Estrogen replacement in middle-aged women: thermoregulatory responses to exercise in the heat.

Thermoregulatory, cardiovascular, and body fluid responses during exercise in the heat were tested in five middle-aged (48 +/- 2 yr) women before and after 14-23 days of estrogen replacement therapy (ERT). The heat and exercise challenge consisted of a 40-min rest period followed by semirecumbent cycle exercise (approximately 40% maximal O2 uptake) for 60 min. At rest, the ambient temperature was elevated from a thermoneutral (dry bulb temperature 25 degrees C; wet bulb temperature 17.5 degrees C) to a warm humid (dry bulb temperature 36 degrees C; wet bulb temperature 27.5 degrees C) environment. Esophageal (Tes) and rectal (Tre) temperatures were measured to estimate body core temperature while arm blood flow and sweating rate were measured to assess the heat loss response. Mean arterial pressure and heart rate were measured to evaluate the cardiovascular response. Blood samples were analyzed for hematocrit (Hct), hemoglobin ([Hb]), plasma 17 beta-estradiol (E2), progesterone (P4), protein, and electrolyte concentrations. Plasma [E2] was significantly (P < 0.05) elevated by ERT without affecting the plasma [P4] levels. After ERT, Tes and Tre were significantly (P < 0.05) depressed by approximately 0.5 degrees C, and the Tes threshold for the onset of arm blood flow and sweating rate was significantly (P < 0.05) lower during exercise. After ERT, heart rate during exercise was significantly lower (P < 0.05) without notable variation in mean arterial pressure. Isotonic hemodilution occurred with ERT evident by significant (P < 0.05) reductions in Hct and [Hb], whereas plasma tonicity remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)