Acidosis (or Tissue Acid Waste) in Muscle Tissues Causes Local and Referred Pain

Acidic Buffer Induced Muscle Pain Evokes Referred Pain and Mechanical Hyperalgesia in Humans

Frey Law Laura A,¹ Sluka Kathleen A,¹ McMullen Tara,¹ Lee Jennifer,¹ Arendt-Nielsen Lars,² and Graven-Nielsen Thomas² Keywords: Fibromyalgia, Myofascitis, Chronic Fatigue Syndrome, Hyperalgesia, Chronic Pain, Trigger Points, Myalgia

Overview

Repeat studies have shown that acidosis (tissue acid waste in body) has multiple negative effects.  It is “well known that some diseased areas, such as cancer nests, inflammation loci (localized inflammation) and infarction areas (heart or other organs that have had the blood supply cut off) are acidified”.  Gene Expressions for Signal Transduction under Acidic Conditions, i  PMCID: PMC3899954

Additional research has also noted – “The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid–base regulation is a vital issue for cell and tissue homeostasis.

Up-regulation and over activity of acid sensors appear to contribute to various forms of chronic pain. . . . “  Handbook of Experimental Pharmacology Volume 194, 2009, pp 283-332, 10 Jun 2009

Acidosis is taking center stage when it comes to many of the afflictions humans are suffering from.  As you will see in this peer- reviewed published paper, it can be the main cause for myofacial, trigger point, fibromyalgia and chronic pain.

This research provides substantial evidence that not only can centralized acidosis in muscle causes localized pain but also referred pain.

I have provided a shortened version of this PubMed paper so that you can get the initial details of the finding of this study.  I have also provided a link to the complete study online.

My Conclusion

In order to reduce or eliminate this localized or body wide acidosis, I suggest utilizing alkaline, negatively charged, ionized water.   Contact me directly for more information about alkaline, negatively charged, ionized water.

Thanks for coming to my site and reviewing this material.

Dr. Noreen Picken, BA, BS, DC

Nutritional Researcher & Longevity Specialist
512-752-8542 or email me at DrNoreenPicken@gmail.com

While tissue acidosis causes local deep-tissue pain, its effect on referred pain and mechanical muscle hyperalgesia is unknown. The aim of this study was to investigate a human experimental acidic muscle pain model using a randomized, controlled, single-blinded study design. 72 subjects (36 female) participated in three visits, each involving one 15 min intramuscular infusion into the anterior tibialis muscle: acidic phosphate buffer (5.2 pH) at 40 ml/hr (N=69) or 20 ml/hr (N=54), normal phosphate buffer (7.3 pH) at 40 ml/hr (N=70), or isotonic saline at 40 ml/hr (N=19). Pain ratings and pressure sensitivity of superficial and deep tissues were assessed before, during, and 20 min after infusion. Acidic buffer produced light to moderate, rate-dependent, muscle pain (not sex-dependent) compared to the control infusions, that referred pain to the ankle in 80% of women and 40% of men. Pain did not vary across self reported menstrual phases. Pressure pain thresholds (PPTs) were reduced over the infused muscle with acidic infusion, defined as primary mechanical hyperalgesia. PPTs decreased at the ankle in those with referred pain in response to acidic buffer, i.e. referred mechanical hyperalgesia, but not at the foot. No pain or changes in PPTs occurred in the contralateral leg.

These results demonstrate muscle acidosis can lead to local and referred pain and hyperalgesia, with significant sex differences in development of referred pain.

More patients seeking medical attention report musculoskeletal pain complaints than any other form of pain [28]. Although multifactorial, one known source of deep-tissue pain is tissue acidosis. Acidosis may lead to pain in muscle trigger points [51]; cardiac muscle [11]; inflammatory conditions [58]; and exercise [30,43]. Accordingly, experimental muscle pain can be induced by the intramuscular infusion of a buffered acidic solution, e.g., ascorbic acid [49] or an acidic phosphate buffer solution [35] in humans. Animal studies have revealed that acidic infusion models activate chemosensitive nociceptors: acid-sensing ion channels (ASICs) and/or the transient receptor potential vanilloid 1  (Capsaicin receptors) (TRPv1), resulting in mechanical hyperalgesia [32,54,55,56].

