Glucagon-Like Peptide-1 Plays a Role in Carotid Chemoreceptor Mediated Sympatho-Excitation and Hypertension

Hypertension is a complex and multifactorial disease that includes components of the vasculature and the autonomic nervous system ^1,2. The etiology of sympathetic nervous activation in hypertension has been studied for years with no clear or comprehensive understanding of the factors that initiate sympatho-excitation. Undoubtedly the central and peripheral nervous systems contribute but the role of specific cardiovascular reflexogenic areas in initiating sympatho-excitation and hypertension remains an enigma. The fact that the metabolic syndrome is characterized, in part, by hyperglycemia, reduced insulin sensitivity, hypertension and sympatho-excitation begs the question, are there humoral factors that can activate sympatho-excitatory areas of the circulation in the pathogenesis of hypertension?

Carotid body sensitization has been implicated as a factor that drives sympathetic nerve activity in both hypertension ³ and heart failure ⁴ even during normoxia. Several mediators of glomus cell sensitivity have been proposed ⁵ and indeed the carotid body has been suggested to act as a glucose sensor ⁶.

Glucagon-like peptide 1 (GLP-1) was originally described as a 30 amino acid molecule derived from intestinal epithelial cells ⁷ but has only recently been shown to have major effects throughout the body and in sympatho-regulatory areas⁸ of the central and peripheral nervous systems. The GLP-1 receptor (GLP-1R) is found on beta cells in the pancreas and in the nervous system ^9,10. It is thus possible that modulation of the GLP-1R can alter sympathetic outflow in disease states.

Having previously shown that carotid body denervation reduces blood pressure in the spontaneously hypertensive rat (SHR)³, Pauza et al.¹¹ in this issue of Circulation Research took a multi-pronged approach to understand the role of the GLP-1R in the carotid body. Comparing carotid bodies from SHR and their normotensive counterpart, the Wistar-Kyoto rat (WKY) they applied quantitative transcriptomic analysis and gene ontology (GO) on 2982 differentially expressed genes. While this analysis identified several metabolically associated transcripts and proteins that were differentially expressed in the SHR they focused on one G-protein coupled receptor known to participate in glucose metabolism, the GLP-1R which was significantly down regulated in SHR. Immunofluorescence histology showed that the GLP-1R was expressed in carotid body glomus and endothelial cells in normotensive Wistar rats but nearly exclusively in carotid body glomus cells in SHR. It was nearly totally absent in vessels of SHR. One of the important mechanistic observations described in this study is the demonstration that the GLP-1R agonist, Exendin-4 when selectively given into the carotid body via the internal carotid artery reduced sympathetic nerve activity in normotensive Wistar rats and less so in SHR. Importantly, GLP-1 positive nerve fibers were visualized near and around glomus cells suggesting a neural source of the peptide close to receptor expression.

Using the isolated heart, lung, brain preparation where perfusion to the carotid body can be precisely controlled, the carotid chemoreflex was activated following sodium cyanide administration into the circuit to evaluate chemoreflex sensitivity with and without Exendin-4 administration. Exendin-4 reduced both carotid sinus nerve activity and sympathetic nerve activity in Wistar rats but this effect was abrogated in SHR. When intracarotid glucose was increased, sympathetic nerve activity increased substantially more in SHR compared to Wistar rats. However, co-administration of Exendin-4 normalized these responses both at baseline and following sodium cyanide.

