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GASTROINTESTINAL, HEPATIC, PULMONARY, AND RENAL

Levcromakalim and MgGDP Activate Small Conductance ATP-Sensitive K⁺ Channels of K⁺ Channel Pore 6.1/Sulfonylurea Receptor 2A in Pig Detrusor Smooth Muscle Cells: Uncoupling of cAMP Signal Pathways

University Department of Pharmacology, Oxford, United Kingdom (S.K., A.F.B.); Department of Cell Physiology, Nagoya University Graduate School of Medicine, Showa-ku, Nagoya, Japan (S.Nak.); Department of Biomedical Sciences, College of Life and Health Sciences, Chubu University, Kasugai, Aichi, Japan (H.A.); and Department of Urology, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan (N.S., S.Nai.)

Pharmacological studies have suggested the existence of ATP-sensitive K⁺ (K_ATP) channel as a therapeutic target in urinary bladders; however, electrical properties have not yet been shown. Patch-clamp techniques were applied to investigate the properties of K_ATP channels in pig detrusor cells. In whole-cell configuration, levcromakalim, a K_ATP channel opener, induced a long-lasting outward current in a concentration-dependent manner. The current-voltage curve of the levcromakalim-induced membrane current intersected at approximately -80 mV. This current was abolished by glibenclamide. Intracellular application of 0.1 mM GDP significantly enhanced the levcromakalim-induced membrane current, whereas cAMP did not. Furthermore, neurotransmitters related to cAMP signaling, such as calcitonin gene-related peptide, vasointestinal peptide, adenosine, and somatostatin, had little effect on the membrane current. In cell-attached configuration, levcromakalim activated K⁺ channels with a unitary conductance of 12 pS. When the patch configuration was changed to inside-out mode, the K⁺ channel activity ran down. Subsequent application of 1 mM GDP reactivated the channels. The openings of the 12 pS K⁺ channels in the presence of 1 mM GDP was suppressed by ATP and glibenclamide. In reverse transcription-polymerase chain reaction, K⁺ channel pore 6.1 and sulfonylurea receptor (SUR)2A were predominant in pig detrusor cells. The 12 pS K⁺ channel activated by levcromakalim in pig detrusor smooth muscle cells is a K_ATP channel. The predominant expression of SUR2A can account for the lack of effect of neurotransmitters related to cAMP.

Address correspondence to: Dr. Shinsuke Nakayama, Department of Cell Physiology, Nagoya University Graduate School of Medicine, 65 Tsuruma-cho, Showa-ku, Nagoya 466-8550, Japan. E-mail: h44673a{at}nucc.cc.nagoya-u.ac.jp