Referred muscle pain has been consistently observed with hypertonic saline muscle pain models, but is muscle dependent. For example, stereotypic referred pain patterns occur with hypertonic saline infusion of the anterior tibialis [24] and infraspinatus [42] muscles in approximately 50% of subjects [22]. Whereas only localized pain is observed with infusion of the triceps brachii or biceps brachii muscles [22]. These distributions are consistent with referred pain patterns associated with muscle trigger points [62,63]. It is not clear whether referred pain similarly occurs with acidic pain models, or may be more clearly expressed due to activation of acid-sensitive nociceptors.

Mechanical hyperalgesia occurs with several clinical pain conditions [7], but it is not consistently observed with the experimental hypertonic saline model in humans [22]. However mechanical hyperalgesia may be stimulus specific, e.g. tissue acidosis.

Animal studies demonstrate that repeated intramuscular bolus injections of acidic saline (pH 4.0) induce cutaneous and muscle mechanical hyperalgesia both ipsilateral and contralateral to the injection site [54,61]. Deep-tissue mechanical hyperalgesia has not been investigated with an acidic pain model in humans, nor are contralateral effects typically considered.

Several musculoskeletal pain conditions, such as fibromyalgia, chronic tension headache and temporomandibular joint syndrome occur more frequently in women [5,27]. However, in various experimental pain models, both greater female pain sensitivity [16,18,65,69] and no sex differences [12,41] are observed. Sex differences have yet to be investigated using an acidic muscle model in humans.

The purpose of this study was to investigate whether acid-evoked muscle pain produces referred pain and/or mechanical hyperalgesia compared to control infusions in men and women. We hypothesized 1) local and referred pain would occur in a dose-dependent manner; 2) mechanical hyperalgesia would occur as observed in animal models; and 3) women would experience greater pain and mechanical hyperalgesia than men.

2.1 Subjects

Seventy-two healthy, pain-free volunteers (36 male, 36 female) were recruited from the University and local community. The mean (SD) age of the participants was 24.3 ± 6.1 yrs (range 18-50 yrs). Mean ± SD height and mass were: 172.3 ± 10.4 cm and 72.4 ± 13.0 kg, respectively.

2.2 Study Protocol

A randomized, controlled, single-blinded study design was used, with each subject serving as their own control. Subjects participated in three visits, each spaced approximately 1 week apart (5-14 days). Each visit involved one 15 min intramuscular (IM) infusion into the mid-belly of the anterior tibialis muscle in a balanced-random order: acidic phosphate buffer (5.2 pH) at 40 ml/hr (acidic 40), acidic phosphate buffer (5.2 pH) at 20 ml/hr (acidic 20), and normal phosphate buffer (7.3 pH) at 40 ml/hr (PB control).

3.1 Local Pain

The acidic infusion model produced muscle pain quality was most frequently described on the McGill Pain Questionnaire (MPQ) as aching, throbbing, cramping and tender across all infusions. Peak pain ratings were not distributed normally, but were positively skewed, thus were log transformed.

3.2 Referred Pain

Referred pain was reported only at the ankle, developing over approximately 4 min, and maintaining a constant intensity during the 15 min infusion (figure 1). Referred pain (≥ 0.5) occurred more frequently during the acidic 40 (62%) than the saline (37%) infusions (p < 0.05), but was not significantly different than the acidic 20 (48%) or PB control (54%) infusions. Significantly more women than men experienced referred pain during the acidic 40 (80% vs. 44%, p < 0.05), acidic 20 (63% vs. 33%, p < 0.05), and PB control (69% vs. 37%, p < 0.05) infusions, respectively. No sex-differences in referred pain were observed for the saline infusion. However, peak referred pain did not vary with menstrual phase across infusions (p = 0.19 to 0.58).

