Effects of Cl^- channel blockers on the cardiac ATP-sensitive K⁺ channel

To explore whether Cl^- channel blockers interact with the ATP-sensitive K⁺ (K(ATP)) channel, I have examined the effect of two common Cl^- channel blockers on the K(ATP) channel activity in isolated rat ventricular myocytes using patch clamp techniques. In inside-out patches, 4,4'-diisothio- cyanatostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid applied to bath solution inhibited the K(ATP) channel activity in a concentration-dependent manner with IC₅₀ of 0.24 and 927 μM, respectively. The inhibitory action of DIDS was irreversible whereas that of niflumic acid was reversible. Furthermore, DIDS-induced block was not recovered despite exposure to ATP (1 mM). In cell-attached and inside-out patches, DIDS blocked the pinacidil- or 2,4-dinitrophenol (DNP)-induced K(ATP) channel openings. In contrast, niflumic acid did not block the pinacidil-induced K(ATP) channel openings in inside-out patches, but inhibited it in cell-attached patches. DIDS and niflumic acid produced additional block in the presence of ATP and did not affect current-voltage relationship and channel kinetics. All these results indicate that DIDS among Cl^- channel blockers specifically blocks the cardiac K(ATP) channel.