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- (Nai+) and extracellular (Nae+) ²³Na⁺ signals using their longitudinal relaxation time constant (T₁) values. Na ⁺-loaded yeast cell (Saccharomyces cerevisiae) suspensions were investigated. Two types of compartmental ²³Na⁺ T ₁ differences were examined: a selective Nae+ T₁ decrease induced by an extracellular relaxation reagent (RR_e), GdDOTP ^5-; and, an intrinsic T₁ 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 23Nae+ T₁ was 2.4 ms and the 23Nai+ T₁ 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 T₁ values, 9.1 (±0.3) ms and 32.7 (±2.3) ms, assigned to ²³Nai+ and ²³Nae+, respectively. The Nai+ content measured was lower, 0.88 (±0.06); while Nae+ was higher, 1.43 (±0.12) compared with MRS/SR_e measures on the same samples. However, the measured efflux rate constant was identical. T₁ MRR potentially may be used for Nai+ determination in vivo and Na⁺ flux measurements; with RR_e for animal studies and without RR_e for humans.