# Are regular activity episodes of the genital tract controlled by pulsatile releases of oxytocin?

Rafael Garcia Villar, P.L. Toutain, D. Schams, Y. Ruckebusch

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Rafael Garcia Villar, P.L. Toutain, D. Schams, Y. Ruckebusch. Are regular activity episodes of the genital tract controlled by pulsatile releases of oxytocin?. Biology of Reproduction, 1983, 29, pp.1183-1188. hal-02726730

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<figure><img src="image_1.png" /></figure>
# Are Regular Activity Episodes of the Genital Tract Controlled by Pulsatile Releases of Oxytocin?

R. GARCIA-VILLAR,¹ P. L. TOUTAIN, D. SCHAMS² and Y. RUCKEBUSCH

Institut National de la Recherche Agronomique
Station de Pharmacologie-Toxicologie
31300 Toulouse, France
and

Department of Physiology of Reproduction and Lactation²
Technische Universität München
3050 Freising-Weidenstephan, West Germany

## ABSTRACT

The possible control by endogenous oxytocin of the regular activity episodes which occur in the ovine genital tract was assessed in 3 cyclic ewes chronically fitted with intraparietal electrodes. Despite the existence of a pulsatile release of the hormone into plasma, no significant relationships were noticed between the motility events of genital tract and plasma concentrations of oxytocin.

## INTRODUCTION

The existence of episodes of electrical and/or mechanical activity lasting 5-10 min and recurring at about hourly intervals in the uterus of the pregnant ewe has been reported by many authors. Different terminologies have been used in the literature to describe these events: "bursts" (Nathanielz et al., 1976; Harding et al., 1982), "trains of action potentials" (Krishnamurti et al., 1979), and "periods of activity" (Bontekoë et al., 1977). Such periodic activities have been described for the oviduct and uterus of the cyclic ewe during the periovulatory period and termed "phases of activity" or "phases of contractions" (Ruckebusch and Buéno, 1976). In a study of uterus and cervix motility, Toutain et al. (1983) suggested that the regular occurrence of active periods in the uterus and cervix, which were termed "regular activity," arose from an underlying ultradian rhythm involving the entire genital tract. This basic rhythm is present in the cervix regardless of the hormonal status (i.e., pregnancy, the entire estrous cycle, seasonal anestrus and after ovariectomy); in contrast, its presence in the uterine horns and oviducts necessitate a permissive action of

estrogens.

In the pregnant ewe, the nearly synchronous appearance of such an activity in different levels of the uterus and in an isolated uterine pouch has been tentatively explained in terms of humoral control or diffuse stimulation by a nerve network (Harding et al., 1982). Among the hormonal compounds which would induce a general activation of the genital tract, oxytocin must deserve attention; the enhancement by exogenous oxytocin of uterine and oviduct motility in either cyclic or pregnant ewes is well documented (Fitzpatrick, 1956; Noonan et al., 1978; Hindson et al., 1969; Glatz et al., 1980). In addition, the cervix remains responsive to exogenous oxytocin, whatever the hormonal status (unpublished observations). A cycle-dependent release pattern of oxytocin has been measured in cycling ewes (Schams et al., 1981; Webb et al., 1981; Sheldrick and Flint, 1981) with an increase during the luteal phase and a decrease at the end of the cycle. After ovariectomy (Schams et al., 1982) only low basal concentrations, similiar to those during pregnancy (Glatz et al., 1980), have been observed.

Furthermore, it has been shown that the nature of oxytocin release in sheep is pulsatile during the estrous cycle, as well as during pregnancy (Mitchell et al., 1982). Therefore, the present study was undertaken to assess the possibility of a correlation between the episodic pattern of oxytocin release and the motility pattern of the uterus and cervix.

<footnote>Accepted August 5, 1983.
Received April 21, 1983.
¹Reprint requests: R. Garcia-Villar, I.N.R.A., Station de Pharmacologie-Toxicologie, 180 chemin de Tournefeuille, 31300 Toulouse, France.</footnote>
# MATERIALS AND METHODS

## *Animals and Surgery*

Three cyclic ewes of mixed breeds weighing from 47 to 64 kg, were used. They were surgically fitted with intraparietal wire electrodes on the uterus and cervix according to a technique described in detail elsewhere (Garcia-Villar et al., 1982). They were then kept in individual cages until the recording session started 3 weeks later.

