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CELLULAR |
1 Ion Channels and Cell Signalling, Division of Basic Medical Sciences, St George's, University of London, London SW17 ORE, UK
Angiotensin II (Ang II) is a potent vasoconstrictor with an important role in controlling blood pressure; however, there is little information on cellular mechanisms underlying Ang II-evoked vasoconstrictor responses. The aim of the present study is to investigate the effect of Ang II on cation conductances in freshly dispersed rabbit mesenteric artery myocytes at the single-channel level using patch-clamp techniques. In cell-attached patches, bath application of low concentrations of Ang II (1 nM) activated cation channel currents (Icat1) with conductances states of about 15, 30 and 45 pS. At relatively high concentrations, Ang II (100 nM) inhibited Icat1 but evoked another cation channel (Icat2) with a conductance of approximately 2 pS. Ang II-evoked Icat1 and Icat2 were inhibited by the AT1 receptor antagonist losartan and the phospholipase C (PLC) inhibitor U73122. The diacylglycerol (DAG) lipase inhibitor RHC80267 initially induced Icat1 which was subsequently inhibited to reveal Icat2. The DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (1 µM) activated Icat1 and Icat2 but inositol 1,4,5-trisphosphate did not evoke either conductance. The protein kinase C (PKC) inhibitor chelerythrine (3 µM) potentiated Ang II-evoked Icat1 and inhibited Icat2 whereas the PKC activator phorbol-12,13-dibutyrate (1 µM) reduced Ang II-induced Icat1 but activated Icat2. Moreover in cell-attached patches pretreated with chelerythrine, application of 100 nM Ang II activated Icat1. These data indicate that PKC inhibits Icat1 but stimulates Icat2. Agents that deplete intracellular Ca2+ stores also activated cation channel currents with similar properties to Icat2. Bath application of anti-TRPC6 and anti-TRPC1 antibodies to inside-out patches inhibited Icat1 and Icat2, respectively. Also flufenamic acid and zero external Ca2+ concentration, respectively, potentiated and reduced Ang II-evoked Icat1. Immunocytochemical studies showed TRPC6 and TRPC1 expression with TRPC6 preferentially distributed in the plasma membrane and TRPC1 expression located throughout the myocyte. These results indicate that Ang II activates two distinct cation conductances in mesenteric artery myocytes by stimulation of AT1 receptors linked to PLC. Icat1 is activated by DAG via a PKC-independent mechanism whereas Icat2 involves DAG acting via a PKC-dependent pathway. Higher concentrations of Ang II inhibit Icat1 by activating an inhibitory effect of PKC. It is proposed that TRPC6 and TRPC1 channel proteins are important components of Ang II-induced Icat1 and Icat2, respectively.
(Received 16 August 2006;
accepted after revision 12 September 2006;
first published online 14 September 2006)
Corresponding author A. P. Albert: Ion Channels and Cell Signalling, Division of Basic Medical Sciences, St George's, University of London, Cranmer Terrace, London SW17 ORE, UK. Email: aalbert{at}sgul.ac.uk
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