Elsevier

Theriogenology

Volume 57, Issue 6, 1 April 2002, Pages 1707-1717
Theriogenology

Mechanisms responsible for increase in circulating inhibin levels at the time of ovulation in mares

https://doi.org/10.1016/S0093-691X(02)00683-0Get rights and content

Abstract

In female mammals, inhibin is secreted by the granulosa cells and selectively inhibits secretion of FSH. Although circulating immunoreactive (ir)-inhibin levels decrease after ovulation as a result of the disappearance of its main source, they abruptly increase at the time of ovulation in mares. To investigate the mechanisms responsible for this increase, 50 ml of equine follicular fluid (eFF) was administered into the abdominal cavity of mares during the luteal phase (eFF, n=4). One hour after treatment, plasma levels of ir-inhibin and inhibin pro-αC (but not estradiol-17β) were significantly higher in eFF-treated mares than in control mares (n=4). The hormone profiles in eFF-treated mares were similar to those in mares with the spontaneous or hCG induced ovulations. The present study demonstrates that the release of follicular fluid into the abdominal cavity when the follicle ruptures is responsible for the ovulatory inhibin surge in the mare. These findings also suggest that circulating inhibin pro-αC may be useful for determining the time of ovulation in the mare.

Introduction

Inhibin is produced by granulosa cells during follicular development in mammals, such as rats [1], [2], hamsters [3], guinea pigs [4], cows [5], sheep [6], monkeys [7] and horses [8], [9]. Plasma concentrations of immunoreactive (ir)-inhibin are higher during the follicular phase than the luteal phase of the estrus cycle in mares, and have an inverse relationship with the levels of FSH [8], [10], [11]. Moreover, passive immunization of mares against inhibin induced significant increases in plasma concentrations of FSH and subsequent ovulation rates [12]. Active immunization of mares against inhibin also resulted in a significant increase in ovulation and embryo recovery rate [13], [14], [15]. These results indicate that inhibin is important in the regulation of FSH secretion and folliculogenesis in mares as well as in other mammals.

Recently, increases in circulating levels of ir-inhibin and inhibin pro-αC, which is the precursor of the inhibin α subunit, have been clearly demonstrated at the time of ovulation in mares [16]. These increases seem to be related to the anatomical characteristics of the equine ovary [17] and to the release of a relatively large amount of ovarian follicular fluid from large ovulatory follicles (4–5 cm in diameter) into the abdominal cavity, although there is no direct evidence of this yet.

To determine the mechanisms responsible for the increase in circulating inhibins at the time of ovulation, two experiments were carried out. The purpose of the first experiment was to confirm that an ovulatory surge of inhibin occurs during both induced and spontaneous ovulation. In the second experiment, the absorption of inhibin was investigated after the administration of equine follicular fluid (eFF).

Section snippets

Animals

The study was performed during May–September in 1999 and 2000. Sixteen thoroughbred mares showing regular estrus cycles, aged 9–17 years and weighing 460–520 kg, were used. These mares were clinically healthy and housed individually. They were allowed to graze together each day, and were fed twice daily on a balanced ration of pelletted feed and hay.

Hormone profile during spontaneous ovulation

Four mares were treated with prostaglandin F2alpha (Pronalgon F, Pharmacia-Upjohn Ltd., Tokyo, Japan) during the luteal phase for induction of

Changes in plasma concentrations of hormones during spontaneous and hCG induced ovulation

Each mare had a single ovulation. Changes in plasma levels of ir-inhibin, inhibin pro-αC, FSH, estradiol-17β and progesterone during the spontaneous ovulation are shown in Fig. 1. Significant increases (P<0.05, n=4) in plasma levels of ir-inhibin and inhibin pro-αC were observed at the time when ovulation was detected (0 h), compared with the preovulatory value (−4 h). These hormone levels fell to the basal values within 12 h after ovulation. No increases in plasma concentrations of estradiol-17β

Discussion

The present studies clearly demonstrated that there were increases in plasma levels of ir-inhibin and inhibin pro-αC, but not estradiol-17β following administration of eFF into the abdominal cavity. These results are similar to those obtained in spontaneous and hCG induced ovulation in mares. These results strongly suggest that inhibin in follicular fluid that is released from the ovulatory follicles to the abdominal cavity is rapidly absorbed by the bloodstream, which explains its presence in

Acknowledgements

We thank Dr. S. Sasamoto, Tokyo University of Agriculture and Technology, for valuable suggestions regarding this study; Drs. J.F. Roser and H. Papkoff, Department of Animal Science, University of California, Davis, CA, USA and Dr. G.D. Niswender, Animal Reproduction and Biotechnology Laboratory, Colorado State University, Fort Collins, CO, USA for providing RIA materials; the Meat Inspection Office in Kumamoto, Japan for the supply of equine ovaries.

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