Effects of hypoxia on the discharge of group III and IV muscle afferents in cats.

The reflex pressor response evoked by static muscular contraction is widely believed to be caused by the stimulation of group III and IV afferents. Although the specific nature of the contraction-induced stimulus to these thin-fiber afferents is unknown, they are thought to be stimulated in part by a condition arising from a mismatch between blood supply and demand in the exercising muscle. Hypoxia, a condition found in skeletal muscle during such a mismatch, may stimulate these afferents. We have therefore tested the hypothesis that perfusion of the triceps surae muscles with hypoxic blood stimulates group III and IV afferents in barbiturate-anesthetized cats. We found that 3-3.5 min of hypoxia with the triceps surae muscles at rest significantly (P < 0.05) increased the average discharge rate of contraction-sensitive group IV afferents but had no effect on the average discharge rate of contraction-sensitive group III afferents. Hypoxia had only trivial effects on the discharge of contraction-insensitive group III and IV afferents. Hypoxia stimulated 4 of 11 contraction-sensitive group IV afferents and 2 of 13 contraction-sensitive group III afferents. The responses of the afferents stimulated by hypoxia were small in magnitude. Hypoxia with the muscles at rest appeared to have no effect on either hydrogen or lactate ion concentrations in the femoral venous blood. In addition, hypoxia increased the responses to contraction in only 3 of 22 group III and 4 of 21 group IV afferents tested. We conclude that muscle tissue hypoxia is a minor stimulus to afferents that sense a mismatch between blood supply and demand during static contraction.