THE ROLE OF MYOENDOTHELIAL CELL CONTACT IN NON-NITRIC OXIDE-MEDIATED,NON-PROSTANOID-MEDIATED ENDOTHELIUM-DEPENDENT RELAXATION OF PORCINE CORONARY-ARTERY

1 Experiments were designed to analyse the requirement of myoendotheli al junctions by bradykinin-induced endothelium-dependent relaxations r esistant to N-G-nitro-L-arginine (L-NOARG) and indomethacin in porcine coronary arteries. 2 Rings of porcine coronary arteries were contract ed with the thromboxane receptor agonist, U46619 and relaxations to br adykinin recorded isometrically. All experiments were performed in the presence of indomethacin. Nitric oxide (NO)-mediated effects were blo cked by the NO synthase inhibitor L-NOARG (250 mu M) and myoendothelia l contacts inhibited by treatment with hypertonic solution containing D-mannitol or sucrose (each 180 mM) or the gap junctional uncoupling a gent 1-heptanol (2 mM). High [K+] solutions (40 mM) were used to probe a possible contribution of endothelium-derived hyperpolarizing factor (EDHF). 3 In the presence of endothelium, bradykinin induced concentr ation-dependent relaxations with a. mean EC(50) of 3.2 nM and a maximu m response of 95 +/- 1% of papaverine-induced relaxation (control curv e). 4 In the absence of endothelium, bradykinin failed to induce relax ations. Addition of cultured porcine aortic endothelial cells to the o rgan bath resulted in some relaxation and restored in part the relaxan t effect of bradykinin. This endothelial cell-mediated relaxant effect was completely abolished in the presence of 250 mu M L-NOARG. 5 Brady kinin-induced relaxations in endothelium-preserved rings were only sli ghtly suppressed by L-NOARG (86% of control). In vessels partially dep olarized by high extracellular [K+] (40 mM) relaxation was reduced to 72% of control. In the presence of L-NOARG, bradykinin failed to relax partially depolarized vessels. 6 In the presence of 2 mM 1-heptanol, 180 mM mannitol or 180 mM sucrose maximum relaxation to bradykinin was reduced to similar to 70%, i.e. to the same extent as in the presence of high [K+]. The remaining relaxation was sensitive to blockade by L -NOARG. 7 Tissue cyclic GMP content which reflects NO activity, was in creased about 4 fold by bradykinin (300 nM). This increase was unaffec ted by high [K+], heptanol or sucrose but blocked by L-NOARG. 8 Our re sults suggest that non-nitric oxide- and non-prostanoid-mediated endot helium-dependent relaxation of porcine coronary artery requires functi onally intact myoendothelial junctions.