[Effect of thyroid hormone on the beta-receptor of cardiac myocytes isolated from rat ventricle].

ABSTRACT

INTRODUCTION Most of the symptoms in hyperthyroidism, such as an augmented metabolism, tachyarrhythmias and increased myocardial contractility, resemble those of increased sympathetic beta-adrenergic receptor are a major concern, but the exact mechanisms of the effects remain to be clarified. In the present study, the direct effects of triiodothyronine(T3) on the spontaneous beating and beta-receptor in relation to protein synthesis of the cultured ventricular myocytes were investigated. METHODS The cardiac myocytes were prepared from neonatal rat ventricle by enzymatic digestion and incubated at 37 degrees C in the atmosphere of 5% CO2 in air. The culture medium used was DME containing 5% fetal bovine serum and was renewed daily. The spontaneous beating frequencies of the myocytes were measured by the use of optical-video system. The cultured myocytes showed spontaneous beating dependent on Ca2+ concentration about 48 hours after the isolation. Although the beating was irregular and unstable at the beginning, it became regular and steady on the 3rd culture day. The experiments measuring spontaneous beating frequency were performed using the cells of this stage. In these experiments, the effects of T3 on the spontaneous beating frequency were studied after a 15-20 minute incubation with various concentrations of T3 (1 x 10(-10)-1 x 10(-7)M). The spontaneous beating was enhanced by isoproterenol and suppressed by Ca antagonists. The cardiac beta-receptors were identified using the hydrophilic radioligand 3H-CGP-12177 which labels specifically cell surface bound beta-receptors. The effects of T3 on protein synthesis in cultured myocytes were studied by measuring 3H-leucine uptake. RESULTS I. The effects of T3 on the spontaneous beating The effects of T3 on the spontaneous beating were investigated in two different ways. Firstly, the direct effect of T3 was studied by incubating the cells in the medium containing variable concentration of T3. T3 increased the beating frequency dose-dependently, and the maximal accelerating action was observed at the concentration of 10(-8)M. T3 at this concentration increased the spontaneous beating frequency from 131 +/- 25 min to 157 +/- 35 min after the treatment. Higher concentration of T3 induced arrhythmias and seemed to be toxic. Therefore, in the following experiments the concentration of 10(-8)M was used. The cardiac myocytes were incubated in either T3 containing or T3 free medium for 10 days. The incubation medium was renewed every day, and the change in the beating frequency was monitored continuously.(ABSTRACT TRUNCATED AT 400 WORDS)