Effects of long‐term exogenous bovine somatotropin on glucose metabolism and the utilization of glucose by the mammary gland in different stages of lactation of crossbred Holstein cattle

Abstract

ABSTRACTThe effects of long‐term administration of recombinant bovine somatotropin (rbST) on glucose turnover and the utilization of glucose in the mammary gland using a continuous infusion of [3‐3H]glucose and [U‐14C]glucose in lactating crossbred Holstein Friesian(HF) cattle were investigated. Glucose turnover of rbST‐treated animals was significantly higher than those of control animals (P < 0.05) in mid lactation, while plasma glucose concentrations were not affected. The utilization of glucose of non‐mammary tissues of rbST‐treated animals significantly increased (P < 0.05) as lactation advanced. The glucose taken up by the mammary gland in early lactation increased flux through the lactose synthesis and the pentose cycle pathway with significant increases in NADPH formation for fatty acid synthesis during rbST administration. The utilization of glucose carbon incorporation into milk appeared to increase in milk citrate and milk triacylglycerol but not for milk lactose as lactation advances in rbST‐treated animals. The stimulant effect for milk yield by rbST treatment was transiently and significantly increased in early lactation and was decreased in late lactation even though there was a high level of udder blood flow. These findings demonstrate that the regulation of biosynthetic capacity within the mammary gland would be influenced more by local than by systemic factors. The proportion of glucose would be metabolized less for lactose synthesis, but metabolized more via the Embden‐Meyerhof pathway and the tricarboxylic acid cycle as lactation advances.