Facilitatory effect of Ins(1,4,5) P 3 on store‐operated Ca 2+ ‐permeable cation channels in rabbit portal vein myocytes

In rabbit portal vein smooth muscle cells, store‐operated Ca 2+ ‐permeable cation channels (SOCs) display multi‐modal gating mechanisms. SOCs are activated by depletion of intracellular Ca 2+ stores but also may be stimulated in a store‐independent manner by noradrenaline acting on α‐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) P 3 ) modulates SOC activity in freshly dispersed rabbit portal vein myocytes with patch pipette recording techniques. Inclusion of 1 μ m Ins(1,4,5) P 3 in the patch pipette solution increased whole‐cell currents evoked by the Ca 2+ ‐ATPase inhibitor cyclopiazonic acid (CPA) by about 3‐fold at −80 mV. In the cell‐attached configuration the cell‐permeable Ca 2+ chelator BAPTA‐AM stimulated SOC activity and after excision of an isolated inside‐out patch bath application of 1 μ m Ins(1,4,5) P 3 increased open channel probability ( NP o ) by approximately 3‐fold. Ins(1,4,5) P 3 also produced a similar increase in NP o 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) P 3 on increasing PDBu‐evoked SOC activity was about 0.4 μ m . The facilitatory effect of Ins(1,4,5) P 3 was also manifest as markedly increasing the rate of activation of SOCs. The synergistic effect of Ins(1,4,5) P 3 was mimicked by the metabolically stable analogue 3‐fluoro‐Ins(1,4,5) P 3 and Ins(1,4) P 2 , a metabolite of Ins(1,4,5) P 3 , but was not inhibited by the classical Ins(1,4,5) P 3 receptor antagonist heparin. Finally Ins(1,4,5) P 3 also increased NP o of SOCs activated by a PKC catalytic subunit. It is concluded that Ins(1,4,5) P 3 facilitates SOC opening via a heparin‐insensitive mechanism at, or close to, the channel protein.