Contribution of non-cardiomyocyte apoptosis to cardiac remodelling that occurs in the transition from compensated hypertrophy to heart failure in spontaneously hypertensive rats

Various alterations in molecular and cellular events have been considered as possibly contributing to the cardiac remodelling that occurs during the transition from compensated hypertrophy to heart failure. The aim of the present study is to clarify (1) whether cardiac apoptosis occurs during the transition from compensated hypertrophy to decompensated heart failure, and (2) whether expression of the genes encoding Bax (an apoptosis inducer) and Bcl-xL and Bcl-2 (apoptosis inhibitors) is altered during this transition. We used 12-month-old and 20-month-old male spontaneously hypertensive rats (SHR₁₂ and SHR₂₀ respectively) and age-matched Wistar–Kyoto rats (WKY₁₂ and WKY₂₀ respectively). These rats were killed after measurement of haemodynamic parameters by transthoracic echocardiography and use of a tipmanometer via the right carotid artery. The expression of bcl-2, bcl-xL and bax was analysed by Northern blotting. Samples were also fixed in 4% paraformaldehyde for in situ nick end-labelling (TUNEL) methods and immunohistochemistry. SHR₁₂ had well compensated left ventricular hypertrophy with normal fractional shortening and normal end-systolic wall stress. In contrast, the hearts of SHR₂₀ developed decompensated dilatation, with a decrease in fractional shortening and an increase in end-systolic wall stress. TUNEL-positive cells were seen exclusively in the hearts of SHR₂₀. The major cell types that showed TUNEL-positive nuclei were non-cardiomyocytes. The expression of bax remained unchanged during the transition to heart failure. However, there was increased expression of bcl-xL in the failing stage, whereas the expression of bcl-2 remained unchanged. Immunohistochemical studies revealed that Bcl-xL protein was up-regulated in the hearts of SHR₂₀. In conclusion, non-cardiomyocyte apoptosis may play a contributory role in the remodelling that occurs in the transition from compensatory hypertrophy to decompensated heart failure. In addition, it is suggested that enhanced expression of bcl-xL plays an important role in the preservation of cardiomyocytes during this transition.