The transient receptor potential protein homologue TRP6 is the essential component of vascular alpha(1)-adrenoceptor-activated Ca2+-permeable cation channel

The Drosophila transient receptor potential protein (TRP) and its mammalian homologues are thought to be Ca2+-permeable cation channels activated by G protein (G(q/11))-coupled receptors and are regarded as an interesting mol ecular model for the Ca2+ entry mechanisms associated with stimulated phosp hoinositide turnover and store depletion. However, there is little unequivo cal evidence linking mammalian TRPs with particular native functions. In th is study, we have found that heterologous expression of murine TRP6 in HEK2 93 cells reproduces almost exactly the essential biophysical and pharmacolo gical properties of alpha (1)-adrenoceptor-activated nonselective cation ch annels (alpha (1)-AR-NSCC) previously identified in rabbit portal Vein smoo th muscle. Such properties include activation by diacylglycerol; S-shaped c urrent-voltage relationship; high divalent cation permeability; unitary con ductance of 25 to 30 pS and augmentation by flufenamate and Ca2+; and block ade by Cd2+, La3+, Gd3+, SK&F96365, and amiloride. Reverse transcriptase-po lymerase chain reaction and confocal laser scanning microscopy using TRP6-s pecific primers and antisera revealed that the level of TRP6 mRNA expressio n was remarkably high in both murine and rabbit portal vein smooth muscles as compared with other TRP subtypes, and the immunoreactivity to TRP6 prote in was localized near the sarcolemmal region of single rabbit portal vein m yocytes. Furthermore, treatment of primary cultured portal Vein myocytes wi th TRP6 antisense oligonucleotides resulted in marked inhibition of TRP6 pr otein immunoreactivity as well as selective suppression of alpha (1)-adreno ceptor-activated, store depletion-independent cation current and Ba2+ influ x. These results strongly indicate that TRP6 is the essential component of the alpha (1)-AR-NSCC, which may serve as a store depletion-independent Ca2 + entry pathway during increased sympathetic activity.