Changes in microarchitecture of atherosclerotic calcification assessed by 18F-NaF PET and CT after a progressive exercise regimen in hyperlipidemic mice

Abstract

Background: Despite the association of physical activity with improved cardiovascular outcomes and the association of high coronary artery calcification (CAC) scores with poor prognosis, elite endurance athletes have increased CAC. Yet, they nevertheless have better cardiovascular survival. We hypothesized that exercise may transform vascular calcium deposits to a more stable morphology.

Methods: To test this, hyperlipidemic mice (Apoe^-/-) with baseline aortic calcification were separated into 2 groups (n = 9/group) with control mice allowed to move ad-lib while the exercise group underwent a progressive treadmill regimen for 9 weeks. All mice underwent blood collections and in vivo ¹⁸F-NaF μPET/μCT imaging both at the start and end of the exercise regimen. At euthanasia, aortic root specimens were obtained for histomorphometry.

Results: Results showed that, while aortic calcification progressed similarly in both groups based on µCT, the fold change in ¹⁸F-NaF density was significantly less in the exercise group. Histomorphometric analysis of the aortic root calcium deposits showed that the exercised mice had a lower mineral surface area index than the control group. The exercise regimen also raised serum PTH levels twofold.

Conclusion: These findings suggest that weeks-long progressive exercise alters the microarchitecture of atherosclerotic calcium deposits by reducing mineral surface growth, potentially favoring plaque stability.

Keywords: 18F-NaF PET/CT imaging; PTH; Treadmill exercise; aortic; calcification; hyperlipidemia; microarchitecture.

Figure 1:. Experimental timeline.

Schematic of the timing of interventions for control and exercise groups. The diet was started at 10 weeks of age.

Figure 2:. In vivo imaging analyses of the effects of exercise on aortic calcium deposits.

(A) In vivo imaging analyses of the effects of exercise on aortic calcium deposits. (A) Vascular calcium deposits (white arrows) imaged by microCT and microPET in control and exercise mice at the start (pre) and end (post) of the 9-week period. (B) MicroCT analysis of total aortic calcium content (left and middle) and fold change in aortic calcium content (right) over the intervention period. (C) MicroPET analysis of the fold change in the ¹⁸F-NaF density (left) and total ¹⁸F-NaF content (right) over the intervention period.

Figure 3:. Histological analyses of the effects of exercise on aortic calcium deposits.

(A-B) Alizarin red staining of representative aortic root sections. Arrowheads indicate small deposits (“spotty”) of calcification, and the arrow indicates a larger coalesced deposit of calcium mineral. Scale bar, 500 μm. (C) Mineral surface area index (MSI) of the control and exercise groups.

Figure 4:. Effects of exercise on serum PTH levels.

(A-B) Serum PTH was measured before (Pre) and after (Post) the intervention period.

Figure 5:. A diagram illustrating the concept of surface area index or ¹⁸F-NaF density.

As seen in this illustration, an increase in the size of round particles in a given space results in a local decrease in the surface area per volume of particles, i.e., surface area index (because volume increases more rapidly than area as radius increases). Conversely, a decrease in the size of particles has the opposite effect (not illustrated).