Apelin Alters Vasomotor Tone in Cerebral Arteries by Inhibiting Endothelium‐Dependent Relaxations that are Mediated by Activation of Large Conductance, Calcium‐Activated K (BK Ca ) Channels

Plasma levels of the adipokine, apelin, are elevated in obesity, which is a known risk factor for cardiovascular disease. Apelin has complex vasoactive effects inasmuch as the peptide may cause either vasodilation or vasoconstriction depending on the vascular bed and experimental conditions. The vasomotor effects of apelin in cerebral arteries are unknown. We previously reported that apelin inhibits BK Ca channel currents in cerebral artery smooth muscle cells (PLoS One 8(12):e83051, 2013). Several endogenous vasodilators, including nitric oxide (NO) and endothelium‐derived hyperpolarizing factors(s) (EDHF), activate BK Ca channels to cause relaxation of vascular smooth muscle cells. Here, we tested the hypothesis that apelin inhibits endothelium‐dependent relaxation of cerebral arteries. Rat isolated cerebral arterial rings were suspended in myographs and contracted with 5‐HT (10 −7 M). Apelin (10 −8 M × 10 −6 M) itself had no direct vasomotor effect in cerebral arteries, with or without endothelium (n=5); however, bradykinin (10 −9 M‐10 −6 M) and A23187 (10 −9 M × 10 −5 M), a calcium ionophore, each caused endothelium‐dependent relaxation. Bradykinin‐induced relaxation was markedly inhibited by the NO synthase inhibitor, nitro‐l‐arginine (NLA; 3 × 10 −5 M). The response to A23187 was unaffected by either NLA or the cyclooxygenase inhibitor, indomethacin (10 −5 M), but was attenuated by MS‐PPOH (10 −5 M), a selective inhibitor of cytochrome p450 catalyzed synthesis of epoxyeicosatrienoic acids (EETs) from arachidonic acid. The NO‐dependent response to bradykinin was inhibited in the presence of apelin (10 −7 M) (pD 2 =8.02 ± 0.3 vs 7.04 ± 0.2, and E max = 83 ± 4 vs 55 ± 9% relaxation, in the absence and presence of apelin, respectively; n=5; p<0.05), as was the NO‐independent, EDHF‐like response to A23187 (7.06 ± 0.2 vs 6.37± 0.1, in the absence and presence of apelin, respectively; n=6; p<0.05). Iberiotoxin (10 −7 M), a selective BK Ca channel blocker, had inhibitory effects that were similar to apelin on relaxations induced by bradykinin and A23187 (n=5). The APJ receptor antagonist, F13A (10 −7 M), abolished the effects of apelin on bradykinin‐ and A23187‐induced relaxations. RT‐qPCR and immunoblot analyses confirmed the expression of APJ receptors in rat cerebral arteries (n=3–5). Immunofluorescence staining and confocal microscopy demonstrated that APJ receptors are present on smooth muscle cells in the cerebral arterial wall (n=3). Taken together, the results indicate that apelin regulates vascular tone in cerebral arteries by inhibiting relaxations evoked by endothelium‐derived NO, as well as an EDHF‐like mediator (possibly an EET). The inhibitory effect of apelin is likely mediated by activation of APJ receptors and inhibition of BK Ca channels present on vascular smooth muscle cells. These inhibitory effects of apelin could be expected to increase the risk of cerebrovascular dysfunction during conditions when plasma levels of apelin are elevated. Support or Funding Information Supported by grant HL124338 from the National Institutes of Health