Plasma sex steroid-binding protein in mature heifers: effects of reproductive status, nutritional level, and porcine growth hormone, and estradiol-17 beta treatments

In heifers, the specific 5 alpha-dihydrotestosterone (5 alpha-DHT) binding to sex steroid-binding protein (SBP) was 53 nM in midluteal phase and was 20% lower (p less than 0.05) in early luteal phase. In ovariectomized heifers on a high (H) or a low (L) energy diet, SBP bound testosterone with high specificity and estradiol-17 beta (E2) with a lower specificity; the affinity constant with 5 alpha-DHT was similar in the two groups independent of diet (Ka = 1.0 and 1.3 x 10(9) x M-1 for H and L heifers, respectively). Electrophoretic mobility of SBP was not affected by undernutrition. The specific 5 alpha-DHT binding to SBP in anestrous heifers compared to heifers in early luteal phase was 17% lower 18 days after cessation of ovarian activity (p less than 0.05) and 35% lower after 78 days of anestrus (p less than 0.001). Specific 5 alpha-DHT binding to SBP was 33% lower (p less than 0.001) in L heifers than in H heifers (40 nM). Porcine growth hormone (pGH) and E2 injections increased specific 5 alpha-DHT binding to SBP in both H and L heifers, but the effect depended on diet. Simultaneous injections of pGH and E2 increased specific binding in H heifers (p less than 0.02), whereas it had no effect in L heifers. We conclude that SBP binding capacity varies with ovarian activity in heifers and that nutritional status is one factor of regulation of SBP binding capacity because it affects binding and modifies the SBP response to stimulating factors such as pGH and E2.