Membrane-bound and cytosolic forms of heterotrimeric G proteins in young and adult rat myocardium: Influence of neonatal hypo- and hyperthyroidism

Membrane and cytosolic fractions prepared from ventricular myocardium of yo ung (21-day-old) hypo- or hyperthyroid rats and adult (84-day-old) previous ly hypo- or hyperthyroid rats were analyzed by immunoblotting with specific anti-G-protein antibodies for the relative content of G(s)alpha, G(i)alpha /G(o)alpha, G(q)alpha /G(11)alpha, and G beta. All tested G protein subuni ts were present not only in myocardial membranes but were at least partiall y distributed in the cytosol, except for G(o)alpha2, and G(11)alpha. Cytoso lic forms of the individual G proteins represented about 5-60% of total cel lular amounts of these proteins. The long (G(s)alpha -L) isoform of G(s)alp ha prevailed over the short (G(s)alpha -S) isoform in both crude myocardial membranes and cytosol. The G(s)alpha -L/G(s)alpha -S ratio in membranes as well as in cytosol increased during maturation due to a substantial increa se in G(s)alpha -L. Interestingly, whereas the amount of membrane-bound G(i )alpha /G(o)alpha and G(q)alpha /G(11)alpha proteins tend to lower during p ostnatal development, cytosolic forms of these C proteins mostly rise. Neon atal hypothyroidism reduced the amount of myocardial G(s)alpha and increase d that of G(i)alpha /G(o)alpha proteins. By contrast, neonatal hyperthyroid ism increased expression of G(s)alpha and decreased that of G(i)alpha and G (11)alpha in young myocardium. Changes in C protein content induced by neon atal hypo- and hyperthyroidism in young rat myocardium were restored in adu lthood. Alterations in the membrane-cytosol balance of C protein subunits a ssociated with maturation or induced by altered thyroid status indicate phy siological importance of cytosolic forms of these proteins in the rat myoca rdium. J. Cell. Biochem. 82: 215-224, 2001. (C) 2001 Wiley-Liss, Inc.