Blood sampling techniques for studying rapidly turning over metabolic fuels in mice

Experiments were carried out in control and Ehrlich ascites carcinomatous mice to determine whether orbital venous sinus blood could be used to reflect blood in the systemic circulation (decapitation blood) in the case of a rapidly turning over metabolic fuel such as free fatty acids. The early time course of intravenously injected, labeled free fatty acids was measured using (9, 10-(3)H) palmitic acid and (1-(14)C) linoleate complexed to mouse serum. No significant differences between decapitation and orbital sinus blood were found at early times in either group of mice. The orbital sinus clearly contains blood that is not stagnant and is replaced so rapidly that it is suitable for studying very rapidly turning over, circulating metabolites.

Equally rapid activation of lipogenesis in nibbling and gorging mice

Minimal average rates of exogenous glucose-C-conversion to whole body, total lipid fatty acids were measured in nibbling and gorging mice. Gorgers trained to eat 1 meal/day (8-10 am) were fasted 22-24 hr and given [14C]glucose with pure glucose, 30% glucose in water, or a 58% glucose, fat-free diet. Conversion of glucose-C to total lipid fatty acids increased from 0.6 (fasted) to approximately 20 mug/min/20 g body weight during 40 min after glucose feeding using each test meal. Dietary amino acids were not required for activation of lipogenesis in gorgers. Exogenous glucose-C was incorporated into fatty acids as fast in nibbling mice as in gorgers. This was true after varying all of the following conditions: training period, number of meals gorged, previous fasting time, and diet composition. The total rate of fatty acid synthesis from body glucose-C during absorption of a glucose load was also estimated in absorption of a glucose load was also estimated in previously fasted nibbling and gorging mice. These estimates were based on composite, serial measurement of both plasma glucose specific activities and 14C-labeled fatty acids. The total rate of fatty acid synthesis from both exogenous and endogenous glucose-C was only 15% higher than the rate from exogenous glucose-C between 10 and 40 min. No significant differences between nibblers and gorgers were found.

Net changes in intermuscular fat before and during rapid lipogenic activation in mice

We have attempted to measure net changes in lipid content in a discrete "intermuscular" fat pad during rapid lipogenic activation that occurs after a previously fasted mouse nibbles a glucose-rich test meal for several minutes. The popliteal fat pad was chosen for the study since it has been shown to be about an order of magnitude more active than the epididymal fat pad in the synthesis of fatty acids from glucose carbon in fasted-refed mice. We found a highly reproducible net loss in the popliteal fat pad's weight and lipid content during fasting. Net deposition of lipid occurred when 24-h fasted mice were allowed to eat a fat-free, 58% glucose diet for several minutes. In two out of three experiments lipid repletion was complete after one brief period of nibbling. Significant decreases in the net amounts of each majority fatty acid, 16:0, 16: 1, 18:1 and 18:2, were found to occur in the popliteal fat pad during a 24-h fast. After nibbling their test meal for several minutes, previously fasted mice restored their major essential fatty acid, linoleic acid, to the original fed level within 2 h, even though total lipid repletion was incomplete. Highly significant net increases in each major non-essential fatty acid were also found after brief ingestion of the test meal; however, in one experiment (incomplete repletion) only about half of the depleted fatty acids was restored. When two successive glucose test meals were eaten (2-h interval), popliteal fat converted glucose carbon to fatty acids more than twice as fast after the second feeding as after the first. However, no significant additional increment either in tissue weight or in total lipid fatty acids was found after the second test meal. Based on these findings, the possible significance of intermuscular fat in the utilization and deposition of fat is discussed.

Contribution of intermuscular fat to lipogenesis from dietary glucose carbon in mice

