Effects of age and hyperlipidemia on prostacyclin receptor receptor-mediated relaxations

Abstract

The role of prostacyclin receptors (IP) in the consequences of ageing and hyperlipidemia on vascular responsiveness was investigated in the aorta of apolipoprotein E knockout (ApoE-/-; a well-established hyperlipidemic animal model) mice and their wild-type counterparts (C57BL/6 mice). ApoE-/- mice and age-matched wild-type mice were fed a normal or a high-fat highcholesterol diet for 29 weeks (starting at five weeks of age). Aortic rings were contracted with phenylephrine and relaxed with cumulative additions of increasing concentrations of iloprost (IP receptor agonist; 10-9-10-6 M), acetylcholine (muscarinic agonist;10-10-10-5 M) or UK14304 (α2-adrenergic agonist; 10-10-10-5 M). In young (five weeks old) wild-type mice, iloprost caused IP receptor-, endothelial nitric oxide (NO) synthase (eNOS)-, and soluble guanylyl cyclase (sGC)-dependent relaxations. Ageing (from five to 34 weeks) did not alter responses to iloprost but reduced relaxations to acetylcholine and UK14304 without altering the sensitivity of the smooth muscle to the NO donor detaNONOate. Western immunoblotting indicated that the IP and thromboxane-prostanoid (TP) receptor presences were comparable in aortae of young and older mice. While ageing combined with high-fat diet did not affect the relaxations to acetylcholine and UK14304, responses to iloprost were shifted from relaxations to contractions; the latter were due to activation of TP receptors, coupled with reduced IP receptor presence. Apolipoprotein E (ApoE) was present in the aortae of wild-type but not ApoE-/- mice and this presence was not affected by ageing but augmented by the high fat diet. With deletion of ApoE, relaxations to iloprost were potentiated but relaxations to acetylcholine and UK14304 were inhibited; the greater iloprost-induced relaxations were not associated with changes in protein presence of IP and TP receptors. Deletion of ApoE also unmasked an L-NAME-resistant response to iloprost in the ageing group and prevented the impact of high fat diet on relaxations to IP receptor stimulation. The latter is likely through inhibition of TP receptor activation since the presence of IP receptors was not different between wild-type and ApoE-/- mice after 29 weeks of high-fat diet.