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 domina nt follicles (DFs) after induced heat stress, Non-lactating dairy cows expr essing estrus at normal intervals were allocated randomly to heat stress (H S; n = 8) and control (C; n = 8) groups. Cows received GnRH (100 mug, i.m.) on Day 0, a progesterone CIDR-B device on Day 4 and prostaglandin (PGF(2 a lpha): 25 mg, i.m.) on Day 7 upon removal of the CIDR device. The DF and fo llicles >5 mm were aspirated on Day 8, and GnRH(100 mug) injected following aspiration, to initiate a new follicular wave. In this manner, a DF was as pirated every 8 days (one "follicular cycle") for 10 cycles. After the firs t follicular cycle. HS cows were placed in environmental chambers for 7 day s during the second follicular cycle (8 h per day at 43.3 degrees 'C set po int and 16 h per day at 24 degreesC for 4 days, and 8 h per day at 43.3 deg reesC set point and 16h per day at 32.2 degreesC set point for 3 days; rela tive humidity, 40%) and thereafter maintained outdoors with control cows at a mean ambient temperature (18.51 degreesC; range 12.7-26 degreesC). Recta l temperature increased(P < 0.001) in HS as compared with C cows (39.28 +/- 0.01 degreesC versus 38.78 +/- 0.01 degreesC), Concentrations of estradiol (E-2; 1662 +/- 189 versus 1493 +/- 188 ng/ml) and progesterone (P-4; 44.7 +/- 5 versus 54.1 +/- 5.1 ng/ml) in follicular fluid (FF) of DF did not dif fer between C and HS treatments, respectively. Total FF protein concentrati on was greater(P < 0.05) in HS (99.7 +/-2.3 mg/ml) than in C (92.7 +/-2.3 m g/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 a nd 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 Lo wer molecular weight IGFBP-2 indicative of a poor quality follicle. Plasma P-4 and E-2 concentrations did not differ between C and HS groups. The numb er 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. H eat stress appeared to induce a decrease in follicular dominance, but GnRH- induced follicular cycles resulted in development of healthy preovulatory f ollicles in both groups. (C) 2001 Published by Elsevier Science B.V.