Effects of chronic heat stress on plasma concentration of secreted heat shock protein 70 in growing feedlot cattle.

Sixty Angus steers (449.2±11.0 kg) with implanted body temperature (BT) transmitters were used in a 110-d study to determine the effect of chronic stress (housing, diet, and climate) on extracellular heat shock protein 70 (eHsp70) concentration in plasma. The steers were a subset of a larger study involving 164 steers. Before the start of the study (d -31), 63 steers were implanted with a BT transmitter between the internal abdominal muscle and the peritoneum at the right side flank. Steers were housed in 20 pens (10 with shade and 10 without). Within each pen, 3 steers had a transmitter, and BT was recorded at 30-min intervals throughout the study. On d 0, 30, 60, 90, and 110, steers were weighed, BCS assessed (1 to 9 scale in which 1=emaciated and 9=obese), and 10 mL of blood from the coccygeal vein was collected for determination of inducible heat shock protein 70 (Hsp70) concentration by ELISA. Climatic variables (ambient temperature, relative humidity, solar radiation, black globe temperature, and wind speed) were obtained every 30 min from an on-site weather station. The relationship between the climatic variables and Hsp70 concentration were examined. As we failed to detect an effect of shade, all data were pooled. Mean BT over the duration of the study was 39.6±0.10°C. Mean BT was lowest (38.7±0.10°C) on d 0 and highest on d 110 (40.2°C±0.10). The Hsp70 concentration was least on d 0 (2.33±0.47 ng/mL) and greatest on d 30 (8.08±0.78 ng/mL). The Hsp70 concentration decreased from d 30 but remained above the d-0 concentrations on d 60, 90, and 110. There was a strong relationship between Hsp70 concentration and ambient temperature (r2=0.86; P<0.0001) and Hsp70 concentration and photoperiod (r2=0.94; P<0.0001) and no relationship with BT (r2=0.06; P<0.0001). When assessed with both BCS and BT, the relationship was moderate (r2=0.48; P<0.001). The relationship between Hsp70 and change in BT (BTΔ) above 38.6°C was also moderate (r2=0.54; P<0.0001). The BT at a given time does not appear to be related to Hsp70 concentration. However, Hsp70 expression may be a useful indictor for BTΔ when BT>38.6°C. The Hsp70 concentration is a reliable indicator of chronic stress but is not a reliable indicator of a single stressor when animals are exposed to multiple chronic stressors.