In vivo 13C spectroscopy in the rat brain using hyperpolarized [1-(13)C]pyruvate and [2-(13)C]pyruvate

Abstract

The low sensitivity of 13C spectroscopy can be enhanced using dynamic nuclear polarization. Detection of hyperpolarized [1-(13)C]pyruvate and its metabolic products has been reported in kidney, liver, and muscle. In this work, the feasibility of measuring 13C signals of hyperpolarized 13C metabolic products in the rat brain in vivo following the injection of hyperpolarized [1-(13)C]pyruvate and [2-(13)C]pyruvate is investigated. Injection of [2-(13)C]pyruvate led to the detection of [2-(13)C]lactate, but no other downstream metabolites such as TCA cycle intermediates were detected. Injection of [1-(13)C]pyruvate enabled the detection of both [1-(13)C]lactate and [13C]bicarbonate. A metabolic model was used to fit the hyperpolarized 13C time courses obtained during infusion of [1-(13)C]pyruvate and to determine the values of VPDH and VLDH.

Figure 1

Three-compartment brain/blood/body metabolic model used to fit the hyperpolarized ¹³C time courses of lactate and bicarbonate and to determine values of V_{PDH brain} and V_{LDH brain} and rate of transport of pyruvate through blood-brain-barrier (V_{tr Pyr brain}) (gray balloons). V_{gly body/blood/brain} – glycolysis fluxes in body/blood/brain V_{tr Pyr/Lac/HCO3}- _body/brain⁺/ V_{tr Pyr/Lac/HCO3}- _body/brain^-– transport rates of pyruvate/lactate/bicarbonate to body/brain V_{LDH body/blood/brain}⁺/ V_{LDH body/blood/brain}^- – lactate dehydrogenase fluxes in body/blood/brain V_{PDH body/brain} – pyruvate dehydrogenase fluxes in body/brain Pyr – pyruvate, Lac – lactate, HCO₃^- – bicarbonate

Figure 2

In vivo time courses measured after injection of hyperpolarized [1-¹³C]pyruvate solution into femoral vein with the r.f. coil placed over the rat head. (A) Representative time course. T_R = 0.75 s, pulse-acquire, 4.5° pulse at the center of the coil, N_EX = 60, 7-Hz line-broadening. (B) Average and standard deviation of five time courses of [1-¹³C]pyruvate, [1-¹³C]pyruvate hydrate, and [1-¹³C]lactate (integral of each of the species was scaled by average of external reference's (formic acid) integral). 4 animals, pulse-acquire, 4.5° pulse at the center of the coil, T_R = 0.75 s, N_EX = 160. ext ref – external reference

Figure 3

(A) In vivo time course of bicarbonate measured in one representative animal after injection of hyperpolarized [1-¹³C]pyruvate into femoral vein of the rat. Pulse-acquire, 43° pulse at the center of the coil, 161.5 to 162.5 ppm, T_R = 1.5 s, N_EX = 60, 5-Hz line-broadening. Experimental hyperpolarized ¹³C time course of lactate (B) and bicarbonate (C) fitted with a three-compartment metabolic model with (continuous line) and without (dashed line) pyruvate transport through the blood-brain-barrier.

Figure 4

Spatial origin of hyperpolarized signals. Single-shot in vivo ¹H decoupled ¹³C spectra acquired 9 s after beginning of injection of hyperpolarized solution with (A) LASER sequence from 216 μL (9 × 1.6 × 16 mm³) voxel containing muscle and subcutaneous tissue (insert: RARE image of a rat brain showing the position and size of the localized volume), (B) LASER sequence from 405 μL (9 × 5 × 9 mm³) voxel containing only brain tissue (insert: RARE image of a rat brain showing the position and size of the localized volume), (C) small-flip angle pulse-acquire (4.5° at the center of the coil), (D) 90° adiabatic BIR4 pulse-acquire. All spectra are shown with 2-Hz line-broadening. The lactate to pyruvate ratio for each spectrum is reported on the right side. ala = alanine

Figure 5

In vivo time courses measured after injection of hyperpolarized [2-¹³C]pyruvate solution into femoral vein with r.f. coil placed over the rat head. (A) Representative spectrum obtained 12 s after beginning of injections (7-Hz line-broadening). (B) Time course of [2-¹³C]pyruvate, [2-¹³C]pyruvate hydrate, and [2-¹³C]lactate obtained from sum of four dissolutions (integral of each of the species was scaled by average of external reference's (formic acid) integral). 3 animals, 4 dissolutions, pulse-acquire, 4.5° pulse at the center of the coil, T_R = 0.75 s, N_EX = 128.

Figure 6

¹H decoupled localized ¹³C spectra obtained from a 405 μL (9 × 5 × 9 mm³) voxel containing brain tissue 18 s after beginning of injection of hyperpolarized [2-¹³C]pyruvate solution into the femoral vein. Representative spectra acquired from (A) non-fasted and (B) fasted animals. ¹³C-LASER, T_E = 29 ms, transmitter offset = 175 ppm, 7-Hz line-broadening.