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Phosphoenolpyruvate carboxykinase (PEPCK) isoforms (c, cytosolic; m, mitochondria) are expressed in the
liver and mammary gland. PEPCK-c is a rate-controlling enzyme for gluconeogenesis and glyceroneogenesis; its activity
is decreased by insulin. PEPCK-m expression is constitutive and functions to channel lactate toward gluconeogenesis. We
hypothesized that the increase in milk protein but decrease in milk lactose and fat when a hyperinsulinemic-euglycemic
clamp (HIEC) is applied to dairy goats is due to decreased expression of mammary PEPCK-c mRNA. Late lactation goats
(n = 4; 150 ± 30 days in milk) were subjected to saline infusion and HIEC (104 μg insulin/h) for 4-day periods in a 2 × 2
crossover design. On day 4 of each period, a mammary biopsy (~1 g) was taken from an udder half to determine expression
of PEPCK-m and PEPCK-c mRNA by real-time RT-PCR. Plasma insulin increased 3.5-fold (P< 0.002) due to the
HIEC and euglycemia was maintained. The HIEC decreased (P< 0.0001) dry matter intake (28%) and milk yield (26%).
While milk fat content was not affected, HIEC increased (P< 0.001) milk protein content (2.82% vs. 3.09%) but decreased
(P< 0.001) milk lactose content (4.22% vs. 4.03%). Expression of PEPCK-m mRNA was 9-fold higher (P< 0.004) than
that of PEPCK-c. The HIEC decreased (P< 0.03) PEPCK-c mRNA 7-fold but tended to increase (P< 0.236) PEPCK-m
mRNA 1.3-fold. These results demonstrate that insulin regulates mRNA expression of mammary PEPCK isoforms, and
this may underlie the changes in milk-component synthesis observed when a HIEC is applied.