Similar Distribution of Gonadotropin Isohormones Is Maintained in the Pituitary Throughout Sexual Maturation in the Heifer

Our working hypotheses for this study were that 1) the profile of intrapituitary LH and FSH isoforms would be shifted toward acidic forms as sexual maturation progresses in the bovine female; and 2) concentration of 17β-estradiol (E2) in circulation during sexual maturation would be a major factor modulating the percentage of the more acidic isoforms. In addition, the biological: immunoreactive (B:l) ratios of each isoform of LH were evaluated at selected stages of sexual maturation. Heifers (7 mo of age) were assigned to one of three treatment groups: 1) ovariectomized (OVX; n = 16); 2) OVX and administered E2 (OVXE; n = 16); or 3) ovary-intact (INTACT; n = 14). Pituitaries were collected from heifers in each group at an estimated 120 days (prepubertal) of 25 days before puberty (peripubertal). A fourth group of 6 heifers remained intact (postpubertal INTACT) to determine time of puberty during the experimental period. Pituitaries of heifers assigned to the postpubertal INTACT group were collected during the follicular phase of the first or second estrous cycle postpuberty. Pituitaries were used for determination of relative amounts of gonadotropin isohormones. Tissue extracts of the pituitaries were chromatofocused on pH 10.5-4.0 gradients. The LH of all pituitaries resolved into thirteen isoforms that were designated isoforms A-L, and S, with isoform A the most basic form. Isoforms F and G (basic pH range) were the predominant isoforms of each chromatofocusing profile and comprised 50- 60% of the immunoreactive LH. Isoforms J and K were the major isoforms eluting in the acidic pH range. Decreases in the acidic isoforms J and K were observed in heifers from the prepubertal and peripubertal OVX groups when compared to those observed in age-matched heifers from the INTACT of OVXE groups. Ovariectomy caused a concomitant increase in the more basic isoforms (B-E) in heifers from the OVX groups when compared to age-matched heifers from the INTACT or OVXE groups. The distributions of UI isoforms in all heifers of the INTACT groups (regardless of stage of sexual maturation) were similar. Isoform F had the greatest B:1 ratio, whereas isoform A (most basic) and isoforms I, J, K, L, and S (most acidic) had the lowest biological potency. The isoforms of FSH were coded with Roman numerals beginning with the most basic form (1-IX). Heifers from the prepubertal and peripubertal OVX groups had lower relative amounts of the most acidic FSH isoform (IX) as compared to age-matched INTACT heifers. Heifers from the OVX group showed an increase in the relative amount of the most basic FSH isoform as compared to age-matched INTACT heifers. In summary, removal of the ovary causes a change in the distribution of isoforms of both LH and FSH, and administration of physiological levels of E2 to OVX heifers helps restore the acidic isoforms of the gonadotropins to levels seen in INTACT heifers. Our hypotheses were not supported in that there was no change in the distribution of LH or FSH isoforms occurred during sexual maturation in intact bovine females