Long-term follicular dynamics and biochemical characteristics of dominant follicles in dairy cows subjected to acute heat stress

The objective of this study was to examine the quality of successive dominant follicles (DFs) after induced heat stress. Non-lactating dairy cows expressing estrus at normal intervals were allocated randomly to heat stress (HS; n=8) and control (C; n=8) groups. Cows received GnRH (100 μg, i.m.) on Day 0, a progesterone CIDR-B device on Day 4 and prostaglandin (PGF2α; 25 mg, i.m.) on Day 7 upon removal of the CIDR device. The DF and follicles >5 mm were aspirated on Day 8, and GnRH (100 μg) injected following aspiration, to initiate a new follicular wave. In this manner, a DF was aspirated every 8 days (one “follicular cycle”) for 10 cycles. After the first follicular cycle, HS cows were placed in environmental chambers for 7 days during the second follicular cycle (8 h per day at 43.3°C set point and 16 h per day at 24°C for 4 days, and 8 h per day at 43.3°C set point and 16 h per day at 32.2°C set point for 3 days; relative humidity, 40%) and thereafter maintained outdoors with control cows at a mean ambient temperature (18.5°C; range 12.7–26°C). Rectal temperature increased (P<0.001) in HS as compared with C cows (39.28±0.01°C versus 38.78±0.01°C). Concentrations of estradiol (E2; 1662±189 versus 1493±188 ng/ml) and progesterone (P4; 44.7±5 versus 54.1±5.1 ng/ml) in follicular fluid (FF) of DF did not differ between C and HS treatments, respectively. Total FF protein concentration was greater (P<0.05) in HS (99.7±2.3 mg/ml) than in C (92.7±2.3 mg/ml). Heat shock protein 90 (Hsp 90) in FF was not altered by heat stress. IGF-II ligand blots were conducted with FF samples (n=79) from four HS and four C cows. There was a predominance of IGFBP-3 in 76 of 79 FF samples, indicating healthy follicular status, and only three FF samples had the lower molecular weight IGFBP-2 indicative of a poor quality follicle. Plasma P4 and E2 concentrations did not differ between C and HS groups. The number of class 1 and 3 follicles increased during and just after heat stress, but the number of class 2 follicles did not differ between C and HS cows. Heat stress appeared to induce a decrease in follicular dominance, but GnRH-induced follicular cycles resulted in development of healthy preovulatory follicles in both groups.