Endocrine influence on the distribution and biological activity of gonadotropin isohormones in the bovine female

The pituitary gonadotropins exist as isohormones which vary according to apparent pI and associated structural differences. Synthesis and secretion of the gonadotropins is dependent on the stimulation of the pituitary gonadotropes by Luteinizing Hormone Releasing Hormone (LHRH) from the hypothalamus. Gonadal steroids are known to modulate the distribution isohormones for the gonadotropins. The gonadal steroids also modulate the secretory pattern of LHRH and the gonadotropins. The hypothesis tested in the first experiment was that in the absence of the ovarian steroids, a decreased hypothalamic stimulation of the pituitary would alter the distribution of isoforms for bovine LH in the pituitary. Increased amounts of LH isoform F were observed in ovariectomized cows actively immunized against LHRH as compared to ovariectomized control cows. However, pituitary concentrations of LH were similar among cows in these two groups. There are subtle changes in the distribution of isoforms for bovine LH, in the absence of ovarian steroids, with decreased hypothalamic stimulation of the pituitary. The hypothesis tested in the second experiment was that 17$\beta$-estradiol (E$\sb2$) would cause an increased amount of acidic isoforms for gonadotropins as sexual maturation progressed in heifers. Removal of the ovaries before puberty caused an increased percentage of the more basic isoforms of LH and most basic isoform of Follicle-Stimulating Hormone (FSH) compared with intact heifers. Also, ovariectomy caused a decrease in the acidic isoforms of LH and the most acidic isoform of FSH compared with intact heifers. Administration of physiologic concentrations of E$\sb2$ to ovariectomized heifers restored the percentage of acidic isoforms for LH, but not FSH to levels similar to age-matched intact heifers. The most basic and all acidic isoforms of LH displayed the lowest bioactivity. Isoform F (mid-alkaline pH) had the greatest bioactivity and was the abundant biologically active isoform of LH in the pituitary of heifers. The distribution of LH and FSH isohormones was not altered as sexual maturation progressed in ovary intact females. Thus E$\sb2$ did not increase the relative amounts of acidic gonadotropins during sexual maturation in heifers