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 (Ap₄A) and diadenosine pentaphosphate (Ap₅A), 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 Ap₄A and Ap₅A. Addition of UDP, which on its own did not affect spontaneous openings of K_ATP channels, prevented Ap₄A and Ap₅A 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 Ap₄A and Ap₅A. 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.