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Tissue-Type Plasminogen Activator in Rat Oocytes: Expression during the Periovulatory Period, after Fertilization, and during Follicular Atresia^*

THOMAS A. BICSAK, STEFAN B. CAJANDER, XIAO-RONG PENG, TOR NY, PHILIP S. LAPOLT, JOHN K. H. LU, PETER KRISTENSEN, ALEX TSAFRIRI and AARON J. W. HSUEH

Department of Reproductive Medicine, University of California-San Diego (T.A.B., S.B.C., X.-R.P., T.N., A. T., A.J. W.H.) La Jolla, California 92093
Departments of Obstetrics and Gynecology and Anatomy, University of California School of Medicine (P.S.L., J.K.H.L.) Los Angeles, California 90024-1740

Address all correspondence and requests for reprints to: Dr. Aaron J. W. Hsueh, Department of Reproductive Medicine, University of California-San Diego, M-025, La Jolla, California 92093.

Abstract

The regulation of tissue-type plasminogen activator (tPA) in rat oocytes during the periovulatory period, in early embryos, and in oocytes during induced follicular atresia was studied using a quantitative chromogenic substrate assay. Oocytes and early embryos were collected from three ovulation models: 1) intact immature female rats treated with PMSG, followed by hCG 48 h later; 2) hypophysectomized immature rats treated with PMSG, followed by a GnRH agonist (GnRHa) 56 h later; and 3) adult cyclic rats on the mornings of proestrus and estrus and up to 5 days after fertilization. In addition, follicular atresia was induced by either withdrawal of diethylstilbestrol (DES) for 2 days or injection of GnRHa for 2 days in hypophysectomized DES-implanted immature rats. Treatment with PMSG alone did not increase oocyte tPA content (5–20 µIU/oocyte) in either immature rat model, but treatment with either hCG or GnRHa induced meiotic maturation and ovulation and increased tPA activity to 80 and 140 µIU/oocyte 24 h after hCG and GnRHa treatment, respectively. Northern blot analysis of total RNA extracted from oocytes of PMSG-treated rats indicated the presence of a specific tPA message at 22S. tPA levels were low in preovulatory oocytes obtained on proestrus morning and increased in ovulated oocytes on estrus morning. After fertilization, tPA levels remained high in the embryos on days 1–4 of pregnancy, but dropped dramatically on day 5. Furthermore, oocytes from atretic follicles of hypophysectomized DES-implanted rats after either DES withdrawal or GnRHa treatment contained elevated levels of tPA, coincident with germinal vesicle breakdown (GVBD). Immunohistochemical staining revealed tPA antigen only in those oocytes that had undergone apparent meiotic maturation, as confirmed by GVBD. Thus, oocytes contain tPA mRNA and synthesize the active protease under a variety of stimuli which result in GVBD. The observed periovulatory increase in oocyte tPA activity, its maintenance until day 5 of pregnancy, and expression of tPA in nonovulatory oocytes of atretic follicles suggest diverse functions for the oocyte and embryo tPA. (Endocrinology 124: 187–194, 1989)

Footnotes

^* This work was supported by NIH Grants HD-14084 and AG-04810 and a grant from the Andrew W. Mellon Foundation.

Recipient of NIH Postdoctoral Fellowship HD-06939.

Permanent address: Department of Pathology, Umea University S-90187, Umea, Sweden. Supported by grants from the Swedish Medical Research Council, the Magnus Bergvall Foundation, and the Medical Faculty of the University of Umea. Recipient of Fritz O. Fernstrom Fellowship 35495.

Received July 22, 1988.

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