Phospholipase C (PLC) and the small G protein RhoA are vital elements for the contraction of vascular smooth muscle cells. The available evidence points to altered PLC-delta1 activity as an element determining enhanced vascular tone in hypertension; however, the factor(s) responsible for increased PLC activity remains unknown. There is the data indicating that RhoA inhibits PLC-delta1 and factors downmodulating RhoA activate phospholipase. In the present study, we explore an impact of a newly identified human ARHGAP6 protein possessing GTPase stimulating activity for RhoA on the catalytic properties of PLC-delta1. Under in vitro conditions, ARHGAP6 protein activated PLC-delta1. ARHGAP6 protein bound PLC-delta1 and regulated its activity by masking the binding sites for inhibitory phospholipids. Moreover, ARHGAP6 increased the V(max) of PLC-delta1 and enhanced its response to Ca(2+) stimulation. A Western blot of immunoprecipitates from Cos-7 cells transfected with pcDNA3-ARHGAP6 and pcDNA3-PLCdelta1 showed the presence of ARHGAP6/PLC-delta1 complexes. The activity of PLC in cells overexpressing ARHGAP6 increased approximately 6-fold compared to control cells. The examination of ARHGAP6 expression in mononuclear cells isolated from the blood of patients with hypertension showed increased ARHGAP6 mRNA and protein levels compared to age-matched normotensive subjects. Enhanced expression of ARHGAP6 was associated with an elevated level of PLC activity and increased levels of IP(3) (1.6-fold) and DAG (2.3-fold). In summary, our data indicate that ARHGAP6 protein binds to and up regulates PLC-delta1 both under in vitro and in vivo conditions. Moreover, the elevated expression of ARHGAP6 provides possible explanation for the altered activity of PLC-delta1 in hypertension.