Endothelium-dependent contractions in SHR: a tale of prostanoid TP and IP receptors

Abstract

In the aorta of spontaneously hypertensive rats (SHR), the endothelial dysfunction is due to the release of endothelium-derived contracting factors (EDCFs) that counteract the vasodilator effect of nitric oxide, with no or minor alteration of its production. The endothelium-dependent contractions elicited by acetylcholine (ACh) involve an increase in endothelial [Ca(2+)](i), the production of reactive oxygen species, the activation of endothelial cyclooxygenase-1, the diffusion of EDCF and the subsequent stimulation of smooth muscle cell TP receptors. The EDCFs released by ACh have been identified as PGH(2) and paradoxically prostacyclin. Prostacyclin generally acts as an endothelium-derived vasodilator, which, by stimulating IP receptors, produces hyperpolarization and relaxation of the smooth muscle and inhibits platelet aggregation. In the aorta of SHR and Wistar-Kyoto rats, prostacyclin is the principal metabolite of arachidonic acid released by ACh. However, in SHR aorta, prostacyclin does not produce relaxations but activates the TP receptors on vascular smooth muscle cells and produces contraction. The IP receptor is not functional in the aortic smooth muscle cells of SHR as early as 12 weeks of age, but its activity is not reduced in platelets. Therefore, prostacyclin in the rule protects the vascular wall, but in the SHR aorta it can contribute to endothelial dysfunction. Whether or not prostacyclin plays a detrimental role as an EDCF in other animal models or in human remains to be demonstrated. Nevertheless, because EDCFs converge to activate TP receptors, selective antagonists of this receptor, by preventing endothelium-dependent contractions, curtail the endothelial dysfunction in diseases such as hypertension and diabetes.

Figure 1

Endothelial dysfunction and endothelium-dependent contractions in SHR aorta. (A) In contracted rings of WKY and SHR aorta, acetylcholine induces endothelium-dependent relaxations, which at higher concentrations are blunted in the arteries of the hypertensive strain. (B) In quiescent aortic rings of SHR, acetylcholine induces endothelium-dependent contractions (in the presence of the nitric oxide synthase inhibitor L-nitro-arginine, L-NA) that are blocked by the antagonist of the TP receptor, S18886, the preferential COX-1 inhibitor, valeryl salicylate, but only partially affected by the preferential COX-2 inhibitor, NS-398. Modified from Yang et al.(Br J Pharmacol, 2002). COX, cyclooxygenase; SHR, spontaneously hypertensive rats; WKY, Wistar-Kyoto rats.

Figure 2

Bioassay of endothelium-derived contracting factor in SHR aortic rings: layered or ‘sandwich’ preparation. Acetylcholine (ACh) produces contraction of the bioassay strip (without endothelium) only if endothelial cells are present on the donor aortic strip. L-nitro-arginine (L-NA) and tetrahydrobiopterin (BH₄) are present to optimize endothelium-derived contracting factor-mediated responses. Papaverine (Papav.) produces complete relaxation of the bioassay strip. Modified from Vanhoutte et al.(Br J Pharmacol, 2005).

Figure 3

Acetylcholine-induced prostaglandin release in WKY and SHR aorta. Acetylcholine produces the endothelium-dependent release of prostacyclin (A, as measured as its stable metabolite 6-keto-PGF_1α), thromboxane A₂ (B, as measured as its stable metabolite thromboxane B₂), PGE₂ (C) and PGF_2α (D). Prostacyclin is by far the most abundant prostaglandin released, and its generation is markedly higher in SHR than in WKY aortic rings. Modified from Gluais et al.(Br J Pharmacol, 2005). SHR, spontaneously hypertensive rats; WKY, Wistar-Kyoto rats.

Figure 4

Thromboxane A₂ generation and acetylcholine-induced endothelium-dependent contractions. Dazoxiben, the thromboxane synthase inhibitor, does not affect acetylcholine-induced release of prostacyclin (A), PGE₂ (C), PGF_2α (data not shown), but abolishes that to thromboxane A₂ (B). However, acetylcholine-induced endothelium-dependent contraction in SHR aortic rings is not influenced by dazoxiben (D). Modified from Gluais et al.(Br J Pharmacol, 2005).

