UC Irvine

The effect of 10 min of high-intensity cycling exercise on circulating growth hormone (GH), insulin-like growth factors I and II (IGF-I and -II), and insulin-like growth factor binding protein 3 (IGF BP-3) was studied in nine eumenorrheic women (age 19-48 yr) at two different phases of the menstrual cycle. Tests were performed on separate mornings corresponding to the follicular phase and to the periovulatory phase of the menstrual cycle, during which plasma levels of endogenous estradiol (E2) were relatively low (272 +/- 59 pmol/l) and high (1,112 +/- 407 pmol/l), respectively. GH increased significantly in response to exercise under both E2 conditions. Plasma GH before exercise (2.73 +/- 2.48 vs. 1.71 +/- 2.09 micrograms/l) and total GH over 10 min of exercise and 1-h recovery (324 +/- 199 vs. 197 +/- 163 ng) were both significantly greater for periovulatory phase than for follicular phase studies. IGF-I, but not IGF-II, increased acutely after exercise. IGF BP-3, assayed by radioimmunoassay, was not significantly different at preexercise, and exercise, or at 30-min recovery time points and was not different between the two study days. When assayed by Western blot, however, there was a significant increase in IGF BP-3 30 min after exercise for the periovulatory study. These findings indicate that the modulation of GH secretion associated with menstrual cycle variations in circulating E2 affects GH measured after exercise, at least in part, by an increase in baseline levels. The acute increase in IGF-I induced by exercise appears to be independent of the GH response and is not affected by menstrual cycle timing.