The Woodruff School

SUMMARY

Alterations in myocyte calcium (Ca2+) handling appear to be centrally involved in the dysfunctional characteristics of the failing heart. Ca2+ enters cells primarily via L-type voltage-gated Ca2+ channels, with Ca2+ efflux occurring via the sodium calcium exchanger (NCX) and plasma membrane Ca2+-ATPase. During ischemia, intracellular calcium overload occurs which can result in cell death. Manganese (Mn2+) has been widely used as a surrogate marker for intracellular Ca2+ due to a comparable ionic radius and similar biochemical properties to Ca2+. Mn2+ is known to enter cells via voltage gated calcium channels, and its shortening of the T1 relaxation time allows it to serve as an MRI contrast agent. Quantitative imaging of the mouse heart with a T1-mapping manganese-enhanced MRI (MEMRI) protocol allows for in vivo estimates of the Mn2+ content, thus providing information regarding Ca2+ handling. This thesis applies T1-mapping MEMRI to study the efflux of Mn2+ from both healthy mice and mice post-myocardial infarction (MI) surgery. Changes in relaxation rates due to the uptake of Mn2+ (∆R1) can be calculated from T1 maps obtained both pre- and post-MnCl2 infusion. By acquiring T1 maps at multiple times post-infusion the temporal trend of ∆R1 in the heart can be observed. This in vivo trend can then be related to the absolute Mn content in the myocardium by performing elemental analysis on sample mice hearts. The relative importance of individual efflux mechanisms in healthy mice can be determined by inhibiting the NCX with SEA0400, following infusion of MnCl2. Regional alterations in Mn2+ efflux can also be studied in the infarcted heart to potentially identify salvageable myocardium in the peri-infarcted zone surrounding the necrosed tissue. Application of pharmacokinetic models to in vivo and elemental analysis data from both the healthy and MI mice groups can be used to estimate contributions from different influx and efflux mechanisms, and to predict alterations in Mn2+ handling due to the disease condition. Studying Mn2+ efflux using these protocols could provide a pre-clinical model for examining alterations in relative Ca2+ fluxes and to potentially monitor disease progression.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Benjamin Waghorn

TIME:	Friday, November 6, 2009, 1:00 p.m.

PLACE:	Neely Building, Class

TITLE:	Assessing Alterations in Myocardial Mn2+ Fluxes Following Myocardial Infarction in a Murine Model Using T1- Mapping Manganese-Enhanced MRI

COMMITTEE:	Dr. Tom Hu, Co-Chair (MCG) Dr. Farzad Rahnema, Co-Chair (NRE/MP) Dr. Sang Cho (NRE/MP) Dr. Autumn Schumacher (MCG) Dr. Chris Wang (NRE/MP) Dr. Nathan Yanasak (MCG)

SUMMARY Alterations in myocyte calcium (Ca2+) handling appear to be centrally involved in the dysfunctional characteristics of the failing heart. Ca2+ enters cells primarily via L-type voltage-gated Ca2+ channels, with Ca2+ efflux occurring via the sodium calcium exchanger (NCX) and plasma membrane Ca2+-ATPase. During ischemia, intracellular calcium overload occurs which can result in cell death. Manganese (Mn2+) has been widely used as a surrogate marker for intracellular Ca2+ due to a comparable ionic radius and similar biochemical properties to Ca2+. Mn2+ is known to enter cells via voltage gated calcium channels, and its shortening of the T1 relaxation time allows it to serve as an MRI contrast agent. Quantitative imaging of the mouse heart with a T1-mapping manganese-enhanced MRI (MEMRI) protocol allows for in vivo estimates of the Mn2+ content, thus providing information regarding Ca2+ handling. This thesis applies T1-mapping MEMRI to study the efflux of Mn2+ from both healthy mice and mice post-myocardial infarction (MI) surgery. Changes in relaxation rates due to the uptake of Mn2+ (∆R1) can be calculated from T1 maps obtained both pre- and post-MnCl2 infusion. By acquiring T1 maps at multiple times post-infusion the temporal trend of ∆R1 in the heart can be observed. This in vivo trend can then be related to the absolute Mn content in the myocardium by performing elemental analysis on sample mice hearts. The relative importance of individual efflux mechanisms in healthy mice can be determined by inhibiting the NCX with SEA0400, following infusion of MnCl2. Regional alterations in Mn2+ efflux can also be studied in the infarcted heart to potentially identify salvageable myocardium in the peri-infarcted zone surrounding the necrosed tissue. Application of pharmacokinetic models to in vivo and elemental analysis data from both the healthy and MI mice groups can be used to estimate contributions from different influx and efflux mechanisms, and to predict alterations in Mn2+ handling due to the disease condition. Studying Mn2+ efflux using these protocols could provide a pre-clinical model for examining alterations in relative Ca2+ fluxes and to potentially monitor disease progression.