THYROID-HORMONE REGULATION OF TRANSMEMBRANE SIGNALING IN NEONATAL RAT VENTRICULAR MYOCYTES BY SELECTIVE ALTERATION OF THE EXPRESSION AND COUPLING OF G-PROTEIN ALPHA-SUBUNITS

Thyroid hormone exerts profound effects on the activity of the hormone-sensitive adenylate cyclase system in the heart. Distinct guanine nucleotide-binding regulatory proteins (G-proteins) mediate stimulatory and inhibitory influences on adenylate cyclase activity. To examine whether the effects of thyroid hormone on adenylate cyclase involve specific changes in G-protein subunit expression, the influence of tri-iodothyronine (T-3) on the biosynthesis and activity of G-proteins in neonatal rat ventricular myocytes was determined. In myocytes challenged with T-3 for 5 days, G(s) alpha levels increased by 4+/-0.5-fold, whereas G(i)2 alpha levels declined by more than 80%. T-3 down-regulated G(i)2 alpha mRNA by 60% within 3 days, but had no effect on G(s) alpha mRNA. The basis for the decline in G(i)2 alpha mRNA was the T-3-mediated suppression of G(i)2 alpha gene transcription by 80+/-9% within 4 h. The decline in G(i)2 alpha mRNA in response to T-3 produced a 2-fold decrease in relative rate of synthesis of G(i)2 alpha but not in its half-life (46+/-7 h). G(s) alpha synthesis was not altered by T-3, but the half-life of G(s) alpha increased from 50+/-6 h in control cells to 72+/-8 h in T-3-treated cells. In addition, T-3 provoked the translocation of G(s) alpha from the cytoplasmic to the membranous compartment. Membranous G(s) alpha increased from 30+/-6% to 61+/-7% of total cellular G(s) alpha, whereas cytoplasmic G(s) alpha declined from 68+/-6% to 33+/-8% within 1 day of exposure to T-3. T-3-mediated up-regulation of G(s) alpha enhanced the activation of myocardial adenylate cyclase by the stimulatory pathway whereas the down-regulation of G(i)2 alpha attenuated the deactivation of myocardial adenylate cyclase by the inhibitory pathway.