[Beta-adrenergic receptor-mediated changes in subcellular localization of G protein beta subunits in perfused rat hearts].

G proteins serve as transducers between cell surface receptors and intracellular effectors. They consist of three subunits, termed alpha, beta, and gamma. Recently, it has been recognized that the beta gamma subunits play an active role such as activation of beta-adrenergic receptor kinase (beta ARK). The desensitization and down-regulation of beta-adrenergic receptors have been observed in the heart failure. beta ARK is one of the components involved in desensitization of beta-adrenergic receptor and it is reported, recently, that G protein beta gamma subunits bind beta ARK through the pleckstrin homology domain. Therefore, we investigated the effects of beta-adrenergic receptor stimulation on steady-state level of G protein beta subunits (G beta) in the rat heart. The whole rat heart was preliminarily perfused for 10 min by Langendorff's technique at 60 mmHg of hydrostatic pressure with Krebs-Henseleit bicarbonate buffer, and then perfused for 30 min in the same buffer with or without 10 microM isoproterenol (ISO), 0.1mM epinephrine (EPI), 10 microM ISO with 0.1mM propranolol (PROP), or 10 microM ISO with 10 microM CGP20712A (CGP). Immunoblotting using isoform-specific antisera against G protein beta subunits revealed that the rat heart contains at least three G protein beta subunits, beta 1, beta 2 and beta 3 at molecular weight of between 35,000 and 37,000. The level of G beta 3 in the cytosol dramatically decreased in the presence of ISO alone or ISO with CGP. G beta 3 decreased in the presence of EPI as well. Propranolol could block ISO-induced decrease of G beta 3 in the cytosol. In contrast, the levels of G beta 1 and G beta 2 didn't change in the presence of ISO or EPI. On the other hand, in membrane fractions the level of G beta 3 significantly increased in the presence of ISO or EPI. ISO with PROP or ISO with CGP did not change the level of G beta 3 in membrane fractions. The levels of G beta 1 and G beta 2 did not change in the presence of ISO or EPI in membrane fractions. Taken together, beta-adrenoceptor agonist might induce isoform-specific translocation of G beta 3 from the cytosol to the membrane.