ROLE OF GABA AND OPIOIDS IN THE REGULATION OF THE VASOPRESSIN RESPONSE TO PHYSICAL EXERCISE IN NORMAL MEN

The present study was undertaken in order to establish the possible involvement of GABAergic and/or opioid pathways in the mechanism underlying the arginine-vasopressin (AVP) response to physical exercise. After fasting overnight, seven subjects were tested on four mornings at least 1 week apart. Exercise was performed on a bicycle ergometer. The workload was gradually increased at 3 min intervals until exhaustion and lasted about 15 min in all subjects. Tests were carried out under administration of placebo, the opioid antagonist naloxone (10 mg as an i.v. bolus injection), the GABAergic agonist sodium valproate (600 mg in three divided doses orally) or naloxone plus sodium valproate. Plasma AVP levels rose 4-fold during exercise. The administration of naloxone did not modify, whereas sodium valproate completely abolished the plasma AVP rise during exercise. When naloxone was given together with sodium valproate, AVP rose 3-fold in response to exercise. These data suggest the involvement of a GABAergic mechanism in regulation of the AVP response to physical exercise in men. Furthermore, the data argue against a role of naloxone sensitive endogenous opioids in the control of AVP during exercise, whereas they suggest a partial opioid mediation of the GABAergic inhibitory action.