Effect of recombinant bovine somatotropin on follicular growth and quality of oocytes in cattle

The effect of exogenous bovine recombinant somatotropin (bST) on the recruitment of follicles and quality of oocytes was investigated in the bovine. Single injections of recombinant depot form of bST were given to nine heifers and seven cows at the time of synchronized estrus (day 0). Nine heifers and six cows served as controls, Three days after bST injection the animals were slaughtered, their ovaries dissected and follicles separated into two categories according to diameter (2-5 and 5-10 mm). A total of 196 (53.2%) healthy oocyte-cumulus complexes (OCC) was isolated and selected from 369 follicles (diameter 2-5 mm) of bST treated heifers and 137 (49.8%) OCC from 275 follicles of bST treated cows. After 25-28 h of culture, 164 (44.4%) and 124 (45.1%) oocytes from follicles of heifers and cows, respectively, had developed to the metaphase of the second meiotic division (MII). From control heifers, 122 (35.5%) OCC derived from 344 follicles (2-5 mm) and from control cows 87 (39.7%) OCC derived from 219 follicles were similarly selected for maturation in vitro and 109 (31.7%) acid 78 (35.6%) oocytes in control heifers and cows, respectively, developed to the MII stage, The numbers of follicles developed in the experimental and control groups were similar (P > 0.05) but differences were evident in the healthy OCC and oocytes maturing in the treated versus control groups of heifers and cows (chi(2) detected at 1% and 5% levels, respectively). The injection of bST resulted in significantly increased levels of growth hormone (GH) in peripheral blood and follicular fluid (FF), the GH levels in blood plasma correlating with the levels in FF. The level of insulin-like growth factor I (IGF-I) in sera from bST treated animals was significantly higher than in sera from control animals (353.6 vs, 155.5 ng ml(-1)). In conclusion, recombinant bST positively affected (at day 3) the proportion of OCC and oocytes maturing to MII.