Prolactin as an Indicator of Impending Fatigue during Uncompensable Exertional Heat Stress

Abstract

Changes in circulating prolactin (PRL) concentrations have been suggested as a marker of impending fatigue due to its strong relationship with core temperature (Tre) and the purported interplay between brain serotonergic and dopaminergic activity. Yet, comprehensive examination of PRL between groups with different fitness levels and during repeated exertional heat stress (EHS) has not been reported. PURPOSE: To examine the use of PRL as an indicator of impending fatigue in endurance trained (TR) and untrained (UT) males during acute (Study 1) and repeated (Study 2) EHS. METHODS: Subjects walked to exhaustion (EXH) on a treadmill (4.5 km·h-1, 2% incline) in 40°C wearing combat clothing and a protective hooded overgarment for acute and repeated (9 days over 11 days) sessions. Study 1 included 12 TR and 11 UT (Mean ± SE: VO2peak = 70 ± 2 and 50 ± 1 mL·kgLBM-1·min-1) and study 2 included 9 UT (VO2peak = 51.7 ± 1.9 mL·kgLBM-1·min-1) males. Venous blood samples were obtained at rest (PRE; prior to heat) and at a Tre of 38, 38.5, 39, 39.5, 40°C and/or EXH. PRL was measured by chemiluminescence. RESULTS: In Study 1, the TR tolerated a higher Tre and longer tolerance time (39.7 ± 0.1°C, 165 ± 11 min) than the UT (39.0 ± 0.1°C, 106 ± 10 min). In Study 2, although Tre EXH was not different (p < 0.06) from Day 1 (38.8 ± 0.1°C) to 9 (39.1 ± 0.1°C), time to EXH increased from Days 1 (95 ± 8 min) to 9 (145 ± 13 min). PRL exhibited a strong relationship with increases in Tre, responding to relative thermal strain, in both studies. In Study 1, increases in PRL were directly related to the change in Tre for both the UT and TR, reaching similar values at EXH (55.4 ± 7.2 and 56.7 ± 4.5 ug·L-1, respectively) despite the 0.7°C higher Tre tolerated for TR. Similarly, PRL values at EXH were similar on Days 1 (48.4 ± 7.2 ug·L-1) and 9 (51.1 ± 8.6 ug·L-1) in Study 2. In addition, PRL was moderately correlated to perceptual strain (PeSI) in Study 1 (r = 0.51) and 2 (r = 0.46), which was also similar between groups or days at EXH. CONCLUSION: Findings suggest that as aerobic fitness increases PRL values will reflect the lower PeSI at a given Tre and respond to the relative, rather than the absolute, thermal strain. PRL is an important marker of impending fatigue given its responsiveness to progressive increases in thermal strain. Funded by DRDC Technology Investment Fund and US Office of Naval Research.