Responses of tissues to insulin as affected by homeorhetic state in dairy cattle.

In vivo insulin (INS) challenges were used to evaluate peripheral tissue responses to insulin in the late prepartum (PRE), early postpartum (POST) and midlactation (ML) periods in Holstein cows. Periparturient (n=7) cows were studied during 4 periods (-30 to -26 d and -12 to -8 d PRE; 3 to 7 d and 26 to 30 d POST) and ML (n=5) cows were studied from d 148 to 152 DIM. During each period, cows were administered a randomized sequence of 5 doses of INS (0.2, 0.4, 0.8, 1.2 and 1.6 μg/kg BW) by i.v. challenge conducted once daily. Blood was sampled frequently via jugular catheter from 30 min pre-challenge to 2 h post-challenge. Plasma glucose and insulin responses to 0.8 and 1.6 μg INS/kg BW were used to estimate tissue sensitivity and responsiveness, respectively. Plasma glucose (mg/dl*min^-1) area under the curve (AUC) decreased and plasma insulin (ng/ml*min^-1) AUC increased dose-dependently following INS challenge. Plasma glucose AUC after 0.8 μg INS/kg BW was lower during PRE than during d 3 to 7 POST and ML; however, glucose AUC following 1.6 μg INS/kg BW was similar among PRE and POST and lower than ML. Plasma insulin AUC after 0.8 μg INS/kg BW was greater during d -12 to -8, d 3 to 7, and d 26 to 30 compared with d -30 to -26 and ML and was greatest after 1.6 μg INS/kg BW during d -12 to -8 and intermediate during d 3 to 7 POST compared to other study periods. Collectively, these data suggest that peripheral tissue responses to INS are greatest during ML and least during the immediate prepartum period. [ABSTRACT FROM AUTHOR]