Effect of Ovarian Follicles on Luteal Regression in Heifers

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Abstract
Our objectives were to determine whether or not ovarian follicles contribute to spontaneous luteal regression in heifers and, if so, when during diestrus do follicles exert their effect. Thirty-one Holstein heifers having displayed at least one estrous cycle (19 to 21 d) were assigned, as available, to randomized blocks for a factorial experiment. Reproductive organs were exposed through a midventral incision on d 9, 12 or 15 postestrus (estrus = d 0). Visible follicles were electrocauterized and both ovaries were x-irradiated (1,500 rads) in treated heifers, whereas ovaries of controls were exteriorized but follicles were not destroyed and ovaries were not x-irradiated. In two additional heifers, the ovary containing the corpus luteum was exteriorized and x-irradiated on d 15 postestrus, but follicles were not electrocauterized. Jugular blood was collected before and every 8 h after surgery until d 24 postestrus. All heifers were ovariectomized on d 24 postestrus to inventory follicles and to weigh corpora lutea. No follicles (> 1 mm diameter) were observed in ovaries from treated animals and concentrations of estradiol-17β did not change over time, whereas different numbers of follicles were observed in ovaries from controls and concentrations of estradiol-17β increased (P<.05) during proestrus. Hence, treatment destroyed follicles and prevented follicular development. On d 24 postestrus, corpora lutea from treated heifers (5.5 ± .5 g) were heavier (P<.001) than corpora lutea from controls (1.1 ± .1 g), independent of day when follicles were destroyed. From surgery to d 24 postestrus concentrations of progesterone in serum were sustained in treated heifers but declined (P<.01) in controls on d 16 to 17 postestrus. In treated heifers, concentrations of luteinizing hormone (LH) increased (P<.01) after surgery, confirming that follicular products exert a negative influence on secretion of LH in presence of functional corpora lutea. In control animals, mean concentrations of LH did not change over time, which indicates that reduced secretion of LH is not required for luteolysis. In animals in which the ovary bearing a corpus luteum was selectively x-irradiated, luteal weights (1.3 ± .3 g) and profiles of progesterone did not differ from the controls. Thus, x-rays did not have a direct effect on the corpus luteum. Luteal function was extended after destruction of ovarian follicles and inhibition of follicular growth on d 9, 12 or 15 postestrus, demonstrating that secretions of ovarian follicles are required for spontaneous luteal regression. Furthermore, it was determined that the luteolytic effect of follicles is exerted within a short interval after d 15 postestrus. Copyright © 1985. American Society of Animal Science . Copyright 1985 by American Society of Animal Science

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