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1 Pharmacology and Clinical Pharmacology, Department of Basic Medical Sciences, St George's Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK
In rabbit portal vein smooth muscle cells, store-operated Ca2+-permeable cation channels (SOCs) display multi-modal gating mechanisms. SOCs are activated by depletion of intracellular Ca2+ stores but also may be stimulated in a store-independent manner by noradrenaline acting on 
-adrenoceptors and by diacylglycerol (DAG) via protein kinase C (PKC). In the present study we have investigated whether inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) modulates SOC activity in freshly dispersed rabbit portal vein myocytes with patch pipette recording techniques. Inclusion of 1 µM Ins(1,4,5)P3 in the patch pipette solution increased whole-cell currents evoked by the Ca2+-ATPase inhibitor cyclopiazonic acid (CPA) by about 3-fold at –80 mV. In the cell-attached configuration the cell-permeable Ca2+ chelator BAPTA-AM stimulated SOC activity and after excision of an isolated inside-out patch bath application of 1 µM Ins(1,4,5)P3 increased open channel probability (NPo) by approximately 3-fold. Ins(1,4,5)P3 also produced a similar increase in NPo of SOCs stimulated by the phorbol ester, phorbol 12,13-dibutyrate (PDBu) in inside-out patches and these channel currents had a unitary conductance of about 2 pS. The equilibrium constant of Ins(1,4,5)P3 on increasing PDBu-evoked SOC activity was about 0.4 µM. The facilitatory effect of Ins(1,4,5)P3 was also manifest as markedly increasing the rate of activation of SOCs. The synergistic effect of Ins(1,4,5)P3 was mimicked by the metabolically stable analogue 3-fluoro-Ins(1,4,5)P3 and Ins(1,4)P2, a metabolite of Ins(1,4,5)P3, but was not inhibited by the classical Ins(1,4,5)P3 receptor antagonist heparin. Finally Ins(1,4,5)P3 also increased NPo of SOCs activated by a PKC catalytic subunit. It is concluded that Ins(1,4,5)P3 facilitates SOC opening via a heparin-insensitive mechanism at, or close to, the channel protein.
(Received 8 April 2005;
accepted after revision 21 April 2005;
first published online 28 April 2005)
Corresponding author A. P. Albert: Pharmacology and Clinical Pharmacology, Department of Basic Medical Sciences, St George's Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK. Email: aalbert{at}sghms.ac.uk
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