Peak referred pain and the area under the referred pain curve (pain-time integral, figure 3A) were greater for the acidic 40 than the remaining three infusions (p < 0.001) across all subjects. When considering only those individuals with referred pain, peak referred pain intensity (mean Borg CR10, SEM) remained significantly greater for the acidic 40: 1.8 (0.3), than the acidic 20, PB control, and saline infusions: 1.4 (0.2), 1.2 (0.1), and 0.9 (0.1), respectively (p < 0.01). Thus, the difference in referred pain intensity between the acidic 40 versus the other conditions was maintained and not merely a result of the difference in referred pain incidence. Women had significantly greater referred pain intensities than men for the acidic 40, acidic 20, and PB control infusions (figure 3C, p < 0.05).

Referred pain intensity was moderately associated with local pain intensity during the acidic 40, acidic 20, and PB control infusions (figure 4, p < 0.001), but not during the saline control infusion. However, these relationships were largely driven by stronger associations in women than men. Peak local – referred pain correlations were typically high in women: 0.73 (p < 0.0001), 0.66 (p < 0.0001), 0.56 (p < 0.0001) and 0.40 (p = 0.33); whereas in men the correlations were relatively low: 0.13 (p = 0.48), 0.50 (p = 0.008), 0.29 (p = 0.10), and -0.07 (p = 0.85) for the acidic 40, acidic 20, PB control and saline infusions, respectively. Referred pain ratings between infusions revealed significant correlations between all combinations of the acidic 40, acidic 20 and PB control infusions (table 1).

Acidic muscle pain evokes rate-dependent local pain and referred pain, with ipsilateral deep mechanical hyperalgesia, but no contralateral pain or mechanical hyperalgesia. Sex differences were observed for select pain measures: with more females experiencing referred pain, females exhibiting a stronger correlation between local and referred pain, and lower baseline PPTs in females.

4.1 Local and Referred Pain

As hypothesized, a rate-dependent pain response was observed, with peak local and referred pain more intense for the higher rate infusion, consistent with previous reports [35]. Similar referred muscle pain patterns and overall incidence were produced with the acidic and the hypertonic saline models [22,23,24]. Distinct referred pain occurred only at the ankle (figure 1C), not merely an enlargement of the local pain region. However, the acidic buffer infusion produced relatively stable pain, whereas constant-rate hypertonic saline models typically produce an initial peak with a gradual decay over time [22].

Several theories have been proposed to explain the referred pain phenomenon, with the convergence-projection theory the most widespread. Input from different tissue types (i.e. muscle, viscera, skin) converge on the same dorsal horn neurons [70]. After injury, increased nociceptive input from the injured muscle, for example, is transmitted supraspinally and misinterpreted at the cortical level as pain from other tissues. However, this theory in isolation does not explain referred pain directionality, e.g. cardiac pain refers to the shoulder, but the reverse is uncommon. Central changes are likely involved as well [1,20,22]. Dorsal horn neurons sensitize after injury resulting in increased receptive field size that would further contribute to the referred pain area. In fact, intramuscular injection of the inflammatory irritant, bradykinin, results in newly developed receptive fields of dorsal horn neurons in rats [29]. These authors conclude that pathways exist to produce referred pain and hyperalgesia but aren’t functional until the appropriate nociceptive stimulation is present. Indeed, intramuscular injections of acid result in widespread hyperalgesia with sensitization of dorsal horn neurons and activation of supraspinal pathways [55,61]. Similarly, in numerous patient populations referred pain areas are enlarged in response to hypertonic saline infusions [3,25,37,42,53,59], supporting that central sensitization involving spinal and supraspinal pathways is involved in referred pain.

