Effect on arterial pressure of rhythmically contracting the hindlimb muscles of cats

Although static contraction of the hindlimb muscles of anesthetized cats is known to reflexly increase arterial pressure and heart rate, the cardiovascular effects of rhythmic contractions of these muscles is unclear. To help clarify this issue, we determined, in chloralose-anesthetized cats, the effects on arterial pressure and heart rate of rhythmically contracting the hindlimb muscles at a frequency of 5 Hz. In addition, we determined the effect of rhythmic contractions on the impulse activity of group III and IV muscle afferents whose activation is known to increase cardiovascular function. We found that rhythmic contractions increased arterial pressure (from 108 +/- 8 to 134 +/- 9 mmHg; P less than 0.05) and heart rate (from 192 +/- 13 to 208 +/- 10 beats/min; P less than 0.05) in 10 cats and decreased arterial pressure (from 107 +/- 8 to 93 +/- 9 mmHg; P less than 0.05) but did not change heart rate in 9 other cats. The increases were reflex, because they were prevented by cutting the spinal roots innervating the contracting hindlimb. The decreases, however, were not reflex, because they persisted after spinal root section. The differences in the arterial pressure responses to rhythmic contractions may have been partly due to individual differences in the level of anesthesia, because in three cats the pressor responses to this maneuver were converted to depressor responses after giving the cats additional chloralose. Rhythmic contractions of the triceps surae muscles stimulated 8 of 10 group III afferents and 9 of 16 group IV afferents. We conclude that rhythmic contraction is capable of reflexly increasing cardiovascular function in cats provided that the effect is not depressed by anesthesia.