Compression-induced ATP release from rat skeletal muscle with and without lengthening contraction

Adenosine triphosphate (ATP) is well known to be released from injured or inflamed tissues, and to excite/sensitize nociceptors in response to heat and mechanical stimulation. To determine whether muscle releases ATP when it is compressed, we measured ATP release from the extensor digitorum longus muscle (EDL). In addition, we investigated whether there is any difference in ATP release from the EDL of rats 2 days after lengthening contraction (LC), since the condition of the muscle is different, i.e., mechanically hyperalgesic and swollen. The EDL was put in a small chamber and superfused with Krebs–Henseleit solution equilibrated with a gas mixture of 95% oxygen and 5% carbon dioxide. The muscle was quantitatively stimulated with a servo-controlled mechanical stimulator. Reproducibility of ATP release was examined with stimulation using a 20g force. Stimulus intensity-dependency of ATP release was also examined with 5 time compression with intensities of 5, 10, 20 and 40g force. Bioluminescent determination by the luciferin–luciferase method was used to quantify ATP in the sample. The ATP release was decreased by repetitive mechanical stimulation of the EDL with 30 min intervals, and it was stimulus intensity (5–40g force)-dependent. The amount of ATP released from the muscle preparations was not different between the non-treated control and the LC group. These results provide clear evidence that ATP is released from rat skeletal muscle by compression.