Myorelaxant action of fluorine-containing pinacidil analog, flocalin, in bladder smooth muscle is mediated by inhibition of l-type calcium channels rather than activation of K_ATP channels

Abstract

Flocalin (FLO) is a new ATP-sensitive K⁺ (K_ATP) channel opener (KCO) derived from pinacidil (PIN) by adding fluorine group to the drug’s structure. FLO acts as a potent cardioprotector against ischemia-reperfusion damage in isolated heart and whole animal models primarily via activating cardiac-specific Kir6.2/SUR2A K_ATP channels. Given that FLO also confers relaxation on several types of smooth muscles and can partially inhibit l-type Ca²⁺ channels, in this study, we asked what is the mechanism of FLO action in bladder detrusor smooth muscle (DSM). The actions of FLO and PIN on contractility of rat and guinea pig DSM strips and membrane currents of isolated DSM cells were compared by tensiometry and patch clamp. Kir6 and SUR subunit expression in rat DSM was assayed by reverse transcription PCR (RT-PCR). In contrast to PIN (10 μM), FLO (10 μM) did not produce glibenclamide-sensitive DSM strips’ relaxation and inhibition of spontaneous and electrically evoked contractions. However, FLO, but not PIN, inhibited contractions evoked by high K⁺ depolarization. FLO (40 μM) did not change the level of isolated DSM cell’s background K⁺ current, but suppressed by 20 % l-type Ca²⁺ current. Determining various Kir6 and SUR messenger RNA (mRNA) expressions in rat DSM by RT-PCR indicated that dominant K_ATP channel in rat DSM is of vascular type involving association of Kir6.1 and SUR2B subunits. Myorelaxant effects of FLO in bladder DSM are explained by partial blockade of l-type Ca²⁺ channel-mediated Ca²⁺ influx rather than by hyperpolarization associated with increased K⁺ permeability. Thus, insertion of fluorine group in PIN’s structure made the drug more discriminative between Kir6.2/SUR2A cardiac- and Kir6.1/SUR2B vascular-type K_ATP channels and rendered it partial l-type Ca²⁺ channel-blocking potency.