Cardiac ATP-sensitive K⁺ channel: A target for diadenosine 5′,5″-P¹,P⁵-pentaphosphate

In numerous studies the intracellular mononucleotide-dependent gating of ATP-sensitive K⁺ (K_ATP) channels has been demonstrated. However, it is not known whether dinucleotide polyphosphates, a family of endogenous compounds structurally-related to ATP, could also modulate this ion conductance. Therefore, in the present study we assessed the direct effect of diadenosine 5′,5″-P¹,P⁵-pentaphosphate (Ap₅A) on cardiac K_ATP channel activity using the inside-out configuration of the patch-clamp technique. Addition of Ap₅A (50 μM) to the internal side of membrane patches, excised from guinea-pig ventricular cells, strongly inhibited K_ATP channel activity. The estimated NP_o (where N is the number of channels in the patch and P_o the open probability of each channel) was 4.16 ± 0.50 in the absence and 0.85 ± 0.30 in the presence of Ap₅A (50 μM). This effect of Ap₅A was partially reversible, and the NP₀ was 2.26 ± 0.60 after washout of Ap₅A. Exposure of K_ATP channels to increasing concentrations of Ap₅A revealed that the Ap₅A-induced inhibition is concentration-dependent with the half-maximal effective concentration of 16 μM (Hill coefficient: 1.6). On the basis of these results, we conclude that Ap₅A is a potent antagonist of the K_ATP channel activity. This represents a previously unrecognized property of Ap₅A, as well as the discovery of a potentially novel endogenous ligand of myocardial K_ATP channels.