Acetylcholine mediated vasodilatation in the microcirculation of patients with CFS 2004 Spence, Khan et al

Abstract

The aetiology of chronic fatigue syndrome (CFS) remains controversial and a number of hypotheses have been put forward to explain it. Research into the condition is hindered by the considerable heterogeneity seen across patients but several reports have highlighted disturbances to cholinergic mechanisms in terms of central nervous system activity, neuromuscular function and autoantibodies to muscarinic cholinergic receptors. This paper examines an altogether separate function for acetylcholine and that is its role as an important and generalized vasodilator. Most diseases are accompanied by a blunted response to acetylcholine but the opposite is true for CFS. Such sensitivity is normally associated with physical training so the finding in CFS is anomalous and may well be relevant to vascular symptoms that characterise many patients. There are several mechanisms that might lead to ACh endothelial sensitivity in CFS patients and various experiments have been designed to unravel the enigma. These are reported here.

Paywall, https://www.plefa.com/article/S0952-3278(04)00013-4/fulltext

 

Abridged —

Introduction
CFS is currently defined exclusively by a number of non-specific symptoms that are common to many conditions but there has been speculation that many of the neurological symptoms might be cholinergically mediated. This conjecture is supported by findings of increased levels of free choline in the central nervous system of CFS patients. As well as these neurological findings, however, there has been a recent report of autoantibodies specifically to muscarinic receptors in many CFS patients suggesting that there might well be subgroups within the CFS construct that are associated with autoimmune abnormalities of cholinergic, muscarinic receptors.

Apart from its neurotransmitter functions acetylcholine is, of course, a well established and prominent vasodilator whose action is dependent upon an intact layer of functioning endothelial cells that line the lumen of all blood vessels.

Acetylcholine (ACh) but not nitric oxide (NO) sensitivity in CFS patients
A common test of endothelial integrity is the response of blood vessels to both endothelial-dependent vasodilators like acetylcholine and endothelial-independent vasodilators like nitric oxide (NO) via an NO donor like sodium nitroprusside.

In most medical conditions associated with cardiovascular disease there is a blunted response to acetylcholine. However, we have reported increased responses to a cumulative dose regime of ACh delivered by iontophoresis into the cutaneous microcirculation of 22 CFS patients when compared with 22 age and gender matched control subjects. In the same patients the response to iontophoretic application of sodium nitroprusside, a donor of NO, was normal compared to controls. There are many possible explanations for endothelial sensitivity to ACh ranging from sensitivity of the G-protein muscarinic (m3) receptor on the surface of the endothelial cell, low levels of the enzyme acetylcholinesterase (AChE) as expressed on endothelial cells and up-regulation of one of many of the postreceptor signaling mechanisms. Since the response to NO was normal we assumed that smooth muscle cell biology was not implicated in the sensitivity to ACh.

Testing the sensitivity of the endothelial ACh muscarinic receptor and other similar G-protein receptors, namely, bradykinin (BK) and substance P (SP) in CFS patients
In a further set of experiments, single doses of 1% ACh, 0.06% bradykinin (BK), 0.15% substance P (SP) [all endothelial-dependent vasodilators] and 1% sodium nitroprusside (SNP) [endothelial independent] were delivered by iontophoresis for 80 s using a 0.1 mA anodal current into the forearm skin of 30 well-defined CFS/ME patients and 30 well-matched control subjects.

The results show that resting skin perfusion was normal and the peak responses to the single doses of ACh, BK and NO were also normal. There was a significantly increased response to SP in CFS patients and this was often accompanied by a spreading flare and localised oedema: a finding not observed in control subjects. It is possible that the increased response to SP is a consequence of a heightened sensitivity to SP in terms of its histamine releasing properties. [...] We propose, therefore, that there is no widespread upregulation of endothelial G protein receptor function in CFS patients.

 

Cont'd —

Prolongation of the acetylcholine-mediated blood flow response and the relation to blood cholinergic enzymes
A further study demonstrated that ACh sensitivity in CFS patients might be explained by prolonged action of the vascular response to ACh. We tested this by recording the dynamics of the ACh-stimulated blood flow response for up to 30 min to allow the rate of decay of the response to be calculated. Two points were determined, t75 and t50; corresponding to the times taken for the blood flow response to return to 75% and 50%, respectively, of the peak response to ACh minus the baseline response. Also, in the same study, we determined levels of red blood cell acetylcholinesterase (AChE) and plasma butrylcholinesterase (BChE) to check any relationship between blood cholinesterase measurements and the endothelial responses to ACh.