Figure 5

Prostacyclin induces relaxation in WKY aortic rings but not in those of SHR. (A) In aortic rings with endothelium of young 12-week-old WKY, prostacyclin induces relaxations that are inhibited by the specific IP receptor antagonist CAY 10441. The presence of the TP receptor antagonist, Triplion®, enhances the relaxations to prostacyclin. (B) In aortic rings with endothelium of young 12-week-old SHR, prostacyclin does not provoke relaxations but only contractions at higher concentrations. These contractions are blocked by the TP receptor antagonist, Triplion®. SHR, spontaneously hypertensive rats; WKY, Wistar-Kyoto rats. Modified from Gomez et al. (AM J Physiol 2008).

Figure 6

Prostacyclin-induced contractions and acetylcholine-induced endothelium-dependent contractions. (A) Original trace showing acetylcholine-induced concentration and endothelium-dependent contractions in SHR aortic rings (presence of L-nitro-arginine, L-NA). The modest amplitude of the contractions and their transient nature can be visualized in comparison with the reference contraction produced by 60 mmol·L⁻¹ KCl (upper trace). In aortic rings with endothelium and in the presence of L-NA (or in rings without endothelium, data not shown), prostacyclin induces transient concentration-dependent contractions (lower trace). (B) SHR aortic rings are more sensitive than that of WKY to the contractile effect of prostacyclin. In both strains, these contractions are blocked by TP receptor antagonists (data not shown). Modified from Gluais et al.(Br J Pharmacol, 2005). SHR, spontaneously hypertensive rats; WKY, Wistar-Kyoto rats.

Figure 7

Acetylcholine-induced concentration-dependent and endothelium-dependent prostacyclin release and contractions. (A) WKY aortic rings with and without endothelium. (B) SHR aortic rings with and without endothelium. Prostacyclin release is represented on the left Y-scale and endothelium-dependent contractions on the right Y-scale (presence of L-nitro-arginine, L-NA). Modified from Gluais et al.(Br J Pharmacol, 2005). SHR, spontaneously hypertensive rats; WKY, Wistar-Kyoto rats.

Figure 8

Thromboxane A₂ and endothelium-dependent contractions in SHR aortic rings. (A) The calcium ionophore, A23187, produces a significantly larger endothelial release of thromboxane A₂ than acetylcholine. (B) The endothelial release of thromboxane A₂ by A23187 is blocked by dazoxiben, the thromboxane synthase inhibitor. (C) Dazoxiben partially inhibits the endothelium-dependent contraction to A23187 (in the presence of L-nitro-arginine), indicating the contribution of thromboxane A₂ in the endothelium-dependent contractions elicited by the calcium ionophore. (D) In SHR aortic rings with endothelium, the dazoxiben-sensitive production of thromboxane A₂ is significantly correlated to the dazoxiben-sensitive component of the endothelium-dependent contractions. SHR, spontaneously hypertensive rats.

Figure 9

Mechanisms of endothelium-dependent contractions in WKY and SHR aortic rings. The sequence of events occurring during endothelium-dependent contractions firstly requires the intracellular accumulation of calcium, which then most likely induces the phospholipase A2-dependent mobilization of arachidonic acid, COX-1 activation and the production of reactive oxygen species along with that of EDCF(s). Reactive oxygen species inactivate NO, can be involved in a positive feedback loop on the endothelial cells by further activating COX-1 and can diffuse towards the vascular smooth muscle cells and produce contraction. PGH₂ along with the various prostaglandins produced by the PG-synthases, prostacyclin (PGI₂), thromboxane A2 (TXA₂) and possibly under some circumstances PGE₂ or PGF_2α, converge towards the TP receptors located on the vascular smooth muscle cells and produce contraction. In the SHR, a functional impairment of the smooth muscle IP receptors contributes to the endothelial dysfunction. COX, cyclooxygenase; EDCF, endothelium-derived contracting factor; NOS, nitric oxide synthase; SHR, spontaneously hypertensive rats; WKY, Wistar-Kyoto rats.