We assessed the contribution of various tissues to the synthesis of fat from glucose carbon in mice during rapid lipogenic activation induced by a glucose test meal. Nibbling and gorging mice were maintained on a 58% glucose, fat-free diet. The mice were fasted 22 h and refed 5-10 muCi [U-14C]glucose (120 mg/20 g body weight) either by gastric intubation or as a test meal (58% glucose diet). The muscular carcass in both nibblers an gorgers contained more than 75% of the total radioactivity in the fatty acids derived from glucose; liver and epididymal fat pad accounted for only a small percentage. Mort than half the carcass activity was in the "muscular" tissue as neutral lipid acids. We could isolate a discrete fat body in the popliteal region of the leg muscle ("muscle fat", "intermuscular fat"). The popliteal fat converted glucose carbon to fatty acids an order of magnitude faster, per unit weight, than the epididymal fat pad or skeletal muscle. The fatty acid moiety of the triacylglycerols had the major portion of the label in the popliteal fat 2 and 6 h after ingestion of the glucose test meals. The diacylglycerol pool was active at 2 h and its activity faded at 6 h implicating its intermediary role in lipid metabolism similar to published findings in epididymal fat pad. These results indicate that fat cells associated with the muscular carcass may play a major role in the de novo synthesis of fat from dietary carbohydrate in mice.

The significance of electrode surface area and stimulating thresholds in permanent cardiac pacing

This paper reports a long-term clinical study of evolving chronic thresholds of cardiac stimulation in patients with implanted pacemakers and two types of permanent prevenous unipolar pacing leads. A comprehensive collection of threshold measurements is presented and analyzed. These data were accumulated over a period of more than 4 years by measuring stimulation thresholds at each of five stimulus pulse durations ranging from 0.5 to 2.5 msec. Measurements were made at initial implantation and at first (21 month) and second (42 month) reimplantations.

Tumor extracellular triglycerides in mice during growth of Ehrlich ascites carcinoma

Our earlier work with Swiss-Webster mice has shown that most of the lipid in Ehrlich ascites tumor extracellular fluid is in the form of free fatty acids. This finding is in direct contradiction to earlier and subsequent reports from another laboratory that has found free fatty acids to be a very minor component and triglycerides to be the major lipid of Ehrlich ascites tumor extracellular fluid. In light of these contradictory reports, we have carried out a study patterned after that of other workers, but using our Swiss-Webster mice. As predicted from our earlier study, we have found very little triglyceride in Ehrlich ascites tumor extracellular fluid. Although we could demonstrate a significant, transient hypertriglyceridemia during tumor growth, maximum plasma triglyceride concentrations were an order of magnitude lower than those reported by other workers. In addition, and again in contrast to other reports, we found that plasma triglyceride and tumor extracellular fluid triglyceride levels in tumorous mice fell significantly with fasting. Thus, interesting differences in triglyceride metabolism between mouse and/or tumor strains seem to exist. Our present findings suggest, but do not prove, that triglycerides in the tumor extracellular fluid probably are not a major source of the rapidly turning over, tumor extracellular fluid free fatty acid in our mice.

Novel interference in thiobarbituric acid assay for lipid peroxidation

The thiobarbituric acid test for lipid peroxidation, when applied to a mixture of acetaldehyde and sucrose, produces a 532 nm aborbing chromogen which is indistinguishable from that formed by malonaldehyde and thiobarbituric acid. Unless special procedures are adopted to correct for this effect, the combined action of acetaldehyde and sucrose interferes seriously with the assay of lipid peroxidation reactions, notably those implicated in alcohol-induced liver injuries. However, this unusual thiobarbituric acid effect also can be used as a sensitive method for the detection of acetaldehyde.

Flux of free fatty acids among host tissues, ascites fluid, and Ehrlich ascites carcinoma cells

The role of plasma free fatty acids (FFA) in the transport of fatty acids from host tissues to Ehrlich ascites carcinoma in mice was studied. [9,10-(3)H] Palmitate complexed to mouse serum (albumin) was injected either intraperitoneally or intravenously into unanesthetized tumor-bearing mice. The incorporation of radioactivity into tumor extracellular fluid FFA, tumor cell FFA, neutral lipid, phospholipid, water-soluble material in cells and fluid, plasma FFA, host carcass total lipid fatty acids, and water-soluble (i.e., nonlipid) material was measured. In addition, the quantity of fatty acid in each of the above lipid fractions was determined. The data were analyzed by multicompartmental analysis (SAAM) using a digital computer, and fractional rate constants of FA movement within and out of the host-tumor system were calculated. These rate constants and pool size measurements were used to estimate the corresponding fluxes. Although FFA in the tumor's extracellular fluid were replaced rapidly, almost none of the newly formed fluid FFA was derived from plasma FFA. Moreover, the transfer of FFA from the tumor extracellular fluid FFA to plasma FFA was virtually negligible. We suggest that the net amount of FFA required to replace the fluid FFA utilized for tumor energy and growth may be derived from direct transfer of FFA from host tissues to the ascitic fluid and that plasma FFA is not an intermediate in this transport process. The transport of FFA from the host to tumor cell lipids through the tumor extracellular fluid was about 26-fold greater than that required to account for net lipid accumulation during growth.

