Considering Sex as a Biological Variable in Cardiovascular Research

The incidence of diastolic dysfunction increases with age in both humans and mice. This is characterized by increased passive stiffness and slower relaxation of the left ventricle. The stiffness arises at least partially from progressively increased interstitial collagen deposition because of highly secretory fibroblasts. In the past, we demonstrated that AMPK activation via the drug 5-aminoimidazole-4-carboxamide riboside (AICAR) in middle-aged mice reduced adverse remodeling after myocardial infarction. Therefore, as an attempt to normalize the fibroblast phenotype, we used 21-mo-old male and female mice and treated them with AICAR (0.166 mg/g body wt) where each mouse was followed in a functional study over a 3-mo period. We found sex-related differences in extracellular matrix (ECM) composition as well as heart function indices at baseline, which were further accentuated by AICAR treatment. AICAR attenuated the age-related increase in left atrial volume (LAV, an indicator of diastolic dysfunction) in female but not in male hearts, which was associated with reduced collagen deposition in the old female heart, and reduced the transcription factor Gli1 expression in cardiac fibroblasts. We further demonstrated that collagen synthesis was dependent on Gli1, which is a target of AMPK-mediated degradation. By contrast, AICAR had a minor impact on cardiac fibroblasts in the old male heart because of blunted AMPK phosphorylation. Hence, it did not significantly improve old male heart function indices. In conclusion, we demonstrated that male and female hearts are phenotypically different, and sex-specific differences need to be considered when analyzing the response to pharmacological intervention. NEW & NOTEWORTHY The aging heart develops diastolic dysfunction because of increased collagen deposition. We attempted to reduce collagen expression in the old heart by activating AMPK using AICAR. An improvement of diastolic function and reduction of cardiac fibrosis was found only in the female heart and correlated with decreased procollagen expression and increased degradation of the transcription factor Gli1. Male hearts display blunted AICAR-dependent AMPK activation and therefore this treatment had no benefits for the male mice.