CIRCULATING LEVELS OF INHIBIN AND FSH DURING THE ESTROUS-CYCLE IN 4 GENOTYPES OF SHEEP WITH DIFFERENT REPRODUCTIVE PERFORMANCES

Two relatively fecund genotypes (Flemish Milksheep and KU-Lovenaar-an F-2 cross-bred originating from pure-bred Suffolk and Flemish Milkshee p) and two meat-type breeds (Texel and Suffolk) were compared regardin g their plasma inhibin and FSH levels. Oestrus was synchronised by pro gestagen-impregnated sponges. Blood sampling, starting 12 h after the sponge removal, continued twice daily for 21 consecutive days. To loca lise the pre-ovulatory LH peak and to determine the cycle length, samp ling at 2 h intervals was carried out during the first 3 days and the last 4 days of the 21-day period. Immunoreactive inhibin was measured by a 5-24 alpha-subunit inhibin assay as well as by an assay based on an antiserum against native bovine 31-kDa inhibin (Monash assay). Chan ges in plasma concentration of immunoreactive FSH and LH during the oe strous cycle agree with the literature data. The profiles generated by the two different inhibin RIAs were different and showed a negative c orrelation during the periovulatory phase and on days 9-11. No charact eristic pattern could be observed during the oestrous cycle. Significa nt breed differences were observed for 5-24 alpha-subunit inhibin as w ell as for inhibin, measured by the Monash assay. The significantly hi gher (P < 0.05) 5-24 alpha-subunit inhibin and lower inhibin (Monash a ssay) concentration in Texel ewes resulted in a differential ratio of both inhibin measurements for the Texel in comparison with the other b reeds. No significant breed differences for FSH were observed during t he oestrous cycle. It is concluded that, although significant breed di fferences in inhibin concentration are observed, the FSH-inhibin inter action is not the only factor responsible for breed differences in ovu lation rate. (C) 1998 Elsevier Science B.V. All rights reserved.