Effect of diet and glucocorticoid administration of liver phosphoenolpyruvate carboxykinase activity in the dairy cow

Cloning and characterization of bovine cytosolic and mitochondrial PEPCK during transition to lactation

The cytosolic (C) and mitochondrial (M) forms of phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) are encoded by two different nuclear genes in mouse, human, and chicken. Our objective was to clone the two forms of PEPCK for bovine and determine their expression during the immediate periparturient interval in dairy cows. Bovine PEPCK-C cDNA contains 2,592 nucleotides and contains 84% similarity to the coding sequence of human PEPCK-C cDNA. A 449-nt partial clone of the 3' end of PEPCK-M is 76% similar to the corresponding sequence of human PEPCK-M. The coding sequence of bovine PEPCK-C and coding sequence of the partial PEPCK-M clone were 58% similar but the similarities in the 3'-untranslated regions were negligible. Northern blot analysis revealed single transcripts of 2.85 and 2.35 kb for PEPCK-C and PEPCK-M, respectively. The transition to lactation did not alter PEPCK-M transcript expression, but expression of PEPCK-C mRNA was transiently increased during early lactation, indicating that enhanced hepatic gluconeogenesis during this period may be tied to enhanced capacity for cytosolic rather than mitochondrial formation of phosphoenolpyruvate.

Effects of diet, pregnancy, and lactation on enzyme activities and gluconeogenesis in ruminant liver

Changes of activities of liver enzymes and of metabolism during pregnancy and lactation have been documented in studies with rodents. Therefore, a study of activities of ruminant liver enzymes and metabolism was undertaken. Several changes of activities of enzymes involved in lipogenesis and gluconeogenesis were observed. However, magnitudes of enzyme change were less than expected from major changes in liver function that accompany lactation. Also, a number of enzymatic changes expected from rodent data were absent. Effects of breed and diet on enzyme activities in cow livers were minimal or absent. Changes in rates of glucose metabolism from lactation were greater than implied by enzyme adaptations. Analysis of kinetic data according to Michaelis-Menten kinetics indicated that apparent maximal velocities of conversion of lactate and propionate to glucose by liver slices from cattle ranged from 10 to 30 mumoles/(g.h) and apparent coefficients were 1.8 to 2.0- mM. Rates of gluconeogenesis with lactate plus propionate were not additive at saturating concentrations. The background data on enzymatic activities, glucose metabolism, lipogenesis, and gluconeogenesis in ruminant liver indicate that sophisticated studies with isolated hepatocytes from ruminants will be required to clarify metabolic adaptations, metabolic patterns, and regulatory mechanism in this tissue.

Milk fat as related to vitamin B12 status

Two experiments examined whether vitamin B12 status of liver affects milk fat percentage or yield. In Experiment 1 B12 concentration in liver, measured at approximately 8 and 15 wk postpartum in 35 cows, was not correlated significantly with milk fat percentage or yield. In Experiment 2 twenty-two cows were fed a ratio similar to one used to depress milk fat percentage. The B12 status of 12 of these cows was enhanced by intramuscular injection of vitamin B12 while the other 10 served as controls. During wk 2 to 8 of lactation, milk fat percentages were almost identical for the two groups. A difference in milk fat yield in favor of the B12-treated group resulted from a difference in milk yield. The data do not support a recent hypothesis of the metabolic cause of milk fat depression.

Gluconeogenesis in cattle: significance and methodology

Gluconeogenesis is a continual process that is of great importance in ruminants because almost all dietary carbohydrates are fermented to volatile fatty acids in the rumen. In turn, propionate is the only major volatile fatty acid that contributes to gluconeogenesis. Many different techniques and analytical procedures are involved in studying ruminant gluconeogenesis. Glucose kinetics can be examined by single-injection or continuous-infusion isotope dilution techniques with a variety of glucose labels. Correcting for recycling of label is an important consideration. Absorption of glucose from the gut can be measured by combining arterial-venous differences and flow rates of portal blood or can be estimated by determining the amount of glucose plus alpha-glucose polymers passing into the small intestine. Production of propionate in the ruminoreticulum can be measured by isotope dilution techniques. Quantitating the conversion of propionate to glucose requires the use of [carbon-14]propionate with careful corrections for propionate carbon entering the citric acid cycle before incorporation into glucose; The same fundamental techniques used with propionate are required to quantitate the contributions of amino acids and other precursors to glucose. In vitro studies of gluconeogenic enzymes, and cellular, tissue, or organ preparations provide valuable insights into the gluconeogenic processes and controls but must be validated by in vivo experiments. Progress has been considerable in understanding some aspects of ruminant gluconeogenesis, but many more studies will be required to obtain a complete understanding.

Effect of a synthetic corticoid on milk yield and composition and on blood metabolites and hormones in dairy cows

Twenty-five dairy cows were assigned randomly by breed to flumethasone treatment (10 mug/day, 13 cows) or placebo control (12 cows) from 4 to 44 wk of lactation. Lactation means and trends with stage of lactation were treatment responses. Mean milk yields of cows supplemented with flumethasone were not significantly different from controls. Mean metabolite (glucose, nonesterified fatty acids, and total esterified fatty acids) and hormone concentrations (corticoids, insulin, and prolactin) of blood plasma and their trends throughout lactation were unaffected by supplementation. Data were pooled to determine effects of stage of lactation, temperature, pregnancy, and month on these variables. Changes during lactation were systematic for feed intake, body weight, milk yield, milk components except somatic cells, blood metabolites, and prolactin. At environmental temperatures above 18 C, effects were consistently negative for milk yield and composition, but only above 26 C for feed intake. Prolactin of plasma increased with increasing temperature to 18.2 C and then decreased. No other plasma metabolite or hormone was affected by days pregnant, age, or temperature. Months affected feed intake, milk yield, milk fat percentages, all blood metabolites, and prolactin. Prolactin concentrations increased as daylight hours increased. Our inability to augment established lactation by feeding a supplement of synthetic glucocorticoid to lactating cows is consistent with the view that a lack of avialable corticoids does not limit persistency of lactation in the cow.

Effects of starvation on intermediary metabolism in the lactating cow. A comparison with metabolic changes occurring during bovine ketosis

1. The purpose of this study was to determine the nature of the metabolic changes associated with carbohydrate and fat metabolism that occurred in the blood and liver of lactating dairy cows during starvation for 6 days. 2. During starvation, the blood concentrations of the free fatty acids and ketone bodies increased, whereas that of citrate decreased. After an initial increase, the blood concentration of glucose subsequently declined as starvation progressed. Starvation caused a significant decrease in the plasma concentration of serine and a significant increase in that of leucine. 3. After 6 days of starvation the hepatic concentrations of oxaloacetate, citrate, phosphoenolpyruvate, 2-phosphoglycerate, 3-phosphoglycerate, glucose, glycogen, ATP and NAD(+) had all decreased, as had the hepatic activities of phosphopyruvate carboxylase (EC 4.1.1.32) and pyruvate kinase (EC 2.7.1.40). 4. The above metabolic changes are similar to those previously found to occur in cows suffering from spontaneous ketosis (Baird et al., 1968; Baird & Heitzman, 1971). 5. Milk yield decreased progressively during starvation. 6. There were marked differences in the ability of individual animals to resist the onset of severe starvation ketosis.