The rank order of potency of a series of benzopyran and cyanoguanidine K+ channel openers (KCOs) for causing relaxation of the PGF2 alpha-precontracted porcine coronary artery was determined. Glyburide, an inhibitor of KATP channels, showed an apparent competitive inhibition of the vasorelaxant activity of the KCOs. The pA2 values of glyburide when cromakalim and CGP 14877 (P1060) were used as vasorelaxants were 7.66 and 7.83, respectively. Charybdotoxin (40 nM), an inhibitor of BKCa channels, also caused a significant inhibition of the cromakalim mediated relaxation of the porcine coronary artery. In order to clarify the site of action of these KCOs, we identified a K+ channel current in single porcine coronary arterial cells and measured channel activity in the presence of these compounds. The prominent K+ ion current in these cells had characteristics typical of the conventional large Ca(2+)-activated K+ channel (BKCa) present in other smooth muscle cells. Using symmetrical K+ concentrations, the channel had a conductance of 214 pS and was found to be sensitive to [Ca2+]i and membrane potential. The KCOs were found to reversibly increase the open probability (P(o)) of the channel without changing channel conductance. The potency of the KCOs to increase K+ channel opening was similar to the potency of these compounds to cause coronary artery relaxation. These results indicate that the porcine coronary artery contains the BKCa channel and that this channel, along with other types of K+ channels (KATP), mediate the vasorelaxant effects of K+ channel openers.