Influence Of Slow Versus Fast Increases In Core Temperature On Prolactin And TNF-alpha

Abstract

Prolactin (PRL), a pituitary hormone regulated by neurons in the hypothalamus, has been suggested as an indicator of fatigue during exercise-induced hyperthermia (EIH) given its strong relationship with body core temperature (Tco). However, the strength of this relationship during different rates of Tco increase and subsequent recovery are unknown. In addition, given the influence that systemic inflammatory cytokines, such as Tumor necrosis factor (TNF)-α, have on the pituitary gland; it would be of interest to determine the relationship between PRL and TNF-α during EIH. PURPOSE: To examine the PRL and TNF-α heat stress responses during low (slow heating) and moderate (fast heating) exercise intensities and subsequent resting or cold water immersion recovery. METHODS: Seven trained individuals (5 males, 2 females, mean ± SE: 27.0 ± 3.3 yrs, 57.9 ± 2.2 mL·kg-1nim·-1, 18.5 ± 2.4 % fat) underwent 4 EIH sessions (40°C, 30% R.H.) on a treadmill to a rectal temperature (Tre), measured continuously, of 39.5°C while wearing an impermeable rain suit followed by upright seated resting or 2°C ice-water immersion recovery. Venous blood was obtained at rest (PRE; prior to exercise), during exercise (Tre 38, 39, 39.5°C), the start of recovery (5 min post 39.5°C), and subsequent recovery (Tre 39, 38°C). PRL and TNF-α (high sensitivity) were measured by an ELISA and corrected for changes in plasma volume. RESULTS: PRL exhibited a strong relationship with Tre during exercise (r >0.75), where no differences were observed between the slow and fast trials at a given Tre. During recovery, PRL remained elevated at 39°C (46 ng·mL-1) and 38°C (37 ng·mL-1) during the cooling compared to resting (28 and 16 ng·mL-1, respectively). TNF-α was elevated throughout cooling compared to resting recovery (0.76-0.82 vs. 0.58-0.69 pg·mL-1), the fast but not the slow EIH. CONCLUSION: PRL remained elevated during the cooling compared to resting recovery, likely due to the shorter recovery and reduced clearance from a decrease in blood flow via cutaneous vasoconstriction. It appears that the influence of TNF-α on the pituitary gland does not interfere with the Tre, or stronger, stimulus for the secretion of PRL. Support: Natural Sciences and Research Council of Canada and Canada Foundation for Innovation (held by G.P. Kenny).