Exercise-Induced Blood Prolactin Variations in Trained Adult Males: A Thermic Stress More than an Osmotic Stress

Abstract

Blood prolactin (PRL) variations have been linked to temperature and osmotic changes in several species. The latter factors are here explored to better understand blood PRL responses frequently induced during physical exercise. Since body heat generated by exercise can lead to marked body fluid shifts, it was postulated that PRL changes observed during exercise could be associated with variations in body temperature and/or blood osmolality (OSM). A wide range (38.5–40.5°C) of rectal temperatures (Tr; used here to appreciate core temperatures) were theoretically selected and randomly assigned as targets to male runners. Measured by thermistor probe, target Tr were obtained by a combination of factors: (a) ↑ heat production by treadmill running, and (b) ⇓ heat losses by appropriate clothing (⇓ evaporation) in warmed (⇓ radiation) and hypo ventilated (⇓ convection) laboratory conditions. For each subject, target Tr was attained not prior to 30 min after initiation of running, and had to be maintained for at least 10 min, for a mean ( ± SD) running time of 52.6 ± 10.0 min. In a first protocol, hypohydration was provoked in 26 runners (23.9 ± 4.7 years) by total restriction of water intake. In a second protocol (10 different runners: 22.3 ± 3.3 years), euhydration was maintained by water intake (20 ml/kg body weight). Venous blood was sampled at rest before and immediately after the run. PRL was assayed by RIA; OSM was measured by freezing point depression; sodium was analyzed by flame photometry. At rest, before the heat-producing exercise, mean PRL values were 9.4 ± 3.4 ng/ml for both eu/hypohydrated groups. In the hypohydrated runners, exercise-induced hyperthermia was significantly (r = 0.82; p < 0.0005) associated with blood PRL responses. Moreover, these changes in Tr were also significantly (r = 0.54; p < 0.0025) related to changes in OSM, the latter variations being mostly explained (78 %) by the accompanying hypernatremia. In the euhydrated group of runners, the hyperthermic exercise failed to induce significant changes in OSM (r = 0.22; p > 0.15) and, as expected, variations in blood sodium levels were also not significant under these conditions. However, hyperthermic running in these iso-osmolar conditions did not prevent blood PRL levels from rising (r = 0.77; p < 0.0005). It was thus concluded that, in male trained runners, exercise-induced blood PRL responses could be derived more from thermic than from osmolar stresses.

Ambient Temperature and the Pituitary Hormone Responses to Exercise in Humans

“…Likewise, heart rate and ventilation were higher in the heat but it seems unlikely that an increase of between 7 and 10 beats min _1 should make much difference when, in the cool condition, heart rate had increased by 2.5 times above the resting level in the first 30 min with very little prolactin production. However, there is considerable evidence that rising body temperature, as a result of exercise or passive warming, is an important stimulus and a linear relationship between circulating prolactin concentration and core temperature during low-intensity exercise has been reported (Brisson et al 1986;Melin et al 1988). Our own results tend to show a more biphasic relationship between rectal temperature and prolactin release (Fig.…”

“…There have been a number of studies investigating the release of prolactin both during exercise (Brisson et al 1986(Brisson et al , 1991 and at rest (Brisson et al 1991), and it is clear that changes in blood pH and osmolality are not responsible for stimulating release (Brisson et al 1986). Likewise, heart rate and ventilation were higher in the heat but it seems unlikely that an increase of between 7 and 10 beats min _1 should make much difference when, in the cool condition, heart rate had increased by 2.5 times above the resting level in the first 30 min with very little prolactin production.…”

“…Experimental Physiology (2003) 88.5, 627-635. 2593 Publication of The Physiological Society * Corresponding author: m.w.bridge@bham.ac.uk the skin is relatively cool (Brisson et al 1986(Brisson et al , 1991Bridge et al 1999).…”

Effect of a 12-Month Randomized Clinical Trial of Exercise on Serum Prolactin Concentrations in Postmenopausal Women

“…Acute exercise exerts physical stress on the body, such as increasing heart rate and hypoxia (21), which can, in turn, increase prolactin levels. Specifically, increased prolactin during exercise may occur through several mechanisms including decreased hepatic and renal blood output (12), increasing core body temperature (22)(23)(24), or increasing lactate concentrations and hypoxia (14,25,26). The underlying biological pathways for these mechanisms are unclear, although the prolactin increase during exercise has been correlated to norepinephrine in one study (27) and ACTH in another (28).…”

Responses to exercise in the heat related to measures of hypothalamic serotonergic and dopaminergic function

“…Changes in prolactin levels in the blood therefore provide a useful marker for changes in central 5-HT activity as 5-HT is a prominent excitatory neurotransmitter for prolactin release (Struder and Weicker 2001). Circulating plasma prolactin concentrations rise during fatiguing exercise, largely in response to increases in core and skin temperature (Brisson et al 1986(Brisson et al , 1991Bridge et al 1999).…”