Data from this study provide four major findings. First, male and female middle-aged rats with a transmural MI of the comparably large size undergo a relatively similar pattern of global LV chamber remodeling and reactive myocardial hypertrophy; however, the myocardial content of fibrillar collagen, especially in periarteriolar regions, increases more in female than in male rats. Second, the transverse growth of cardiac myocytes in the LV free wall is limited to the endomyocardium of female post-MI rats, whereas male rats experience a marked enlargement of cardiac myocyte in both epimyocardial and endomyocardial regions. Third, post-MI rats of both sexes showed that significant adaptive angiogenesis occurs only in the areas of the LV free wall associated with the cross-sectional expansion of cardiac myocyte greater than 50%. Finally, a significant expansion of the coronary arteriolar tree occurs in both the LV free wall and the septum of post-MI male rats, whereas in females, adaptive arteriogenesis is limited only to the free wall. Taken together, these findings support our conclusion that significant regional sex-related differences characterize the post-MI adaptive responses of various structural components in surviving LV myocardium, especially cardiac myocytes and coronary microvessels.
Does sex/gender affect the pattern of global post-MI LV remodeling and a regional hypertrophic response of the cardiac myocytes?
Although the structural alterations in LV architecture, termed ventricular remodeling, which occur in response to a large transmural MI, have been studied for the last three decades in both humans and experimental animals [1–3, 30], the findings related to sex/gender-specific differences during this process remain obscure [11, 31, 32]. Therefore, our study is the first to conduct a side-by-side comparison of the global and regional patterns of LV remodeling between post-MI male and female middle-aged rats. We focused specifically on middle-aged animals because this age group, along with older individuals, corresponds to a human population, including both sexes, which are most vulnerable for myocardial ischemia and infarction [17, 18, 33, 34]. We also chose to evaluate the hearts at the end of the first post-MI month because it usually represents a compensated phase of the infarct-induced LV remodeling process. At this stage, the left ventricle contains a formed scar , hypertrophied cardiac myocytes, and significantly expanded cavity , but yet preserves the thickness of the noninfarcted myocardial wall  and the external cross-sectional chamber dimensions . In agreement with these observations, we found that although the LV cavity of post-MI male and female middle-aged rats was expanded, the LV weight-to-body weight ratio, the external cross-sectional dimensions of the LV chamber, and the thickness of both the septal and noninfarcted free wall myocardium remained similar to shame values, suggesting a compensated stage of ventricular remodeling. Most importantly, we established the fact that despite a significant difference in the size of the left ventricles, the post-MI middle-aged male and female rats had virtually identical values of the remodeling indices and the LV cavity diameter to septum thickness ratio, indicative of a similar pattern of global LV remodeling between the sexes.
However, our data are in contrast to previous observations by others who studied young adult post-MI rats [12, 14, 31]. These studies noted a significant difference in the LV remodeling pattern between male and female animals. One study found a greater increase in the thickness of noninfarcted free wall in males compared to females , whereas another reported that males had a smaller cross-sectional expansion of the LV chamber than females . We believe that discrepancy between our findings and those reported previously are attributable to the significant difference in the age of rats studied. This argument is supported by findings showing that LV parameters such as cavity dimensions, the size of cardiac myocytes, and the scale of myocardial fibrosis differ significantly between young and aged rodents (mice and rats) both at the onset of MI and during LV remodeling [19, 20]. These data suggest that the high level of myocardial fibrosis in the hearts of sham-operated middle-aged rats, particularly males, is probably age-related since data from younger animals did not reveal extensive fibrosis . Yet, our data are consistent with the fact that LV myocardium of the female rats, even after a large MI, revealed a lesser amount of interstitial collagen than males . To a certain extent, this phenomenon can be attributed to an antifibrotic effect of estrogen , the level of which remains fairly high in middle-aged rats . However, we found a significant interaction between the effects of sex and the experimental model on perivascular collagen content in both LV free wall and the septum. Particularly, we determined that post-MI remodeling in middle-aged female rats causes a much greater accumulation of interstitial and especially perivascular collagen than in males.
Since the compensatory recovery of LV weight in post-MI male and female middle-aged rats occurred without noticeable thickening of the myocardial wall, it is likely that in both sexes, most of the compensatory myocardial growth occurred via cardiac myocyte lengthening, similar to that reported previously by Chen et al.  in young adult rats. However, in contrast to the latter study, we detected the marked regional differences in transverse myocyte areas, primarily in the noninfarcted free wall myocardium. One potential explanation for such discrepancy might be the fact that Chen and colleagues  analyzed cardiac myocytes isolated from both the free wall and the septum together, while in our study, we measured the myocyte cross-sectional areas separately in three LV regions (free wall epi- and endomyocardium; and septal endomyocardium). Besides, our data documenting post-MI compensatory growth of cardiac myocytes in the LV free wall are consistent with numerous previous studies [5, 12, 13, 38, 39]. However, in addition to these findings, our data revealed a significant interaction between the effects of sex and the experimental model on myocyte cross-sectional area in the epimyocardial region. Specifically, we established the fact that post-MI female rats, in contrast to males, had no reactive enlargement of cardiac myocytes in the epimyocardial region. Such distinction in myocyte hypertrophic response suggests the existence of a sex-specific difference in regional epicardial wall stress [40, 41] between the sexes. Taking into consideration the fact that regionally elevated wall stress can cause pronounced focal degradation of the extracellular matrix , it feasible to speculate that such areas of post-MI male hearts may be more prone to side-by-side slippage of myocyte bundles [42, 43], increased apoptotic myocyte death  and, hence, exaggerated concentric hypertrophy of surviving cardiac myocytes .
