Cardiac mRNA levels during the development and growth of the rat

Preservation of TSPO by chronic intermittent hypobaric hypoxia confers antiarrhythmic activity

Abnormal activation of mitochondrial translocator protein (TSPO) contributes to arrhythmogenesis during cardiac metabolic compromise; however, its role in the antiarrhythmic activities of chronic hypoxia adaptation remains unclear. Our results demonstrated that 80% of normoxic rats developed ischaemic VF, whereas this condition was seldom observed in rats with 14 days of chronic intermittent hypobaric hypoxia (CIHH). TSPO stimulation or inhibition affected the arrhythmias incidence in normoxic rats, but did not change the CIHH-mediated antiarrhythmic effects. Abrupt and excessive elevation of TSPO activity was positively linked to ischaemic VF, and CIHH preserved TSPO activity during ischaemia. The preservation of TSPO activity by CIHH also contributed to the maintenance of intracellular Ca homeostasis. These results suggest that the blunt sensitivity of TSPO to ischaemic stress may be responsible for the antiarrhythmic effects by CIHH.

Mitochondrial benzodiazepine receptors mediate cardioprotection of estrogen against ischemic ventricular fibrillation

The cardioprotective effects of estrogen remain controversial in clinical practice. Previous reports have shown that cardioprotective mechanisms converge on the mitochondria, but the role of mitochondria in estrogen's actions on cardiac arrhythmias is unclear. Here, we report that stimulation or inhibition of mitochondrial benzodiazepine receptors (mBzR) affected ventricular fibrillation (VF) almost in an "all-or-none" manner in an in vitro rat heart model of ischemic VF. Low concentrations of estrogen did not provide antiarrhythmic effects; however, the combination of mBzR activator and estrogen reduced VF incidence in hearts from either gender. Such synergistic actions also enabled cardiomyocytes to resist metabolic stress-induced intracellular [Ca(2+)](i) overload. Ligand binding experiments revealed that estrogen itself did not affect mBzR activity under basal conditions but promoted its up-regulation under myocardial ischemia. Our results suggest that mBzR may be an important molecule for ischemic arrhythmia and may act as a molecular switch for estrogen's antiarrhythmic effects. This finding provides a clue for elucidating the conflicting results regarding estrogen's cardiac effects in clinical studies and also suggests potential new strategies for hormone treatment in the female population.

Heterotopic cardiac transplantation decreases the capacity for rat myocardial protein synthesis

Heterotopic cardiac isografts are vascularly perfused hearts that maintain structural and functional integrity for prolonged periods of time. When placed in an infrarenal location, the heart is hemodynamically unloaded and undergoes negative growth, leading to cardiac atrophy. At 7 and 14 days after transplantation, the transplanted heart was decreased in size compared with the in situ heart (p less than 0.001). To assess the possible mechanism(s) to account for this reduction in size we studied in vivo rates of total left ventricular (LV) protein synthesis, total LV RNA content, and 18S ribosomal RNA content by nucleic acid hybridization. The LV protein synthetic rate was 4.7 and 5.3 mg/day in the in situ heart and was significantly decreased to 2.9 and 2.7 mg/day in the transplanted hearts at 7 and 14 days, respectively. LV RNA content of the transplant declined to 53% and 48% of the in situ value at 7 and 14 days, respectively. Hybridization studies revealed that LV 18S ribosomal subunit content was reduced proportionately to total RNA in the heterotopic hearts. As a result of these changes, there was no significant difference in the efficiency of total LV protein synthesis between the in situ and transplanted hearts. The present studies demonstrate that the hemodynamic unloading and cardiac atrophy that is characteristic of heterotopic cardiac transplantation is accompanied by a decrease in LV total RNA content and 18S RNA, resulting in a decreased capacity for myocardial protein synthesis.