Sex difference in the human peripheral plasma beta-endorphin and beta-lipotropin levels.

Human plasma beta-endorphin (beta-EP) and beta-lipotropin (beta-LPH) were measured simultaneously by radioimmunoassay in 4 women with regular menstrual cycle (4 follicular phase and 4 luteal phase), 5 normal men and 10 term fetuses. Mean plasma beta-EP level in the total samples of 8 women was 30.7 +/- 5.8 pg/ml (+/- SE), significantly (p less than 0.03) higher than the beta-EP level in men (12.2 +/- 2.0 pg/ml). There was no significant sex difference in fetal plasma beta-EP level. These data suggest that the existence of sex differences for beta-EP secretion and release, and opioid peptides secretion and release mechanism might change with sexual maturation, due to changing sex steroids secretion.