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Cyclic GMP protein kinase activity is reduced in thyroxine-induced hypertrophic cardiac myocytes

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1. We tested the hypothesis that the cGMP-dependent protein kinase has major negative functional effects in cardiac myocytes and that the importance of this pathway is reduced in thyroxine (T4; 0.5 mg/kg per day for 16 days) hypertrophic myocytes.

2. Using isolated ventricular myocytes from control (n = 7) and T4-treated (n = 9) rabbit hypertrophic hearts, myocyte shortening was studied with a video edge detector. Oxygen consumption was measured using O2 electrodes. Protein phosphorylation was measured autoradiographically.

3. Data were collected following treatment with: (i) 8-(4-chlorophenylthio)guanosine-3′,5′-monophosphate (PCPT; 10−7 or 10−5 mol/L); (ii) 8-bromo-cAMP (10−5 mol/L) followed by PCPT; (iii) β-phenyl-1,N2-etheno-8-bromoguanosine-3′,5′-monophosphorothioate, SP-isomer (SP; 10−7 or 10−5 mol/L); or (iv) 8-bromo-cAMP (10−5 mol/L) followed by SP.

4. There were no significant differences between groups in baseline percentage shortening (Pcs; 4.9 ± 0.2 vs 5.6 ± 0.4% for control and T4 groups, respectively) and maximal rate of shortening (Rs; 64.8 ± 5.9 vs 79.9 ± 7.1 µm/ s for control and T4 groups, respectively). Both SP and PCPT decreased Pcs (−43 vs−21% for control and T4 groups, respectively) and Rs (−36 vs−22% for control and T4 groups, respectively), but the effect was significantly reduced in T4 myocytes. 8-Bromo-cAMP similarly increased Pcs (28 vs 23% for control and T4 groups, respectively) and Rs (20 vs 19% for control and T4 groups, respectively). After 8-bromo-cAMP, SP and PCPT decreased Pcs (−34%) and Rs (−29%) less in the control group. However, the effects of these drugs were not altered in T4 myocytes (Pcs −24%; Rs −22%). Both PCPT and cAMP phosphorylated the same five protein bands. In T4 myocytes, these five bands were enhanced less.

5. We conclude that, in control ventricular myocytes, the cGMP-dependent protein kinase exerted major negative functional effects but, in T4-induced hypertrophic myocytes, the importance of this pathway was reduced and the interaction between cAMP and the cGMP protein kinase was diminished.
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Keywords: cAMP; cGMP; cGMP protein kinase; cardiac myocyte; rabbit; thyroxine-induced hypertrophy

Document Type: Research Article

Affiliations: Heart and Brain Circulation Laboratory, Departments of Physiology & Biophysics and Surgery, University of Medicine and Dentistry of New Jersey – Robert Wood Johnson Medical School, Piscataway, New Jersey, USA

Publication date: December 1, 2003

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