Endothelium-dependent contractions: when a good guy turns bad!

Abstract

Endothelial cells can induce contractions of the underlying vascular smooth muscle by generating vasoconstrictor prostanoids (endothelium-dependent contracting factor; EDCF). The endothelial COX-1 isoform of cyclooxygenase appears to play the dominant role in the phenomenon. Its activation requires an increase in intracellular Ca(2+) concentration. The production of EDCF is inhibited acutely and chronically by nitric oxide (NO), and possibly by endothelium-dependent hyperpolarizing factor (EDHF). The main prostanoids involved in endothelium-dependent contractions appear to be endoperoxides (PGH(2)) and prostacyclin, which activate thromboxane-prostanoid (TP) receptors of the vascular smooth muscle cells. Oxygen-derived free radicals can facilitate the production and/or the action of EDCF. Endothelium-dependent contractions are exacerbated by ageing, obesity, hypertension and diabetes, and thus are likely to contribute to the endothelial dysfunction observed in older people and in essential hypertensive patients.

Figure 1

Upper panel: a donor strip is stitched onto the bioassay tissue creating a ‘sandwich’-like layered preparation. Isometric tension is recorded from the bioassay strip and the donor tissue does not directly contribute to the recorded response. Lower panel: acetylcholine-induced contractions only occurred when the donor strip contained endothelium. The experiment was performed in the presence of nitro-l-arginine and tetrahydrobiopterin to optimize the EDCF-mediated response (reproduced from Vanhoutte et al. 2005, with permission).

Figure 2

The mRNA expression of COX-1, measured by RT-PCR in freshly isolated endothelial cells, was significantly higher in 36-week-old SHR compared to 36-week-old WKY (n = 6). There was no difference in gene expression of COX-1 in smooth muscle cells between 36-week-old WKY and SHR (n = 8). Data are means ±s.e.m.*P < 0.05 (data from Tang & Vanhoutte, 2008b, reproduced with permission).

Figure 3

A, representative merged images taken by confocal microscopy showing responses to infusion of acetylcholine (3 × 10⁻⁶m) or A23187 (10⁻⁶m) on cytosolic calcium of aortic endothelial cells from WKY and SHR. The addition of acetylcholine caused a rapid increase in intracellular calcium in aortic endothelial cells of both WKY and SHR (indicated by an increase in green fluorescence and a decrease of red fluorescence), which was greater in the latter. The calcium ionophore A23187 caused a comparable increase in intracellular calcium in preparations from WKY and SHR. B, the increase in fluorescence ratio in endothelial cells from WKY and SHR in response to acetylcholine (3 × 10⁻⁶m) (left) or A23187 (10⁻⁶m) (right) is expressed in percentages of the baseline values. Data are shown as means ±s.e.m.; n = 5. *P < 0.05 WKY versus SHR. Acetylcholine caused greater calcium increase in aortic endothelial cells of SHR than WKY, while A23187 caused a comparable response in the two strains. The increase of calcium was not affected by treatment with indomethacin, tiron plus diethyldithlocarbonate acid (DETCA; an inhibitor of superoxide dimutase) or N^G-nitro-l-argine methylestes (L-NAME; an inhibitor of nitric oxide synthase) (reproduced from Tang et al. 2007, with permission). The experiment was performed in the presence of S18886, a TP-receptor antagonist, to prevent contraction of the smooth muscle.

Figure 4

The chain of events leading to the occurrence of endothelium-dependent contractions first involves an abnormal increase in intracellular calcium (which can be evoked by receptor-dependent agonists, such as acetylcholine or ADP, or mimicked with calcium-increasing agents, such as the calcium ionophore A23187) that presumably activates phospholipase A₂ to release arachidonic acid. The endothelial COX-1 isoform metabolizes the fatty acid into endoperoxides which per se are EDCF or are transformed predominantly into prostacyclin that subsequently causes contraction by activating the TP receptors of the underlying vascular smooth muscle cells. Reactive oxygen species generated in the endothelium may reach the smooth muscle layer by passive diffusion or through myoendothelial gap junctions and serve to amplify TP receptor-mediated contractions by activating the cyclooxygenase of the vascular smooth muscle. AA, arachidonic acid; ACh, acetycholine; ADP, adenosine diphosphate; m, muscarinic receptors; P, purinergic receptors; PGD₂, prostaglandin D₂; PGE₂, prostaglandin E₂; PGF_2α, prostaglandin F_2α; PGI₂, prostacyclin; PLA₂, phospholipase A₂; ROS, reactive oxygen species; TXA₂, thromboxane A₂.