Cyclooxygenase blockade attenuates responses of group IV muscle afferents to static contraction

Cyclooxygenase products of arachidonic acid might be some of the substances that accumulate in contracting muscle to cause the reflex increases in arterial pressure and ventilation that are evoked by exercise. Recently, cyclooxygenase blockade has been shown to attenuate the reflex cardiovascular responses to static muscular contraction in anesthetized cats. Group IV afferents are believed to comprise part of the afferent arm of the reflex arc, the activation by which static muscular contraction causes these cardiovascular effects. We therefore examined the effects of indomethacin and aspirin, two cyclooxygenase-blocking agents, on the responses to static contraction of group IV afferents with endings in the triceps surae muscles of anesthetized cats. We found that indomethacin (5 mg/kg iv) decreased the responses to contraction of each of eight group IV afferents tested. Likewise, aspirin (50 mg/kg iv) decreased the responses to contraction of each of four group IV afferents tested. On the other hand, we found that arachidonic acid (2 mg) injected into the femoral artery did not increase the responses to contraction of four group IV afferents that were stimulated by this maneuver. In addition, arachidonic acid injection did not cause any of seven group IV afferents not stimulated by static contraction to become responsive to this maneuver. Nevertheless, arachidonic acid injection with the muscle at rest stimulated five of seven contraction-insensitive and two of four contraction-sensitive group IV afferents. Our data suggest that cyclooxygenase metabolites of arachidonic acid are needed for the full expression of the responses of group IV muscle afferents to static contraction. Our data also suggest that exogenous administration of arachidonic acid neither increases the responses to contraction of group IV afferents stimulated by this maneuver nor affects group IV afferents that are not stimulated by contraction.