Central interaction between carotid baroreceptors and skeletal muscle receptors inhibits sympathoexcitation

To determine the potential of an inhibitory interaction between the carotid sinus baroreflex (CSB) and the exercise pressor reflex (EPR), both pathways were activated to produce sympathoexcitation. It was hypothesized that, under conditions when the baroreflex increased sympathetic outflow, the interaction between CSB and EPR would be inhibitory. Bilateral carotid occlusion (BCO), electrically induced muscle contraction (EMC), and passive muscle stretch (PMS) were used to evoke sympathoexcitation. BCO decreased sinus pressure 50 ± 5 mmHg, and the levels of muscle tension generated by EMC and PMS were 7 ± 2 and 8 ± 1 kg, respectively. This resulted in significant increases in mean arterial pressure (MAP) of 55 ± 9, 50 ± 7, and 50 ± 6 mmHg (P = not significant, BCO vs. EMC vs. PMS) and in heart rate (HR) of 7 ± 2, 19 ± 4, and 17 ± 2 beats/min (P < 0.05, BCO vs. EMC and PMS). When BCO was combined with EMC or PMS, the reflex increase in MAP was augmented (80 ± 8 and 79 ± 10 mmHg; BCO+EMC and BCO+PMS, respectively; P < 0.05). However, summation of the individual MAP responses was greater than the response evoked during coactivation (106 ± 11 and 103 ± 12 mmHg, respectively, P < 0.05). Because summing the individual blood pressure responses exceeded the response during coactivation, the net effect was that the CSB and EPR interacted in an occlusive manner. In contrast, summation of the individual chronotropic responses was the same as the response evoked during coactivation. Moreover, there was no difference in summation of the individual MAP or HR responses when muscle afferents were activated by either EMC or PMS. In conclusion, the interaction between the CSB and the EPR in control of MAP was occlusive when both reflexes were stimulated to evoke sympathoexcitation. However, summation of the reflex changes in HR was simply additive.