We and other groups have reported that
endothelin (ET)-1 expression in the heart is altered in the setting of
heart diseases. We have also reported that myocardial ET-1 is involved in the progression of
heart failure, and that an ET receptor antagonist improves long-term survival in
heart failure (Nature 384: 353-355, 1996). However, the role of myocardial ET-2 in disease states are not known. To characterize the role of ET-2, we used a) the failing hearts of rats with
heart failure caused by
myocardial infarction, and b) primary cultured cardiomyocytes subjected to
hypoxia. In the failing heart in vivo, ET-1
mRNA increased by 390% compared with that in the non-failing heart, while ET-2
mRNA drastically decreased by 88%. Thus, gene expression of ET-1 and ET-2 was reciprocally altered in the failing heart in vivo. In in vitro studies, reciprocal alterations in ET-1 and ET-2 gene expression were also observed in isolated primary cultured cardiomyocytes, subjected to
hypoxia. Specifically, acute hypoxic stress induced a significant increase (360% of the basal level) in ET-2
mRNA expression compared with that in normoxic cells, whereas it decreased ET-1
mRNA expression by 62% in primary cultured cardiomyocytes. Although these two crucial conditions, i.e.,
heart failure in vivo and acute hypoxic stress in vitro, are pathophysiologically distinct from each other, reciprocal alteration of ET-1 and ET-2 gene expression was observed in both cases. To further investigate the regulatory mechanism of the altered gene expression,
luciferase analysis was performed using primary cultured cardiomyocytes. ET-2 promoter, which is the 5'-flanking region of preproET-2 gene (5'ET-2), showed a marked increase in
luciferase activity during acute
hypoxia. In contrast, the
luciferase activity of 5'ET-1 (ET-1 promoter) did not change in response to hypoxic stress. The present study suggests that there are transcriptionally distinct regulatory mechanisms for ET-1 and ET-2 expression in cardiomyocytes, and therefore this study may provide a new aspect of cardiac ET system that not only ET-1 but also ET-2 can be participated in the pathophysiological conditions.