Identification of a specific binding site for K+ channel openers in rat aorta.

The K+ channel openers (KCOs) form a structurally heterogeneous group of compounds which relax smooth muscle by opening K+ channels in the plasmalemma. At present it is not known whether these drugs open the same K+ channel in smooth muscle and, if so, whether they bind to the same site of this channel. To address these questions we present the first binding study with KCOs in a smooth muscle preparation. In intact rat aortic strips, the novel tritiated KCO, 3H-P1075 (N-cyano-N'-(1,1-dimethyl[2,2,3,3(3)H]propyl)-N"-3-pyridylguanidine++ +), a potent pinacidil analogue, showed saturable specific binding of high affinity (KD = 6 +/- 1 nM, Bmax = 23 +/- 3 fmol/mg tissue wet weight). Specific binding of 3H-P1075 was inhibited stereospecifically by representatives from all major families of K+ channel openers, with a rank order of potency that correlated well with the potencies for vasorelaxation in rat aorta. The sulfonylurea glibenclamide, a relatively specific blocker of ATP-sensitive K+ channels and an inhibitor of the effects of KCOs, also inhibited 3H-P1075 binding and increased the rate of dissociation of 3H-P1075 from the tissue in a concentration-dependent manner. Lowering temperature (from 37 degrees C to 2 degrees C) and decreasing intracellular ATP levels by metabolic poisoning, diminished specific 3H-P1075 binding by reducing Bmax. However, depolarization (KCl = 55 mM) or lowering pH from 7.4 to 6.0 did not influence binding. The data demonstrate the existence of a specific binding site for the KCO 3H-P10756 in rat isolated aorta, which seems to be of functional relevance.(ABSTRACT TRUNCATED AT 250 WORDS)