Introduction Manganese is a potent MRI contrast agent and can enter myocytes through l-type Calcium channels, making it an interesting imaging probe for Ca2 +channel activity.1 2 Mn2 +accumulates in the normal myocardium, reducing the longitudinal (T1) relaxation rate and increasing the relaxivity (R1) of the tissue. Hence, we hypothesised that Mn2 +mediated changes in T1 could reflect intracellular Ca2 +changes in the myocardium acutely after myocardial infarction (MI). By preloaded the heart with Mn2 +prior to coronary occlusion, we aimed to assess the efficacy of T1 mapping-Manganese-enhanced MRI (MEMRI) in quantifying intracellular Ca2 +response to ischaemic injury in a mouse model of MI.
Materials/methods In the MI group, adult male C57B1/6 mice received intraperitoneal injections of 0.10 mmol/kg MnCl2 40 min before left anterior descending (LAD) coronary artery occlusion. T1-mapping was used to monitor dynamic alterations of calcium influx at 1, 2 and 3 hours after LAD occlusion and at 2 days, with Mn2 +injected 100 min prior to MRI (Mn2 +loading time similar to the 1 hour post-MI group). Imaging was performed in the mid-short axis view as described3 4 using a Look-locker inversion recovery sequence: TE/TR=0.99/3 ms, effective TR=~3 s, TI=~100 ms intervals, FOV=25.6 mm, matrix=128, slice thickness=1.0 mm using a 9.4T MRI Agilent system. T1 maps were generated using MATLAB.5 For analysis R1 values (1/T1=the relaxivity of the tissue) were analysed from 3 tissue types: Infarcted hearts were classified into area at risk segments (AAR-MI, n=12) and viable segments (Viable-MI, n=12): These were compared to naïve control heart data (Viable-Naïve, n=12). R1 values of the LV blood pool were also measured.
Results As soon as 1 hour after LAD occlusion R1 values were significantly higher in the Viable-MI tissue compared with AAR-MI tissue (p<0.0001), allowing early delineation of the infarct region (figure 1, 2 and table 1). R1 in the Viable-MI tissue continued to rise at 2 and 3 hours post-MI (p=0.02 and p=0.01), but remained constant in the AAR-MI. When compared to naïve control animals, R1 values were slightly, but significantly increased in the AAR-MI tissue at 1 and 2 hours after occlusion (p=0.006 and p=0.03), while R1 was elevated to an even greater degree in the Viable-MI tissue (figure 1, 2 and table 1). The R1 values of LV blood pool in MI and Naïve groups did not change over the 3 hours. When the same animals were imaged at 2 days post-MI, R1 values were still significantly higher in the Viable-MI tissue compared with AAR-MI tissue (p=0.03). However at this point Viable-MI tissue had a similar R1 to naïve hearts, while R1 in the infarcted AAR-MI was lower than controls (p=0.03) (figure 1, 2 and table 1).
Discussion Acutely after ischaemic injury a large increase in R1 (reflecting increased Mn2 +uptake) occurred in the Viable-MI myocytes, while a small Mn2 +increase was also present in the AAR-MI. Increased R1 in the viable-MI myocytes is likely due to elevated catecholamine levels acutely post-MI leading to increased cardiac work and thus Ca2+/Mn2 +uptake. The increased R1 at 1 and 2 hours post-MI in the AAR-MI segments following ischaemic injury may be a result of calcium overload in the ischaemic tissue, which reduces over time as cell integrity is lost. By 2 days the catecholamine storm has passed and R1 levels in the surviving myocardium had normalised, while Mn2 +uptake in the dead infarct region was reduced due to lack of functional myocytes.
Conclusion The present study shows that T1-Mapping MEMRI allows sensitive in vivo detection of subtle changes of Mn2+in viable myocytes in the early phase after myocardial injury. Preloading of the myocardium with Mn2+prior to infarction makes this a sensitive approach which could be used to monitor imbalance in Ca2+homeostasis.
. Huijuan Y, Ramamoorthy H, Abraham P, Isaac B. Decreasing expression of α1C calcium L-type channel subunit mRNA in rat ventricular myocytes upon manganese exposure. Journal Biochem. Mol. Toxicol 2006;20(4).
. Nordhøy W, Anthonsen HW, Bruvold M, Jynge P, Krane J, Brurok H. Manganese ions as intracellular contrast agents: Proton relaxation and calcium interactions in rat myocardium. NMR Biomed April 2003;16(2):82–95.
. Price AN, Cheung KK, Lim SY, Yellon DM, Hausenloy DJ, Lythgoe MF. Rapid assessment of myocardial infarct size in rodents using multi-slice inversion recovery late gadolinium enhancement CMR at 9.4T. J. Cardiovasc. Magn. Reson 2011;13(1):44.
. Stuckey DJ, McSweeney SJ, Thin MZ, Habib J, Price AN, Fiedler LR, Gsell W, Prasad SK, Schneider MD. T1 mapping detects pharmacological retardation of diffuse cardiac fibrosis in mouse pressure-overload hypertrophy. Circ. Cardiovasc. Imaging 2014;7:240–249.
. Jackson LH, Vlachodimitropoulou E, Shangaris P, Roberts TA, Ryan TM, Campbell-Washburn AE, David AL, Porter JB, Lythgoe MF, Stuckey BJ. Non-invasive MRI biomarkers for the early assessment of iron overload in a humanised mouse model of β-thalassemia. Sci. Rep. February 2017;7:43439.
Statistics from Altmetric.com
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.