title: Dual regulation of the ATP-sensitive potassium channel by caffeine.
abstract: ATP-sensitive potassium (K(ATP)) channels couple cellular metabolic status to changes in membrane electrical properties. Caffeine (1,2,7-trimethylxanthine) has been shown to inhibit several ion channels; however, how caffeine regulates K(ATP) channels was not well understood. By performing single-channel recordings in the cell-attached configuration, we found that bath application of caffeine significantly enhanced the currents of Kir6.2/SUR1 channels, a neuronal/pancreatic K(ATP) channel isoform, expressed in transfected human embryonic kidney (HEK)293 cells in a concentration-dependent manner. Application of nonselective and selective phosphodiesterase (PDE) inhibitors led to significant enhancement of Kir6.2/SUR1 channel currents. Moreover, the stimulatory action of caffeine was significantly attenuated by KT5823, a specific PKG inhibitor, and, to a weaker extent, by BAPTA/AM, a membrane-permeable Ca(2+) chelator, but not by H-89, a selective PKA inhibitor. Furthermore, the stimulatory effect was completely abrogated when KT5823 and BAPTA/AM were co-applied with caffeine. In contrast, the activity of Kir6.2/SUR1 channels was decreased rather than increased by caffeine in cell-free inside-out patches, while tetrameric Kir6.2LRKR368/369/370/371AAAA channels were suppressed regardless of patch configurations. Caffeine also enhanced the single-channel currents of recombinant Kir6.2/SUR2B channels, a nonvascular smooth muscle K(ATP) channel isoform, although the increase was smaller. Moreover, bidirectional effects of caffeine were reproduced on the K(ATP) channel present in the Cambridge rat insulinoma G1 (CRI-G1) cell line. Taken together, our data suggest that caffeine exerts dual regulation on the function of K(ATP) channels: an inhibitory regulation that acts directly on Kir6.2 or some closely associated regulatory protein(s), and a sulfonylurea receptor (SUR)-dependent stimulatory regulation that requires cGMP-PKG and intracellular Ca(2+)-dependent signaling.
authors.affiliation: Dept. of Physiology, University of California, Davis, CA 95616, USA.
authors.forename: Xia
authors.initials: X
authors.lastname: Mao
authors.forename: Yongping
authors.initials: Y
authors.lastname: Chai
authors.forename: Yu-Fung
authors.initials: YF
authors.lastname: Lin
mesh_terms: D012368:4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002227:Carbazoles / Q000494:pharmacology; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D015640:Ion Channel Gating / Q000187:drug effects*; D007515:Islets of Langerhans / Q000187:drug effects; D007546:Isoquinolines / Q000494:pharmacology; D009130:Muscle, Smooth / Q000378:metabolism; D010179:Pancreas / Q000378:metabolism; D015221:Potassium Channels / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms; D011688:Purinones; D051381:Rats; D013449:Sulfonamides / Q000494:pharmacology
chemical_list: D002227:Carbazoles; D007211:Indoles; D007328:Insulin; D007546:Isoquinolines; D015221:Potassium Channels; D020033:Protein Isoforms; D011688:Purinones; D013449:Sulfonamides; C073601:KT 5823; D012368:4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone; D002110:Caffeine; D000255:Adenosine Triphosphate; C011145:zaprinast; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide