Corpus Luteum function in the Bovine: in Vivo in Vitro Evidence for Both a Seasonal and Breedtype Effect

Abstract

Breed type and season appear to modify the reproductive capacity of cattle. This study was designed to elucidate the effect of these factors on the bovine corpus luteum. Six Brahman (B) and six Hereford × Holstein (H) heifers were bled via coccygeal vessel puncture on d 2, 4, 6, 8, 10, 12 and 13 postestrus within 20 d of both the summer and winter solstices. On d 13 postestrus, the corpus luteum (CL) was surgically removed from each heifer following blood sampling. Serum progesterone, total progesterone content of each CL and in vitro progesterone (P4) production of dispersed luteal cells was determined. Serum progesterone on d 2 through 13 did not vary with breedtype or season. However, the CL weights differed (P>.05) between breeds (H heavier than B), and there was a tendency for CL weight to be greater during the summer than winter among both B and H heifers. CL concentrations of P4 (µg/g) also differed between B and H. Concentrations of P4 in B were greater (P>.10) during winter than during summer, but H concentrations of P4 did not differ between seasons. Total P4 content was greater (P>.01) in the winter than in summer and in H than in B. Luteal cell viability did not vary with breedtype, season or LH dosage. Breedtype (P>.001) and season (P>.001) influenced the capacity of incubated luteal tissue to release progesterone in response to luteinizing hormone (LH). Luteal tissue from H released a greater amount of progesterone than incubated B tissue. Further, the release of progesterone by winter CL tissue in the LH-challenged cultures was diminished. The minimal LH dosage required to elevate P4 above concentrations observed at 0 ng LH did not differ between breeds or seasons (5 ng LH). However, the amount of P4 released at 5 ng LH dosage was highest for tissue from H during the winter, followed by H during the summer, B during the summer and B during winter. In contrast, maximal release of P4 was associated with the 500 ng LH dosage for H (summer and winter), the 100 ng LH dosage for B during summer and the 50 ng LH dosage for B during winter. At LH dosages above 10 ng, P4 concentrations were consistently greater in H than B, with summer greater than winter values in both breedtypes. The greatest change in P4 release (highest P4 concentrations minus P4 concentration at 0 ng LH) followed the same trends as above. Significant (P>.001) interaction terms were also detected for breed × season and breed × dose. These data indicate that luteal tissue from H had a greater capacity to release P4 in vitro than luteal tissue from B. Further, CL removed during the winter had a lessened capability to release P4 than CL removed during the summer.