Oscillation of pain intensity during adenosine infusion. Relationship to beta-endorphin and sympathetic tone

High-dose adenosine infusion provokes oscillations of chest pain without correlation to opioid modulation: A double-blind controlled study

Adenosine is a neuromodulator with both excitatory and inhibitory effects in different organs. High-dose adenosine infusion provokes chest pain in patients and healthy volunteers. This study examines the nature of chest pain and whether it is modified by the mu opioid receptor agonist (beta-endorphin) or a nonselective opioid antagonist (naloxone). Ten healthy volunteers with a mean age of 26 +/- 3 years participated in the study. The study was performed during 3 sessions. During every session the subjects were given a high dose of adenosine infusion (140 microg/kg/min) for 22 minutes. After 5 minutes, the subjects were randomized in a double-blind order on 1 of the 3 categories: (1) as placebo, NaCl bolus for 2 minutes followed by NaCl infusion for 15 minutes; (2) beta-endorphin bolus followed by infusion; and (3) naloxone bolus followed by NaCl infusion. Hemodynamic and pain parameters were monitored. During adenosine infusion, all volunteers experienced chest pain with oscillations of pain intensity. The oscillations continued during beta-endorphin and naloxone. There were no significant differences between hemodynamic and pain parameters during beta-endorphin or naloxone compared to adenosine infusion. High-dose infusion of adenosine provokes chest pain with oscillations of algesia and pain-free intervals. Peripheral opioid administration did not influence the adenosine-provoked chest pain.

PERSPECTIVE

Adenosine-induced oscillations of pain and pain-free intervals could theoretically be a sign of neuronal reflex activity dependent on spatiotemporal summation of adenosine excitatory and inhibitory properties. This could contribute to the complex nature of angina pectoris. Peripheral opioid receptors might not be involved in the oscillations.

Role of Adenosine and Opioid-Receptor Mechanisms for Pain in Patients with Silent Myocardial Ischemia or Angina Pectoris: A Double-Blind, Placebo-Controlled Study

Patients with silent myocardial ischemia have similar prognosis but fewer primary care and emergency care visits than patients with angina pectoris. Silent myocardial ischemia has been associated with an increased pain threshold because of an increased endogenous opioid receptor activity. In this double-blind, placebo-controlled study, we tested whether patients with silent myocardial ischemia would have less sensitivity to the ischemic pain messenger adenosine compared with angina pectoris patients and healthy controls. In addition, we tested whether this effect might be due to an increased opioid receptor activity.

MATERIALS AND METHODS

Thirteen male patients with silent myocardial ischemia (mean age 58 +/- 10 years with BMI 25 +/- 3) were compared with 10 male patients with angina pectoris (mean age 57 +/- 9 years with BMI 29 +/- 5). Healthy volunteers (mean age 52 +/- 7 years with BMI 25 +/- 3, n = 10), acted as controls. Increasing doses of adenosine were injected rapidly into an antebrachial vein in a double-blind, randomized order. Central chest pain was provoked and quantified using psychophysical methods. The procedure was repeated after an injection of 0.4 mg of the non-selective opioid antagonist, naloxone.

RESULTS

Patients with silent myocardial ischemia exhibited higher pain threshold than patients with angina pectoris and healthy volunteers. After naloxone, healthy volunteers and patients with angina pectoris tended to have more pain than patients with silent myocardial ischemia.

CONCLUSION

In conclusion, patients with silent myocardial ischemia had a decreased sensitivity to adenosine-provoked chest pain compared with patients with angina pectoris. The decreased pain sensitivity was not related to opioid receptor activity.

Experimental pain by ischaemic contractions compared with pain by intramuscular infusions of adenosine and hypertonic saline

Deep tissue pain can be related to reduced muscle blood flow, which comprises the metabolic demand under muscle work. The tissues and receptors involved in nociception after ischaemic muscle contractions are not known. The concentration of adenosine is increased after ischaemic contractions and might act as an algesic substance. In 15 subjects, adenosine, hypertonic saline (algesic), and isotonic mannitol (placebo) were infused into the tibialis anterior muscle and compared with the pain caused by ischaemic contractions. The muscle pain intensity (visual analogue scale; VAS), distribution, and quality were assessed. Pressure pain thresholds were recorded to assess the deep tissue sensitivity. Adenosine did not induce more pain than the placebo. The maximal VAS score after hypertonic saline and ischaemic contractions was higher compared with adenosine/placebo infusions. The duration and area of pain were significantly increased after hypertonic saline infusions compared with ischaemic contractions. Higher scores on the McGill pain questionnaire were given to the "stabbing", "burning", "heavy", and "exhausting" word categories after ischaemic contractions, and "cramping" was rated higher during hypertonic saline-induced muscle pain compared with ischaemic contractions. During hypertonic saline infusions, the pressure pain threshold was decreased compared with before and immediately after the pain had vanished. The present study shows that pharmacological levels of adenosine in skeletal muscle did not induce pain. Excitation of muscle nociceptors by hypertonic saline evoked hyperalgesia, larger areas of pain, and a different quality of pain compared with ischaemic contractions, suggesting that the pain after ischaemic contractions is mediated by other populations of nociceptors in muscle and/or other tissues than excited by hypertonic saline.