Secretion of PGF2α 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 PGF2α 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°C, 20% RH) or HS (42°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 PGF2α (PGFM) and OT. During the 7-h experiment, body temperature of HS heifers reached 41.2°C as compared to 38.5°C in control heifers. Plasma concentrations of PGFM increased (P