Thyroid hormone regulation of transmembrane signalling 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 (T3) on the biosynthesis and activity of G-proteins in neonatal rat ventricular myocytes was determined. In myocytes challenged with T3 for 5 days, Gs alpha levels increased by 4 +/- 0.5-fold, whereas Gi2 alpha levels declined by more than 80%. T3 down-regulated Gi2 alpha mRNA by 60% within 3 days, but had no effect on Gs alpha mRNA. The basis for the decline in Gi2 alpha mRNA was the T3-mediated suppression of Gi2 alpha gene transcription by 80 +/- 9% within 4 h. The decline in Gi2 alpha mRNA in response to T3 produced a 2-fold decrease in relative rate of synthesis of Gi2 alpha but not in its half-life (46 +/- 7 h). Gs alpha synthesis was not altered by T3, but the half-life of Gs alpha increased from 50 +/- 6 h in control cells to 72 +/- 8 h in T3-treated cells. In addition, T3 provoked the translocation of Gs alpha from the cytoplasmic to the membranous compartment. Membranous Gs alpha increased from 30 +/- 6% to 61 +/- 7% of total cellular Gs alpha, whereas cytoplasmic Gs alpha declined from 68 +/- 6% to 33 +/- 8% within 1 day of exposure to T3. T3-mediated up-regulation of Gs alpha enhanced the activation of myocardial adenylate cyclase by the stimulatory pathway whereas the down-regulation of Gi2 alpha attenuated the deactivation of myocardial adenylate cyclase by the inhibitory pathway.
Bahouth
S W
SW
Department of Pharmacology, College of Medicine, University of Tennessee, Memphis 38163, USA.
D000818:Animals; D000831:Animals, Newborn; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects; D015151:Immunoblotting; D046911:Macromolecular Substances; D009206:Myocardium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D014284:Triiodothyronine / Q000494:pharmacology* / Q000502:physiology*; D016276:Ventricular Function
D046911:Macromolecular Substances; D014284:Triiodothyronine; D000242:Cyclic AMP; D019204:GTP-Binding Proteins