R.S. Emery

Metabolism of long chain fatty acids by ruminant liver

The primary source of fatty acids processed by ruminant liver is nonesterified fatty acids (NEFA) from blood. Uptake is regulated by concentration of NEFA and blood flow. Blood NEFA concentration increases with negative energy balance. Blood flow increases with energy intake. Uptake and secretion of triacylglycerol between blood and the liver is limited. The reason for limited hepatic secretion of triacylglycerol-rich lipoprotein is unclear but probably involves the secretory process, not synthesis of triacylglycerol or apolipoprotein. Oxidation of fatty acids and ketogenesis are inhibited by malonyl-CoA and propionic acid. The stress of late gestation and early lactation increases NEFA supply to the liver, where they cause deposition of fat. Ketogenesis and oxidation in the liver increase but not sufficiently to prevent an accumulation of fat, which may contribute to decreased feed intake in the peripartum period.

Food intake and serum insulin responses to intraventricular infusions of insulin and IGF-I

Previous studies reported that intracerebroventricular (ICV) infusion of insulin decreased food intake in rats and baboons. Insulin can bind to insulin-like growth factor I (IGF-I) receptors and mimic the response of IGF-I. Our objective was to determine the effects of ICV infused-insulin or IGF-I on food intake in sheep. In the present study, a 6-day ICV infusion of insulin (123 ng/kg of body weight/day) but not of IGF-I (123 ng/kg of body weight/day) decreased food intake by 40% (p less than 0.003) and body weight (p less than 0.015) compared with control sheep. In addition, sheep that received ICV insulin or IGF-I had only half the concentration of insulin in serum as compared with controls. Our results support the hypothesis that ICV insulin does not decrease food intake through IGF-I receptors. Nevertheless, apparently both insulin and IGF-I in the brain can influence the concentration of insulin in blood.

Energy balance and body condition influence luteal function in Holstein heifers.

A factorial experiment was conducted to determine influence of energy balance (EB) and body condition (BC) on luteal function in heifers. Heifers with moderate (MBC) or fat (FBC) BC were fed individually to sustain positive EB (PEB) or to cause negative EB (NEB). Intake of feed was measured daily and body weight weekly. Progesterone was quantified daily in serum for 3.5 estrous cycles. On days 9, 10, or 11 after fourth estrus, blood was sampled every 15 min for 12 hr to quantify luteinizing hormone (LH), growth hormone (GH), insulin and non-esterified fatty acids (NEFA). The next day, luteal cells were incubated and proportions of small to large cells were determined. After fourth estrus, area of progesterone profiles in serum for 10 days postestrus was reduced in all heifers relative to MBC-PEB heifers. But, luteal weight from FBC-PEB and MBC-NEB heifers was less than MBC-PEB heifers and FBC-NEB heifers were intermediate. Secretion of progesterone in vitro was increased by LH for PEB but not NEB heifers. MBC-NEB heifers had increased ratios of small to large luteal cells. Independent of BC, NEB decreased concentrations of insulin and increased GH and NEFA. Secretion of progesterone was not associated with LH, GH or insulin, but was correlated negatively with NEFA. We conclude that reduced concentrations of progesterone in serum of FBC-PEB and MBC-NEB heifers is due to impaired luteal development. But, reduced concentrations of progesterone in serum of NEB heifers is due also to reduced basal (MBC) and LH-induced (MBC and FBC) secretion of progesterone by luteal cells. Body condition at onset of NEB may determine when effects of NEB on progesterone are detected.

Therapy of Diseases of Ruminant Intermediary Metabolism

Diseases of intermediary metabolism include ketosis and fatty liver of dairy cattle and pregnancy toxemia of ewes. These conditions occur when there is a failure of the homeostatic mechanisms regulating the mobilization of fats and the conservation of carbohydrates. The therapeutic approach is to reestablish the normal homeostatic patterns of fuel utilization. Suppression of excessive ketogenesis is the most important factor in reestablishing homeostasis. Ketogenesis can be suppressed by a number of therapeutic agents that act either by suppressing the mobilization of fatty acids or by inhibiting the transport of fatty acids into the hepatic mitochondria, the site at which fatty acids are converted to ketone bodies. Useful therapies include bolus glucose infusions, glucose precursors, and glucocorticoids.

Characteristics of fatty acid esterification by homogenates of bovine mammary tissue.

Fatty acid esterifying activity of homogenates of bovine mammary tissue was associated with the particulate fraction of the cell, was strongly dependent upon ATP, CoA,d,l-glycerol-3-phosphate, and Mg2+, and was stimulated by NaF, dithiothreitol and bovine serum albumin. The system made phospholipids, mono-, di- and triglycerides but did not esterify butyrate. The inability to form greater than 58% triglyceride suggested some factor(s) was limiting the acylation of di-to triglyceride. The results were consistent with glyceride synthesis by the α-glycerophosphate pathway.Fatty acid esterifying activity of homogenates of bovine mammary tissue was associated with the particulate fraction of the cell, was strongly dependent upon ATP, CoA,d,l-glycerol-3-phosphate, and Mg2+, and was stimulated by NaF, dithiothreitol and bovine serum albumin. The system made phospholipids, mono-, di- and triglycerides but did not esterify butyrate. The inability to form greater than 58% triglyceride suggested some factor(s) was limiting the acylation of di-to triglyceride. The results were consistent with glyceride synthesis by the α-glycerophosphate pathway.

