Characterization of G proteins in rat myometrium. A differential modulation of Gi2 alpha and Gi3 alpha during gestation

Myometrial membranes, obtained from estrogen-dominated (day 0) rat uteri, were immunoblotted with antiserum (SG1), which recognizes the alpha subunits of both Gi1 and Gi2, with antiserum (LE2) specific for Gi2 alpha, and with I3B antiserum, specific for Gi3 alpha. The data revealed the absence of detectable levels of Gi1 alpha and the simultaneous presence of Gi2 alpha and Gi3 alpha as Gi subunits in rat myometrium. The expression of Gi proteins during gestation (days 0, 12, 21) was studied with the above antibodies. No qualitative change in the nature of Gi alpha species was observed during gestation: Gi1 alpha remained undetectable, Gi2 alpha and Gi3 alpha were both present on days 12 and 21. Of significance was the increase (160%) in the amount of Gi2 alpha at midgestation (day 12) compared to days 0 and 21. A different pattern was observed with Gi3 alpha, which decreased with advancing gestation (day 0 greater than 12 greater than 21). Immunodetection of beta subunits of G proteins indicated the presence of a 35/36 kDa doublet on days 0, 12 and 21, with an increase at midgestation. The simultaneous increase in Gi2 alpha and beta subunits may provide an explanation for the previously demonstrated alteration in adenylate cyclase stimulability detected at midgestation.