Figure 4. Graded muscular metabolic response monitored with hyperpolarized fMRS.

(a) Multi-bolus delivery paradigm whereby hyperpolarized ¹³C MRS acquisitions are interleaved with either variable-frequency or variable-amplitude stimulations. (b) MtoC trajectories displayed by the injected ¹³C₁-pyruvate and the resulting ¹³C₁-lactate monitored as a function of varying frequency and amplitude electrical muscle stimulations. Experiments were carried out as described in Figure 2, with epochs I-IV initiated by injecting 100 µL/2s boluses of 60 mM ¹³C-pyruvate, separated by ca. 30 s of data acquisition. (c) Rate constants reflecting pyruvate's disappearance (k _Pyr) and lactate's appearance (k _Lac) as a function of the stimuli's frequency and voltage, arising by fitting the kind of fMRS data summarized in panel (b) for n = 4 animals.