Does sex/gender influence a scale of coronary microvessels (capillary and arterioles) adaptation during post-MI remodeling?
Over the past several decades, a large body of accumulating evidence has established the fact that the coronary vasculature undergoes structural adaptations, including angiogenesis and arteriogenesis, in response to postinfarction LV remodeling . However, most of the published experimental studies used either male [39, 47–49] or, rarely, female  animals only. Most surprising is the fact that, with the exception of our recent studies on middle-aged rats [6, 21, 22, 26], the majority of earlier observations involving murine myocardium have been done on young or young adult mice or rats, which are known to have a more elaborated and highly adaptable coronary vascular network than the hearts of older animals [51–53]. Therefore, our current study is the first to address a sex-specific pattern of coronary microvessel adaptations in the left ventricle of post-MI middle-aged rats.
Consistent with the previous reports on younger animals [39, 47, 49, 54], we found that post-MI hearts of middle-aged rats demonstrated a noticeable reduction in numerical density of coronary capillaries mainly in the regions of the noninfarcted free wall in which the remaining cardiac myocytes underwent substantial concentric enlargement, i.e., in the epi- and endomyocardium of males and endomyocardium of female rats. However, in contrast to males, post-MI female rats had a smaller reduction in capillary density in free wall endomyocardium, despite the fact that both sexes had the noticeable concentric growth of cardiac myocytes in this region. This finding indicates a better regional adaptive expansion of capillaries in the subendocardium of female compared to male rats. Such observation was associated with a significant interaction between the effects of sex and the experimental model on the capillary-to-cardiac myocyte ratio in free wall endomyocardium.
In addition, we found that in both sexes the capillary-to-myocyte ratio increased dramatically only in the areas of free wall associated with enlargement of myocyte cross-sectional area greater than 50% of shame values. Considering the data reported previously on young adult male rats , it is feasible to suggest that such a significant increase in the capillary-to-myocyte ratio in these hypertrophic areas of post-MI male and female middle-aged rats, indicates augmented regional compensatory angiogenesis, which could be caused by a variety of factors, including increased wall stress and myocardial stretch, or impaired myocyte oxygenation . Though, the effect of more prominent side-by-side myocyte slippage, which might substantially modify myocyte numerical density in these regions, cannot be ruled out either [38, 43].
While the capillary network is important for oxygen delivery to cardiac myocytes, the coronary arteriolar bed is critical for a distribution of blood between capillary domains. Post-MI hypertrophy of the LV myocardium requires structural modifications of the arteriolar tree in order to provide an adequate blood supply to the overloaded myocytes within noninfarcted LV regions [8, 9, 48]. Unfortunately, nearly all previous experimental studies that addressed arterioles in post-MI myocardium were done exclusively on male animals, including our own earlier studies on middle-aged rats [6, 21, 22, 26]. However, according to our current findings and previous observations on mice of both sexes , female hearts have a significantly smaller arteriolar bed than do males. The evident sex-specific differences in the size of the arteriolar bed seen in our study are consistent with the data published by others, regardless of the fact that most of these studies have had the extent of the coronary arteriolar bed reported either for male [56, 57] or female [50, 58] animals only. The existence of sex/gender related differences in the size of coronary arterial beds has also been confirmed in human studies , revealing that women have smaller coronary artery size than men even after adjusting for either body size  or left ventricular mass .
Despite the fact that arteriolar length and volume densities were significantly greater in male than in female middle-aged rats as indicated by sham values, both sexes demonstrated a relatively similar degree of adaptive arteriolar growth in the LV free wall during post-MI remodeling. However, we found a significant interaction between the effects of sex and the experimental model on the arteriolar diameter and the frequency distribution of arterioles in ‘LV free wall. Specifically, we established the fact that in females, the growth was limited to small diameter vessels only. It is important to note that previous studies done separately on male and female animals have also confirmed the fact that while in post-MI male rats the arterioles grew throughout the entire vascular tree of the remaining LV free wall myocardium , in females, the only small size arterioles showed a tendency to expansion . In addition, we uncovered a significant interaction between the effects of sex and the experimental model on arteriolar density in the septum. This finding was associated with the lack of the noticeable compensatory growth of the arterioles in the septum of post-MI female rats as compared to male counterparts. The exact nature of these regional sex-related differences in the responses of male and female coronary arteriolar trees to post-MI remodeling remains to be clarified. We can only speculate that such discrepancy in the pattern of arteriolar bed reorganization within the septum as opposed to the LV free wall of the female post-MI heart, in comparison to males, might be caused by a variety of factors, including the regional difference in wall stress  or temporary composition of growth-stimulating milieu .
The current study examined the sex-related differences in hypertrophic response of cardiac myocytes and adaptive expansion of coronary microvessels only at one time point during MI-induced LV remodeling. Therefore, it is difficult to corroborate whether continuing ventricular remodeling would further modify a scale of adaptive responses. Furthermore, because only rats with a large transmural MI were used, the effect of the moderate or small size infarcts on a degree of adaptive processes remained unknown. In addition, it is important to emphasize that this experimental study on middle-aged rats was not intended to investigate the role of sex hormones during a post-MI remodeling process; instead, we attempted to evaluate the responses of different structural components in the remodeled male and female left ventricle under the natural circumstances that might occur in middle-aged humans.