Plasma β-Endorphin, the Menstrual Cycle and Exercise

Abstract

<p>The underlying mechanism(s) of exercise-associated amenorrhea are not well understood. The discovery of β-endorphin, an endogenous opioid peptide that was shown to inhibit LH release during the menstrual cycle and become elevated during intense exercise, led to the postulate that this peptide may play a role in menstrual dysfunction observed in athletes.</p> <p>The hypothesis for this thesis was that plasma β-endorphin concentrations are increased in vigorously exercising women with amenorrhea. At the time this hypothesis was generated, current radioimmunoassays to measure plasma β-endorphin were unreliable and insensitive. Therefore, the first objective was to develop a sensitive, specific and precise radioimmunoassay.</p> <p>Two studies were then conducted to test this hypothesis. 1. Resting plasma β-endorphin concentrations were measured in non-exercising (n=10) and exercising (n=11) women during a regular menstrual cycle and in a group of exercising women with amenorrhea (n=11). Blood samples were obtained 3 times/week for one complete menstrual cycle or for 4 weeks in the amenorrheic subjects. Measurements were made of plasma β-endorphin, estradiol, progesterone, LH and FSH. 2. Plasma β-endorphin and LH concentrations were measured every 20 minutes for 8 hours in these 3 groups (n=3/group). Studies were conducted during the mid-follicular and mid-luteal phase of the cycle in ovulatory subjects.</p> <p>Results from this investigation indicate that 1. Significant increments of β-endorphin occur during the early follicular, early luteal and late luteal phase of the cycle in both non-exercising and exercising subjects, however the latter group demonstrate β-endorphin increments of greater magnitude during times. Gonadal steroid modulation may be responsible for the early luteal phase rise of β-endorphin, but as yet the increases seen at the other times remain to be explained. 2. Plasma β-endorphin concentrations in amenorrheic athletes were similar to those observed in normal menstruating athletes, but no pattern was evident.</p> <p>3. β-endorphin is released into the circulation in a pulsatile fashion. Pulse frequency appears to be reduced in the luteal phase of both non-exercising and exercising subjects, while amenorrheic subjects exhibit pulse frequency similar to that found in the follicular phase. Pulse amplitude did not vary between phases of sedentary or athletic subjects, but pulse amplitude was greater in the athletes. Amenorrheic athletes had greater pulse amplitudes when compared to the other 2 groups.</p> <p>The significance of pulsatile secretion of β-endorphin is yet to be determined but the greater amplitude observed in amenorrheic athletes may suggest that LH secretion is being altered in this group. In conclusion, the hypothesis was confirmed that amenorrheic athletes have higher concentrations of β-endorphin and that these may play a role in exercise-associated amenorrhea.</p>