It is not clear why referred pain is observed in only a portion of the population. The incidence of referred pain following infusion of acidic buffer (60%) is consistent with hypertonic saline models [22]. With the acidic model, women develop referred pain more frequently (80%) than men (40%). Further, referred and local pain intensities were associated in women only. Although not always investigated, similar sex-differences in referred pain incidence occur with hypertonic saline, 67.4% versus 37.5%, and electrically-induced muscle pain, 32.3% versus 7.7%, for women and men respectively, despite no differences in local pain [21]. An acidic-evoked esophageal pain model results in greater referred pain in females [46]. However, contrary to our findings, men exhibited greater esophageal mechanical hyperalgesia.

The mechanisms underlying sex differences in referred pain prevalence are not clear. The pain does not appear to vary across the menstrual cycle, consistent with a recent review of the literature [52]. Response bias or gender-role expectations [47] may be a factor; however, local infusion-site pain and mechanical hyperalgesia did not differ between sexes.

Sex differences in referred pain may be a result of spinal or supraspinal mechanisms. Temporal summation, also believed to be centrally-mediated, typically occurs at a higher rate in women in response to thermal [13,19,48,50] and mechanical stimuli [50]. However, the activation of supraspinal pain modulation systems, such as diffuse noxious inhibitory controls (DNIC), do not generally differ between men and women [2,14,16,45]. Although one study observed greater DNIC in males than females [16]. Clinically, many chronic musculoskeletal pain conditions have a female predominance, i.e. fibromyalgia, temporal mandibular disorder, chronic fatigue syndrome, arthritis [5,27]; thus the enhanced likelihood for development of referred pain in females may provide an underlying explanation for this phenomenon. However, the mechanisms remain elusive.

4.2 Mechanical Hyperalgesia

The mechanical hyperalgesia observed with this acidic infusion model is consistent with cutaneous acidic models, producing mechanical hyperalgesia to von Frey stimulation [57]. However, it has not been typically observed with other deep-tissue pain models, including intramuscular electrical stimulation [38] and hypertonic saline injection [17,22,26]. Accordingly, the acidic infusion may provide a means to study mechanical hyperalgesia not readily available with these other models.

Referred pain and mechanical hyperalgesia is likely a result of central mechanisms; while local hyperalgesia is largely peripherally mediated [70]. Nevertheless, referred hyperalgesia and central sensitization can clearly be driven by peripheral nociceptive sensitization after injury, requiring initial input from nociceptors. As mentioned above, sensitization of dorsal horn neurons with expansion of receptive fields could underlie the referred pain and hyperalgesia [29,55]. These spinal changes could be driven by supraspinal sites since blockade of brainstem sites not only reduces hyperalgesia but also reduces spinal release of the excitatory neurotransmitter glutamate after tissue injury [15,44,61].

Clinically, mechanical hyperalgesia occurs in referred pain regions in patients with musculoskeletal [39] or visceral [66] pain origins. A prolonged pain experience may increase the likelihood of developing referred pain [22], partially explaining the discrepancy often observed between experimental and clinical pain conditions, but may also be dependent on the underlying algesic stimulus. Thus, mechanical hyperalgesia in the referred pain region may be associated with the local activation of specific acid-sensitive nociceptors, and not readily induced via hypertonic saline or electrical stimulation models.

4.3 Acidic Pain and Hyperalgesia

The acidic solution produces the greatest pain response, suggesting proton-activated nociceptors are involved. However, the PB control infusion produces equivalent pain to the lower-rate acidic 20 infusion, possibly a result of the hypotonicity of the solution, or differences in Ca²⁺ chelation between solutions. ASIC activation by protons competes with Ca²⁺, so that changes in extracellular Ca²⁺ from the infusion can alter channel kinetics [33]. Thus, the pain associated with the PB control infusion may be a result of different nociceptive activation than the acidic infusion. The isotonic saline infusion produces minimal pain that began to decay prior to the end of the infusion, suggesting limited mechanical sources of pain. Thus, the mechanisms contributing to the deep tissue pain in this model are largely chemo-mediated: primarily proton activation, possibly Ca²⁺ mediated, and minimally from tissue distension.

