**4. Discussion**

This study investigated transcriptional adaptation in LV and RV following ischemia/reperfusion in vivo up to 120 days. A higher gene expression level has been attributed to better postinfarct adaptation in female versus male mice [11]. Here, differences were found between both ventricles in the very early phase after ischemia/reperfusion. A significant downregulation of the transcription factor GATA-4 was found specifically in the LV 24 h after reperfusion confirming similar findings after 2 h [11]. According to this observation, the expressions of GATA4-dependent genes (MHC-α, eNOS, and VEGF) were also downregulated. GATA-4 can exert cell survival signaling in cardiac myocytes and delivery of GATA-4 locally to the infarct border zone induces multiple local effects resulting in beneficial remodeling [12, 13]. Regardless of these initial differences, similar changes, mainly induction of gene expression, were found in both ventricles after 3 days. In both ventricles, repression of cardiac gene expression was more prominent rather than induction at later time points, specifically after 4 months. At that time point, the expression profile significantly differed between both ventricles with a more adaptive phenotype in the RV. Again, at this later time point, reduced expression of GATA4 seems to account for some of the differentially downregulated genes in the LV. In summary, this study highlights the differential expression of GATA4 in LV and RV after successful reperfusion and correlates the expression of GATA4 with main differences in molecular adaptation between both ventricles. Regardless of the different molecular adaptation, LV and RV developed a drop of function although the infarct area and inflammation was specifically located at the left ventricle.

The cardiac ventricle is able to respond to ischemic stress with changes in steady-state mRNA levels within 24 h. In principle, steady-state levels of mRNA are the sum of mRNA formation and degradation and the quantification of steady-state levels does not necessarily identify mechanisms that cause these changes. However, in the current study, the main changes in ventricular expression that occurred within 24 h in the LV corresponded to genes that are known to obtain a GATA-4 promoter responsive element. Please note that only the LV is exposed to ischemia/ reperfusion. Moreover, GATA-4 itself was also downregulated. It should be noted that within the first few hours after the ischemic event, transcriptional changes were mainly present in the LV.

The response of the heart to LV ischemia and reperfusion significantly differed at day 3. At that time, strong molecular adaptations were obtained in both ventricles. In most cases, these were linked to an upregulation of genes indicating an active adaptation to the postischemic stress. Interestingly, most genes that were upregulated in the LV were also regulated in the RV. An exception to this rule was Nkx.2a. This cardiacspecific transcription factor was below the level of detection in the RV but induced in the LV, probably indicating an active regenerative process. This may indicate cardiac differentiation of cells infiltrated into the infarct-affected ventricle (possibly circulating stem cells), or a cardiac differentiation of cells that were located in the LV and that still have a potential to differentiate (cardiac progenitor cells). Alternatively, it may indicate an active repair process of terminally differentiated cardiomyocytes (hypertrophy). However, the more important finding was the observation that a couple of genes were specifically upregulated in the RV that are not induced in the LV. As such decorin, an endogen inhibitor of the profibrotic cytokine TGF-β1, eNOS, increasing the bioavailability of nitric oxide, and Nrf-1, improving mitochondrial biogenesis, were identified. Each of these factors is a marker of compensatory hypertrophy.

One week after the ischemic event, the initial activation of gene transcription is normalized and replaced by downregulation of many genes in LV and

**17**

*Right Heart Adaptation to Left Ventricular STEMI in Rats*

of the RV seems to be more favorable than that of the LV.

RV. Interestingly among them are endothelial cell markers and genes linked to cardiac metabolism. Thus, the molecular adaptation switches into a profile of maladaptation. In concert with this view, biglycan, a factor that favors fibrosis, was induced in the LV. In the RV, the expression of profibrotic genes was downregulated as well as the expression of inflammatory markers. Again, the molecular adaptation

Four months after the ischemic event, the cardiac function was reduced in both ventricles. At that time, decorin was downregulated in both ventricles. Moreover, the RV was characterized by downregulation of elastin and fibronectin. Both molecules are required for a proper cardiac function. In summary, although the molecular adaptation of the RV to LV myocardial infarction differs from that of the LV, the corresponding molecular adaptation of the RV leads to dysfunction of

