Diadenosine polyphosphate-induced inhibition of cardiac K-ATP channels: Operative state-dependent regulation by a nucleoside diphosphate

It has been proposed that the regulatory action of mononucleotides, such as ATP and UDP, on cardiac ATP-sensitive K+ (K-ATP) channels is determined by the state of the channel. Recently, dinucleotides, such as diadenosine tetraphosphate (Ap(4)A) and diadenosine pentaphosphate (Ap(5)A), have been recognized as novel intracellular ligands of cardiac K-ATP channels. However, it is not known whether the state of K-ATP channels also determines the response of the channel to dinucleotides. Therefore, we examined the action of diadenosine polyphosphates on K-ATP channel activity during different operative channel states, using the inside-out patch clamp technique applied to patches excised from guinea-pig ventricular myocytes. Spontaneous openings of K-ATP channels (or operative condition 1) were inhibited by Ap(4)A and Ap(5)A. Addition of UDP, which on its own did not affect spontaneous openings of K-ATP channels, prevented Ap(4)A and Ap(5)A to inhibit spontaneous K-ATP channel activity. In contrast, following ''run-down'' of spontaneous channel activity, UDP induced K-ATP channel openings (or operative condition 2), but could no longer antagonize the inhibitory effect of Ap(4)A and Ap(5)A. We conclude that the inhibitory action of diadenosine polyphosphates on K-ATP channels can be blocked by UDP only when K-ATP channels are in operative condition 1, but not in operative condition 2. Thus, the operative condition of K-ATP channels determines the UDP-mediated regulation of the diadenosine polyphosphate-dependent inhibitory channel gating. This finding further suggests that the operative state of the cardiac K-ATP channel protein is a critical determinant of the regulatory action of an intracellular ligand.