Cardiac magnetic resonance imaging of mice: Assessment of left-ventricular function, and development of manganese as a functional contrast agent.

摘要

The main objective of this thesis was two-fold. The first was to use in vivo cardiac MRI to determine whether there are differences in cardiac function between mice that lack both muscle and mitochondrial creatine kinase isoforms (DKO-CK) compared with wild type. The second goal was to develop MRI techniques sensitive to calcium influx. Manganese was chosen for use as an agent in these studies, because divalent manganese ions can enter excitable cells via voltage-gated calcium channels. Since manganese is paramagnetic, this property can be utilized as an MRI contrast agent to monitor calcium influx into the heart.;CK is an abundant cardiac enzyme involved in energy metabolism. Recent studies of perfused mouse hearts lacking muscle and mitochondrial creatine kinase did not show any contractile alterations. To extend the analysis of these hearts in vivo, MRI was used to measure cardiac function in normal and DKO-CK hearts. There was no difference in ejection fraction during resting and dobutamine stress in normal and DKO-hearts. A detailed examination of contraction during the cardiac cycle revealed that in DKO-CK hearts peak systole was longer and relaxation to diastole was quicker during dobutamine stress than in normal hearts. This result was verified in perfused hearts, which further enabled intracellular calcium measurements to be made using fluorescence changes in the calcium sensitive probe Rhod-2. Interestingly, while dobutamine increased intracellular calcium in normal hearts there was little effect in DKO-CK hearts. This result indicates that the mechanism whereby the DKO-CK heart generates force during dobutamine inotropy has been altered and this may explain the alterations in the cardiac cycle in DKO-CK hearts.;MRI techniques to monitor calcium dynamics in the heart would be very useful. Due to the slow clearance of manganese and its ability to enter cells on voltage gated calcium channels, kinetic studies of cardiac MRI signal enhancement in the presence of Mn2+ might measure calcium influx. To test this, the effects of MnCl2 on cardiac MRI were studied in control hearts, hearts stimulated with dobutamine and hearts given a calcium channel blocker. The rate and extent of Mn2+ enhancement were higher with dobutamine and lower with a calcium channel block then control hearts. These results are consistent with the hypothesis that Mn2+ enhancement is sensitive to calcium influx. Furthermore, manganese was used to assess calcium influx in transgenic mice expressing troponin I lacking protein kinase C phosphorylation sites (TNI-PKC) and in a larger canine model of ischemia. In both cases, results indicate that Mn2+ will be a useful functional MRI contrast agent to monitor aspects of calcium dynamics in vivo.