Characteristics of the cholesterol-esterifying activity in normal and atherosclerotic rabbit aortas

Esterification of cholesterol with [1- 14 C]palmityl-CoA by an atherosclerotic cell-free homogenate was approximately 16-50-fold greater than that by a normal cell-free homogenate for a given amount of protein in the homogenate. This difference was due to hyperactivity of the cholesterol-esterifying system in the atherosclerotic cell-free homogenate rather than to depletion of radioactive palmityl-CoA in the reaction mixture containing normal homogenate. Neither an activator of cholesterol esterification in the soluble fraction of the atherosclerotic aortic homogenate nor an inhibitor in the soluble fraction of the normal aortic homogenate could be demonstrated. The pH optimum within the pH range covered for esterification and the apparent K m values were approximately the same in normal and atherosclerotic microsomes, suggesting that the enzymes were probably the same. The results suggested a higher concentration or a higher activity of the enzyme in or on atherosclerotic microsomes. An alternative possibility is that high concentrations of free cholesterol in the atherosclerotic microsomes were responsible for the augmented cholesterol esterification. This possibility seems unlikely, because the observed 2.3-fold increase in the free cholesterol concentration should not produce a 25-fold increase in cholesterol esterification. The rate of cholesterol esterification by atherosclerotic microsomes varied with the substrate: oleyl-CoA > palmityl-CoA > linoleyl-CoA.

Specific role of glucose in rapid lipogenic activation in vivo

Lipogenesis from glucose C was previously found to be rapidly activated as soon as mice nibbled a fat-free, glucose-rich diet. We have studied here whether such rapid activation is a specific effect of dietary glucose. The flux of endogenous glucose C to total lipid fatty acids (TLFA) in mice fasted for 1 day was compared with the minimal average flux of exogenous dietary glucose to TLFA during a 40-min period after the ingestion of various glucose-rich test meals by previously fasted mice. The fasted mice were injected intravenously with [U-(14)C]glucose, and the flux of glucose C to TLFA and to all "end products" was estimated from serial plasma glucose specific activity measurements and (14)C incorporation into TLFA 30 min after (14)C injection. Only 0.6 to 0.8 micro g of glucose C/min/20 g body wt was converted to TLFA, whereas 208 +/- 16 micro g of glucose C/min/20 g body wt was converted to all "end products" in the fasted animals. Previously fasted mice were fed [(14)C]glucose in small test meals as a neat solid, as a 30% aqueous solution, or as a fat-free, 58% glucose diet. During the next 40 min, the average flux of glucose C into TLFA increased at least 50- to 60-fold, regardless of the form in which glucose was fed; however, when glucose was fed as part of a complete fat-free diet, glucose was utilized at a much lower plasma glucose level than in mice fed either pure solid glucose or an aqueous glucose solution. Rapid activation of lipogenesis from glucose requires only glucose as a dietary constituent.

Lipogenic activation after nibbling and gorging in mice

Lipogenic activation was studied in mice that had been restricted to a single large meal once a day rather than being allowed to eat at frequent intervals throughout the night. Mice were injected intravenously with [U-(14)C]glucose, and the flux of glucose C to total lipid fatty acids (TLFA) and to all "end products" was estimated from serial plasma glucose specific activities and measurements of incorporation of (14)C into TLFA of hepatic and extrahepatic tissues. Tracer studies were carried out in mice fasted for 1 day and at various times after the mice ate one or two small test meals or a single large test meal. Test meals consisted of a fat-free, 58% glucose diet. The flux of glucose C to TLFA increased by an order of magnitude within an hour after mice nibbled a test meal for several minutes. After ingestion of two small test meals or a single large test meal, the flux of glucose C to TLFA increased from a fasting rate of 0.5 to 35 and 87 micro g of glucose C/min/20 g body wt, respectively. Although trained meal eaters are thought to have abnormally increased lipogenesis, their lipogenic response to a single test meal was the same as that previously reported for untrained nibbling mice. Most of the newly synthesized fatty acids were found in extrahepatic tissues. Ingestion of a first test meal completely prevented the expected hyperglycemic response following ingestion of a second test meal even though the latter contained over 10 times more glucose than that in the total body glucose pool.

