Activation of APJ Receptors by CMF-019, but not Apelin, Causes Endothelium-Dependent Relaxation of Spontaneously Hypertensive Rat Coronary Arteries.

Hypertension is a major risk factor for coronary artery disease, but the pathophysiological mechanisms linking these diseases are not completely understood. Apelin is a novel vasoactive adipokine. Receptors for apelin (APJ receptors) are G-protein-coupled receptors and are widely expressed throughout the cardiovascular system. APJ receptors can also signal via G-protein-independent pathways, including G-protein-coupled-receptor kinase 2 (GRK2), which inhibits nitric oxide synthase (eNOS) activity and nitric oxide (NO) production in endothelial cells. Apelin causes endothelium-dependent, NO-mediated relaxation of coronary arteries from normotensive animals, but the effects of activating APJ receptor signaling pathways in hypertensive coronary arteries are unknown. Since endothelium-dependent relaxations are often blunted in arteries from hypertensive animals, we tested the hypothesis that apelin-induced relaxation is impaired in coronary arteries from spontaneously hypertensive rats (SHR). Moreover, we hypothesized that endothelium-dependent relaxations in response to CMF-019, an APJ receptor G-protein biased agonist with little effect on GRK2, would be unaffected in SHR coronary arteries. Western blot and RT-qPCR analysis demonstrated that APJ receptor protein and mRNA expression did not differ between normotensive WKY (Wistar-Kyoto) and SHR coronary arteries. eNOS protein expression was also similar between SHR and WKY coronary arteries, but GRK2 expression was significantly increased in SHR coronary endothelial cells, as compared to WKY. In myograph studies, apelin (10 -10 M) caused endothelium-dependent relaxation of isolated WKY coronary arterial rings (pD = 7.00 ± 0.11; E = 54 ± 4% relaxation; n=6), but had no effect in arteries from SHR. By contrast, CMF-019 (10 -10 M)-induced relaxation did not differ between normotensive and hypertensive coronary arteries. The relaxation response to CMF-019 was abolished by the APJ receptor antagonist, F13A (10 M), by removal of the endothelium, and by inhibition of eNOS with nitro- L-arginine (3 x 10 M). Moreover, the GRK2 inhibitor, CMPD 101 (3 x 10 M), partially restored the relaxation response to apelin in SHR coronary arteries (pD = 6.56 ± 0.22; E = 46 ± 6% relaxation; n=6). Immunoblot analysis of SHR coronary endothelial cells demonstrated that CMF-019 (10 M) significantly increased eNOS phosphorylation whereas apelin (10 M) had no effect. We conclude that apelin failed to cause relaxation in hypertensive coronary arteries, likely due to impaired eNOS activity resulting from APJ receptor signaling via the GRK2 pathway and decreased eNOS phosphorylation. APJ receptor activation by the biased agonist, CMF-019, caused similar endothelium-dependent relaxations in WKY and SHR coronary arteries. Thus, APJ receptor-biased agonists, such as CMF-019, may be more effective than apelin in causing vasodilation of hypertensive coronary arteries.