The data demonstrated that the dynamics of the ACh stimulated blood flow response is significantly different in CFS patients compared with control subjects in that the action of ACh is prolonged in the CFS patients—the blood flow recovery half times (t75 and t50 ) were 13.7 and 8.8 min and 24.5 and 15.1 min for control subjects and CFS patients respectively (P<0.03 for t75 and t50). We have postulated that this prolonged ACh response might be related to inhibition of endothelial expression of the enzyme acetylcholinesterase (AChE) possibly via a viral mechanism since herpes virus is known to inhibit AChE within cholinergically sensitive cells

It is apparent, however, that in control subjects a high BChE activity is normally associated with a quicker decrease in hyperaemic blood flow response but the opposite is true for AChE activity. This may be explained by the fact that AChE is inhibited by its own substrate, ACh, such that large amounts of exogenously delivered ACh may disproportionately swamp local AChE activity. This effect may also be disproportionately greater in those with the highest residual levels of AChE since the maximal rate of hydrolysis of ACh by AChE is obtained at the lowest concentrations of ACh. No such relationship was seen for AChE or BChE activity and recovery of the hyperaemic blood flow response in CFS patients and this is one further indication of disruption to cholinergic pathways in these patients.

Acetylcholine sensitivity is specific to a sub-group of patients within the CFS construct
We had hypothesised that farm-workers who developed a CFS like syndrome when exposed to organophosphate cholinesterase inhibitors present in sheep dip might also be ACh sensitive. We also speculated that those with Gulf War syndrome who had taken the cholinesterase inhibitor pyridostigmine bromide as a nerve protection agent and who had been exposed to organophosphate de-lousing powder might be ACh sensitive. [...] The results demonstrated that only the CFS/ME patients were sensitive to ACh.

Our second aim was to test further the hypothesis that vascular hypersensitivity in CFS was due to a reduction in endothelial cholinesterase activity. We tested this hypothesis by assessing the blood flow responses to MCh, a vasodilator that is almost identical to acetylcholine, but which is much less influenced by the action of cholinesterase. We predicted that if our ACh results were a consequence of reduced cholinesterase activity we would expect to find no difference between the vascular responses to ACh and MCh. In control subjects MCh responses were indeed significantly greater than those for ACh and this was also true for those with CFS/OP and CFS/GWS conditions. In the CFS/ME group there were no significant differences between ACh and MCh responses. These results point to a problem with AChE under-expression on the vascular endothelium of CFS patients.

 

Cont'd

The fatty acid hypothesis and the ACh vasodilator pathway
While the current data we have might point to underexpression of AChE on the endothelium of CFS patients there are, in fact, several possible pathways for the blood vessel to dilate following stimulation of the ACh muscarinic receptor through to the vascular smooth muscle.

Under normal circumstances with an intact endothelium it is thought that the calcium activated NO pathway is responsible for the major component of acetylcholine-mediated vasodilatation. Phospholipase A2 activated prostacylcin (PGI2) also contributes to vasodilatation and the contribution of this pathway to total relaxation can be determined by blocking with cyclo-oxygenase inhibitors such as aspirin. Under pathological conditions in which the NO pathway is disrupted, however, endothelium-derived hyperpolarising factor (EDHF) may also be a prominent vasodilator and this is activated by a cytochrome P450 epoxygenase such as epoxyeicosotrienoic (EET) acid which is a metabolic derivative of arachadonic acid (AA). What contribution each of these pathways makes to the enhanced sensitivity to ACh in CFS patients is intriguing and needs to be determined. Such research is entirely feasible and urgently required.

Conclusion
In three separate studies we have demonstrated abnormalities of the ACh endothelium-dependent vasodilator pathway in CFS patients. Sensitivity to ACh seems to be restricted to those patients within the CFS construct who fit descriptions for ME and PVFS but not those for gulf war syndrome, those exposed to organophosphate compounds or, indeed, those with fibromyalgia.

We are, therefore, confident that the findings of increased sensitivity to acetylcholine in CFS patients are robust and unusual. Increased sensitivity to ACh is normally associated with trained athletes while CFS patients are characterised by having ‘a substantial reduction [...]’. Our results are important [...] and may be relevant to the problems of orthostatic instability that is so evident in most CFS patients.

Recent evidence in a very small group of patients suggested that CFS might be the consequence of a cholinergic dysautonomia and that treatment with cholinesterase inhibiting agents might well be therapeutic. Such a hypothesis is in direct contrast with the findings reported here so great caution is needed in treating an illness with such obvious heterogeneity.

 

Do you like it @SNT Gatchaman?

 

I thought given 20 yrs on, and that Andy had posted it, it was worth liberally quoting as it summarises their work. Definitely worthy of further investigation as Hutan points out above and Ravn in the other thread. That didn't happen. We'll have to see how newer studies (esp now with LC) might support or progress.

 

Thanks @SNT Gatchaman!
I feel optimistic about the possibility of this avenue.