Discrimination of intra- and extracellular 23Na+ signals in yeast cell suspensions using longitudinal magnetic resonance relaxography

This study tested the ability of MR relaxography (MRR) to discriminate intra- (Na-i(+)) and extracellular (Na-e(+)) Na-23(+) signals using their longitudinal relaxation time constant (T-1) values. Na+-loaded yeast cell (Saccharomyces cerevisiae) suspensions were investigated. Two types of compartmental Na-23(+) T-1 differences were examined: a selective Na-e(+) T-1 decrease induced by an extracellular relaxation reagent (RRe), GdDOTP5-; and, an intrinsic T-1 difference. Parallel studies using the established method of Na-23 MRS with an extracellular shift reagent (SRe). TmDOTP5-, were used to validate the MRR measurements. With 12.8 mM RRe, the Na-23(e)+ T-1 was 2.4 ms and the Na-23(i)+ T-1 was 9.5 ms (9.4T, 24 degrees C). The Na+ amounts and spontaneous efflux rate constants were found to be identical within experimental error whether measured by MRR/RRe or by MRS/SRe. Without RRe, the Na+-loaded yeast cell suspension Na-23 MR signal exhibited two T-1 values, 9.1 (+/-0.3) ms and 32.7 (+/-2.3) ms, assigned to Na-23(i)+ and Na-23(e)+, respectively. The Na-i(+) content measured was lower, 0.88 (+/-0.06); while Na-e(+) was higher, 1.43 (+/-0.12) compared with MRS/SRe measures on the same samples. However, the measured efflux rate constant was identical. T-1 MRR potentially may be used for Na-i(+) determination in vivo and Na+ flux measurements; with RRe for animal studies and without RRe for humans. (C) 2010 Elsevier Inc. All rights reserved.