Autonomic cardiovascular responses to hypercapnia in conscious rats: the roles of the chemo- and baroreceptors

Shigeru Oikawa, Haruhisa Hirakawa, Tatsumi Kusakabe, Yasuhide Nakashima, Yoshiaki Hayashida Autonomic Neuroscience: Basic and Clinical 117 (2005) 105– 114 The level of CO2 is tightly regulated by the chemoreceptor reflex. We know that this reflex is activated when in response to hypercapnia leading to an increase in blood pressure and sympathetic nerve activity. Heart rate (HR) tends to decrease in response to hypercapnia; however there is evidence to support the opposite effect. This particular study wanted to investigate the role of chemoreceptors and baroreceptors in response to hypercapnia in the conscious rats. Chemo denervation did not have any effect on baroreflex function. They found that in response to hypercapnia there was increases in BP and renal sympathetic nerve activity (RSNA) and a decrease in HR in the intact and chemo denervated rats. Hypercapnia did not affect baroreceptor function in the intact rats. I response to atropine the decrease in HR observed during hypercapnia was not seen. Interestingly, when the aortic depressor nerve was bilateral removed the HR response was not seen. These data demonstrate that the increase in BP and RSNA is due sympatho-exicitatory effect on the central nervous system. Furthermore, the peripheral chemoreceptors are not necessary for activation the sympathetic nervous system. Because after the atropine administration there was no change in HR this shows that there was activation of parasympathetic nervous system. So hypercapnia leads to increases in SNA through central chemoreceptor activation and a decrease in in HR due to activation of the parasympathetic system. Also the chemo-receptors are not playing a major in the cardiovascular response to hypercapnia in conscious but the aortic depressor nerve may play a role in mediating the PSNA response.-MD