FSH-induced resumption of meiosis in mouse oocytes: effect of different isoforms

The ability of different isoforms of follicle stimulating hormone (FSH) to induce the resumption of meiosis in cultured mouse oocytes was evaluated. O ocytes were cultured in the presence of hypoxanthine to prevent spontaneous resumption of meiosis. Using serial dilutions of the isoform fractions rep resenting less acidic isoforms (pl 6.43-5.69), mid-acidic (pl 5.62-4.96) an d acidic (pl 4.69-3.75), the concentration which caused 50% of the oocytes to resume meiosis and undergo germinal vesicle breakdown (GVBD) after a cul ture period of 24 h (i.e. ED50% GVBD) was determined. The FSH concentration of the isoform fractions was determined by radioimmunoassay, radio-recepto r assay or through cAMP release in a Chinese hamster ovary-cell line expres sing the human FSH-receptor. Determined by radioimmunoassay, the (ED50% GVB D) values were: less acidic 6.4 +/- 0.3 IU/I (mean +/- SD), mid-acidic 6.1 +/- 0.7 IU/I and acidic 12.2 +/- 0.7 IU/I. The less and mid-acidic isoforms were significant lower than the acidic (P < 0.0005). Similar relationships between the isoform fractions were obtained by the two other FSH assays. T he results demonstrate that FSH isoforms with a pl of >5.0 induced resumpti on of meiosis significantly more efficiently than acidic isoforms, Less and mid-acidic isoforms may exert an important physiological function by induc ing the resumption of meiosis in oocytes from pre-ovulatory follicles durin g the mid-cycle gonadotrophin surge.