Activating of ATP-Dependent K⁺ Channels Comprised of K_ir 6.2 and SUR 2B by PGE₂ Through EP₂ Receptor in Cultured Interstitial Cells of Cajal from Murine Small Intestine

Abstract

The interstitial cells of Cajal (ICC) are pacemaker cells in gastrointestinal tract and generate an electrical rhythm in gastrointestinal muscles. We investigated the possibility that PGE₂ might affect the electrical properties of cultured ICC by activating ATPdependent K⁺ channels and, the EP receptor subtypes and the subunits of ATP-dependent K⁺ channels involved in these activities were identified. In addition, the regulation of intracellular Ca²⁺ ([Ca²⁺]i) mobilization may be involved the action of PGE₂ on ICC. Treatments of ICC with PGE₂ inhibited electrical pacemaker activities in the same manner as pinacidil, an ATPdependent K⁺ channel opener and PGE₂ had only a dose-dependent effect. Using RT-PCR technique, we found that ATP-dependent K⁺ channels exist in ICC and that these are composed of K_ir 6.2 and SUR 2B subunits. To characterize the specific membrane EP receptor subtypes in ICC, EP receptor agonists and RT-PCR were used: Butaprost (an EP₂ receptor agonist) showed the actions on pacemaker currents in the same manner as PGE₂. However sulprostone (a mixed EP₁ and EP₃ agonist) had no effects. In addition, RT-PCR results indicated the presence of the EP₂ receptor in ICC. To investigate cAMP involvement in the effects of PGE₂ on ICCs, SQ-22536 (an inhibitor of adenylate cyclase) and cAMP assays were used. SQ-22536 did not affect the effect of PGE₂ on pacemaker currents, and PGE₂ did not stimulate cAMP production. Also, we found PGE₂ inhibited the spontaneous [Ca²⁺]i oscillations in cultured ICC. These observations indicate that PGE₂ alters pacemaker currents by activating the ATP-dependent K⁺ channels comprised of K_ir 6.2-SUR 2B in ICC and this action of PGE₂ are through EP₂ receptor subtype and also the activation of ATP-dependent K⁺ channels involves intracellular Ca²⁺ mobilization.

5-Hydroxytryptamine Generates Tonic Inward Currents on Pacemaker Activity of Interstitial Cells of Cajal from Mouse Small Intestine

“…In our previous study, we showed prostaglandin E2 and F2α have actions on pacemaker activity in ICC [24,29]. Whether or not the effect of 5-HT on ICC is by COX induction was not proven in the current study, thus further studies are needed.…”

“…It is well-known that inositol 1,4,5-triphosphate receptor plays an important role for generating spontaneous electrical activity in pacemaker cells [22] ]i oscillations in ICC from mouse ileum [23], and these actions in ICC are considered to be the primary pacemaker activity in the gut. Furthermore in our previous reports, we showed spontaneous [Ca 2＋ ]i oscillations in ICC and found that carbachol and PGE2 regulated the [Ca 2＋ ]i oscillations [24,25], suggesting that [Ca 2＋ ]i in ICC is the important element regulating pacemaker activity. In the studies conducted on 5-HT, the intracellular calcium was increased and returned back to normal conditions showing a similar effect as electrophysiologic studies.…”

The Inhibitory Effects of Hydrogen Sulfide on Pacemaker Activity of Interstitial Cells of Cajal from Mouse Small Intestine

“…Activation of ATP-sensitive K ＋ channels produces hyperpolarization of the cell membrane [11]. Previously, we reported that ATP-sensitive K ＋ channels existed in mouse intestinal ICC [12]. Related to this, there are some reports that the activation of ATP-sensitive channels by NaHS was proposed as the consequence of ATP depletion due to the inhibition by sulfide of the oxidative phosphorylation [10], with partial involvement of ATP-sensitive K ＋ channels on NaHS-induced contraction and relaxation in rat and mouse aorta [13].…”

(-)-Epigallocatechin Gallate Inhibits the Pacemaker Activity of Interstitial Cells of Cajal of Mouse Small Intestine

“…Activation of ATP-sensitive K ＋ channels produces hyperpolarization of cell membrane, thus decreasing Ca 2＋ influx and inhibiting cell excitability (Rodrigo and Standen, 2005). We also reported that ATP-sensitive K ＋ channels exist in murine intestinal ICC (Choi et al, 2006). Furthermore EGCG has been reported to inhibit ATP-sensitive K ＋ channel which is widely distributed in human tissues, and also all subtypes of ATP-sensitive K ＋ channel (Baek et al, 2005).…”