RESUMO
Study objectives were to evaluate hepatic gluconeogenic enzyme gene expression in recombinant bovine somatotropin (rbST)-treated lactating dairy cattle during heat stress (HS) or in thermal-neutral, pair-fed (PF) animals. Twenty-two multiparous (99 d in milk, 656 kg of BW) Holstein cows were subjected to 3 consecutive experimental periods (7 d each): (1) thermal neutral, (2) HS or PF, and (3) HS or PF with rbST (Posilac, administered on d 1 of period 3). Liver biopsies were obtained on the final day of each period. Heat stress conditions progressively decreased dry matter intake for the first 5 to 6 d during period 2 before stabilizing (a decrease of 6.15 kg; 30%) on d 6 and 7, and feed intake remained stable and not different from period 2 during period 3. Cytosolic phosphoenolpyruvate carboxykinase mRNA abundance increased during PF, but was unaffected by HS or bST. Pyruvate carboxylase gene expression increased during HS and PF, and administering bST decreased pyruvate carboxylase mRNA abundance during both HS and PF. Insulin-like growth factor-I gene expression increased following bST administration during HS and PF, confirming hepatic bST responsiveness. Exposure to HS leads to a change in hepatic gluconeogenic enzyme profile that appears to be dependent on plane of nutrition.
