Effects of hormones and osmolarity in the culture medium on germinal vesicle breakdown of porcine oocytes

The present study was conducted to examine effects of hormones and osmolari ty on germinal vesicle breakdown (GVBD) and histone H1 kinase (H1K) activit y in porcine oocytes cultured in vitro. The basic medium used for culture o f oocytes was modified Tyrode's solution in which the osmolarity was adjust ed to 134 to 495 mOsm by changing the concentration of sodium chloride (NaC l). When the hormones were present, osmolarity of medium that allows GVBD o f oocytes was less than 400 mOsm. However, the range of osmolarity of mediu m that allows meiotic maturation of oocytes was 210 to 362 mOsm. On the oth er hand, without hormonal supplement, the incidence of GVBD in oocytes decr eased as the osmolarity of the medium increased in the rage of 210 to 362 m Osm. By increasing the osmolarity of the medium from 210 to 362 mOsm by add ition with sorbitol instead of NaCl, the incidence decreased from 89.1% to 13.3%. In oocytes cultured in medium of 210 mOsm without hormones, the perc entage of oocytes that underwent GVBD and had increased H1K activity 20 h a fter culture was significantly higher (P<0.05) than those of oocytes cultur ed in the same medium supplemented with hormones or medium of 362 mOsm. The se results indicate that in vitro induction of GVBD in porcine oocytes is s trongly affected by osmolarity of the medium in the absence of hormones. Th e results also suggest that, under low osmolarity (210 mOsm), GVBD is accel erated with rapid increase of H1K activity. (C) 1999 by Elsevier Science In c.