Proceedings of The Physiological Society

University College Cork (2004) J Physiol 560P, C13



Gladwell,valerie Frances; Fletcher,Janine ; Rangarajan,Balaji ; Coote,John H;

1. Biological Sciences, University of Essex, Colchester, United Kingdom. 2. School of Applied Sciences, University of Wolverhampton , Wolverhampton, United Kingdom. 3. Department of Physiology, University of Birmingham, Birmingham, United Kingdom.

During isometric contraction of limb muscles, heart rate (HR) and vasoconstrictor tone are differentially regulated (Gladwell & Coote, 2002). At the onset of contraction, HR increases before a blood pressure (BP) change. At the end of contraction, if the circulation remains occluded (post-exercise circulatory occlusion, PECO) HR recovers rapidly to control whereas BP remains elevated (Bull et al., 1989). This is in part due to baroreceptor reflex control of cardiac vagal tone. In this study we attempted to test this by reducing the input from the carotid baroreceptors with neck pressure (NP) applied bilaterally to the carotid bifurcation (Raine & Cable, 1999). We proposed that NP at the initiation of a voluntary contraction of triceps surae would reduce the excitatory drive from the baroreceptors to the cardiac vagal neurones (CVN), thus increasing the HR response to the contraction. Further, we considered that if NP was applied during PECO, immediately at the cessation of contraction, the usual rapid fall in HR would be diminished. Following South Birmingham Health Authority ethics committee approval, in agreement with the Declaration of Helsinki, healthy human volunteers were instrumented to record BP (FINAPRES), ECG, respiration and force of contraction during two studies. The effect of the 2 conditions (with and without NP) was statistically tested using paired t-tests (p≤0.05). Values given as mean±standard deviation. In study 1, (n=5, mean age 29±6 years) NP was applied at the initiation of 30% MVC. The change in HR to contraction without NP applied was very small (-0.6±1.6 beats/min (bpm)) whereas with NP the HR increase was significantly greater (p≤0.02) (4.0 ±0.4 bpm). In study 2, (n=6, mean age 26±7 years), the application of NP at the cessation of 40% MVC with PECO, significantly altered the fall of HR from peak value at end of contraction with 3.2 ±1.5 bpm, compared to7.9±1.4 bpm with no NP (p=0.003). There were no significant differences in SBP or DBP in the two conditions in either of the studies (p≥0.5). These data support the idea that during contraction that central command and/or muscle mechanoreceptors are the main factors that control baroreceptor excitation of CVN.

Where applicable, experiments conform with Society ethical requirements