Receptor-induced depletion of phosphatidylinositol 4,5-bisphosphate inhibits inwardly rectifying K~+ channels in a receptor-specific manner

摘要

Phosphatidylionsitol 4,5-bisphosphate (PIP_2), a substrate of phos-pholipase C, has recently been recognized to regulate membrane-associated proteins and act as a signal molecule in phospholipase C-linked Gq-coupled receptor (GqPCR) pathways. However, it is not known whether PIP_2 depletion induced by GqPCRs can act as receptor-specific signals in native cells. We investigated this issue in cardiomyocytes where PIP_2-dependent ion channels, G protein-gated inwardly rectifying K~+ (GIRK) and inwardly rectifying background K~+ (IRK) channels, and various GqPCRs are present. The GIRK current was recorded by using the patch-clamp technique during the application of 10 μM acetylcholine. The extent of receptor-mediated inhibition was estimated as the current decrease over 4 min while taking the GIRK current (I_(GIRK)) value during a previous stimulation as the control. Each GqPCR agonist inhibited I_(GIRK) with different potencies and kinetics. The extents of inhibition induced by phenylephrine, angiotensin Ⅱ, endothelin-1, prosta-glandin F2α, and bradykinin at supramaximal concentrations were (mean ± SE) 32.1 ± 0.6%, 21.9 ± 1.4%, 86.4 ± 1.6%, 63.7 ± 4.9%, and 5.7 ± 1.9%, respectively. GqPCR-induced inhibitions of I_(GIRK) were not affected by protein kinase C inhibitor (calphostin C) but potentiated and became irreversible when the replenishment of PIP_2 was blocked by wortmannin (phosphatidylinositol kinase inhibitor). Loading the cells with PIP_2 significantly reduced endothelin-1 and prostaglandin F2α-induced inhibition of I_(GIRK). On the contrary, GqPCR-mediated inhibitions of inwardly rectifying background K~+ currents were observed only when GqPCR agonists were applied with wortmannin, and the effects were not parallel with those on I_(GIRK). These results indicate that GqPCR-induced inhibition of ion channels by means of PIP_2 depletion occurs in a receptor-specific manner.