EFFECT OF STAGE OF LACTATION AND DIET ON SERUM INSULIN LEVEL AND MONONUCLEAR LEUKOCYTE INSULIN RECEPTOR CHARACTERISTICS IN DAIRY COWS

Abstract

Blood was collected from and measurements were made of body weight and ultrasonic backfat (US) in ten 2–6 yr old Holstein cows at 8 wk prepartum, 3 d postpartum and at 4, 6 and 12 wk of lactation. Cows were fed a low-fat (LF) total mixed ration containing 3.0% either extract and 13.6% acid detergent fiber ad libitum until day 15 postpartum when some of the diet concentrate was replaced by whole sunflower seeds (four cows) or sunflower seed hulls, sunflower seed meal and tallow (two cows) to yield diets high in fat (6.7 and 6.9%, respectively) and acid detergent fiber (18.1 and 17.9%, respectively). All diets were formulated to have a NE1 of 7.07 MJ kg−1 dry matter and were isonitrogenous. Milk yield and composition were measured daily during each week of blood collection. Radioimmunoassay for insulin was performed on eight serum samples collected at 0.5-h intervals commencing at 0730 h. Specific cell binding (SB) of insulin as well as number of and affinity constants for high affinity (N1, K1, respectively) and low affinity (N2, K2, respectively) insulin receptors were determined on mononuclear leukocytes (MNL) isolated from 250 mL of whole blood. Throughout the trial the US measurement in cows on the LF diet was greater than in cows on the high-fat (HF) diets. Body weight, dry matter intake and milk yield were similar in HF and LF fed cows. Serum insulin concentration was lower 3 d postpartum than prepartum and had increased by 4 wk of lactation. The increase was greater with HF feeding. Insulin SB to MNL was negatively correlated to body weight and N1 was positively correlated to US measurement. Percent insulin SB to MNL at 12 wk of lactation (8.4 ± 1.6) was lower than that found for all other stages of lactation. There was no effect of diet or stage of lactation on K1 and K2 values. At 4 wk of lactation N2 (1.44 ± 0.15 × 104 sites cell−1) was 100 and 50% greater than at 6 and 12 wk of lactation, respectively. Prepartum N2 (1.21 ± 0.17 × 104) did not differ from N2 at any other stage of lactation. At 4 wk of lactation N1 (5.79 ± 0.73 × 102 sites cell−1) tended (P < 0.056) to be greater than that found for all other stages of lactation. There was no significant (P ≥ 0.05) effect of diet on N1, N2, K1, K2 or SB. Key words: Insulin, receptor, mononuclear leukocyte, lactation, dairy cow