Clin Res Cardiol (2022). https://doi.org/10.1007/s00392-022-02002-5

A novel approach to characterize cardiac sodium storage - a combined fluorescence photometry & MRI setup
M. Christa1, F. Dithmar2, T. Weinaus2, M. Kohlhaas2, F. Gutjahr3, I. Elabyad2, C. Maack2, W. R. Bauer1
1Medizinische Klinik und Poliklinik I, Universitätsklinikum Würzburg, Würzburg; 2Deutsches Zentrum für Herzinsuffizienz, Universitätsklinikum Würzburg, Würzburg; 3Experimental Physics 5, Universität Würzburg, Würzburg;

Background and Aim:

Alterations of sodium homoeostasis play an important role in cardiovascular diseases. Assignment of sodium storage to the intra- and extracellular space (e.g. in the myocardium) would be of paramount importance for our pathophysiological understanding of electromechanical dysfunction and potential treatment.

We combined fluorescence dye-based photometry (e.g. IonOptix) - a standard to quantify intra-cellular sodium levels in basic research – with dedicated 23Na/1H MRI techniques assessing total sodium and the extracellular volume fraction.

Combined the optical and MR technique allow to deduct extracellular sodium content and to solve the question which compartment is affected in different diseases and treatments. 

 

Methods:

Male mice (B6/N) underwent sham, TAC or MI surgery at an age of 10 weeks. Eight weeks later, functional cardiac MRI and T1 mapping are performed at our department’s 7T small animal MRI system(Bruker BioSpin, Ettlingen). Additionally total cardiac sodium content (TSC) was determined by using 23Na-MRI and a custom-made multi-echo UTE (MEUTE) sequence (TE: 0.345ms; TR 100ms, number of echos: 7, FA 90°, effective resolution of 0.625x0.625mm/Voxel; 100 averages; acquisition time 45 min.)

Two vials with known sodium concentration (50 and 100 mmol/l) attached to the coil were used as reference. ROIs for ECV and TSC were both placed in the (remote) myocardial septum.

 

Isolated cardiac myocytes from sham, TAC and MI mice were exposed to different stimulation frequencies (0.5, 2 & 4 Hz). We used an automatic microscope (IonOptix/CytoCyfer) to determine diastolic and systolic [Ca2+] by Indo1-AM and intracellular sodium concentrations with SBFI. 

 

Extracellular sodium content (CNa,ext) was determined by using the available data – TSC, Intracellular Natrium (CNa,int) and Extracellular Volume (ECV) – by solving the equation: 
TSC = ECV x CNa,ext + (1-ECV) x CNa,int 

with (1-ECV) representing the intracellular volume

 

Results:

Preliminary analyses showed significantly (p <0.012) increased cardiac TSC in MI compared to SHAM and TAC mice(Sham vs TAC vs MI: 43.4± 2.4 mmol/l vs 49.1±1.8 mmol/l vs 67.1±4.9 mmol/l). Intracellular sodium was significantly increased in cardiomyocytes isolated from TAC (p=0.034) and MI (p=0.020) mice compared to Sham (Sham vs TAC vs MI: 16.9± 0.2 mmol/l vs 22.3±0.8 mmol/l vs 23.1±2.4 mmol/l). Using intracellular sodium levels and Proton-MRI derived ECV-Values, we could calculate extracellular sodium levels. Mean CNa,ext was 138.5± 13.9 mmol/l in Sham mice, 139.8±12.5 mmol/l in TAC mice and significantly increased  with 223.0±10.0 mmol/l in MI mice compared to the other groups(p<0.005). Thus, we can explain the slightly increased TSC content in TAC mice mainly by the increased intracellular sodium levels and opposing to this, increased TSC levels in the myocardium of MI are mainly caused by an increased extracellular (assumedly) water-free sodium deposit.

 

Conclusion:

The combination of sodium MRI and fluorescence dye-based photometry measurements allows to allocate the changes of TSC to either the intra-, or extracellular sodium or both. This can be used to investigate effects of diseases or treatments on different tissue sodium compartments. Additional information such as cardiac mass and function can be extracted from MRI measurements, whereas fluorescence dye-based photometry provides information on Ca2+-handling, redox-status and cell contractility.


https://dgk.org/kongress_programme/jt2022/aP1937.html