Modifications in characteristics and activities of beta-adrenergic receptors and certain parameters of the cyclic nucleotide systems were observed in the hypertrophied heart of the rat chronically treated with T4. These include: 1) an increased number of beta-adrenergic receptors without a change in their affinity, as determined by binding of (-)-[3H]dihydroalprenolol to the membrane; 2) increased sensitivity and magnitude of stimulation of adenylate cyclase in homogenates by isoproterenol, without a change in the basal or NaF-stimulated (total) enzyme activity; 3) decreased formation of cAMP and decreased activation of cAMP-dependent protein kinase in the minced heart stimulated by isoproterenol, probably due to decreased myocardial ATP concentration; 4) decreased activity of cAMP phosphodiesterase in the particulate fraction; 5) decreased activity of cGMP-dependent protein kinase in both the soluble and particulate fractions, accompanied by decreased activity of cAMP-dependent protein kinase in the particulate fraction; 6) decreased activity of the stimulatory modulator of cGMP-dependent protein kinase and, conversely, increased activity of the inhibitory modulator of cAMP-dependent protein kinase; and 7) increased sensitivity accompanied by decreased maximum tension development of the ventricular strip to contract in response to isoproterenol. These alterations largely disappeared upon regression of the hyperthyroid state. It is suggested that the above changes, many of which were the opposite of those reported earlier for the desensitized and hypertrophied rat heart caused by isoproterenol, may in part consitute the molecular basis for the reputed catecholamine supersensitivity of the heart in the hyperthyroid state.

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1. We tested the hypothesis that increasing myocardial cyclic GMP levels would reduce myocardial O2 consumption and areas of low O2 supply/consumption balance, using zaprinast, a selective cyclic GMP-phosphodiesterase inhibitor. 2. The study was conducted in three groups (vehicle, 10(-3) and 3 x 10(-3) mol/L zaprinast) of anaesthetized open-chest New Zealand white rabbits (n = 24). Coronary blood flow (radioactive microspheres), arterial and venous O2 saturation (microspectrophotometry), O2 consumption, cyclic GMP content (competitive binding) and cyclic GMP-phosphodiesterase activity (conversion of 3H-cyclic GMP to 3H-GMP) were determined. 3. Agents were applied to a patch on the myocardial surface and did not cause significant haemodynamic changes, except for bradycardia in the vehicle and low dose group. 4. The total myocardial cyclic GMP-phosphodiesterase activity was 148 +/- 14 while the zaprinast (10 mumol/L) inhibitable activity averaged 63 +/- 8 pmol/mg protein per min. Cyclic GMP content was increased with increasing doses of zaprinast (vehicle, 4.308 +/- 0.349 pmol/g; low dose zaprinast, 4.803 +/- 0.279 and high dose zaprinast, 7.938 +/- 1.304 pmol/g). 5. Coronary blood flow was not different after treatment (198 +/- 11, 209 +/- 10 and 153 +/- 9 mL/min per 100 g for the vehicle, low and high dose zaprinast, respectively). 6. Under control conditions, 48% of the small veins had O2 saturations below 50%. With zaprinast, this value was reduced to 19% for the low and 24% for the high dose. 7. Average venous O2 saturation increased with zaprinast (49 +/- 2%, 61 +/- 3% and 59 +/- 1%).(ABSTRACT TRUNCATED AT 250 WORDS)

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