In this study, we analyzed also the expression of intermedin and the receptor activation modifier proteins (RAMP). They have been analyzed with respect to cardiac regulation in the context of pressure-induced hypertrophy but data on the expression in ischemia and reperfusion are lacking. Intermedin (=adrenomedullin-2) potentially stabilized cardiac function by binding to CGRP receptors that are linked to RAMP-1, -2, or -3. This study shows a downregulation of intermedin in the RV at day 3 and in the LV at day 7. Thus, it is differentially regulated during the subsequent molecular adaptation after ischemia/reperfusion but not as a direct response to the ischemic event itself. Significant downregulation of RAMP-2 and RAMP-1 days after myocardial infarction in both ventricles (RAMP-2) and specifically in the RV (RAMP-1) suggests an impairment of receptor signaling during CGRP-receptors during the subsequent remodeling process. As the development of cardiac dysfunction occurred in both ventricles within 4 months, this observation requires future work because this may be a likely candidate for the subsequent

Finally, it should be mentioned that this study has of course some limitations. Firstly, data are restricted to genes which mRNA steady-state levels are different between sham and ischemia/reperfusion at a p value of 0.05. This does not exclude the possibility that genes that are strongly regulated but below a p value of 0.05 due to a higher individual variation are not relevant for the molecular adaptation. Secondly, the level of significance is a variable of the n number of animals that are investigated. Here, we used eight rats per group. A lower n reduces the number of genes identified as significantly regulated. However, these limitations are counterbalanced by the quantitative real-time RT-PCR protocol and even more important by the high number of genes under investigation and the analysis of groups of genes linked to specific adaptations (i.e., linked to fibrosis, apoptosis, etc.). This allows a

In conclusion, the study identifies a time-dependent difference in the response of LV and RV to STEMI. In the early phase of LV remodeling, GATA-4-dependent downregulation was dominant. The novel and important finding from this study is, however, that a delayed but significant molecular adaptation of the RV. This RV adaptation in the absence of necrosis seems to be more adaptive but still not sufficient to preserve the function. The regulation of RAMP-2 in both ventricles may be

We thank Nadine Woitasky and Peter Volk for excellent technical support. The

data of this study are in part results of the thesis of Pia Weber.

*DOI: http://dx.doi.org/10.5772/intechopen.84868*

the RV as well.

development of heart failure.

more general view on the adaptation process.

one candidate for future research.

**Acknowledgements**

*Visions of Cardiomyocyte - Fundamental Concepts of Heart Life and Disease*

This study investigated transcriptional adaptation in LV and RV following ischemia/reperfusion in vivo up to 120 days. A higher gene expression level has been attributed to better postinfarct adaptation in female versus male mice [11]. Here, differences were found between both ventricles in the very early phase after ischemia/reperfusion. A significant downregulation of the transcription factor GATA-4 was found specifically in the LV 24 h after reperfusion confirming similar findings after 2 h [11]. According to this observation, the expressions of GATA4-dependent genes (MHC-α, eNOS, and VEGF) were also downregulated. GATA-4 can exert cell survival signaling in cardiac myocytes and delivery of GATA-4 locally to the infarct border zone induces multiple local effects resulting in beneficial remodeling [12, 13]. Regardless of these initial differences, similar changes, mainly induction of gene expression, were found in both ventricles after 3 days. In both ventricles, repression of cardiac gene expression was more prominent rather than induction at later time points, specifically after 4 months. At that time point, the expression profile significantly differed between both ventricles with a more adaptive phenotype in the RV. Again, at this later time point, reduced expression of GATA4 seems to account for some of the differentially downregulated genes in the LV. In summary, this study highlights the differential expression of GATA4 in LV and RV after successful reperfusion and correlates the expression of GATA4 with main differences in molecular adaptation between both ventricles. Regardless of the different molecular adaptation, LV and RV developed a drop of function although the infarct area and inflammation was specifically located at

The cardiac ventricle is able to respond to ischemic stress with changes in steady-state mRNA levels within 24 h. In principle, steady-state levels of mRNA are the sum of mRNA formation and degradation and the quantification of steady-state levels does not necessarily identify mechanisms that cause these changes. However, in the current study, the main changes in ventricular expression that occurred within 24 h in the LV corresponded to genes that are known to obtain a GATA-4 promoter responsive element. Please note that only the LV is exposed to ischemia/ reperfusion. Moreover, GATA-4 itself was also downregulated. It should be noted that within the first few hours after the ischemic event, transcriptional changes

