SOCs - store-operated channels in vascular smooth muscle?

Abstract

Unravelling the mechanisms of noradrenaline's vasoconstrictor action is a fascinating business, and an important example of the rich texture of signalling within smooth muscle cells. Noradrenaline's stimulation of α1-adrenoceptors sets off complex signalling cascades that act to initiate and maintain vascular smooth muscle contraction. One early discovery was that noradrenaline releases Ca2+ from intracellular stores. Whether this is a graded, all-or-nothing or oscillatory response, the stores must at some stage refill. This can occur partly by uptake of cytosolic Ca2+ but, because released Ca2+ is extruded from the cell, Ca2+ replenishment from the extracellular fluid is also necessary. Voltage-gated Ca2+ channels can contribute, but vascular smooth muscle cells - like other cell types - would also seem to have specialised Ca2+ channels for this function. Twenty-one years ago in The Journal of Physiology, Casteels & Droogmans published results obtained using 45Ca2+ as a tracer. Depletion of intracellular stores stimulated the rate of 45Ca2+ uptake from the extracellular solution. The pathway appeared to be tightly coupled to stores because the Ca2+ uptake failed to cause contraction. It was resistant to conventional Ca2+ antagonists and inhibited by K+-induced depolarisation. Manganese ions blocked the pathway. These data were the first suggesting the existence of a store-operated channel (SOC) that opens to allow Ca2+ entry when Ca2+ stores deplete. The evidence for SOCs in vascular smooth muscle is good. Blockade of Ca2+ reuptake into stores by inhibitors (e.g. cyclopiazonic acid) of the store's Ca2+-ATPase (SERCA) or chelation of Ca2+ leaking from stores by BAPTA both cause store depletion while circumventing complexities of α-adrenoceptor-mediated signalling. Both also activate Ca2+-permeable non-selective cationic channels in the plasma membrane of aortic and portal vein smooth muscle cells (Trepakova et al. 2000; Albert & Large, 2002a). The portal vein channels differ from those in aorta, for example showing greater Ca2+-selectivity, but neither is as Ca2+-selective as the calcium-release-activated Ca2+ (CRAC) channels of lymphocytes. Nevertheless, both vascular channels are coupled to store depletion. SERCA's fundamental role in intracellular Ca2+ handling and in the superficial buffer barrier of vascular smooth muscle can complicate experiments with SERCA inhibitors, as can background Ca2+-flux pathways. The use of manganese flux as an indicator of SOC activity may be undermined by manganese block of SOCs. But, by-and-large, such experiments have supported the existence of SOCs in vascular smooth muscle. Further evidence has also emerged. The SOCs of aortic myocytes are activated by calcium influx factor (CIF), a substance of unknown chemical identity released from store-depleted - but not control - platelets (Trepakova et al. 2000). There is also TRPC1 protein at the plasma membrane of vascular smooth muscle cells. TRPC1 has sequence homology to voltage-gated ion channel α-subunits. Nine independent laboratories now have data showing it is a functional component of SOCs. A TRPC1 peptide-specific antibody attenuates SOC-mediated Ca2+ entry in vascular smooth muscle (Xu & Beech, 2001). Intriguingly, in platelets, TRPC1 co-immunoprecipitates with inositol trisphosphate receptor, but only when stores are depleted (Rosado & Sage, 2001). Therefore, molecular elements necessary for SOCs are present and active in vascular smooth muscle. In this issue of The Journal of Physiology, Albert & Large (2002b) report patch-clamp data on the mechanism coupling α1-adrenoceptors to SOCs in portal vein smooth muscle cells. They provide compelling evidence that SOCs can be activated independently of store depletion. In cell-attached patches noradrenaline activates channels that are biophysically very similar to those activated by cyclopiazonic acid - i.e. they both appear to activate the same SOC. Strikingly, SOC activation by noradrenaline, cyclopiazonic acid or BAPTA is prevented by protein kinase C inhibitors. In excised outside-out patches, noradrenaline and protein kinase C activators stimulate SOCs - but cyclopiazonic acid has no effect, as if the store-depletion mechanism is absent. Therefore, although noradrenaline depletes Ca2+ stores it seems it can also activate the portal vein SOCs independently of store-depletion, via a classical phospholipase C-diacylglycerol-protein kinase C pathway. The physiological importance of this effect compared with the store-operated mechanism will need to be worked out, as will the apparent dual role of protein kinase C in the store-independent and -dependent pathways. We can look forward to more intrigue to come.