Systemic Bone Loss Following Myocardial Infarction in Mice is Mitigated by Treatment with a β3 Adrenergic Receptor Antagonist

Myocardial infarction (MI) and osteoporotic fracture (Fx) are leading causes of morbidity and mortality worldwide, and there is epidemiological evidence linking their incidence, suggesting possible crosstalk. MI can exacerbate underlying atherosclerosis through sympathetic nervous system activation and β adrenoreceptor-mediated release of hematopoietic stem cells (HSCs), leading to monocytosis. We hypothesized that this same pathway may initiate systemic bone loss following MI, since osteoclasts differentiate from monocytes. In this study, MI was performed in 12-week-old male mice (n=24), and mice were randomized to treatment with a β-adrenergic receptor antagonist (SR 59230A) or no treatment for 10 days post-operatively; additional mice (n=21, treated and untreated) served as un-operated controls. Bone mineral density (BMD), bone mineral content (BMC), and body composition were quantified at baseline and 10 days post-MI using DXA; at 10 days post-MI circulating monocyte and neutrophil levels were quantified, and the L5 vertebral body and femur were analyzed with micro-computed tomography. We found that MI led to increases in circulating monocyte and neutrophil levels, but contrary to our initial hypothesis, β-antagonist treatment further increased circulating monocytes compared to untreated mice. BMD and BMC of MI mice decreased at the femur and lumbar spine (−6.9% femur BMD, -3.5% lumbar BMD); β-antagonist treatment diminished this bone loss response (−5.3% femur BMD, -1.2% lumbar BMD). Similarly, trabecular bone volume was decreased in MI mice compared to control mice, and this bone loss was partially attenuated by β-antagonist treatment. These results suggest that MI leads to bone loss at both the axial and appendicular skeleton, and that the sympathetic nervous system may be a modulator of this response. This study is the first to mechanistically show bone loss following an ischemic injury; these findings suggest that bone loss and increased fracture risk may be important clinical co-morbidities following MI or other ischemic injuries.