1. The effect on R-R interval of a brief hindlimb contraction, elicited by electrical stimulation of L7 ventral roots, was investigated in decerebrate cats. The first series of experiments was performed at both low and high carotid sinus pressure to vary the level of vagal tone. When carotid sinus pressure was elevated to increase vagal tone, contraction commenced 1 s later. 2. The change in R-R interval at low carotid sinus pressure was expressed as the difference between the mean of the five R-R intervals immediately preceding contraction and the mean of the last five R-R intervals at the end of a 5 s contraction. At high carotid sinus pressure, the change was expressed as the difference between the mean of the last five R-R intervals at the end of a 5 s contraction and the mean of five R-R intervals at an equivalent time after raising pressure alone. 3. Hindlimb contraction at low carotid sinus pressure produced a significant reduction in R-R interval from 359 +/- 25 (mean +/- S.E.M. n = 8) to 336 +/- 24 ms (P less than 0.005). At high carotid sinus pressure the response was enhanced with contraction producing a reduction in R-R interval from 474 +/- 45 to 419 +/- 47 ms (P less than 0.001). 4. The shortening of R-R interval produced by hindlimb contraction at high carotid sinus pressure, 55 +/- 8 ms, was significantly greater than that observed at low sinus pressure, 23 +/- 5 ms (P less than 0.001, n = 8, paired t test). This pattern of response was also seen at stimulation frequencies as low as 10 Hz. 5. In a second series of experiments, designed to determine the latency of the cardiac acceleration, the minimum latency between the onset of L7 ventral root stimulation and the end of the first shortened R-R interval was 687 +/- 29 ms (n = 5). 6. Atropine (0.4 mg kg-1, I.V.) prevented a 5 s contraction from producing any change in R-R interval. 7. These results indicate that afferent information originating from receptors in contracting muscles is responsible for producing an immediate shortening of R-R interval, which is mediated by vagal withdrawal. The possibility that the shortening of R-R interval at the start of contraction is linked to a reduction in arterial baroreceptor reflex sensitivity, possibly via inhibitory effects on neurones forming the central pathway of the baroreceptor reflex, is discussed.
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