Plasma vasopressin and oxytocin in response to exercise and during a day-night cycle in man

Plasma vasopressin (AVP) and oxytocin (OXT) were determined by radioimmunoassay in healthy young men before, during and after a running exercise until exhaustion. In four of the five test subjects (one subject was underloaded) physical load resulted in a marked increase in plasma AVP with peak values of 11.7 to 57.8 pg/ml at exhaustion. Plasma OXT increased in three test subjects with peak values of 4.5 to 23.9 pg/ml. Within 1 h of recovery, both neuropeptides returned to basal plasma levels. The relationship between plasma AVP and osmolality (p less than 0.001) suggests that changes in osmolality play a dominant role in regulating the secretion of AVP. Besides peripheral effects, the increased levels of AVP and/or OXT during and immediately after the exercise, respectively, might induce a favourable action on the brain function. Determination of plasma AVP and OXT during a day-night cycle in the same test subjects reveals increased AVP levels from 02.00 to 08.00 h as compared to the other time periods chosen (18.00--24.00 h, p less than 0.001; 08.00--16.00 h, p = 0.031). In contrast, OXT does not differ between these time periods but decreased from 08.00 to 16.00 h (p less than 0.01). Accordingly, no correlation exists between both neuropeptides suggesting an independent secretion. It is unclear whether the circadian variations shown are able to influence the neuropeptide response to exercise.