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 young (21‐day‐old) hypo‐ or hyperthyroid rats and adult (84‐day‐old) previously hypo‐ or hyperthyroid rats were analyzed by immunoblotting with specific anti‐G‐protein antibodies for the relative content of G s α, G i α/G o α, G q α/G 11 α, and Gβ. All tested G protein subunits were present not only in myocardial membranes but were at least partially distributed in the cytosol, except for G o α2, and G 11 α. Cytosolic forms of the individual G proteins represented about 5–60% of total cellular amounts of these proteins. The long (G s α‐L) isoform of G s α prevailed over the short (G s α‐S) isoform in both crude myocardial membranes and cytosol. The G s α‐L/G s α‐S ratio in membranes as well as in cytosol increased during maturation due to a substantial increase in G s α‐L. Interestingly, whereas the amount of membrane‐bound G i α/G o α and G q α/G 11 α proteins tend to lower during postnatal development, cytosolic forms of these G proteins mostly rise. Neonatal hypothyroidism reduced the amount of myocardial G s α and increased that of G i α/G o α proteins. By contrast, neonatal hyperthyroidism increased expression of G s α and decreased that of G i α and G 11 α in young myocardium. Changes in G protein content induced by neonatal hypo‐ and hyperthyroidism in young rat myocardium were restored in adulthood. Alterations in the membrane‐cytosol balance of G protein subunits associated with maturation or induced by altered thyroid status indicate physiological importance of cytosolic forms of these proteins in the rat myocardium. J. Cell. Biochem. 82: 215–224, 2001. © 2001 Wiley‐Liss, Inc.