Activation of the human, intermediate-conductance, Ca2+-activated K+ channel by methylxanthines.
This study demonstrated that the methylxanthines, theophylline, IBMX and caffeine, activate the human, intermediate-conductance, Ca2+-activated K+ channel (hIK) stably expressed in HEK-293 cells. Whole-cell voltage-clamp experiments showed that the hIK current increased reversibly and voltage independently after the addition of methylxanthines. In current-clamp experiments, theophylline dose-dependently hyperpolarised the cell membrane from a resting potential of -18 mV to -56 mV. The methylxanthines did not affect large-conductance (BK) or small-conductance (SK2), Ca2+-activated K+ channels, demonstrating that the effects were not secondary to a rise in intracellular Ca2+. However, the activation of hIK by theophylline required an intracellular [Ca2+] above 30 nM. The hIK current was insensitive to 8-bromoadenosine cyclic 3',5'-monophosphate (8-bromo-cAMP), forskolin, 8-bromoguanosine cyclic 3',5'-monophosphate (8-bromo-cGMP) and sodium nitroprusside. Moreover, in the presence of inhibitors of protein kinase A (PKA) or protein kinase G (PKG) theophylline still activated the current. Finally, mutation of the putative PKA/PKG consensus phosphorylation site (Ser334) had no effect on the theophylline-induced activation of hIK. Since the observed activation is independent of changes in PKA/PKG-phosphorylation and of fluctuations in intracellular Ca2+, we suggest that the methylxanthines interact directly with the hIK protein.
Schrøder
R L
RL
NeuroSearch A/S, Ballerup, Denmark.
Jensen
B S
BS
Strøbaek
D
D
Olesen
S P
SP
Christophersen
P
P
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D006152:Cyclic GMP / Q000031:analogs & derivatives* / Q000494:pharmacology; D017869:Cyclic GMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015870:Gene Expression; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009599:Nitroprusside / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects* / Q000235:genetics / Q000502:physiology*; D011994:Recombinant Proteins / Q000378:metabolism; D013806:Theophylline / Q000008:administration & dosage / Q000494:pharmacology; D014162:Transfection; D014970:Xanthines / Q000494:pharmacology*
D004791:Enzyme Inhibitors; D015221:Potassium Channels; D011994:Recombinant Proteins; D014970:Xanthines; D009599:Nitroprusside; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; C008514:methylxanthine; C016276:8-bromocyclic GMP; D002110:Caffeine; D013806:Theophylline; D017868:Cyclic AMP-Dependent Protein Kinases; D017869:Cyclic GMP-Dependent Protein Kinases; D006152:Cyclic GMP; D008274:Magnesium; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine