Two novel cardiac atrial K⁺ channels, I_K.AA and I_K.PC

Two K⁺-selective channels in neonatal rat atrial cells activated by lipophilic compounds have been characterized in detail. The arachidonic acid-stimulated channel (I_K.AA) had a slope conductance of 124 ± 17 pS at +30 mV in symmetrical 140 mM potassium and a mean open time of ∼1 ms, and was relatively voltage independent. I_K.AA activity was reversibly increased by lowering pH to 6.0. Arachidonic acid was most effective in activating this channel, although a number of lipophilic compounds resulted in activation. Surprisingly, choline, a polar molecule, also activated the channel. A second K⁺ channel was activated by 10 μM phosphatidylcholine applied to the intracellular surface of inside-out atrial patches. This channel (I_K.PC) had a slope conductance of 60 ± 6 pS at +40 mV and a mean open time of ∼0.6 ms, and was also relatively voltage independent. Fatty acids are probably monomeric in the membrane under the conditions of our recording; thus detergent effects are unlikely. Since a number of compounds including fatty acids and prostaglandins activated these two channels, an indirect, channel-specific mechanism may account for activation of these two cardiac K⁺ channels.