## *Recording Session and Blood Sampling*

Each ewe was recorded continuously throughout 3 complete estrous cycles. Three periods were distinguished: 1) the periovulatory period, including estrus, recognized on the basis of myoelectrical events (Day 0); 2) the early luteal phase (from Day 5 to Day 7); and 3) the late luteal phase (from Day 11 to Day 15). During each of these 3 periods, blood samples were collected for 2 to 3 consecutive hours every 10 or 15 min, and at the 1st and 4th min after the onset of a regular activity episode (in the uterus and cervix during the periovulatory period or only the cervix during the luteal phase). Blood was collected in chilled heparinized tubes (10 ml) and centrifuged at 4500 rpm within 2-3 min after sampling at the jugular vein by either direct venipuncture or via an indwelling catheter placed at least 2 h before the experiment.

Plasma samples were immediately stored at -20°C until assayed for oxytocin.

## *Hormone Assay*

Oxytocin was determined radioimmunologically according to a procedure described elsewhere (Schams et al., 1979). The method was improved (Schams, 1983) by using an extraction method with SEP-PAK C₁₈ cartridges, which also allows concentration of the sample, an antiserum with a higher sensitivity to standard oxytocin and preparation of the standard curve in buffer. The antiserum used was raised in a rabbit against synthetic oxytocin coupled to bovine thyroglobulin and showed no cross-reaction with related peptides such as vasopressin or anterior pituitary hormones. The lower limit of assay sensitivity was 0.25 pg/tube, corresponding to 0.25-1.0 pg/ml plasma, depending on the amount of plasma extracted. Mean recovery of oxytocin added to plasma, or only buffer, after extraction was 71.3 ± 8.1%. For each assay, correction for recovery was made. The intraassay coefficient of variation (CV) of two control samples taken at the beginning, middle and end of the assay was 7.9 ± 2.6% (n=18 assays). The interassay CV of 5 control samples with low and high oxytocin concentrations varied between 10.8 - 17.3%. The 50% intercept was 2.5 ± 0.3 pg. The reference preparation (synthetic oxytocin kindly donated by Sandoz,

<figure><img src="image_2.png" /><figcaption>(a)</figcaption></figure>
Switzerland, 200 IU/ml) had the same potency as the biological and immunological well-defined 4th International Standard from the National Institute for Biological Standards and Control, London.

### Statistical Analyses

Statistical analyses were performed using a paired *t* test.

## RESULTS

The electromyograms recorded for each ewe were similar to those previously described for

each period of the estrous cycle (Garcia-Villar et al., 1982), ie., the presence of the so-called regular activity episodes in the cervix, regardless of the stage of the cycle, but only during the periovulatory stage in the uterus (see Fig. 1).

It appeared that oxytocin concentrations fluctuated rapidly within the different periods, however, no relationships were evident between oxytocin plasma levels and the activity of the genital tract (Fig. 1). Individual oxytocin concentrations are reported in Table 1. Within each stage of the estrous cycle, and for each

