Iptakalim modulates ATP-sensitive K⁺ channels in dopamine neurons from rat substantia nigra pars compacta

Iptakalim, a novel cardiovascular ATP-sensitive K⁺ (K _ATP) channel opener, exerts neuroprotective effects on dopaminergic (DA) neurons against metabolic stress-induced neurotoxicity, but the mechanisms are largely unknown. Here, we examined the effects of iptakalim on functional K_ATP channels in the plasma membrane (pm) and mitochondrial membrane using patch-clamp and fluorescence-imaging techniques. In identified DA neurons acutely dissociated from rat substantia nigra pars compacta (SNc), both the mitochondrial metabolic inhibitor rotenone and the sulfonylurea receptor subtype (SUR) 1-selective K_ATP channel opener (KCO) diazoxide induced neuronal hyperpolarization and abolished action potential firing, but the SUR2B-selective KCO cromakalim exerted little effect, suggesting that functional K_ATP channels in rat SNc DA neurons are mainly composed of SUR1. Immunocytochemical staining showed a SUR1- rather than a SUR2B-positive reaction in most dissociated DA neurons. At concentrations between 3 and 300 μM, iptakalim failed to hyperpolarize DA neurons; however, 300 μM iptakalim increased neuronal firing. In addition, iptakalim restored DA neuronal firing during rotenone-induced hyperpolarization and suppressed rotenone-induced outward current, suggesting that high concentrations of iptakalim close neuronal K_ATP channels. Furthermore, in human embryonic kidney 293 cells, iptakalim (300-500 μM) closed diazoxide-induced Kir6.2/SUR1 K_ATP channels, which were heterologously expressed. In rhodamine-123-preloaded DA neurons, iptakalim neither depolarized mitochondrial membrane nor prevented rotenone-induced mitochondrial depolarization. These data indicate that iptakalim is not a K_ATP channel opener in rat SNc DA neurons; instead, iptakalim is a pm-K_ATP channel closer at high concentrations. These effects of iptakalim stimulate further pharmacological investigation and the development of possible therapeutic applications.