997). Therefore, the role of the different endothelin receptor subtypes in response to endothelin stimulation is still not fully elucidated.