Presence of an intracellular endometrial inhibitor of prostaglandin synthesis duing early pregnancy in the cow

Abstract

Previous studies have detected reduced endometrial secretion of prostaglandins during pregnancy in cattle. The present experiment tested the hypothesis that reduced secretion of prostaglandins is caused by induction of anintracellular endometrial inhibitor of prostaglandin synthesis. The microsomal fraction of parturient bovine cotyledons was utilized as a source of enzymes for prostaglandin synthesis. Endometrial tissues collected at Day 17 of the estrous cycle ( n=12) and pregnancy ( n=12) were homogenized and subjected to differential centrifugation for preparation of microsomes and a highspeed (100,000 × g) cytosolic supernatant. Endometrial intracellular preparations were then examined for the ability to modulate prostaglandin synthesis by cotyledonary microsomes from parturient cows. Endometrial intracellular preparations from cyclic cows decreased (P< 0.05) PGF synthesis by cotyledonary microsomes to a slight extent (supernatant, 21% reduction; microsomes, 11% reduction), while preparations from pregnant cows markedly decreased (P< 0.01) PGF synthesis (supernatant, 63% reduction, microsomes, 28% reduction; supernatants vs microsomes, P< 0.01. Regardless of the amount of arachidonic acid available as substrate (25–400 μg) endometrial supernatant from pregnant cows (pooled sample) caused a 50% inhibition (IC 50) of prostaglandin synthesis at a tissue equivalent of 270 ± 9.1 mg. The mechanism of inhibition by endometrial high-speed supernatant from pregnant cows appears to be non-competitive with respect to arachidonic acid. The inhibitor(s) may be proteinacous (70–75 kDa and 25–35 kDa) and can be precipitated by 20% saturated amonium sulfate. In conclusion, early pregnancy in cattle appears to be associated with increased amounts of an intracellular endometrial inhibitor of prostaglandin synthesis.