Theophylline decreases pain in the ischaemic forearm test

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Abstract
To study the hypothesis that endogenous adenosine is a mediator of the ischaemic pain sensation, the effect of the adenosine receptor blocker theophylline (5.5 mg of the ethylendiamine salt .cntdot. kg-1 intravenously) was tested in a placebo controlled double blind cross overy study (placebo/theophylline/placebo or placebo/placebo/theophylline) in five healthy volunteers. Ischaemic work was performed with a spring loaded hand ergometer (1 Hz). The pain sensation was continuously reported using the Borg scale. Blood flow was measured by occlusion plethysmography. Pain was reported 18 (SEM 2.4) s after starting the ischaemic work and increased continuously to a maximum after 129(18) s (placebo). Theophylline at a plasma concentration of 75(7) .mu.mol .cntdot. litre-1 decreased the pain sensation in relation to working time. With theophylline, 12(3)% more work (p < 0.01) was performed for the same reported pain estimate. Blood flow increased from a basal level of 52(9) to 495(55) ml .cntdot. min-1 .cntdot. 100 ml-1 30 s after work and returned to normal within 30-40 min. Theophylline did not affect blood flow. In conclusion, theophylline has a small but significant inhibitory effect on the ischaemic pain sensation compatible with a hyperalgesic effect of adenosine.