In vivo assessment of pyruvate dehydrogenase flux in the heart using hyperpolarized carbon-13 magnetic resonance

Abstract

The advent of hyperpolarized (13)C magnetic resonance (MR) has provided new potential for the real-time visualization of in vivo metabolic processes. The aim of this work was to use hyperpolarized [1-(13)C]pyruvate as a metabolic tracer to assess noninvasively the flux through the mitochondrial enzyme complex pyruvate dehydrogenase (PDH) in the rat heart, by measuring the production of bicarbonate (H(13)CO(3)(-)), a byproduct of the PDH-catalyzed conversion of [1-(13)C]pyruvate to acetyl-CoA. By noninvasively observing a 74% decrease in H(13)CO(3)(-) production in fasted rats compared with fed controls, we have demonstrated that hyperpolarized (13)C MR is sensitive to physiological perturbations in PDH flux. Further, we evaluated the ability of the hyperpolarized (13)C MR technique to monitor disease progression by examining PDH flux before and 5 days after streptozotocin induction of type 1 diabetes. We detected decreased H(13)CO(3)(-) production with the onset of diabetes that correlated with disease severity. These observations were supported by in vitro investigations of PDH activity as reported in the literature and provided evidence that flux through the PDH enzyme complex can be monitored noninvasively, in vivo, by using hyperpolarized (13)C MR.

Fig. 1.

In vivo spectra from the heart of a male Wistar rat. (A) Single representative spectrum acquired at t = 10 s showing resonances attributed to the injected pyruvate (and its equilibrium product pyruvate hydrate), as well as the metabolic products, lactate, alanine, and bicarbonate. (B) Time course of spectra acquired every second over a 60-s period after injection. The arrival and subsequent decay of the injected pyruvate signal can be seen, along with the generation of lactate, alanine, and bicarbonate. (C) Example time course of the fitted peak areas of the pyruvate, lactate, alanine, and bicarbonate resonances. The pyruvate area has been reduced by a factor of 10 to improve the visualization.

Fig. 2.

Bicarbonate:pyruvate ratio in fed rats compared with fasted rats (n = 6; *, P < 0.001). Representative single spectra, acquired at t = 10 s, illustrate the difference in bicarbonate production.

Fig. 3.

Bicarbonate:pyruvate ratio in rats before and 5 days after induction of type 1 diabetes with STZ injection (n = 5; *, P < 0.02). Representative single spectra, acquired at t = 10 s, illustrate the difference in bicarbonate production.

Fig. 4.

Relationship between blood glucose and bicarbonate:pyruvate ratio in six rats, 5 days after induction of type 1 diabetes with STZ (r = −0.93). In rats with the highest blood glucose, the increased severity of the disease further inhibited PDH-mediated bicarbonate production.