THE IMPACT OF HEAT EXPOSURE AND REPEATED EXERCISE ON CIRCULATING STRESS HORMONES

To determine if heat exposure alters the hormonal responses to moderat e, repeated exercise, 11 healthy male subjects [age = 27.1 (3.0) years ; maximal oxygen consumption, (V) over dot O-2max = 47.6 (6.2) ml.kg.m in(-1); mean (SD)] were assigned to four different experimental condit ions according to a randomized-block design. While in a thermoneutral (23 degrees C) or heated (40 degrees C, 30% relative humidity) climati c chamber, subjects performed either cycle ergometer exercise (two 30- min bouts at approximate to 50% (V) over dot O-2max, Separated by a 45 -min recovery interval, CEx and HEx conditions), or remained seated fo r 3 h (CS and HS conditions). Blood samples were analyzed for various exercise stress hormones [epinephrine (E), norepinephrine (NE), dopami ne, cortisol and human growth hormone (hGH)]. Passive heating did not alter the concentrations of any of these hormones significantly. Durin g both environmental conditions, exercise induced significant (P < 0.0 01) elevations in plasma E, NE and hGH levels. At 23 degrees C during bout 1: E = 393 (199) pmol.1(-1) (CEx) vs 174 (85) pmol.1(-1) (CS), NE = 4593 (CEx) vs 1548 (505) pmol.1(-1) (CS), and hGH = 274(340) pmol.1( -1) (CEx)vs 64(112)pmol.1(-1) (CS). At 40 degrees C, bout l:E = 596 (3 46) pmol.1(-1) (HEx) vs 323 (181)pmol.1(-1) (HS), NE = 7789 (5129) pmo l.1(-1) (HEx) vs 1527 (605) pmol.1(-1) (HS), and hGH = 453 (494) pmol. 1(-1) (HEx) vs 172 (355) pmol.1(-1) (HS). However, concentrations of p lasma cortisol were increased only in response to exercise in the heat [HEx = 364 (168) nmol.1(-1) vs HS = 295 (114) nmol.1(-1)). Compared t o exercise at room temperature, plasma levels of E, NE and cortisol we re all higher during exercise in the heat (P < 0.001 in all cases). Th e repetition of exercise did not significantly alter the pattern of ch ange in cortisol or hGH levels in either environmental condition. Howe ver, repetition of exercise in the heat increased circulatory and psyc hological stress, with significantly (P < 0.001) higher plasma concent rations of E and NE. These results indicate a differential response of the various stress hormones to hear exposure and repeated moderate ex ercise.