Regulation of the muscarinic K+ channel by extracellular ATP through membrane phosphatidylinositol 4,5-bisphosphate in guinea-pig atrial myocytes

摘要

The present study was designed to examine the functional role of membrane phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) in the regulation of the muscarinic K+ channel (IK,ACh) by extracellular ATP and adenosine in guinea-pig atrial myocytes, using the whole-cell patch-clamp method.Bath application of ATP in micromolar concentrations typically evoked a transient activation of IK,ACh; a rapid activation phase was consistently followed by a progressive decline even to the baseline level despite the continued presence of ATP. This progressive decline of IK,ACh was significantly attenuated either by blockade of phospholipase C (PLC) with compound 48/80 (100 μM) or by addition of PtdIns(4,5)P2 (50 μM) to the cell inside, suggesting that depletion of membrane PtdIns(4,5)P2 via PLC activation is mainly, if not totally, responsible for the progressive decline of IK,ACh during the presence of ATP.When atrial myocytes were exposed to wortmannin (50 μM) following ATP (50 μM) application to impair the resynthesis of PtdIns(4,5)P2, the activation of IK,ACh evoked by subsequently applied ATP (50 μM) was greatly reduced. Activation of IK,ACh by adenosine (100 μM) was partially reduced by pretreatment of atrial myocytes with ATP (100 μM) and was largely abolished by a further addition of wortmannin (50 μM) in the presence of ATP (100 μM). These results support the view that the activation of IK,ACh by ATP and adenosine depends on membrane PtdIns(4,5)P2 that is subject to reduction by extracellular ATP.The present study thus provides functional evidence to suggest that extracellular ATP activates PLC and thereby depletes membrane PtdIns(4,5)P2 that is critically involved in the activation process of IK,ACh by its agonists ATP and adenosine in guinea-pig atrial myocytes.