D. Wolfenson et al., SECRETION OF PGF2-ALPHA AND OXYTOCIN DURING HYPERTHERMIA IN CYCLIC AND PREGNANT HEIFERS, Theriogenology, 39(5), 1993, pp. 1129-1141
The effects of acute heat stress (HS) and oxytocin (OT) injection on p
lasma concentrations of PGF2alpha and OT were examined in cyclic (C; n
= 15) and pregnant (P; n = 11) dairy heifers. On Day 17 of synchroniz
ed estrous cycles, animals were randomly assigned to either thermoneut
ral (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 colle
cted 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. P
lasma samples were assayed subsequently for concentrations of 13,14-di
hydro-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 con
centrations either before or after OT injection. In the P group, PGFM
concentrations following OT injection tended to be higher in HS heifer
s than in TN heifers (peak values of 690 vs. 410 pg/ml). Pregnant TN a
nd HS heifers were further classified as responders or non-responders
to OT challenge according to a cutoff value for PGFM of 193 pg/ml (ove
rall 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 conc
entrations in plasma prior to injection of exogenous OT were not affec
ted by HS or pregnancy status. It is concluded that in C heifers, acut
e HS in vivo does not cause any further rise in PGF2alpha secretion. H
owever, in P heifers, HS appears to antagonize suppressive effects of
the embryo on uterine secretion of PGF2alpha, as indicated by the larg
er proportion of P heifers responding to OT challenge.