Periadventitial fat releases a vascular relaxing factor.

Virtually all blood vessels are surrounded by adventitial fat. Adipocytes produce a host of vasoactive substances that may influence vascular contraction. We tested whether or not perivascular adipose tissue modulates contraction of aortic ring preparations. We studied aortic rings surrounded by periadventitial adipose tissue from adult Sprague-Dawley rats. At a maximum concentration of 300 nM angiotensin II, 6.5 microM serotonin, and 5 microM phenylephrine, the contractile response of intact rings was 95%, 80%, and 30% lower than that of vessels without periadventitial fat. The anticontractile effect of periadventitial fat was reduced by inhibition of ATP-dependent K+ channels with glibenclamide (3 microM) and by the tyrosine kinase inhibitor genistein (10 microM). Blocking NOS, cyclo-oxygenase, cytochrome P450, or adenosine receptors did not restore the vascular response in intact vessels. The anticontractile effect of perivascular fat was present in Zucker fa/fa rats, suggesting that leptin receptors were not responsible. Transferring the bath solution from intact vessels, isolated periadventitial tissue, and cultured rat adipocytes to precontracted vessels lacking periadventitial fat resulted in a rapid relaxation. We suggest that perivascular adventitial adipose tissue releases a transferable adventitium-derived relaxing factor that acts by tyrosine kinase-dependent activation of K+ channels in vascular smooth muscle cells.

Cytochrome P450-dependent eicosapentaenoic acid metabolites are novel BK channel activators.

P450-dependent arachidonic acid (AA) metabolites regulate arterial tone by modulating calcium-activated (BK) potassium channels in vascular smooth muscle cells (VSMC). Because eicosapentaenoic acid (EPA) has been reported to improve vascular function, we tested the hypothesis that P450-dependent epoxygenation of EPA produces alternative vasoactive compounds. We synthesized the 5 regioisomeric epoxyeicosattrienoic acids (EETeTr) and examined them for effects on K(+) currents in rat cerebral artery VSMCs with the patch-clamp technique. 11(R),12(S)-epoxyeicosatrienoic acid (50 nmol/L) was used for comparison and stimulated K(+) currents 6-fold at +60 mV. However, 17(R),18(S)-EETeTr elicited a more than 14-fold increase. 17(S),18(R)-EET and the remaining four regioisomers were inactive. The effect of 17(R),18(S)-EETeTr was blocked by tetraethylammonium but not by 4-aminopyridine. VSMCs expressed P450s 4A1 and 4A3. Recombinant P450 4A1 hydroxylated EPA at C-19 and C-20 and epoxygenated the 17,18-double bond, yielding the R, S- and S, R-enantiomers in a ratio of 64:36. We conclude that 17(R),18(S)-EETeTr represents a novel, potent activator of BK potassium channels. Furthermore, this metabolite can be directly produced in VSMCs. We suggest that 17(R),18(S)-EETeTr may function as an important hyperpolarizing factor, particularly with EPA-rich diets.