SECRETION OF PGF2-ALPHA AND OXYTOCIN DURING HYPERTHERMIA IN CYCLIC AND PREGNANT HEIFERS

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.