Effect of capsaicin-evoked jaw-muscle pain on intramuscular blood-flow

Aim: To investigate effect of capsaicin-evoked masseter-muscle pain on intramuscular blood-flow (BF) at rest and during contractions. Methods: Eight healthy men (22-31 years) participated. BF was measured with Laser Doppler (Moor Instruments, UK) using a single-fibre probe inserted into the right masseter. Three BF probes were attached to the skin above right and left masseter and the right-middle finger. Subjects performed 30 s isometric contractions at 5%, 15%, and 25% of maximal voluntary contraction. After the contractions, capsaicin (0.1 mL, 100 μg/mL) was injected into the right masseter close to the fibre probe. When the pain sensation had disappeared, the series of submaximal contractions were repeated. BF data were sampled continuously, processed in 10 s bins, and analyzed with repeated-measures ANOVAs. Results: Intramuscular BF significantly increased immediately after capsaicin injection (P < 0.050) and rapidly (30 s) decreased to pre-injection values. A significant increase in cutaneous BF above the right masseter was observed (P < 0.050) and lasted for 10 min, while a significant BF decrease in the finger (P < 0.050) was observed. The contractions were associated with increases in intramuscular BF before and after the injection (P < 0.022) and the contraction levels were also associated with increase in intramuscular BF before injection (P = 0.008) but not after injection (P = 0.314). Conclusions: This study demonstrated BF increased by muscle contraction but failed to show effects of contraction levels on BF in a muscle exposed to nociceptive stimuli. Neurogenic inflammation in muscles could possibly be mediated via antidromical effects and local release of vasoactive substances. The decreased BF in the finger could be due to involvement of central regulatory mechanisms.