Bovine luteal secretory proteins of the oestrous cycle and pregnancy

The steroidogenic large luteal cell accounts for 40% of the volume of the bovine corpus luteum. Two populations of large luteal cells have been identified, the a and b. The b appears to be the most steroidogenic ally active as indicated by smooth endoplasmic reticulum that is highly dilated as compared to the a. Both the a and b large luteal cells contain secretory granules that reach their peak in numbers on Days 7 to 11 of the oestrous cycle. In the case of pregnancy the granules are maintained until Day 20 and depleted via exocytosis by Day 30. In both cases of the cycle and early pregnancy the Granules contain oxytocin and neurophysin. Around Day 75 a new population of secretory granules appear in which the contents remain unidentified. In an attempt to characterize those proteins secreted by the corpus luteum across the oestrous cycle and pregnancy, luteal explants were incubated with H-3-Leucine and the secreted proteins submitted to two-dimensional polyacrylamide gel electrophoresis. Fourteen major secreted proteins have been characterized ranging in molecular masses of 20 to 70 kilodaltons and isoelectric points of 5.5 to 8.0. Seven proteins were observed only on Day 3 of the cycle. Four of these proteins were identified through N-terminal amino acid sequencing as isoforms of apolipoprotein E. Although the other seven proteins were not observed on Day 3, they were secreted starting on Day 7 and were thereafter observed throughout the cycle and pregnancy. Three of these proteins were identified as apolipoprotein A-I, tissue inhibitor of metalloproteinases-1, and tissue inhibitor of metalloproteinases-2, respectively. The apolipoproteins E and A-I have a potential paracrine/autocrine role in remodeling of the developing corpus luteum and in transport of cholesterol into the steroidogenic cells for membrane synthesis and steroidogenesis, The tissue inhibitor of metalloproteinases are secreted and their respective mRNAs most highly expressed toward the end of the life-span of the corpus luteum indicating a potential role in the regulation of remodeling of that tissue during regression.