Asymptomatic or mildly symptomatic effort-induced myocardial ischemia: Plasma β-endorphin and effect of naloxone

The hypothesis that endogenous opioids may be involved in reduced exercise-induced ischemic pain or in silent ischemia was tested. Fifteen male patients with coronary artery disease were tested in a randomized, double-blind crossover study. After a preliminary screening effort test they were divided into two groups: the first group of nine patients received an i.m. injection of naloxone 0.4 mg, or saline as placebo, and the second group, comprising six patients, received 4 mg naloxone or saline i.v. Effort testing was performed at weekly intervals on an ergometric bicycle, following the Bruce protocol. ECG, heart rate, blood pressure and pain perception were monitored continually. Blood was sampled through an indwelling venous catheter for β-endorphin determination before, at the peak of, and 10-20 min following exercise. ST depression, heart rate, blood pressure and the double product were similar after naloxone and following saline administration. β-endorphin concentrations in plasma were significantly increased following exercise in the second group of patients. The increase in β-endorphin concentration was larger when the patients were pretreated with naloxone (4 mg) than with placebo. However, chest pain was not significantly altered by either dose of naloxone.