Multiple activation mechanisms of store-operated TRPC channels in smooth muscle cells

Abstract

Store-operated channels (SOCs) are plasma membrane Ca2+-permeable cation channels which are activated by agents that deplete intracellular Ca2+ stores. In smooth muscle SOCs are involved in contraction, gene expression, cell growth and proliferation. Single channel recording has demonstrated that SOCs with different biophysical properties are expressed in smooth muscle indicating diverse molecular identities. Moreover it is apparent that several gating mechanisms including calmodulin, protein kinase C and lysophospholipids are involved in SOC activation. Evidence is accumulating that TRPC proteins are important components of SOCs in smooth muscle. More recently Orai and STIM proteins have been proposed to underlie the well-described calcium-release-activated current (ICRAC) in non-excitable cells but at present there is little information on the role of Orai and STIM proteins in smooth muscle. In addition it is likely that different TRPC subunits coassemble as heterotetrameric structures to form smooth muscle SOCs. In this brief review we summarize the diverse properties and gating mechanisms of SOCs in smooth muscle. We propose that the heterogeneity of the properties of these conductances in smooth muscle results from the formation of heterotetrameric TRPC structures in different smooth muscle preparations.

Figure 1

Store-independent activation of store-operated channel (SOC) activity by noradrenaline and calyculin A in rabbit portal vein myocytes A, bath application of cyclopiazonic acid (CPA) did not evoke SOC activity whereas subsequent bath application of noradrenaline did induce SOC activity in an outside-out patch held at −70 mV. B, bath application of calyculin A stimulated SOC activity in a quiescent outside-out patch at −70 mV. Figures reproduced from Albert & Large (2002b).

Figure 2

Role of PKC and PKA in SOC activation in rabbit portal vein myocytes A, bath application of PDBu activated SOC activity in an inside-out patch at −80 mV which was reversibly inhibited by co-application of 8-Br-cAMP. B shows that PDBu-induced SOC activity is inhibited by co-application of a PKA catalytic subunit in an inside-out patch at −80 mV. C, bath application of H-89 induced SOC activity in a quiescent cell-attached patch at −80 mV. D shows that SOC activity evoked by a PKC catalytic subunit was potentiated by co-application of IP₃ in an inside-out patch at −80 mV. Figures reproduced from Liu et al. (2005a, .

Figure 3

CaM activates SOC activity and CaM kinase II has an inhibitory effect on SOC activity in rabbit portal vein myocytes Aa, bath application of CaM induces SOC activity in an inside-out patch at −80 mV; b shows that CaM-activated SOCs had a slope conductance of 1.8 pS and an extrapolated reversal potential of +28 mV; and c illustrates that CaM-evoked SOC activity is modulated by changing [Ca²⁺]_i. B shows that BAPTA-AM-induced SOC activity was blocked by co-application of the CaM antagonist calmidazolium (CMZ) in a cell-attached patch at −80 mV. C, bath application of CMZ to a quiescent cell activated SOC activity that subsequently declined in a cell-attached patch at −80 mV. D, bath application of the CaM kinase II inhibitor KN-93 evoked sustained SOC activity in a cell-attached patch at −80 mV. E shows that CaM-evoked SOC activity in 100 nm[Ca²⁺]_i was reversibly inhibited by co-application of purified CaM kinase II in an inside-out patch at −80 mV. Aa and b, B, C, D and E were reproduced from Albert et al. (2006b). Ac is authors' previously unpublished data.

Figure 4

TRPC1 and TRPC5 are important components of SOCs in rabbit portal vein and mesenteric artery myocytes Aa and b, bath application of, respectively, anti-TRPC1 and anti-TRPC5 antibodies reversibly inhibited CaM-evoked SOC activity in inside-out patches from portal vein myocytes at −80 mV. Ba and b, bath application of, respectively, anti-TRPC1 and anti-TRPC5 antibodies inhibited Ang II-induced stimulation of SOC activity in inside-out patches from mesenteric artery myocytes at −80 mV. Ba was reproduced from Saleh et al. (2006). Aa and b, and Bb are authors' previously unpublished data.

Figure 5

Schematic diagram of multiple activation mechanisms of TRPC1/TRPC5-mediated SOCs in rabbit portal vein myocytes A, CPA may activate SOCs by depleting Ca²⁺ levels within the SR ([Ca²⁺]_SR) leading to stimulation of PKC/CaM by an unknown pathway (dashed lines) which induces channel opening. However, CPA may also induce SOC activity by evoking a rise in cytosolic Ca²⁺ levels ([Ca²⁺]_c) which leads to activation of a Ca²⁺-sensitive PKC and CaM. B, BAPTA-AM may also activate SOCs by depleting [Ca²⁺]_SR but this agent may produce a reduction in [Ca²⁺]_c to remove the inhibitory action of CaM kinase II (CaMKII) on SOCs leading to stimulation of the channels via the presence of a constitutive driver, e.g. CaM. C, noradrenaline (NA) acting at α₁-adrenoceptors activates SOC activity via both store-dependent and -independent pathways involving stimulation of PKC. In addition lysophospholipids (LPLs) may also activate SOC opening.