Effect of opiate and beta-adrenergic blockers on the gut transit response to mild exercise

Acute mild exercise accelerates transit of material through the gastrointestinal tract via mechanisms that remain entirely unknown. Because exercise increases peripheral sympathetic drive, and can increase circulating opioids, in this study we examined the role that naloxone and propranolol-dependent pathways play in accelerating orocecal transit in exercise. Transit was determined in 11 subjects by H2 detection after ingestion of 0.36 g/kg lactulose in a 240 kcal, 250 ml liquid meal; the exercise was mild walking at 5.6 km/hr. Under naloxone treatment (0.4 mg intravenous bolus followed by infusion at 40 micrograms/kg/hr) transit acceleration in mild exercise was apparent: transit time (mean +/- SE) was 87 +/- 7 min at rest, 63 +/- 5 min in exercise (p < 0.05), a result identical to that seen in previous studies not involving drugs. In contrast, propranolol treatment (160 mg/day) accelerated the resting transit rate to equal the rate during exercise (65 +/- 6 vs. 60 +/- 4 min; p = NS). Under both conditions, exercise elevated oxygen consumption and heart rate, although propranolol predictably produced relative bradycardia at rest and exercise. These results suggest that mild exercise accelerates upper gut transit via an opiate-independent effect that, for gut propulsive motility, yields results similar to beta-adrenergic blockade.