Leukotriene C₄ modulation of muscarinic K⁺ current activation in bullfrog atrial myocytes

The effects of leukotriene C₄ (LTC₄) on activation of muscarinic acetylcholine receptor (mAChR) - stimulated, inwardly rectifying K⁺ current (I_K[ACh]) were examined in single bullfrog atrial myocytes using the whole-cell patch clamp technique. LTC₄ produced a reversible, concentration-dependent increase in steady-state, guanosine-γ-thiotriphosphate (GTPγS)-activated I_K[ACh], with a K_0.5 of 3.1 μM. LTC₄ also increased the rate of GTPγS-mediated I_K[ACh] activation, both in the absence and presence of 1 nM ACh, with comparable K_0.5 values of 4.7 μM under basal conditions and 4.9 μM in the presence of 1 nM ACh. LTC₄ did not alter the relative affinities of the G protein, G_k, for GTPγS and GTP. We hypothesize that all of the effects of LTC₄ on the kinetics of G_k-mediated I_K[ACh] activation are produced at a common site with a K_0.5 of 3-5 μM. The effects of LTC₄ on I_K[ACh] activation are fully reversible in the presence of GTPγS. Under physiological conditions (i.e., intracellular GTP), 10 μM LTC₄ increased the ACh-activated peak I_K[ACh]. Inhibitors of cellular LTC₄ production, including 5,8,11,14-eicosatetraynoic acid, baicalein, cinnamyl-3,4-dihydroxy-α-cyanocinnamate, and α-pentyl-4-(2-quinolinylmethoxy)-benzene methanol, greatly attenuated ACh-dependent I_K[ACh] activation, preventing activation of peak, and producing a lower steady-state I_K[ACh] (when compared with the control response in the same cell). Addition of exogenous LTC₄ was able to overcome the effects of LTC₄ synthesis inhibitors, restoring both the peak and steady-state I_K[ACh] responses. Although the mechanism of LTC₄-mediated modulation of I_K[ACh] activation is not known, our results suggest that endogenously produced lipoxygenase metabolites of arachidonic acid, specifically LTC₄, are involved in the physiological process of I_K[ACh] activation.