ADENYLYL-CYCLASE INHIBITION AND ALTERED G-PROTEIN SUBUNIT EXPRESSION AND ADP-RIBOSYLATION PATTERNS IN TISSUES AND CELLS G(I2)ALPHA- - MICE/

The inhibition of alpha(i2)-/- mouse cardiac isoproterenol-stimulated adenylyl cyclase (AC; EC 4.6.1.1) activity by carbachol and that of al pha(i2)-/- adipocyte AC by phenylisopropyladenosine (PIA), prostagland in E(2), and nicotinic acid were partially, but not completely, inhibi ted. While the inhibition of cardiac AC was affected in all alpha(i2)- /- animals tested, only 50% of the alpha(i2)-/- animals showed an impa ired inhibition of adipocyte AC, indicative of a partial penetrance of this phenotype, In agreement with previous results, the data show tha t G(i2) mediates hormonal inhibition of AC and that G(i3) and/or G(i1) is capable of doing the same but with a lower efficacy, Disruption of the alpha(i2) gene affected about equally the actions of all the rece ptors studied, indicating that none of them exhibits a striking specif icity for one type of G(i) over another and that receptors are likely to be selective rather than specific in their interaction with functio nally homologous G proteins (e,g., G(i1), G(i2), G(i3)) Western analys is of G protein subunit levels in simian virus 40-transfarmed primary embryonic fibroblasts from alpha(i2)+/+ and alpha(i2)-/- animals showe d that alpha(i2) accounts for about 50% of the immunopositive G protei n alpha subunits and that loss of the alpha(i2) is accompanied by a pa rallel reduction in G beta(35) and G beta(36) subunits and by a 30-50% increase in alpha(i3) This suggests that G beta gamma levels may be r egulated passively through differential rates of turnover in their fre e vs, trimeric states, The existence of compensatory increase(s) in al pha(i) subunit expression raises the possibility that the lack of effe ct of a missing alpha(i2) on AC inhibition in adipocytes of some alpha (i2)- /- animals may be the reflection of a more pronounced compensato ry expressionof alpha(i3) and/or alpha(i1).