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

Abstract

Muscarinic receptors of cardiac pacemaker and atrial cells are linked to a potassium channel (I_K.Ach) by a pertussis toxin-sensitive GTP-binding protein^1,2,3. The dissociation of G-proteins leads to the generation of two potential transducing elements, α-GTP and βγ^4–6. I_K.ACH is activated by G-protein α- and βγ-subunits applied to the intracellular surface of inside-out patches of membrane^7–10. 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 I_K.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 I_K.ACH by stimulating the production of lipoxygenase-derived second messengers.