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Gi3 does not contribute to the inhibition of adenylate cyclase when stimulation of an alpha 2-adrenergic receptor causes activation of both Gi2 and Gi3.

ABSTRACT: Agonist occupancy of the alpha 2-C10 adrenergic receptor in a stable clone (1C) of Rat 1 fibroblasts produced by transfection of cells with genomic DNA encoding this receptor causes the activation of both of the pertussis-toxin-sensitive G-proteins Gi2 and Gi3 [Milligan, Carr, Gould, Mullaney & Lavan (1991) J. Biol. Chem. 266, 6447-6455]. An IgG fraction from an antiserum (I3B) which identifies the C-terminal decapeptide of Gi3 alpha only was able to inhibit partially receptor stimulation of high-affinity GTPase activity. An equivalent fraction from an antiserum (AS7) able to identify the C-terminal decapeptide of Gi1 alpha + Gi2 alpha, but not Gi3 alpha, was also able to inhibit partially receptor stimulation of GTPase activity, and the effects of the two antisera were additive. By contrast, agonist-mediated inhibition of forskolin-amplified adenylate cyclase activity was abolished completely by the IgG fraction of antiserum AS7, but was not decreased by treatment with antiserum 13B. Based on the proportion of agonist-stimulated high-affinity GTPase which was prevented by each antiserum and on the measured membrane levels of Gi2 and Gi3, calculations indicated that essentially all of the cellular Gi3, but only 15% of the available Gi2, can be activated by the alpha 2-C10 adrenergic receptor in these cells. These results demonstrate that, although Gi3 is activated by alpha 2-adrenergic agonists in membranes of clone 1C cells, it does not contribute to the transduction of receptor-mediated inhibition of adenylate cyclase.

SUBMITTER: McClue SJ

PROVIDER: S-EPMC1132675 | biostudies-other | 1992 Jun

REPOSITORIES: biostudies-other

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Project description:Two subtypes of beta-adrenoceptors, beta 1 and beta 2, mediate cardiac catecholamine effects. These two types differ qualitatively, e.g., regarding G protein coupling and calcium channel stimulation. Transgenic mice overexpressing human beta 2-adrenoceptors survive high-expression levels, unlike mice overexpressing beta 1-adrenoceptors. We examined the role of inhibitory Gi proteins, known to be activated by beta 2- but not beta 1-adrenoceptors, on the chronic effects of human beta 2-adrenoreceptor overexpression in transgenic mice. These mice were crossbred with mice where G alpha i2, a functionally important cardiac Gi alpha-subunit, was inactivated by targeted gene deletion. Survival of beta 2-adrenoreceptor transgenic mice was reduced by heterozygous inactivation of G alpha i2. Homozygous knockout/beta 2-adrenoreceptor transgenic mice died within 4 days after birth. Heterozygous knockout/beta 2-adrenoreceptor transgenic mice developed more pronounced cardiac hypertrophy and earlier heart failure compared with beta 2-adrenoreceptor transgenic mice. Single calcium-channel activity was strongly suppressed in heterozygous knockout/beta 2-adrenoreceptor transgenic mice. In cardiomyocytes from these mice, pertussis toxin treatment in vitro fully restored channel activity and enhanced channel activity in cells from homozygous G alpha i2 knockout animals. Cardiac G alpha i3 protein was increased in all G alpha i2 knockout mouse strains. Our results demonstrate that G alpha i2 takes an essential protective part in chronic signaling of overexpressed beta 2-adrenoceptors, leading to prolonged survival and delayed cardiac pathology. However, reduction of calcium-channel activity by beta 2-adrenoreceptor overexpression is due to a different pertussis-toxin-sensitive pathway, most likely by G alpha i3. This result indicates that subtype-specific signaling of beta 2-adrenoreceptor functionally bifurcates at the level of Gi, leading to different effects depending on the G alpha isoform.

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Gi3 does not contribute to the inhibition of adenylate cyclase when stimulation of an alpha 2-adrenergic receptor causes activation of both Gi2 and Gi3.

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