SYNERGISTIC EFFECTS OF GLYBURIDE AND U-37883A, 2 STRUCTURALLY DIFFERENT VASCULAR ATP-SENSITIVE POTASSIUM CHANNEL ANTAGONISTS

Glyburide, a sulfonylurea, and U-37883A, a guanidine (4-Morpholinecarb oximidine-N-1-Adamantyl-N' cyclohexylhydrochloride), have been previou sly characterized as antagonists of the vascular ATP-sensitive K+ chan nels (K(ATP)). In this report, the in vitro interaction between these two chemically distinct K(ATP) antagonists was investigated using isol ated rabbit mesenteric artery. The K(ATP) antagonism was functionally studied as the inhibition of vasodilation produced by various K(ATP) o peners as follows: pinacidil (1 muM), minoxidil sulfate (5 muM), croma kalim (0.5 muM) and RP-49356 (1 muM). Glyburide alone produced inhibit ion in the concentration range of 50 to 500 nM with the glyburide IC50 ranging from 72 to 148 nM. U-37883A alone produced inhibition in the concentration range of 0.5 to 5 muM, with the U-37883A IC50 ranging fr om 0.78 to 1.4 muM. In the presence of a threshold U-37883A concentrat ion of 0.5 muM, the glyburide inhibition dose-response curve against p inacidil was significantly shifted to the left and the glyburide IC50 was lowered from 72 to 3.9 nM, representing an 18-fold decrease. Simil arly, in the presence of a threshold glyburide concentration of 50 nM, the U-37883A inhibition dose-response curve against pinacidil was sig nificantly shifted to the left and the U-37883A IC50 was lowered from 780 to 96 nM, representing an eightfold decrease. Thus, glyburide and U-37883A potentiated each other's effects as K(ATP) blockers. This syn ergistic interaction between glyburide and U-37883A was observed indep endently of the pinacidil, cromakalim or minoxidil sulfate used to pro duce vasodilation. Furthermore, the synergistic eff ect of the glyburi de plus U-37883A combination was selective because this combination ha d no significant inhibitory effect when vasodilation was produced by D 600, a calcium channel antagonist, a vasodilator that does not utilize K(ATP) activation as a mechanism for its effect. These data collectiv ely suggest that the synergy between glyburide and U-37883A observed i s associated with the vascular K(ATP). A schematic model is presented suggesting that glyburide and U-37883A produce their synergistic effec t on the vascular K(ATP) either by acting at two different sites on a common protein or by acting at two different proteins that interact cl osely in modulating the function of the K(ATP) channel. An eventual un derstanding of the mechanism by which these compounds potentiate each other's effects may provide insight into the regulation of the vascula r K(ATP) activity.