Abstract
We investigate the potential of multiple quantum filtered (MQF) 23Na NMR to probe intracellular [Na]i in the Langendorff perfused mouse heart. In the presence of Tm(DOTP) shift reagent the triple quantum filtered (TQF) signal originated largely from the intracellular sodium pool with a 32±6% contribution of the total TQF signal arising from extracellular sodium, whilst the rank 2 double-quantum filtered signal (DQF), acquired with a 54.7° flip-angle pulse, originated exclusively from the extracellular sodium pool. Given the different cellular origins of the 23Na MQF signals we propose that the TQF/DQF ratio can be used as a semi-quantitative measure of [Na]i in the mouse heart. We demonstrate a good correlation of this ratio with [Na]i measured with shift reagent at baseline and under conditions of elevated [Na]i. We compare the measurements of [Na]i using both shift reagent and TQF/DQF ratio in a cohort of wild type mouse hearts and in a transgenic PLM3SA mouse expressing a non-phosphorylatable form of phospholemman, showing a modest but measurable elevation of baseline [Na]i. MQF filtered 23Na NMR is a potentially useful tool for studying normal and pathophysiological changes in [Na]i, particularly in transgenic mouse models with altered Na regulation.
Highlights
Intracellular Na concentration [Na]i is a key modulator of cardiac cell function [1]
Using the commonly used shift reagent, Tm(DOTP)5− to distinguish the relative contributions to the multiple quantum filtered (MQF) signal from intra [Na]i and extracellular [Na]e, we were able to quantify [Na]i and thereby compare these to the measurements obtained using the MQF experiments
In the presence of shift reagent we found that the proportion of the total triple quantum filtered (TQF) signal arising from the extracellular [Na]e pool was approximately 32%, indicating that the signal mainly represents [Na]i
Summary
Intracellular Na concentration [Na]i is a key modulator of cardiac cell function [1]. The Na electrochemical gradient across the cell membrane provides the energy for the action potential upstroke, as well as the active transport of many other ions, amino acids and substrates into the cell [2,3]. [Na]i is critical for the control of intracellular calcium [Ca]i via the sodium–calcium exchanger (NCX), thereby determining sarcoplasmic reticulum (SR) Ca content and cardiac contractility [4]. Maintenance of the Na+ gradient is fundamentally important in normal physiology but is critically important in cardiac hypertrophy and heart failure where the elevation of [Na]i contributes to contractile and electrical dysfunction [5,6,7]. Eykyn et al / Journal of Molecular and Cellular Cardiology 86 (2015) 95–101 that TQF and DQF 23Na NMR signals arise from different cellular compartments [18]
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