Previous results have shown that in rat portal vein myocytes the beta gamma
dimer of the G(13) protein transduces the angiotensin II-induced stimulati
on of calcium channels and increase in intracellular Ca2+ concentration thr
ough activation of phosphoinositide 3-kinase (PI3K). In the present work we
determined which class I PI3K isoforms were involved in this regulation. W
estern blot analysis indicated that rat portal vein myocytes expressed only
PI3K alpha and PI3K gamma and no other class I PI3K isoforms. In the intra
cellular presence of an anti-p110 gamma antibody infused by the patch clamp
pipette, both angiotensin II- and G beta gamma -mediated stimulation of Ca
2+ channel current were inhibited, whereas intracellular application of an
anti-p110 alpha antibody had no effect. The anti-PI3K gamma antibody also i
nhibited the angiotensin II- and G beta gamma -induced production of phosph
atidylinositol 3,4,5-trisphosphate. In Indo-1 loaded cells, the angiotensin
II-induced increase in [Ca2+](i) was inhibited by intracellular applicatio
n of the anti-PI3K gamma antibody, whereas the anti-PI3K alpha antibody had
no effect. The specificity of the anti-PI3K gamma antibody used in functio
nal experiments was ascertained by showing that this antibody did not recog
nize recombinant PI3K alpha in Western blot experiments. Moreover, anti-PI3
K gamma antibody inhibited the stimulatory effect of intracellularly infuse
d recombinant PI3K gamma on Ca2+ channel current without altering the effec
t of recombinant PI3K alpha. Our results show that, although both PI3K gamm
a and PI3K alpha are expressed in vascular myocytes, the angiotensin II-ind
uced stimulation of vascular L-type calcium channel and increase of [Ca2+](
i) involves only the PI3K gamma isoform.