G-protein beta Gamma-subunits activate the cardiac muscarinic K+-channel via phospholipase A2

Muscarinic receptors of cardiac pacemaker and atrial cells are linked to a potassium channel (IK.Ach) by a pertussis toxin-sensitive GTP-binding protein1,2,3. The dissociation of G-proteins leads to the generation of two potential transducing elements, α-GTP and βγ4–6. IK.ACH is activated by G-protein α- and βγ-subunits applied to the intracellular surface of inside-out patches of membrane7–10. βγ has been shown to activate the membrane-bound enzyme phospholipase A2 in retinal rods11. Arachidonic acid, which is produced from the action of phospholipase A2 on phospholipids, is metabolized to compounds which may act as second messengers regulating ion channels in Aplysia12. Muscarinic receptor activation leads to the generation of arachidonic acid in some cell lines13. We therefore tested the hypothesis that βγ activates IK.ACH by stimulation of phospholipase A2. When patches were first incubated with antibody that blocks phospholipase A2 activity14, or with the lipoxygenase inhibitor, nordihydroguaiaretic acid, βγ failed to activate I K.ACH· Arachidonic acid and several of its metabolites derived from the 5-lipoxygenase pathway, activated the channel. Blockade of the cyclooxygenase pathway did not inhibit arachidonic acid-induced channel activation. We conclude that the βγ-subunit of G-proteins activates IK.ACH by stimulating the production of lipoxygenase-derived second messengers.