Luteinizing hormone receptors in the bovine corpus luteum during the oestrous cycle and pregnancy

The concentration and affinity of luteinizing hormone (LH) receptors in bov ine luteal tissues during the oestrous cycle and pregnancy were investigate d by Scatchard analysis of the binding of I-125-labeled human chorionic gon adotropin. Corpora lutea (CL) were classified into five stages of the oestr ous cycle and three stages of pregnancy. The concentration of LH receptors sharply increased from the early I stage of the oestrous cycle (Days 2-3; 3 .09 fmol mg(-1) protein) to the early II stage (Days 5-6; 9.44 fmol mg(-1) protein) and then remained constant until the late luteal stage (Days 15-17 ; 8.14-9.56 fmol mg(-1) protein). The LH receptors could not be analysed in the regressed luteal tissue due to the small amounts of binding. There was no significant difference in the concentrations of LH receptors (5.63-9.64 fmol mg(-1) protein) among the three stages of pregnancy. Moreover, the co ncentrations of the receptors in the CL of pregnancy were comparable to tho se in the mid-cycle CL. The binding affinity did not change significantly d uring the oestrous cycle and pregnancy. Based on these results, it is assum ed that the luteal function during the entire period of pregnancy might be regulated at least in part, by LH, which is mediated via its specific recep tors, and that the luteal function during pregnancy seems not to be regulat ed by changes in the binding capacity and affinity of LH receptors. To unde rstand the physiological roles of LH in regulating luteal function in pregn ant cows, further studies are required.