Cocaine inhibits cromakalim-activated K⁺ currents in follicle-enclosed Xenopus oocytes

The effect of cocaine on K⁺ currents activated by the K _ATP channel opener cromakalim was investigated in follicular cells of Xenopus oocytes. The results indicate that cocaine in the concentration range of 3-500 μM reversibly inhibits cromakalim-induced K⁺ currents. The IC₅₀ value for cocaine was 96 μM. Inhibition of the cromakalim-activated K⁺ current by cocaine was noncompetitive and voltage independent. Pretreatment with the Ca²⁺ chelator BAPTA did not modify the cocaine-induced inhibition of cromakalim-induced K⁺ currents, suggesting that Ca²⁺-activated second messenger pathways are not involved in the actions of cocaine. Outward K⁺ currents activated by the application of 8-Br-cAMP or forskolin were also inhibited by cocaine. The EC₅₀ and slope values for the activation of K ⁺ currents by cromakalim were 184±19 μM and 1.14 in the absence of cocaine as compared to 191±23 μM and 1.03 in the presence of cocaine (300 μM). Cocaine also blocked K⁺ currents mediated through C-terminally deleted form of Kir6.2 (KirΔC26) in the absence of sulfonylurea receptor with an IC₅₀ value of 87 μM, suggesting that cocaine interacts directly with the channel forming Kir6.2 subunit. Radioligand binding studies indicated that cocaine (100 μM) did not affect the binding characteristics of the K_ATP ligand, [³H] glibenclamide. These results demonstrate that cromakalim-activated K ⁺ currents in follicular cells of Xenopus oocytes are modulated by cocaine.