title: Secretion of PGF2alpha and oxytocin during hyperthermia in cyclic and pregnant heifers.
abstract: The effects of acute heat stress (HS) and oxytocin (OT) injection on plasma concentrations of PGF2alpha and OT were examined in cyclic (C; n = 15) and pregnant (P; n = 11) dairy heifers. On Day 17 of synchronized estrous cycles, animals were randomly assigned to either thermoneutral (TN; 20 degrees C, 20% RH) or HS (42 degrees C, 60% RH) chambers. The jugular vein of each heifer was cannulated and blood samples collected hourly for 4 h, then every 15 min for an additional 3 h. Oxytocin (100 IU) was injected (IV) 5 h after the start of blood collection. Plasma samples were assayed subsequently for concentrations of 13,14-dihydro-15-keto PGF2alpha (PGFM) and OT. During the 7-h experiment, body temperature of HS heifers reached 41.2 degrees C as compared to 38.5 degrees C in control heifers. Plasma concentrations of PGFM increased (P<0.05) and peaked 30 min after OT injection in C (890 pg/ml) and P (540 pg/ml) heifers. In C heifers, heat stress failed to alter PGFM concentrations either before or after OT injection. In the P group, PGFM concentrations following OT injection tended to be higher in HS heifers were further TN heifers (peak values of 690 vs. 410 pg/ml). Pregnant TN and HS heifers were further classified as responders or non-responders to OT challenge according to a cutoff value for PGFM of 193 pg/ml (overall mean of C heifers minus 1 SD). Five of six HS and one of five TN pregnant heifers were classified as responders (P<0.06). Oxytocin concentrations in plasma prior to injection of exogenous OT were not affected by HS or pregnancy status. It is concluded that in C heifers, acute HS in vivo does not cause any further rise in PGF2alpha secretion. However, in P heifers, HS appears to antagonize suppressive effects of the embryo on uterine secretion of PGF2alpha, as indicated by the larger proportion of P heifers responding to OT challenge.
authors.affiliation: Department of Animal Science, Faculty of Agriculture Hebrew University, Rehovot 76100, Israel.
authors.forename: D
authors.initials: D
authors.lastname: Wolfenson
authors.forename: F F
authors.initials: FF
authors.lastname: Bartol
authors.forename: L
authors.initials: L
authors.lastname: Badinga
authors.forename: C M
authors.initials: CM
authors.lastname: Barros
authors.forename: D N
authors.initials: DN
authors.lastname: Marple
authors.forename: K
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authors.lastname: Cummins
authors.forename: D
authors.initials: D
authors.lastname: Wolfe
authors.forename: M C
authors.initials: MC
authors.lastname: Lucy
authors.forename: T E
authors.initials: TE
authors.lastname: Spencer
authors.forename: W W
authors.initials: WW
authors.lastname: Thatcher