J. MembraneBiol. 148, 287-297 (1995) The Joumal of Membrane Biology 9 Springer-Verlag New York Inc. 1995 Membrane Stretch Augments the Cardiac Muscarinic K + Channel Activity A. Pleumsamran, D. Kim Department of Physiology and Biophysics,Finch University of Health Sciences Chicago Medical School, 3333 Green Bay Road, North Chicago, IL 60064 Received: 5 June 1995/Revised: 8 September 1995 Abstract. Arachidonic acid has been shown to activate K+-selective, mechanosensitive ion channels in cardiac, neuronal and smooth muscle cells. Since the cardiac G protein (Gx)-gated, muscarinic K + (KAch) channel can also be activated by arachidonic acid, we investigated whether the KAch channel was also sensitive to mem- brane stretch. In the absence of acetylcholine (ACh), KAC h channels were not active, and negative pressure failed to activate these channels. With ACh (10 gM) in the pipette, applying negative pressure (0 to -80 mm Hg) to the membrane caused a reversible, pressure-dependent increase in channel activity in cell-attached and inside- out patches (100 BM GTP in bath). Membrane stretch did not alter the sensitivity of the Kac h channel to GTP. When G K was maximally activated with 100 gM GTPyS in inside-out patches, the KAch channel activity could be further increased by negative pressure. Trypsin (0.5 mg/ ml) applied to the membrane caused activation of the KAC h channel in the absence of ACh and GTP; KAC h channel activity was further increased by stretch. These results indicate that the atrial muscarinic K + channels are modulated by stretch independently of receptor/G pro- tein, probably via a direct effect on the channel protein/ lipid bilayer. Key words: Mechanosensitivity -- Arachidonic acid -- Muscarinic K channel -- Pressure -- G protein Introduction Acetylcholine binds to muscarinic receptors in atrial cells and activates an inwardly rectifying K § channel (KAch channel) via a GTP binding protein referred to as G K [1]. Other receptor agonists such as phenylephrine [14], and platelet-activating factor [18] have been re- Correspondence to: D. Kim ported to activate the KAch channel as a result of stim- ulation of phospholipase A a and formation of arachi- donic acid and its metabolites [18, 24]. Activation of the atrial KAC h channel by direct application of arachidonic acid (or its metabolites) has also been demonstrated [9, 13]. The mechanism of activation of the KAC h channel by arachidonic acid or its metabolites is not clearly known. In addition to activation of the Kac h channel in atrial cells, arachidonic acid has been shown to activate K +- selective ion channels in cardiac muscle cells [7], smooth muscle cells [11] and neuronal cells [10, 20]. In cardiac and smooth muscle cells, the K § channel could also be activated by other types of free fatty acids such as my- ristic or linoleic acids which are not substrates for arachi- donic acid-metabolizing enzymes (i.e., lipoxygenases). This suggests that the K + channel is directly modulated by the fatty acid itself [19]. Interestingly, the K + chan- nels modulated by arachidonic acid were also found to be sensitive to membrane stretch [7, 11, 30]. In these elec- trophysiological studies, application of negative pressure to the membrane patch via the pipette caused an increase in K + channel activity in a reversible, pressure-dependent manner. These findings suggest that certain K + channels modulated by arachidonic acid (and perhaps other fatty acids) could also be sensitive to membrane stretch. As the KAC h channel is able to be activated by arachidonic acid, we tested whether the KAC h channel was also sensitive to membrane stretch. We used cell- attached and inside-out patches from neonatal and adult rat atrial cells to study whether the membrane stretch produced by negative pressure applied via the patch pi- pette would augment the open probability of the Kac h channel under different experimental conditions. Since the channel is normally activated through the receptor- coupled G protein pathway, we examined the possible alteration in G protein-KAch channel interaction, and the involvement of the G protein in the stretch-induced changes in KAC h channel function.