1. We examined the effect of ischaemic pain and sustained isometric muscle contraction on plasma immunoreactive γ-lipotropin (γLPH), β-endorphin/β-lipotropin (βEND/βLPH) and corticotropin (ACTH), which are all synthesized from a common precursor (pro-opiocortin), and plasma Cortisol in 10 normal subjects.
2. Experimental pain was produced by inflation to 250 mmHg of a sphygmomanometer cuff, placed above the elbow of the ‘dominant’ arm, after which the subject squeezed a hand dynamometer, loaded to 12 kg, 20 times at 2 s intervals. Blood was drawn before, after 5 and 10 min of pain, and 30 min after release of the cuff. In a control session, the subjects were asked to squeeze the handgrip alone for 5 min at 30% of their maximum strength, a procedure which elevates the blood pressure without causing pain.
3. One subject had unexplained high (30–71 pmol/l) baseline peptide concentrations. Baseline values for the nine other subjects were: ACTH, 7·3 ± 1·9 pmol/l (mean ± sem); γLPH, 18·6 ± 1·0 pmol/l; βEND/βLPH, 10·0 ±1·1 pmol/l; Cortisol, 599 ±55 nmol/l. Neither procedure significantly increased the plasma concentration of ACTH or any other peptide, whereas plasma Cortisol was significantly increased at both 5 min and 10 min. Plasma ACTH was positively correlated with plasma γLPH (r = 0·701; P < 0·001), βEND/βLPH (r = 0·970; P < 0·001) and plasma Cortisol (r = 0·758; P < 0·05).
4. The present study demonstrates that, in normal man, plasma endorphins do not change with experimental ischaemic pain. The rise in plasma Cortisol without concomitant rise in ACTH is not explained, but suggests the action of some other agent at the level of the adrenal cortex.