Characteristics of corpus luteum formed from the first wave dominant follicle following hCG in cattle

Two experiments were conducted to characterize the development and function of corpus luteum (CL) induced by hCG. In Experiment 1, cows (n=18) were randomly assigned either to serve as controls (CONT, n=6) or to receive hCG on Day 7 with (hCG-LUT, n=6), or without (hCG-CONT, n=6) surgical removal of the spontaneous CL on Day 12. The diameters of the hCG-induced and spontaneous CL of similar age did not differ (P>0.05) between Days 1 and 4. At Day 5, the CONT (spontaneous) diameter (29.3 +/- 1.4 mm) was larger (P<0.05) than that of the hCG-LUT (24.5 +/- 1.5 mm) or the hCG-CONT (24.6 +/- 1.7 mm) induced CL. Similarly, induced CL diameter for hCG-LUT and hCG-CONT groups was smaller (P<0.01) than the CONT (spontaneous) CL between Days 10 to 14. Plasma progesterone (P-4) levels were not different (P>0.05) among treatment groups until Day 12. On Day 14, the P-4 concentration of hCG-LUT cows decreased (P<0.01) to 1.1 +/- 0.9 ng/ml, then increased to 3.1 +/- 0.9 ng/ml by Day 18. Comparative values for hCG-CONT and CONT cows were 5.8 +/- 0.8 and 4.2 +/- 0.8; 4.5 +/- 0.8 and 5.5 +/- 0.8 ng/ml, respectively. The onset of regression of CL as well as estrous cycle length were similar (P>0.05) for all treatment groups. In Experiment 2, the effects of intrauterine infusion of indomethacin on the diameter, function and life span of hCG-induced CL were examined. A slight, albeit not significant, suppression of PGFM levels was observed in indomethacin-infused cows (n=4) compared with the controls (n=4) in blood samples obtained once a day during the infusion period. However, in 2 cows from which blood samples were collected every 6 h, the control cow showed several pulses of PGFM while the indomethacin-treated cow exhibited none. Induced CL diameter and lifespan were not affected by indomethacin infusion. However, mean P-4 levels were higher (P<0.05) between Days 16 and 20 in the indomethacin-infused group. In conclusion, the results suggest that 1) hCG-induced CL are functional but appear to be smaller and secrete less P-4 than spontaneous CL of similar age, and 2) the small size and reduced secretary function observed is not necessarily due to PGF(2 alpha) secreted by the uterine endometrium but, probably, to inherent characteristics.