Mechanosensitive activation of K + channel via phospholipase C‐induced depletion of phosphatidylinositol 4,5‐bisphosphate in B lymphocytes

In various types of cells mechanical stimulation of the plasma membrane activates phospholipase C (PLC). However, the regulation of ion channels via mechanosensitive degradation of phosphatidylinositol 4,5‐bisphosphate (PIP 2 ) is not known yet. The mouse B cells express large conductance background K + channels (LK bg ) that are inhibited by PIP 2 . In inside‐out patch clamp studies, the application of MgATP (1 m m ) also inhibited LK bg due to the generation of PIP 2 by phosphoinositide (PI)‐kinases. In the presence of MgATP, membrane stretch induced by negative pipette pressure activated LK bg , which was antagonized by PIP 2 (> 1 μ m ) or higher concentration of MgATP (5 m m ). The inhibition by PIP 2 was partially reversible. However, the application of methyl‐β‐cyclodextrin, a cholesterol scavenger disrupting lipid rafts, induced the full recovery of LK bg activity and facilitated the activation by stretch. In cell‐attached patches, LK bg were activated by hypotonic swelling of B cells as well as by negative pressure. The mechano‐activation of LK bg was blocked by U73122, a PLC inhibitor. Neither actin depolymerization nor the inhibition of lipid phosphatase blocked the mechanical effects. Direct stimulation of PLC by m ‐3M3FBS or by cross‐linking IgM‐type B cell receptors activated LK bg . Western blot analysis and confocal microscopy showed that the hypotonic swelling of WEHI‐231 induces tyrosine phosphorylation of PLCγ2 and PIP 2 hydrolysis of plasma membrane. The time dependence of PIP 2 hydrolysis and LK bg activation were similar. The presence of LK bg and their stretch sensitivity were also proven in fresh isolated mice splenic B cells. From the above results, we propose a novel mechanism of stretch‐dependent ion channel activation, namely, that the degradation of PIP 2 caused by stretch‐activated PLC releases LK bg from the tonic inhibition by PIP 2 .