Synergism between inositol phosphates and diacylglycerol on native TRPC6-like channels in rabbit portal vein myocytes

In rabbit portal vein myocytes noradrenaline activates a non-selective cation current (I_cat) which involves a transient receptor potential protein (TRPC6). Previously we have shown that the diaylglycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) stimulates I_cat via a protein kinase C (PKC)-independent mechanism, and in the present study we have investigated the interaction between inositol phosphates (InsPs) and OAG on I_cat. With whole-cell recording of I_cat from freshly isolated rabbit portal vein myocytes the amplitude and rate of activation of noradrenaline-evoked I_cat were much greater than those of OAG-induced I_cat. Inclusion of inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃) in the pipette solution did not evoke I_cat but greatly potentiated the amplitude and rate of activation of OAG-induced I_cat. With isolated outside-out patches Ins(1,4,5)P₃ markedly increased the rate of activation and the open probability of OAG-evoked channel activity, with no change in unitary conductance, channel mean open times or burst durations. The effects of Ins(1,4,5)P₃ were mimicked by Ins(2,4,5)P₃, 3-F-Ins(1,4,5)P₃ and Ins(1,4 P₂ but not by Ins(1,3,4,5)P₄ and the potentiating effects of InsPs were not inhibited by heparin. Therefore it is concluded that both DAG and InsPs are necessary for full activation of I_cat by noradrenaline and the effect of InsPs is via a heparin-insensitive mechanism and represents a novel action of InsPs.