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Possible Respiratory Modulation of Sympathetic Activity in Rats After Sino‐aortic Denervation

It is well known that several days after sino‐aortic denervation (SAD) rats present normal levels of sympathetic activity and mean arterial pressure (MAP). However, the mechanisms involved in the modulation of the sympathetic activity after SAD remain unclear. Here we hypothesized that the respiratory network may contribute to the modulation of the pre‐sympathetic neurons preventing sustained increase in MAP in SAD rats. To study this mechanism we recorded the activities of phrenic, thoracic sympathetic (tSNA) and cervical vagus nerves (cVN) in male juvenile Wistar rats in the working heart‐brainstem preparation, at high or low respiratory drive induced by hypercapnia or hypocapnia, respectively (SAD, n = 26 and Sham, n = 33). The duration of inspiration significantly increased in SAD rats (1.46 ± 0.05 vs 1.14 ± 0.07 s, P < 0.001). It was also observed that SAD increases cVN inspiratory activity (25 ± 1 vs 32 ± 2%, P < 0.05) and decreases in its post‐inspiratory component (76 ± 1 vs 68 ± 2%, P < 0.05), suggesting an increased upper airway resistance during inspiration in SAD rats. Regarding the baseline tSNA, no changes were observed in the baseline tSNA in all phases of the respiratory cycle in SAD rats, which presented significant reduction in the Traube‐Hering waves magnitude (2.57 ± 0.04 vs 1.19 ± 0.20 mmHg, P = 0.0048). In order to reveal the influence of the respiratory network on the modulation of sympathetic activity rats were exposed to hypercapnia (from 5 to 10 % of CO2) or hypocapnia (from 5 to 3 and 1 % of CO2). High respiratory drive induced by hypercapnia increases tSNA in all phases of respiration in Sham rats (n=13), but in SAD rats (n=14) a significant increase in tSNA was observed only during inspiration (43 ± 3% vs 55 ± 3%, P < 0.01). On the other hand, hypocapnia reduced the respiratory drive and induced sympatho‐inhibition in the SAD (n=8) and in Sham rats (n=10). The data show that in spite of longer inspiratory activity, the baseline sympathetic activity is normal in SAD rats. Whether or not the longer inspiratory activity is driving the sympathetic activity in SAD rats is still matter for further investigation.