Using Synchrotron Radiation Imaging Techniques to Elucidate the Actions of Hexarelin in the Heart of Small Animal Models

Abstract

The majority of the conventional techniques that are utilized for investigating the pathogenesis of cardiovascular disease in preclinical animal models do not permit microlevel assessment of in situ cardiomyocyte and microvascular functions. Therefore, it has been difficult to establish whether cardiac dysfunction in complex multiorgan disease states, such as heart failure with preserved ejection fraction and pulmonary hypertension, have their origins in microvascular dysfunction or rather in the cardiomyocyte. Herein, we describe our approach of utilizing synchrotron radiation microangiography to, first, ascertain whether the growth hormone secretagogue (GHS) hexarelin is a vasodilator in the coronary circulation of normal and anesthetized Sprague-Dawley rats, and next investigate if hexarelin is able to prevent the pathogenesis of right ventricle (RV) dysfunction in pulmonary hypertension in the sugen chronic hypoxia model rat. We show that acute hexarelin administration evokes coronary microvascular dilation through GHS-receptor 1a and nitric oxide, and through endothelium-derived hyperpolarization. Previous work indicated that chronic exogenous administration of ghrelin largely prevented the pathogenesis of pulmonary hypertension in chronic hypoxia and in monocrotaline models. Unexpectedly, chronic hexarelin administration prior to sugen chronic hypoxia did not prevent RV hypertrophy or RV cardiomyocyte relaxation impairment. Small-angle X-ray scattering revealed that super relaxed myosin filaments contributed to diastolic dysfunction, and that length-dependent activation might contribute to sustained contractility of the RV. Thus, synchrotron-based imaging approaches can reveal novel insights into cardiac and coronary functions in vivo.

MRI overestimates articular cartilage thickness and volume compared to synchrotron radiation phase-contrast imaging

“…The X-ray detector field of view was 55 mm, and the X-ray beam height was 2 mm ( Fig 1C and 1D ). Multiple scans were required to image the entire vertical height of each sample, due to the limited beam height, and these scans were stitched together using ImageJ [ 37 ].…”

“…Large animal studies, such as pig, can replicate gross features of human articular cartilage disease progression and are useful in evaluating preclinical therapeutic strategies [48]. Also, a recent development in the field is to image living animals using synchrotron-based techniques, even imaging the same animal multiple times, potentiating longitudinal studies of OA progression from early stages [37,[49][50][51][52][53].…”

“…SR enables high-resolution imaging because it has high photon flux, stable sources, and an energy density 1000 times higher than conventional X-ray absorption imaging [ 34 , 35 ]. Some synchrotron imaging techniques take advantage of the high resolution, including absorption-based micro-CT, or even revealing material properties by small angle X-ray scattering [ 36 , 37 ]. However, the coherent X-rays in SR can reveal phase contrast depending on the refractive index of different tissues, and diffraction-enhanced imaging is even possible [ 38 ].…”

“…Second, how do quantitative image analyses from 3T MRI compare to SR-PCI? Previous proof-of-principle papers using single samples compared SR-PCI to 3T MRI [ 24 , 30 , 37 , 38 ]. Here, for the first time, measurements of multiple samples of human articular cartilage from medial tibial condyles of non-OA donors and OA patients permit statistical analyses to compare these imaging modalities.…”