Abstract
This study tested the ability of MR relaxography (MRR) to discriminate intra- ( Na i + ) and extracellular ( Na e + ) 23Na + signals using their longitudinal relaxation time constant ( T 1) values. Na +-loaded yeast cell ( Saccharomyces cerevisiae) suspensions were investigated. Two types of compartmental 23Na + T 1 differences were examined: a selective Na e + T 1 decrease induced by an extracellular relaxation reagent (RR e), GdDOTP 5−; and, an intrinsic T 1 difference. Parallel studies using the established method of 23Na MRS with an extracellular shift reagent (SR e), TmDOTP 5−, were used to validate the MRR measurements. With 12.8 mM RR e, the 23 Na e + T 1 was 2.4 ms and the 23 Na i + T 1 was 9.5 ms (9.4T, 24 °C). The Na + amounts and spontaneous efflux rate constants were found to be identical within experimental error whether measured by MRR/RR e or by MRS/SR e. Without RR e, the Na +-loaded yeast cell suspension 23Na MR signal exhibited two T 1 values, 9.1 (±0.3) ms and 32.7 (±2.3) ms, assigned to 23 Na i + and 23 Na 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/SR e 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 RR e for animal studies and without RR e for humans.
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