Simultaneous investigation of cardiac pyruvate dehydrogenase flux, Krebs cycle metabolism and pH, using hyperpolarized [1,2-(13)C2]pyruvate in vivo

Abstract

(13)C MR spectroscopy studies performed on hearts ex vivo and in vivo following perfusion of prepolarized [1-(13)C]pyruvate have shown that changes in pyruvate dehydrogenase (PDH) flux may be monitored non-invasively. However, to allow investigation of Krebs cycle metabolism, the (13)C label must be placed on the C2 position of pyruvate. Thus, the utilization of either C1 or C2 labeled prepolarized pyruvate as a tracer can only afford a partial view of cardiac pyruvate metabolism in health and disease. If the prepolarized pyruvate molecules were labeled at both C1 and C2 positions, then it would be possible to observe the downstream metabolites that were the results of both PDH flux ((13)CO(2) and H(13)CO(3)(-)) and Krebs cycle flux ([5-(13)C]glutamate) with a single dose of the agent. Cardiac pH could also be monitored in the same experiment, but adequate SNR of the (13)CO(2) resonance may be difficult to obtain in vivo. Using an interleaved selective RF pulse acquisition scheme to improve (13)CO(2) detection, the feasibility of using dual-labeled hyperpolarized [1,2-(13)C(2)]pyruvate as a substrate for dynamic cardiac metabolic MRS studies to allow simultaneous investigation of PDH flux, Krebs cycle flux and pH, was demonstrated in vivo.

Figure 1

Hyperpolarized ¹³C MRS data acquired from pre-polarized bicarbonate using an interleaved RF excitation scheme that alternate between a 40° spectrally selective RF pulse (for CO₂ only) and a 10° non-selective RF pulse. Stable pH values were obtained temporally (range: 7.38 - 7.43) that agrees well with the pH meter measurement (7.35).

Figure 2

Representative spectra from pre-polarized [1,2-¹³C₂]pyruvate phantom study at 3T.

Figure 3

In vivo cardiac MRS data acquired from one of the pigs following infusion of pre-polarized [1,2-¹³C₂]pyruvate. ¹³C spectrum from one of the time points is shown above ([2-¹³C]pyruvate hydrate at 96.5 ppm is outside the range of display) while the time courses of the substrate and some of the metabolites are plotted blow. ¹³CO₂ signal amplitudes (corrected for tip angle and echo time differences) and time courses from the two interleaves (40° selective RF and 10° non-selective RF) were observed to have good agreement (lower right).

Figure 4

Cardiac pH estimated using hyperpolarized H¹³CO₃⁻ and ¹³CO₂ signals measured in vivo in pig hearts. ¹³CO₂ signals from the 40° selective pulse were used in the data shown (empty circle and solid square). pH measurement from the studies without using the interleaved RF scheme is also shown (pig 1* open diamond and pig 3* open circle).