Fatty acid specificity of glyceride synthesis by homogenates of bovine mammary tissue.

Fatty acid esterification by cell free preparations of bovine mammary tissue was investigated to determine if the type of long chain fatty acid supplied might influence the rate of triglyceride synthesis by that tissue. Homogenates of lactating bovine mammary tissue esterified14C-fatty acids into glycerides at rates dependent upon chain length and degree of unsaturation. Palmitic, stearic, oleic and linoleic acids were esterified at rates consistent with their concentration in milk fat. A comparison of free fatty acid concentrations of mammary tissue with levels saturating esterification suggested that supply of fatty acids does not limit glyceride synthesis. Certain combinations of fatty acids were facilitory, competitive or inhibitory to esterification. Stearic acid complimented esterification of palmitic and oleic acids. Unlabeledtrans-11-octadecenoic acid did not compete with14C-palmitate as efficiently in the esterification process as did unlabeledcis-9-octadecenoic acid, indicating that the mammary gland may preferentially esterify thecis-isomer of C-18∶1. Linoleic acid inhibited esterification of palmitic, stearic and oleic acids.

Milk Fat Depression and the Influence of Diet on Milk Composition

Milk composition depends on inheritance, stage of lactation, age, infection, and diet. Fat is the most variable component of milk, and its concentration depends on the supply of acetate, butyrate, and performed fatty acids to the mammary gland. This supply depends on diet and competition among organs. Adequate, effective fiber is the critical dietary consideration.

Factors affecting fatty acid oxidation in bovine mammary tissue

Oxidation of fatty acids was studied in bovine mammary tissue slices in order to evaluate their potential contribution to energy metabolism. Rates of fatty acid oxidation decreased with increasing chain length: acetate>octanoate>palmitate or oleate. Rates of oxidation of long chain, but not short chain, fatty acids increased over time, which could not be explained by carnitine palmitoyltransferase (CPT) activity. This phenomenon is not an artifact of the incubation system or caused by substrate solubility, as rates of palmitate oxidation were constant in rat kidney cortex slices. Preincubating mammary tissue with or without unlabeled palmitate showed that increasing rates of palmitate oxidation is not caused by use of endogenous fatty acids. Palmitate at 0.26 mM, equivalent to arterial fatty acid concentration, gave maximal rates of oxidation. The β-oxidation enzymes may restrict fatty acid oxidation as oxidation of [1-14C] palmitate exceeded that of [U-14C] palmitate. Acetate inhibited palmitate oxidation (75%) but not esterification, suggesting that acetate inhibits palmitate oxidation by substrate competition at the mitochondrial level or via malonyl-CoA inhibition of CPT. Glucose inhibited palmitate oxidation (67%) and stimulated esterification. Low palmitoyl-CoA levels would favor glyceride synthesis over oxidation, since the apparent Km for palmitoyl-CoA, of the glycerol-3-phosphate acyltransferases is lower than that for CPT. Thus, glucose presumably diverts palmitate from oxidation to glycerolipids. Clofenapate, a glyceride synthesis inhibitor, decreased triacylglycerol formation, and marginally increased palmitate oxidation. We estimated that long chain fatty acids can potentially account for 6–10% of the oxidative metabolism of mammary tissue.

Mammary lipoprotein lipase in plasma of cows after parturition or prolactin infusion

Plasma lipase activity from the mammary vein and a tail blood vessel was measured in periparturient Holstein cows treated in one of three ways: control, CB154 (2-Br-α-ergocryptin) or CB154 plus prolactin. CB154 administration decreased basal serum prolactin concentration by 80% and blocked the normal parturient increase of serum prolactin. In CB154 plus prolactin-treated cows, prolactin was infused continously for six days starting five and eight days prepartum. Plasma lipase activity was not detectable up to 26 hr prepartum in control and CB154-treated cows or before the start of prolactin infusion in CB154 plus prolactin-treated cows. After two hr prepartum, plasma lipase activity was detected in all treatments. In CB154 plus prolactin-treated cows, plasma lipase activity was detected in the presence of high concentrations of serum progesterone four days after the start of prolactin infusion and at least two days before parturition. Plasma lipase activity was four times greater in the mammary vein than in the tail vessel at sampling times at which activity was detected in both vessels. We propose the difference between plasma lipase activity from the mammary vein and tail vessel is due to release of lipoprotein lipase from the mammary gland into blood, and this activity can be induced prepartum by prolactin or at parturition even if the parturient increase in prolactin is suppressed.