Changes in serum somatotropin, somatotropin mRNA, and serum and follicular insulin-like growth factor-I in response to feed restriction in cows actively immunized against growth hormone-releasing factor.

Cyclic cows immunized against growth hormone-releasing factor (GRFi, n = 19), human serum albumin (HSAi, n = 10), or not immunized (CON, n = 18) were used to investigate the effects of feed restriction on serum and pituitary somatotropin (ST), pituitary ST mRNA, and serum and follicular IGF-I. Cows were either fed 2.7 kg/d cottonseed hulls (R) or given ad libitum access to feed (AL) for 15 d. Ovaries bearing the largest follicle and pituitaries were collected on d 14, at 44 to 45 h after injection of prostaglandin F2 alpha. Data from CON and HSAi cows were similar; thus, data were combined (represented as CON). Serum ST (nanograms/milliliter) on d 13 was greater (P < .09) in CON-R (5.3) than in CON-AL (3.9), whereas ST in GRFi-AL (1.1) and GRFi-R (1.1; pooled SE = .4) were similar. Hemipituitary weight (grams) and ST mRNA (arbitrary units) were greater (P < .05) in CON (1.5 +/- .1 and 135 +/- 25) than in GRFi (1.0 +/- .1 and 90 +/- 18) cows. Across immunization, ST mRNA and pituitary ST concentration (mg/100 mg of tissue), respectively, were greater (P < .06) in R (152 +/- 22 and 22.5 +/- 1.9) than in AL (73 +/- 16 and 17.3 +/- 1.8) cows. Immunization and diet decreased (P < .05) serum IGF-I (nanograms/milliliter) on d 13 (CON, 176 +/- 7 vs GRFi, 42 +/- 8; AL, 120 +/- 7 vs R, 98 +/- 8). Concentrations of IGF-I in follicular fluid (FFL) from the largest follicle were lower in GRFi (29 +/- 3) than in CON (102 +/- 6) cows; however, IGF-I in FFL was similar in AL (70 +/- 9) and R (71 +/- 10) cows. In conclusion, GRFi decreased serum ST and IGF-I, and decreased ST mRNA. Feed restriction increased serum ST and ST mRNA, and decreased serum IGF-I. Although feed restriction and GRFi decreased serum IGF-I, concentrations of IGF-I in FFL were decreased only by GRFi.