MRI of Potassium and Sodium Enables Comprehensive Analysis of Ion Perturbations in Skeletal Muscle Tissue After Eccentric Exercise

The aims were to investigate if potassium ((39)K) magnetic resonance imaging (MRI) can be used to analyze changes in the apparent tissue potassium concentration (aTPC) in calf muscle tissue after eccentric exercise and in delayed-onset muscle soreness, and to compare these to corresponding changes in the apparent tissue sodium concentration (aTSC) measured with sodium ((23)Na) MRI. MATERIALS AND METHODS: Fourteen healthy subjects (7 female, 7 male; 25.0 ± 2.8 years) underwent (39)K and (23)Na MRI at a 7 T MR system, as well as (1)H MRI at a 3 T MR system. Magnetic resonance imaging data and blood samples were collected at baseline (t0), directly after performing eccentric exercise (t1) and 48 hours after exercise (t2). Self-reported muscle soreness was evaluated using a 10-cm visual analog scale for pain (0, no pain; 10, worst pain) at t0, t1, and t2. Quantification of aTPC/aTSC was performed after correcting the measured (39)K/(23)Na signal intensities for partial volume and relaxation effects using 5 external reference phantoms. Edema volume and (1)H T(2) relaxation times were determined based on the (1)H MRI data. Participants were divided according to their increase in creatine kinase (CK) level into high (CK(t2) ≥ 10·CK(t0)) and low CK (CK(t2) < 10·CK(t0)) subjects. RESULTS: Blood serum CK and edema volume were significantly increased 48 hours after exercise compared with baseline (P < 0.001). Six participants showed a high increase in blood serum CK level at t2 relative to baseline, whereas 8 participants had only a low to moderate increase in blood serum CK. All participants reported increased muscle soreness both at rest and when climbing stairs at t1 (0.4 ± 0.7; 1.4 ± 1.2) and t2 (1.6 ± 1.4; 4.8 ± 1.9) compared with baseline (0 ± 0; 0 ± 0). Moreover, aTSC was increased at t1 in exercised muscles of all participants (increase by 57% ± 24% in high CK, 73% ± 33% in low CK subjects). Forty-eight hours after training, subjects with high increase in blood serum CK still showed highly increased aTSC (increase by 79% ± 57% compared with t0). In contrast, aTPC at t2 was elevated in exercised muscles of low CK subjects (increase by 19% ± 11% compared with t0), in which aTSC had returned to baseline or below. Overall, aTSC and aTPC showed inverse evolution, with changes in aTSC being approximately twice as high as in aTPC. CONCLUSIONS: Our results showed that (39)K MRI is able to detect changes in muscular potassium concentrations caused by eccentric exercise. In combination with (23)Na MRI, this enables a more holistic analysis of tissue ion concentration changes.