Differential antivasoconstrictor effects of levcromakalim and rilmakalim on the isolated human mammary artery and saphenous vein

It is well established that spasm of an arterial and venous graft conduit may occur during harvesting or after coronary artery bypass grafting (CABG). The antivasoconstrictor effect of levcromakalim and rilmakalim, K+ channel openers (KCOs), was studied in isolated human internal mammary artery (HIMA) and human saphenous vein (HSV) prepared for CABG. HIMA and HSV rings were contracted by electrical field stimulation (EFS, 20 Hz) or with exogenous noradrenaline (NA). Levcromakalim induced a concentration-dependent and equipotent inhibition of contraction of HIMA and HSV preconstricted by EFS and exogenoulsy applied NA, while rilmakalim produced a stronger inhibition of EFS- than NA-evoked contractions. Glibenclamide, a selective ATP-sensitive K+ channel (K-ATP channel) blocker, significantly antagonized levcromakalim-induced inhibition of EFS- and NA-evoked contractions, as well as rilmakalim-induced inhibiton of EFS-evoked contractions on HIMA and HSV. However, glibenclamide failed to antagonize rilmakalim-induced inhibition of NA-evoked contractions. The results suggest that the antivasoconstrictor effect of levcromakalim occurs postsynapticaly by the opening K-ATP channels in the vascular smooth muscle cells. They also suggest that the effect of rilmakalim on EFS-evoked contractions involves K-ATP channels located pre-synaptically. However, the mechanism by which rilmakalim inhibits NA-evoked contraction seems to be K-ATP channel independent and warrants further elucidation.