Intramyocyte glucose concentrations during contractions in rat skeletal muscle<i>in vivo</i>

Background: Muscle glycogen is a preferential substrate for energy production over glucose and its levels are decreased with muscle contractions. Intramyocyte glucose concentration ([Glu] i ) regulates multiple processes involved in glucose metabolism: However, its dynamics during muscle contractions remain to be elucidated. Purpose: In this investigation, we resolved in vivo [Glu] i dynamics during contractions. We tested the hypothesis that, regardless of contraction intensity or type, [Glu] i is increased with muscle contractions, but that this increase does not necessarily relate to a post-contraction glycogen concentration. METHOD: Glucose-sensitive fluorescent protein (Pglu-700μDelta6) was transfected and expressed in the spinotrapezius muscle of male Wistar rats by electroporation of plasmid 7 days prior to the imaging experiment. Rats were divided into two groups: 1) control (CONT, n = 16), and 2) glucose transport inhibition (cytochalasin B: CB, n = 15). On the day of the experiment, the muscle was exposed and subjected to the [Glu] i imaging during twitch (1 Hz, ~ 10 V, 10 min) or tetanic (100 Hz for 700 ms, 50 times, 5 sets with 5 min intervals) contractions. [Glu] i was measured from the fluorescence ratio (R: 535 nm/480 nm) and normalized to the pre-contractions value (R/R pre ). In the CB group, CB (1 mM) was applied from 20 min before contractions. Intramuscular glycogen levels were measured by biochemical analysis from the muscle isolated post-contractions. Results: [Glu] i was significantly increased by twitch contractions (+12.3 ± 1.9%, p &lt; 0.05 vs pre), and following 30 min remained higher than pre-contractions (+7.9 ± 1.7%, p &lt; 0.05 vs pre). With tetanic contractions, [Glu] i was increased within the 1st set (+36.7 ± 6.6%, p &lt; 0.01 vs pre), and remained elevated throughout the 5-set bout (post-5 set: 25.8 ± 8.2%, p &lt; 0.01 vs pre) without significant recovery or additional increase among sets. The contractions-induced [Glu] i increase was significantly blunted by CB (post-twitch: +4.5 ± 2.0%; post-1 set tetanic: +4.5 ± 2.4%, p &lt; 0.05 vs pre). CB treatment precipitated a decrease in relative muscle force in the tetanic (CONT: 68.1 ± 4.5%, CB: 49.8 ± 1.3%, p &lt; 0.01), but not in the twitch contractions. Both CONT and CB exhibited decreased muscular glycogen with twitch contractions (CONT: 19.4 ±1.5 µmol/g, CB: 18.4 ±1.5 µmol/g) and tetanic contractions (CONT: 13.4 ± 1.3 µmol/g, CB: 10.3 ± 1.3 µmol/g) compared to the non-contracting muscle (29.9 ± 2.1 µmol/g, p &lt; 0.01). Conclusion: In this investigation, we demonstrated that, while glucose is taken up and accumulates within the contracting myocyte, it does not appear to contribute to the suppression of muscle glycogen reduction in either twitch or tetanic contractions. Thus, whilst intramyocyte glucose levels are not associated with the reduction of glycogen during muscle contractions observed herein they may be crucial for maintenance of tetanic tension. This study was supported by JSPS KAKENHI Grant (No. 20K20620). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.