<figure><img src="image_3.png" /></figure>

FIG. 1. Plasma oxytocin concentration and concomitant integrated electromyograms of the uterus (*Ut*) and the cervix (*Cx*) during a typical periovulatory period (a) and early luteal phase (b) in one representative ewe. Note the absence of a consistent relationship between the two measured parameters. \* = Episodes of regular spiking activity.
<table><caption>TABLE 1. Plasma oxytocin concentrations (pg/ml; mean ± SD) in the 3 ewes (A, B, C) within 2-3 consecutive estrous cycles (1, 2, 3) during (I) and between (II) regular activity episodes.</caption><thead><tr><th rowspan="2">Animals</th><th rowspan="2">Cycles</th><th colspan="2">Estrus (Day 0)</th><th colspan="2">Early luteal (Days 5-7)</th><th colspan="2">Late luteal (Days 12-14)</th></tr><tr><th>I</th><th>II</th><th>I</th><th>II</th><th>I</th><th>II</th></tr></thead><tbody><tr><td rowspan="3">A</td><td>1</td><td>6.33 ± 1.12<br>(n=7)<sup>a</sup></td><td>6.76 ± 1.53<br>(n=21)</td><td>36.52 ± 6.59<br>(n=5)</td><td>26.69 ± 6.92<br>(n=17)</td><td>ND</td><td>10.71 ± 1.69<br>(n=14)</td></tr><tr><td>2</td><td>5.18 ± 1.12<br>(n=5)</td><td>5.48 ± 1.78<br>(n=9)</td><td>ND<sup>b</sup></td><td>22.15 ± 4.63<br>(n=6)</td><td>17.27 ± 5.52<br>(n=3)</td><td>19.74 ± 8.61<br>(n=7)</td></tr><tr><td>1</td><td>---</td><td>---</td><td>---</td><td>---</td><td>38.20 ± 15.68<br>(n=7)</td><td>33.59 ± 7.73<br>(n=21)</td></tr><tr><td rowspan="3">B</td><td>2</td><td>10.93 ± 1.76<br>(n=7)</td><td>10.77 ± 2.05<br>(n=17)</td><td>ND</td><td>70.35 ± 16.36<br>(n=15)</td><td>48.03 ± 1.12<br>(n=3)</td><td>52.08 ± 6.83<br>(n=5)</td></tr><tr><td>3</td><td>8.01 ± 2.38<br>(n=10)</td><td>9.60 ± 2.56<br>(n=27)</td><td>60.6 ± 27.28<br>(n=4)</td><td>55.91 ± 11.20<br>(n=10)</td><td>---</td><td>---</td></tr><tr><td>1</td><td>---</td><td>---</td><td>7.4<br>(n=1)</td><td>6.83 ± 1.30<br>(n=23)</td><td>ND</td><td>4.80 ± 2.32<br>(n=14)</td></tr><tr><td rowspan="2">C</td><td>2</td><td>3.71 ± 0.58<br>(n=8)</td><td>4.07 ± 0.7<br>(n=25)</td><td>50.40 ± 1.56<br>(n=2)</td><td>44.63 ± 4.86<br>(n=12)</td><td>6.85 ± 1.34<br>(n=2)</td><td>5.35 ± 1.04<br>(n=11)</td></tr></tbody></table>

<sup>a</sup>n=number of samples.

<sup>b</sup>ND=no data.
animal, no significant differences were noticed according to the pattern of genital tract activity (P>0.05). In addition, when the concentrations measured during an episode of regular activity were compared to those obtained just before or just after the regular activity, no significant differences were noticed for all the ewes and all the stages of the estrous cycle (P>0.05).

DISCUSSION

The present results agree with previously published data on short-term and long-term fluctuations of oxytocin plasma concentrations in the ewe: 1) within a period of sampling, oxytocin levels fluctuate rapidly as described by Mitchell et al. (1982), and 2) the levels were lower during the periovulatory period than during the midluteal phase as previously reported by Schams et al. (1982). Despite this fluctuation, no consistent relationships were noticed with the motility events of the genital tract, indicating that uterine or cervical activities are probably not controlled by a systemic pulsatile release of oxytocin.

Nevertheless, no definitive conclusion can be drawn before considering the possible action of oxytocin released locally from the ovary. According to Flint and Sheldrick (1982a), significant venoarterial concentration differences indicate secretion of oxytocin by the ovary bearing a corpus luteum: ovarian venous concentrations exceeded arterial concentrations by a factor of 2.8 during the late luteal phase. In addition, Flint and Sheldrick (1982b) showed that this release was episodic. Therefore, whether or not these pulsatile local releases of oxytocin are able to induce genital tract activity during the luteal phase remains unknown. However, the facts that the cervix continues to display regular activities after ovariectomy and that the entire genital tract displays regular activities during the periovulatory period (i.e., when a functional corpus luteum is absent) argue against the hypothesis of a control of the cyclic activity of the genital tract by local pulsatile releases of oxytocin.

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