FIGURE 5.

Increased [Ca²⁺]i impairs glucose uptake in muscle fibers. [Ca²⁺]_i and glucose uptake were recorded simultaneously in vivo in intact gastrocnemius fibers WT and RyR1-p.R163C and db/db using double-barreled selective Ca²⁺ microelectrodes and the 2-NBDG fluorescent glucose analog. (A) Left panel: Intracellular [Ca²⁺] was higher in RYR1-p.R163C and db/db than in the WT muscle. Right panel: Glucose uptake was significantly reduced in RYR1-p.R163C and db/db compared to WT muscle. All fluorescence signals were normalized to WT. (B) A positive correlation between muscle glucose uptake and [Ca²⁺]_i was found in RyR1-p.R163C and db/db (R = 0.63). An increase in muscle [Ca²⁺]_i caused a decrease in glucose uptake. (C) Show a positive correlation analysis between mice’s blood glucose (mg/dL) and muscle [Ca²⁺]_i (nM) (R = 0.68). Increased muscle [Ca²⁺]_i caused a more elevated blood glucose concentration. (D) Left panel: [Ca²⁺]_i was elevated in RYR1-p.R163C and db/db compared to WT muscle. Dantrolene treatment significantly reduced [Ca²⁺]_i and all genotypes. Right panel: insulin-dependent glucose uptake was reduced in RYR1-p.R163C and db/db compared to WT muscle. Dantrolene improved glucose uptake in the RYR1-p.R163C and db/db muscles, with no significant effect in WT. All fluorescence signals were normalized to WT. The results represent the mean ± S. D from n _cells = 14–52/genotype group, isolated from n _mice = 5–6 per genotype; n _cells = 14–52/genotype group. Statistical analysis was performed as described above. ** = p < 0.01; *** = p < 0.001.