The response of the heart to LV ischemia and reperfusion significantly differed at day 3. At that time, strong molecular adaptations were obtained in both ventricles. In most cases, these were linked to an upregulation of genes indicating an active adaptation to the postischemic stress. Interestingly, most genes that were upregulated in the LV were also regulated in the RV. An exception to this rule was Nkx.2a. This cardiacspecific transcription factor was below the level of detection in the RV but induced in the LV, probably indicating an active regenerative process. This may indicate cardiac differentiation of cells infiltrated into the infarct-affected ventricle (possibly circulating stem cells), or a cardiac differentiation of cells that were located in the LV and that still have a potential to differentiate (cardiac progenitor cells). Alternatively, it may indicate an active repair process of terminally differentiated cardiomyocytes (hypertrophy). However, the more important finding was the observation that a couple of genes were specifically upregulated in the RV that are not induced in the LV. As such decorin, an endogen inhibitor of the profibrotic cytokine TGF-β1, eNOS, increasing the bioavailability of nitric oxide, and Nrf-1, improving mitochondrial biogenesis, were identified. Each of these factors is a marker of compensatory hypertrophy. One week after the ischemic event, the initial activation of gene transcription is normalized and replaced by downregulation of many genes in LV and

**4. Discussion**

the left ventricle.

were mainly present in the LV.

**16**

RV. Interestingly among them are endothelial cell markers and genes linked to cardiac metabolism. Thus, the molecular adaptation switches into a profile of maladaptation. In concert with this view, biglycan, a factor that favors fibrosis, was induced in the LV. In the RV, the expression of profibrotic genes was downregulated as well as the expression of inflammatory markers. Again, the molecular adaptation of the RV seems to be more favorable than that of the LV.

Four months after the ischemic event, the cardiac function was reduced in both ventricles. At that time, decorin was downregulated in both ventricles. Moreover, the RV was characterized by downregulation of elastin and fibronectin. Both molecules are required for a proper cardiac function. In summary, although the molecular adaptation of the RV to LV myocardial infarction differs from that of the LV, the corresponding molecular adaptation of the RV leads to dysfunction of the RV as well.

In this study, we analyzed also the expression of intermedin and the receptor activation modifier proteins (RAMP). They have been analyzed with respect to cardiac regulation in the context of pressure-induced hypertrophy but data on the expression in ischemia and reperfusion are lacking. Intermedin (=adrenomedullin-2) potentially stabilized cardiac function by binding to CGRP receptors that are linked to RAMP-1, -2, or -3. This study shows a downregulation of intermedin in the RV at day 3 and in the LV at day 7. Thus, it is differentially regulated during the subsequent molecular adaptation after ischemia/reperfusion but not as a direct response to the ischemic event itself. Significant downregulation of RAMP-2 and RAMP-1 days after myocardial infarction in both ventricles (RAMP-2) and specifically in the RV (RAMP-1) suggests an impairment of receptor signaling during CGRP-receptors during the subsequent remodeling process. As the development of cardiac dysfunction occurred in both ventricles within 4 months, this observation requires future work because this may be a likely candidate for the subsequent development of heart failure.

Finally, it should be mentioned that this study has of course some limitations. Firstly, data are restricted to genes which mRNA steady-state levels are different between sham and ischemia/reperfusion at a p value of 0.05. This does not exclude the possibility that genes that are strongly regulated but below a p value of 0.05 due to a higher individual variation are not relevant for the molecular adaptation. Secondly, the level of significance is a variable of the n number of animals that are investigated. Here, we used eight rats per group. A lower n reduces the number of genes identified as significantly regulated. However, these limitations are counterbalanced by the quantitative real-time RT-PCR protocol and even more important by the high number of genes under investigation and the analysis of groups of genes linked to specific adaptations (i.e., linked to fibrosis, apoptosis, etc.). This allows a more general view on the adaptation process.

In conclusion, the study identifies a time-dependent difference in the response of LV and RV to STEMI. In the early phase of LV remodeling, GATA-4-dependent downregulation was dominant. The novel and important finding from this study is, however, that a delayed but significant molecular adaptation of the RV. This RV adaptation in the absence of necrosis seems to be more adaptive but still not sufficient to preserve the function. The regulation of RAMP-2 in both ventricles may be one candidate for future research.

## **Acknowledgements**

We thank Nadine Woitasky and Peter Volk for excellent technical support. The data of this study are in part results of the thesis of Pia Weber.
