Regulation of by protein kinases of CFTR chloride current and the acidic pH-activated chloride current in cardiac myocytes

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1. It is well known that extracellular acidosis modulates many types of ion channels in excitable and non-excitable cells. Recently, a novel Cl- current that is activated by acidic pH has been found in rat sertoli cells. A similar current (I_<Cl.pH> was found in ventricular myocytes from mouse and guinea pig. When acidic solution was applied to the cells, I_<Cl.pH> appeared with a delay of 〜1 min, increased gradually, and reached a maximum in 〜5 min. In contrast, the current disappeared rapidly (<1 min) upon resumption of the solution with normal pH (7.4). I_<Cl.pH> was activated in a pH・dependent manner, with a half maximal activation at about pH 6.0. I_<Cl.pH> exhibited a weak time・dependent activation, the current amplitude slightly increasing during depolarizing step pulses. I・V relationship of I_<Cl.pH> showed a strong outward rectification under symmetrical [Cl-] conditions. The anion selectivity of this current was estimated to be I>Cl->Asp. Pharmacological studies showed that I … More _<Cl.pH> was inhibited by several Cl- channel blockers (DIDS, niflumic acid and gliben Cl-amide). Thus, the properties of I_<Cl.pH> differ from those of other cardiac Cl- currents (volume・regulated Cl- current, inwardly rectifying Cl- current, Ca^<2+>・activated Cl- current or CFTR current). I_<Cl.pH> may play a role in the control of the action potential duration under pathological conditions, such as ischemia-related cardiac acidosis.2. The effects of extracellular ATP on β-adrenergic activation of CFTR Cl current (I^<Cl,PKA>) were examined in guinea-pig ventricular cells. The cells were initially exposed to 0.02-1 μM isoproterenol (ISO) for 〜3 min to activate I_<Cl,PKA>, and then to 1.100 μM ATP in the presence of ISO. ATP was found to potentiate I_<Cl,PKA>, in most cells examined. In about 2/<3> of them, however, the potentiation was preceded by an inhibition, I_<Cl,PKA> changing in a biphasic manner. The initial inhibition was due to stimulation of P1-purinoceptor by ATP, since the inhibition was attenuated by the blockers of this receptor type. With 50 μM ATP, the potentiation, on average, resulted in a 1.3 fold increase of the Cl- conductance activated by ISO alone (0.02-1μM). The effects of ADP and ATPγS on I_<Cl,PKA> were similar to those of ATP, while AMP and adenosine never potentiated I_<Cl,PKA>. Thus the potentiation was attributed to a stimulation of P2-purinoceptors. PDBu (0.5 μM), an activator of PKC, facilitated I_<Cl,PKA>, and in the presence of PDBu ATP did not further potentiate I_<Cl,PKA>. When BIM (0.2 μM), an inhibitor of PKC, was present, ATP did not facilitate I_<Cl,PKA>. These findings suggested involvement of PKC in the observed ATP action. When ATP was removed in the presence of ISO, the potentiated ICl,PKA decreased (recovered) only slowly, and, if ATP was reapplied during this slowly recovering phase, the subsequent current potentiation was weak. Thus the stimulation of P_2 purinoceptors by ATP facilitates the β-adrenergic activation of I_<Cl,PKA> through PKC activation, and this potential appears to persist for several min after removal of ATP. Less