Serological cross-reactions between the hemagglutinin subunits of H0N1 and H1N1 influenza viruses detected with "monospecific" antisera

"Monospecific" antisera to the "fragile" hemaglutinnis of H0N1 (PR8) and H1N1 (FM1) influenza viruses detected an asymmetrical cross-reaction between these two strains that could not be explained by a common neuraminidase.

Compartmental and semicompartmental approaches for measuring glucose carbon flux to fatty acids and other products in vivo

We have attempted to estimate the flux of glucose carbon to total body fatty acids and to other metabolic end products in Bar Harbor 129/J mice fasted 5-8 hr. Tracer [U-(14)C]glucose was injected intravenously, and the following data were obtained at various times up to 180 min: plasma glucose C specific activity, plasma glucose concentration, total body glycogen, and (14)C in total body fatty acid, total body lipid, unsaponifiable lipid, expired CO(2), and in hepatic and extrahepatic glycogen. The data were analyzed by three techniques, namely, multicompartmental, semicompartmental, and noncompartmental analyses. All three methods yielded comparable rates of glucose C conversion to total body fatty acids (2-3 micro g of glucose C/min/20 g of body weight). Although the semicompartmental approach is extremely simple (it only requires analyses of plasma glucose specific activity as a function of time and (14)C-labeled fatty acid at one point in time), it gives an apparently valid approximation for the flux of glucose C to fatty acids. Other quantitative aspects of glucose metabolism in postabsorptive mice are also considered.

Rapid activation and inactivation of fatty acid synthesis from glucose in vivo

The flux of glucose carbon to total body fatty acids was measured in unanesthetized mice either after fasting or 50-80 min after they nibbled a small test meal containing 120 mg of glucose (fasted-refed). Flux was calculated from plasma [(14)C]glucose specific activity curves and from total body (14)C-labeled fatty acid 30 min after intravenous injection of tracer [(14)C]glucose. Mobilization of liver glycogen, changes in the body glucose pool size, and total flux of carbon through the glucose pool during periods of fasting and refeeding were defined. Liver glycogen was almost completely depleted 8 hr after food removal. Body glucose pool size fell during fasting and increased after refeeding the test meal. Irreversible disposal rate of glucose C varied directly with body glucose pool size; but flux of glucose C into fatty acids increased exponentially as body glucose concentration increased. Within an hour after nibbling a small test meal, the flux of glucose C into total body fatty acids increased 700% in mice previously starved for 24 hr. However, flux of glucose C into fatty acids in postabsorptive mice (food removed for 2 hr; livers rich in glycogen) was only about 2% of the value calculated from published studies in which the incorporation of an intubated [(14)C]glucose load into total body fatty acid was measured in mice. A possible explanation for this phenomenon is presented.

Glucose: a possible intermediate in the oxidation of the side chain of cholesterol in resting and stimulated rats

The effect of repeated muscular contraction on the rate of oxidation of the side chain of cholesterol was studied in anesthetized rats. The animals received an intravenous pulse-label injection of either cholesterol-26-(14)C, incorporated into rat plasma lipoproteins, or bicarbonate-(14)C. In half the animals of each group, the hind legs were repeatedly stimulated by electrical impulses. A multicompartmental analysis was attempted, based on the disappearance curve of plasma free cholesterol-(14)C and on the excretion rate of expired (14)CO(2), as well as on previously reported rates of bile acid and adrenal steroid secretion. The rate of expired (14)CO(2) originating from cholesterol-26-(14)C was much less than that predicted by the digital computer analysis; cholesterol degradation could not be evaluated since the data were incompatible with a model that assumes direct oxidation of the side chain to CO(2). A revised model was postulated in which an important fraction of the side chain of cholesterol would be converted to CO(2) only after previous conversion to glucose. Direct measurement of plasma glucose-(14)C after the injection of cholesterol-26-(14)C supported this hypothesis.