Capacitative and 1-oleyl-2-acetyl-sn-glycerol-activated Ca2+ entry distinguished using adenylyl cyclase type 8

Although the molecular identity of capacitative Ca2+ entry (CCE) channels remains elusive, transient receptor potential channel (TRPC) family members 3, 6, and 7, which are activated by diacylglycerol (DAG), have been put forward as possible candidates. Because human embryonic kidney (HEK) 293 cells endogenously express these TRP subunits, this cell line is suitable for investigating whether DAG-activated TRP subunits form part of the putative multimeric assemblies that mediate CCE. Adenylyl cyclase type 8 (AC8) is activated by CCE in nonexcitable cells but is not responsive to other forms of Ca2+ entry, such as ionophore- or arachidonate-activated entry through the plasma membrane (J Biol Chem 271: 12438 - 12444, 1996; J Biol Chem 273: 9297 - 9305, 1998; J Biol Chem 274: 31174 - 31178, 1999). In this study, we exploited this unique dependence of AC8 on CCE to determine whether the DAG analog, 1-oleyl-2-acetyl-sn-glycerol (OAG), activates the same subset of Ca2+ channels as store depletion, which triggers CCE. In populations of HEK 293 cells, OAG evoked a faster and greater influx of Ca2+ than CCE. Both pathways of Ca2+ entry could be triggered simultaneously in the same batch of cells, with additive effects. It is striking that OAG-mediated Ca2+ entry, unlike CCE, did not stimulate AC8 activity in populations of cells. In single cells, OAG evoked a highly heterogeneous response, whereas CCE occurred as a smooth and sustained increase in [Ca2+](i). Taken together, our results indicate that, in HEK 293 cells, OAG-activated Ca2+ entry is distinct from CCE. The inability of the OAG- activated Ca2+ entry pathway to regulate AC8 further reinforces the absolute dependence of this enzyme on CCE.