The authors conclude that Exendin-4 inhibited afferent activity and the CB reflex thus GLP1R likely inhibits glomus cell transduction and its loss in SHR would be to disinhibit. i.e. become more excitatory. If chemoreceptor afferents drive sympathetic nerve activity in the SHR, activation of GLP-1Rs may mediate a decrease in sympathetic outflow in hypertension. This helps define some of the multiple abnormalities that contribute to hypertension in the metabolic syndrome. The data support the idea that GLP-1 released from surrounding tissues nearby glomus cells may be unable to active its receptor in the hypertensive state. This is an important finding and helps to knit the mosaic of hypertension together. There are however limitations, some of which are acknowledged by the authors. The SHR is a very well characterized hypertensive model that from its initial development has been shown to exhibit high levels of sympathetic tone¹². While there are clear correlates in human hypertension the role of chemoreflex sensitivity in other forms of hypertension remains to be determined. In fact, these data raise the question as to the effects of hypertension per se on the GLP-1 axis. In the setting of chronic heart failure sympatho-excitation is indeed mediated, in part, by sensitized peripheral chemoreceptor input ^4,13. The role of GLP-1 or the GLP-1R has not been investigated in this sympatho-excitatory state, but based on the current findings this is a high priority study. While low oxygen (pO₂ and O₂ saturation) does not appear to be involved in chemoreflex sensitivity in either hypertension or heart failure during normoxic breathing a shift in the chemoreflex curves is likely to occur if the change in membrane potential resulting in enhanced activity of afferents due to paracrine or endocrine factors, as suggested by this work. A limitation of the current study is the lack of true chemoreflex sensitivity curves relating pO₂ to sympathetic nerve activity or to afferent discharge with and without Exendin-4. Technically, this is a difficult experiment to carry out in the preparation used but should be kept in mind when evaluating these data.

In addition to the above, one should consider alterations in carotid body blood flow as a factor in alterations in the sensitivity of carotid chemoreceptors. This has been shown to occur in experimental heart failure¹⁴. This idea is supported by the observation in this study that GLP-1R was markedly and somewhat exclusively down-regulated in the microvasculature of the CB in SHR rats. GLP-1 or its agonists are known to maintain microvascular function in other tissues (e.g. coronary and renal) and support microvascular blood flow via activation of eNOS-NO-cGMP pathway and anti-oxidant effect by inhibiting angiotensin II- NADPH oxidase production of ROS in endothelial cells. Down regulation of NOS and ROS activation via angiotensin II-NADPH oxidase are known to sensitize and activate CB afferent activity and function in sympatho-excitatory states such as heart failure.

Finally, the molecular mechanisms underlying the downregulated GLP-1R in the carotid body of SHR remain unclear. A hallmark of the SHR model is increased sympathetic outflow to many somatic/visceral organs including the carotid body. A recent study from the same research group showed that unilateral removal of the superior cervical ganglia (SCGx) attenuated the CB-evoked sympatho-excitation in SHR rats but was without effect in Wistar rats. Unilateral SCGx also abolished firing of chemoreceptive petrosal neurons in SHR rats, which became hyperpolarized. In addition, CB injections of phenylephrine enhanced CB-evoked sympatho-excitation In Wistar rats, which was prevented by prazosin in SHR rats. These data suggest that increased sympathetic innervation from SCG contributes to chemoreflex sensitization in SHR rats. It will be intriguing to investigate if increased sympathetic outflow from SCG to CB is a potential upstream contributor to the downregulated GLP-1 signaling in CB of SHR rats. In summary, the study by Pauza et al.¹¹ provides strong data for a chemoreceptor mediated sympatho-excitatory effect in hypertension which is often associated with diabetes. It will be important to determine if chronic Exendin-4 administration reduces sympathetic nerve activity and blood pressure in conscious SHR. These data are highly relevant to the diabetic and hyperglycemic states as evidence supports the view that insulin also stimulates sympathetic outflow and may contribute to hypertension¹⁵. Thus, in the hyperglycemic state sympathetic outflow may be exacerbated by insulin itself. The data presented here suggests that a negative feedback system exists that involves activation of the GLP-1R, ostensibly by GLP-1 released from the intestinal epithelium (Figure). The translational and therapeutic implications of this work are clear. Overall, the study provides much food for thought and sets the stage for additional studies in this field.

Figure:

The gastrointestinal peptide GLP-1 is released from L-type epithelial cells in response to hyperglycemia. GLP-1 may induce several affects at various sites including enhancement of insulin release by beta cells in the pancreas and by activation of the GLP-1R on glomus cells in the carotid body. Insulin possesses sympatho-excitatory effects itself. In addition to circulating GLP-1, it appears that GLP-1 is contained in afferent carotid body. Activation through the GLP-1R reduces sympathetic outflow in a genetic model of hypertension. Therefore, abnormalities in this pathway (i.e. reduced activation or activity of the GLP-1R) may contribute to the pathogenesis of hypertension. (Figure made with BioRender.com).