Nociception resulting from the acidotic environment is likely due to activation of ASICs and/or TRPV1. ASICs (ASIC1, ASIC2 and ASIC3) are located in the periphery and in dorsal root ganglion innervating muscle [67]. ASIC currents are transient, but have varying time constants, with the ASIC1a and ASIC2a maintaining depolarization longer than the ASIC1b or ASIC3 subunits [4,67,68]. It is not entirely clear how transient ASIC currents mediate sustained pain when exposed to a prolonged acidic environment. However, in addition to rapid ASIC depolarization through H+ binding, ASICs may be activated by the unbinding of Ca²⁺, resulting in shallow, sustained currents [34]. Pain recovery following the acidic infusion appears to have two-phases: an initial rapid decay over the first 4 -5 min, followed by a slower, prolonged decay (figure 1), when compared to controls. A two-phase decay may be a result of channel kinetics of two (or more) ASIC subunits or the ASIC binding interactions between H⁺ and Ca²⁺.

Peripherally-located ASICs are critical for the development of mechanical hyperalgesia with deep tissue insult. Specifically, secondary mechanical hyperalgesia does not develop in ASIC3 knockout mice [32,55,56]. Re-expression of ASIC3 in muscle from ASIC3-/- mice restores the development of mechanical hyperalgesia that normally occurs after muscle injury [54]. In animals, intramuscular acid injections result in contralateral mechanical hyperalgesia after two injections, but not after one [54]. No contralateral hyperalgesia was observed in this model, possibly due to: solution volume, pH or buffering capacity, number of injections, and species.

The second candidate for nociception in this pain model is a polymodal receptor, the TRPV1 channel, located on peripheral sensory neurons and activated by capsaicin, protons, heat and endovanilloids [10]. TRPV1 demonstrates pH responsiveness [9], but may not be a primary mediator of acid-evoked pain. Rather, it is sensitized by protons resulting in heightened responses to additional nociceptive stimuli [8,36]. This model (pH 5.2) may involve TRPV1 as capsazepine partially blocks cutaneous acidic pain in human subjects using pH 5.0 but not pH 6.0 [64]. TRPV1 is most noted for its chemical and thermal sensitivity, whereas mechanical sensitivity, consistent with our experimental observations, is more typically associated with ASICs.

In summary, this acidic experimental pain model provides a temporary light to moderate muscle pain that reproduces the common experience of deep-tissue pain in humans. Similar to the more common hypertonic saline model, it produces referred pain at the ankle, but additionally can produce local and referred mechanical hyperalgesia, without the need to increase infusion rate to produce a constant pain.

Women experienced referred pain more than men, providing evidence of sex-dependent central sensitization, but without compelling evidence for specific hormonal effects. Future studies are warranted to further investigate the underlying mechanisms of the observed sex differences and factors contributing to the occurrence of referred pain and hyperalgesia.

This research was supported by the International Association for the Study of Pain, Scan/Design by Inger & Jens Bruun Foundation (LFL, KAS, TGN, LAN); American Pain Society Small Grants Program (LFL); Carver Foundation at the University of Iowa (LFL, KAS); and the National Institutes of Health: K12-HD055931 (LFL), AR052316 (KAS) and AR053509 (KAS). The authors have no conflicts of interest to report. We would like to acknowledge Carol Leigh for her assistance with manuscript preparation.

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Link to complete Pubmed article  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2613646/#R28

The publisher’s final edited version of this article is available at Pain

Pain. Author manuscript; available in PMC Nov 30, 2009.

Published in final edited form as: Pain. Nov 30, 2008; 140(2): 254–264.

Published online Oct 2, 2008. doi:  10.1016/j.pain.2008.08.014

PMCID: PMC2613646  NIHMSID: NIHMS80164

 

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