Inhibitory action of somatostatin on meiotic maturation of cultured porcine follicular ova

Abstract

To investigate the physiological importance of somatotrophin release-inhibiting factor (SRIF), effects in vitro of synthetic SRIF 14 on germinal vesicle breakdown (GVB) of cultured porcine follicular ova were studied. The proportion of ova with GVB decreased gradually and significantly with increasing concentrations of SRIF 14 in the range from 6 \m=x\ 10\m=-\12 to 6 \m=x\ 10\m=-\7 m during a 22 h period of culture. The inhibitory effect was apparent for the period between 14 and 22 h in the course of culture but was reversed by a concomitant addition of anti-SRIF to the medium. Neither synthetic oxytocin, vasoactive intestinal polypeptide nor substance P exerted any inhibitory or stimulatory action on GVB. These results suggest a limited but definite inhibitory action of SRIF on GVB of porcine ova.Although the presence of somatotrophin releaseinhibiting factor (SRIF) in porcine ovaries has been demonstrated in our previous work (Mori et al. 1983), the physiological significance of this peptide is totally unknown at present. Oocyte meiosis inhi¬ bitor (OMI) present in porcine follicular fluid (Tsafriri et al. 1976) is very close to SRIF in molecular weight. However, these two bioactive peptides are different from each other, because SRIF was shown to be degraded readily by proteolytic enzymes in follicular fluid in our previous study, while OMI remains active. Since somatosta¬ tin interacts with calcium in insulin secretion (Taminato et al. 1975), and calcium may be in¬ volved in oocyte maturation (Wasserman & Masui 1976), we have attempted in the present work to examine the effects, if any, of somatostatin on meiosis of follicular ova of the same species. Materials and MethodsCollection and culture of cumulus-coated porcine ova Porcine ovaries from prepubertal gilts weighing approx¬ imately 100 kg were obtained at a local abbatoir and were transported to the laboratory in physiologic saline kept at 35°C. Follicular ova with cumulus cells were aspirated by needle puncture from medium-sized follicles of 2-5 mm in diameter, thoroughly rinsed with sterile physiological saline and were subjected to culture within 2 h of sacri¬ ficing the animals. Only ova with cumulus cells were used and the whole procedure was performed at 37°C.Ova were cultured after the method of Sato 8c Ishibashi (1977) for different periods at 37 ± 0.5°C in an incubator continuously flushed with 5% C02 and 95% air saturated with water, using modified Krebs-Ringer bicar¬ bonate solution, pH 7.2-7.4, as culture medium which was supplemented with 0.25 mM D-glucose, 25.0 mM sodium lactate, 1 mg/ml crystalline bovine serum albu¬ min fraction V (Armour, New York), 100 I U/ml penicil¬ lin and 50 µg/ml streptomycin. A set of 10 ova was pipetted into 0.4 ml culture medium which was placed in plastic Petri dish (4.5 cm in diameter), covered with light paraffin oil, which had been equilibrated with the gas phase in the incubator for 2 h prior to use.

Immunoreactive somatostatin in the rat ovary

“…Our observation of dose-dependent stimulation of meiotic maturation by SRIH contrasts with the inhibi¬ tion reported in the porcine ovary by Mori et al (4).…”

“…female, somatostatin-like immunoreactivity has been detected in porcine ovary (3), where it has been shown to inhibit germinal vesicle breakdown (4). In the one reported study in the rat (5), no effect of somatostatin on germinal vesicle breakdown was demonstrated, but only a small number of oocytes were studied.…”

Mammalian oocyte maturation

“…As more and more pro-hormones, hormones (and their mRNAs, Einspanier et al, 1986) are discovered in the ovary some may be candidates as recently mentioned for the SRIF (pig, Mori et al, 1985) and for the anti-mullerian hormone (AMH) (rat, Takahashi et al, 1986 Lintern-Moore, 1978 ;Kaplan et al, 1982 ;Bachvarova et al, 1985). After the growth period a low rate of RNA synthesis is maintained, even if nucleolus compaction indicates that rRNA transcription is impaired.…”