Regulation of cardiac CFTR Cl(-) channel activity by a Mg(2+)-dependent protein phosphatase.
Dephosphorylation of the CFTR Cl(-) channel is known to be induced by both okadaic-acid- (OA-) sensitive and -insensitive protein phosphatases (PPs). In the present study, the effects of cytosolic free Mg(2+) on the cardiac CFTR Cl(-) current were examined in relation to the latter PP activity in guinea pig ventricular myocytes. Even when maintaining intracellular Mg-ATP at millimolar concentrations under whole-cell patch-clamp mode, cAMP-activated Cl(-) conductance was reversibly suppressed by cytosolic free Mg(2+), with an IC(50) of around 2.5 mmol/l. In contrast, changes in the cytosolic concentration of free Mg(2+) ([Mg(2+)](i)) had no effect on genistein-activated CFTR Cl(-) currents. The Mg(2+) effect on cAMP-activated CFTR Cl(-) conductance was completely reversed by application of anthracene-9-carboxylic acid (9-AC), which was previously shown to inhibit an OA-insensitive PP in cardiac myocytes. A 9-AC-sensitive fraction of endogenous PP activity in the extract of guinea pig ventricle was found to be activated by free Mg(2+) at millimolar concentrations but to be inactive at micromolar concentrations. The intracellular application of OA failed to activate basal Cl(-) conductance at millimolar [Mg(2+)](i). In the presence of OA, however, basal Cl(-) conductance became activated either by reducing [Mg(2+)](i) to micromolar concentrations or by applying 9-AC. Thus, we conclude that a Mg(2+)-dependent PP sensitive to 9-AC plays a key role in the cAMP-mediated regulation of cardiac CFTR Cl(-) channel at physiological [Mg(2+)](i)under both basal and cAMP-activated conditions. Also, it appears that the genistein-activated conformation of the cardiac CFTR channel is not sensitive to the Mg(2+)-dependent PP.
Zhou
S -S
S-
Department of Cell Physiology, National Institute for Physiological Sciences, Okazaki 444-8585, Japan.
Takai
A
A
Okada
Y
Y
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D000873:Anthracenes / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000494:pharmacology; D002712:Chlorides / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019833:Genistein / Q000494:pharmacology; D006168:Guinea Pigs; D007545:Isoproterenol / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D032383:Myocytes, Cardiac / Q000201:enzymology*; D019319:Okadaic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D000071636:Protein Phosphatase 2C
D000873:Anthracenes; D002316:Cardiotonic Agents; D002712:Chlorides; D004791:Enzyme Inhibitors; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D005576:Colforsin; D019319:Okadaic Acid; C010030:9-anthroic acid; D000255:Adenosine Triphosphate; D019833:Genistein; D010749:Phosphoprotein Phosphatases; D000071636:Protein Phosphatase 2C; D008274:Magnesium; D007545:Isoproterenol