Ligand-insensitive State of Cardiac ATP-sensitive K 1 Channels Basis for Channel Opening

The mechanism by which ATP-sensitive K 1 (K ATP ) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac K ATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. “Rundown” of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the K ATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac K ATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for K ATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell. key words: K ATP channel • nucleotide diphosphate • kinetic model • allosteric model • sulfonylurea i n t r o d u c t i o n ATP-sensitive K 1 (K ATP ) channels transduce cellular metabolic events into membrane potential changes (Ashcroft and Ashcroft, 1990; Lazdunski, 1994; Seino et al., 1996; Bryan and Aguilar-Bryan, 1997), which in heart muscle leads to shortening of action potential duration during ischemia (Nichols and Lederer, 1991; Findlay, 1994; Terzic et al., 1995). The defining property of K ATP channels is their inhibition by intracellular ATP (Noma, 1983). In cardiomyocytes, however, the ATP concentration ( z 5–10 mM) exceeds by . 100-fold the IC 50 value for K ATP channel closure. Thus, a change of two orders of magnitude in the ATP concentration would be required for channels to open, which does not occur even under extreme cellular hypoxia (Weiss and Hiltbrand, 1985; Decking et al., 1995, 1997), suggesting that additional modulators of K ATP channel opening are important. In this regard, intracellular nucleotide diphosphates are of particular importance since they favor opening of K ATP channels even within a cytosolic environment of high ATP concentration (Ashcroft and Ashcroft, 1990; Nichols and Lederer, 1991; Weiss and Venkatesh, 1993; Findlay, 1994; Terzic et al., 1994 d ; Elvir-Mairena et al., 1996). However, the mechanism of this action of nucleotide diphosphates remains controversial. A conventional assumption has been that nucleotide diphosphates competitively antagonize ATP at an inhibitory binding site on the channel protein (Dunne and Petersen, 1986; Kakei et al., 1986; Misler et al., 1986; Findlay, 1987; Bokvist et al., 1991; Nichols and Lederer, 1991; Ueda et al., 1997). However, this mechanism cannot fully explain K ATP channel opening since altered concentrations of cytosolic ATP and/or nucleotide diphosphates are not readily detectable, nor do they correlate with changes in K ATP channel function. Moreover, nucleotide diphosphates, such as ADP or UDP, induce channel opening in the absence of ATP (Findlay, 1988; Lederer and Nichols, 1989; Tung and Kurachi, 1991; Allard and Lazdunski, 1992; Forestier and Vivaudou, 1993; Terzic et al., 1994 a ) and can lose their ability to antagonize ATP-dependent channel inhibition under certain operative conditions of the channel (Deutsch and Weiss, 1993; Terzic et al., 1994 a ). Such nonuniform regulation of K ATP channel opening by nucleotide diphosphates has also been observed with other inhibitory ligands including sulfonylurea drugs (Venkatesh et al., 1991; Brady et al., 1996 b , 1998) and diadenosine polyphosphates (Jovanovic et al., 1996, 1997). These findings suggest that an operative condiAddress correspondence to Andre Terzic, M.D., Ph.D., Guggenheim 7, Mayo Clinic, Rochester, MN 55905. FAX: (507) 284-9111; E-mail: [email protected] on Jne 2, 2017 D ow nladed fom Published February 1, 1998