Modulation of uterine prostaglandin biosynthesis by pregnant and nonpregnant cows at day 17 post-estrus in response to in vivo and in vitro heat stress

Abstract

Lactating pregnant and nonpregnant dairy cows were used to determine if summer heat stress alters responsiveness of the uterus to an oxytocin-induced secretion of prostaglandin F2α. Cows were assigned to thermoneutral (evaporatively-cooled barn), or heat stress (exposure to summer environment) treatments for 7.5 h on Day 17 post-estrus. Blood samples were collected during the first 6 h. Oxytocin (100 IU) was then administered and blood collected for a further 1.5 h. Conceptuses were recovered nonsurgically to determine pregnancy status. Prior to oxytocin treatment, plasma concentrations of 13,14-dihydro-15-keto PGFM2α (PGFM) were similar among cows, regardless of treatment or pregnancy status. Oxytocin increased mean PGFM concentrations, but the increase was higher for nonpregnant and pregnant cows with embryos retarded in development than for pregnant cows with normal embryos. Heat stress increased oxytocin-induced PGFM concentrations in nonpregnant cows, but had no effect on PGFM in pregnant cows. Endometrial explants from cyclic and pregnant cows were used to determine if heat-induced endometrial secretion of prostaglandin (PG, PGF and PGE2) is regulated by an intracellular inhibitor of PG synthesis. Endometrial explants were incubated at 39 or 42°C, and conditioned medium analyzed for PGF and PGE2. Cotyledonary microsomes from parturient cows were utilized as a PG generating system for detection of endometrial inhibitors of PG synthesis. Endometrial synthesis of PGE at 39°C was greater for cyclic than pregnant cows, while PGE2 synthesis was similar between statuses. Heat stress increased PGF and PGE2 synthesis by endometrium from cyclic and pregnant cows. Synthesis of PGF by the generating system was decreased 44% when incubated with endometrium of pregnant cows cultured at 39°C and this decrease was 35.3% less with cytosol from endometrium of pregnant cows incubated at 42°C. Decreased uterine PGF production by pregnant cows, carrying viable embryos, in response to oxytocin in vivo and by endometrial explants from pregnant cows, indicates that the conceptus attenuates release of uterine PGF. Heat stress amplified PG synthesis by nonpregnant cows and in endometrial explants from cyclic and pregnant cows. Decreased responsiveness of the pregnant uterus to oxytocin and heat stress suggests a conceptus-induced factor attenuates responsiveness of the uterine PG generating system to stimulation by heat stress.