Introduction
Cardiomyopathy, heart failure, and coronary artery disease are the main reasons for heart diseases being the leading causes of death in western society [
1]. Fundamental in vitro and in vivo molecular research aims to find new translational diagnostics and therapeutic perspectives to lower mortality and improve patients' quality of life. Especially in vivo animal models are most suitable for transferring clinical obstacles, such as myocardial infarction due to coronary artery disease into a research setting to decipher underlying mechanisms and prevent further deteriorating events. For instance, murine models of myocardial infarction aim to reduce the myocardial damage and improve the cardiac remodelling to preserve the cardiac function after irreversible loss of heart muscle cells.
Standard animal models, including mice, rats, rabbits, and pigs, are used to broaden our understanding of diseases. Out of these, mice models are the most common due to short reproduction time, comfortable breeding, feasible logistics, and costs.
Most of the current cardiac injury models are performed with male mice [
2,
3]. Nevertheless, several murine studies indicate that cardiovascular injury models, depending on mouse gender, differ in their results [
4,
5].
For instance, mice deficient of the G-protein coupled receptor 30 (GPR30), a membrane-bound estrogen receptor involved in the estradiol signalling, demonstrate that male but not female mice develop impaired left-ventricular cardiac function [
6]. The overexpression of the cardiac sodium-calcium exchanger increases the susceptibility to ischemia–reperfusion injury in male but not female mice [
7]. In a diabetic mouse model, the induced adverse cardiac remodelling was observed earlier in female mice than in their male counterparts [
8]. These publications further state several underlying molecular and functional differences in the hearts of male and female mice.
Moreover, mice are used for cardiac injury models at different age stages, commonly starting at eight weeks up to 18 weeks.
Functional imaging using positron emission tomography (PET) facilitates serial non-invasive measurements of the same individuum at different time points [
9,
10]. ECG-gated functional PET imaging with 2-deoxy-2-[
18F]fluoro-D-glucose (18F-FDG) not only enables the assessment of cardiac viability but also estimating left ventricular function parameters (end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV) left ventricular ejection fraction (EF%)) [
11], as already demonstrated in several cardiac stress models, e.g., in myocardial infarction model and transaortic constriction model for induction of heart hypertrophy [
12,
13].
Researchers conduct in vivo studies either in one sex or less often distinguish between sexes in laboratory animal studies. Besides using various experiment ages for mice, there is still a lack of knowledge, for the most suitable time frame to perform cardiac injury models and avoid growth-dependent biases.
Therefore, this study aimed to evaluate metabolic and cardiac function parameters using serial micro PET measurements in ageing male and female mice to provide researcher data about cardiac gender differences at different age stages.
Discussion
In this study, we provide evidence for age-dependent gender-specific differences in murine hearts by small-animal PET imaging.
By primarily assessing the left ventricular metabolic volume, we used a standardized method that provides an efficient and proven surrogate marker of murine heart mass [
13]. Our results indicate that male hearts compared to female hearts are bigger in LVMV and that growth of male hearts continues after ten weeks of age. Here we show that the female heart increases only mildly and not statistically significant in the LVMV. Our results mirror human data showing that males' heart mass is up to 15–30% higher [
23]. It is known as well that male and female hearts show hypertrophy during ageing [
24]. Yet our study did only document the murine early to mid-adulthood.
Regarding the clinically relevant cardiac parameters, the ESV in male increases over time. The SV remains stable, which is in line with the observation of growing male hearts [
24]. While the male hearts could become more durable, they are in theory, not dependent on the former ventricle volume to maintain the body perfusion. Therefore, as a consequence, the ejection fraction in male mice would drop. While the female hearts do not increase in LVMV at these stages of age, body mass increases over time. They could depend on more ESV reserve, and therefore, the EF% could increase for adequate organ perfusion.
The %IA/g, also described as %ID/g in previous publications resembles the ratio between the activity of the tracer detected in the tissue, and the total tracer activity injected [
14,
19,
25]. Interestingly, the %IA/g in male mice tends to decrease over follow-up time. In contrast, the cardiac %IA/g remains stable in the female mice, which could be partially attributed to the moderate increase in body weight and the constant cardiac mass. Interestingly, our data indicate a positive correlation of %IA/g and the measured heart rate, which could illustrate a higher cardiac demand for glucose at higher heart rates. However, the %IA/g could depend on multiple distinct and yet undetermined variables masking a direct affiliation.
Of note, the term %ID/g or more precisely %IA/g was primarily described in post-myocardial injury studies.
These recent studies could demonstrate that the cardiac %IA/g in myocardial infarction is elevated in the acute phase on day five in humans [
25]. This increased accumulation could be associated with the inflammatory response, and the invading immune cells and subsequent higher glucose consumption in the tissue [
11,
14]. Herein, we could demonstrate that the uptake of 18F-FDG also positively correlates with murine heart rate. Our results also support the notion that increasing heart mass in healthy hearts, represented by LVMV, correlates with reduced heart rates. The documented murine heart rate and correlation towards the cardiac surrogate marker LVMV, even with isoflurane narcotic, is in line with published studies [26][27]. Of note, we cannot exclude sex differences in response to anaesthesia, since the standard mouse model is male and more research is warranted to provide insight.
A limitation of the 18F-FDG PET scan could be the usage of isoflurane, as discussed in [
16]. Concerning the usage of 18F-FDG, cardiac uptake and metabolism could be modified by various systemic factors, e.g., insulin and glucagon that were not evaluated in this study [
17].
This study provides insight into the cardiac homeostasis at 10 weeks, 14 weeks, and 18 weeks of age in male and female mice using the innovative design of serial non-invasive PET imaging. Our results could help research groups determine the correct age choice for their murine injury model in both sexes and avoid growth-related biases.
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