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Crepitus of the knee may mirror structural and functional changes in the joint during motion. Although the magnitude of these sounds increases with greater cartilage damage, it is unclear whether knee joint sounds also reflect joint loading. Twelve healthy volunteers (mean 26 (SD 3.6) years, 7 females) participated in the randomized-balanced crossover study. Knee joint sounds were recorded (linear sampling, 5512 Hz) by means of two microphones, one placed on the medial tibial plateau and one on the patella. Two activities of daily living (standing up from/sitting down on a bench; descending stairs) and three open kinetic chain knee extension-flexion cycles (passive movement, 10% and 40% loading of the individual one repetition maximum) were performed. Each participant carried out three sets of five repetitions and three sets of 15 steps downwards (stairs), respectively. For data analysis, the mean sound amplitude and the median power frequency for each loading condition were determined. Friedman test and Bonferroni-Holm adjusted post-hoc test were performed to detect differences between conditions. We obtained significant differences between joint sound amplitudes for all movements, both measured at the medial tibial plateau (Chi2 = 20.7, p < 0.001) and at the patella (Chi2 = 27.6, p < 0.001). We showed a significant difference in the median power frequency of the patella between all movements (Chi2 = 17.8, p < 0.5). Overall, the larger the supposed knee joint loading was, the louder was the recorded knee crepitus. Consequently, vibroarthrographically assessed knee joint sounds can differ across knee joint loading conditions. • Knee joint sounds differ depending on knee joint loading conditions. • The larger the supposed knee joint loading, the louder the recorded knee crepitus. • Excessive friction may lead to accelerated wear of the articular cartilage. • Customized sports and exercise therapy may reduce non-physiological joint loading. [ABSTRACT FROM AUTHOR]

Crepitus of the knee may mirror structural and functional changes in the joint during motion. Although the magnitude of these sounds increases with greater cartilage damage, it is unclear whether knee joint sounds also reflect joint loading. Twelve healthy volunteers (mean 26 (SD 3.6) years, 7 females) participated in the randomized-balanced crossover study. Knee joint sounds were recorded (linear sampling, 5512 Hz) by means of two microphones, one placed on the medial tibial plateau and one on the patella. Two activities of daily living (standing up from/sitting down on a bench; descending stairs) and three open kinetic chain knee extension-flexion cycles (passive movement, 10% and 40% loading of the individual one repetition maximum) were performed. Each participant carried out three sets of five repetitions and three sets of 15 steps downwards (stairs), respectively. For data analysis, the mean sound amplitude and the median power frequency for each loading condition were determined. Friedman test and Bonferroni-Holm adjusted post-hoc test were performed to detect differences between conditions. We obtained significant differences between joint sound amplitudes for all movements, both measured at the medial tibial plateau (Chi2 = 20.7, p < 0.001) and at the patella (Chi2 = 27.6, p < 0.001). We showed a significant difference in the median power frequency of the patella between all movements (Chi2 = 17.8, p < 0.5). Overall, the larger the supposed knee joint loading was, the louder was the recorded knee crepitus. Consequently, vibroarthrographically assessed knee joint sounds can differ across knee joint loading conditions. • Knee joint sounds differ depending on knee joint loading conditions. • The larger the supposed knee joint loading, the louder the recorded knee crepitus. • Excessive friction may lead to accelerated wear of the articular cartilage. • Customized sports and exercise therapy may reduce non-physiological joint loading. [ABSTRACT FROM AUTHOR]

The activation of Toll-like receptors (TLRs) by lipopolysaccharide or other ligands evokes a proinflammatory immune response, which is not only capable of clearing invading pathogens but can also inflict damage to host tissues. It is therefore important to prevent an overshoot of the TLR-induced response where necessary, and here we show that extracellular ATP is capable of doing this in human monocytes. Using reverse transcription-PCR, we showed that monocytes express P2Y1, P2Y2, P2Y4, P2Y11, and P2Y13 receptors, as well as several P2X receptors. To elucidate the function of these receptors, we first studied Ca2+ signaling in single cells. ATP or UTP induced a biphasic increase in cytosolic Ca2+, which corresponded to internal Ca2+ release followed by activation of store-operated Ca2+ entry. The evoked Ca2+ signals stimulated Ca2+-activated K+ channels, producing transient membrane hyperpolarization. In addition, ATP promoted cytoskeleton reorganization and cell migration; however, unlike chemoattractants, the migration was non-directional and further analysis showed that ATP did not activate Akt, essential for sensing gradients. When TLR2, TLR4, or TLR2/6 were stimulated with their respective ligands, ATPγS profoundly inhibited secretion of proinflammatory cytokines (tumor necrosis factor-α and monocyte chemoattractant protein-1) but increased the production of interleukin-10, an anti-inflammatory cytokine. In radioimmune assays, we found that ATP (or ATPγS) strongly increased cAMP levels, and, moreover, the TLR-response was inhibited by forskolin, whereas UTP neither increased cAMP nor inhibited the TLR-response. Thus, our data suggest that ATP promotes non-directional migration and, importantly, acts as a ‘host tissue damage’ signal via the Gs protein-coupled P2Y11 receptor and increased cAMP to negatively regulate TLR signaling. [ABSTRACT FROM AUTHOR]