Muscle lipoprotein lipase activity in voluntarily exercising rats

Voluntary exercise of rats in freely rotating work wheels has been extensively used, but muscle adaptations that result from such exercise training are poorly documented. The purpose of this study was to determine whether the exercise performed by voluntarily active rats would increase succinate dehydrogenase or lipoprotein lipase activities in the soleus muscle (SM) or the red portion of the vastus lateralis muscle (RV). In SM the activities of these two enzymes were not increased after 7 or 16 wk of voluntary exercise. Succinate dehydrogenase activity in RV was moderately increased after 7 and 16 wk of voluntary activity (P less than 0.05). Substantial increases occurred in RV lipoprotein lipase activity (P less than 0.01). The increase in RV lipoprotein lipase activity was positively related to distance run by the rats. The results indicate that only small muscle-dependent increases in mitochondrial enzymes occur in rats allowed to exercise voluntarily in rodent work wheels. Voluntary exercise training induced a selective increase in lipoprotein lipase activity in a muscle containing a high percentage of fast-twitch red fibers, a response absent in a muscle containing a predominance of slow-twitch red fibers. It is unlikely that this differential response can be explained by exercise-induced changes in plasma hormone concentrations involved in the regulation of lipoprotein lipase.     

Tissue-specific alterations in lipoprotein lipase activity in copper-deficient rats

Copper deficiency results in alterations in lipid metabolism that include elevations in serum cholesterol and triglycerides and a decrease in whole-body respiratory quotient. Copper-deficient animals are also leaner even though electron micrographs of the myocardium present increased lipid droplet accumulation. To address whether a compromised copper status impacts triglyceride deposition in a tissue-specific manner, the activity of lipoprotein lipase was measured in adipose tissue and cardiac and skeletal muscle. Weanling rats fed a copper-restricted diet (<1 ppm) for 6 wk demonstrated a greater than twofold increase in cardiac lipoprotein lipase activity concomitant with a significant reduction in adipose tissue lipoprotein lipase activity. Skeletal muscle lipoprotein lipase activity was not altered by the copper-deficient state. The results of this study suggest that copper deficiency may induce a tissue-specific alteration in lipoprotein lipase activity in rats, which may contribute to the notable deposition of lipid substance in myocardium and the concomitant general body leanness.     

Increased lipoprotein lipase activity of skeletal muscle in cold-acclimated rats

The activity of lipoprotein lipase (LPL) in the heart, diaphragm, and soleus muscles was markedly increased in cold-acclimated rats and it was even greater in rats treated with oxytetracycline (OTC) while exposed to cold. Other skeletal muscles studied had low and variable activities which were not significantly increased by cold acclimation or by cold plus OTC treatment. It appears therefore that, apart from the heart and the muscles involved in respiratory movements, LPL activity is primarily associated with those muscles which contain a predominance of slow-twitch oxidative fibers, and that the enzyme in muscle, heart, and diaphragm responds to cold acclimation and cold plus OTC treatment in a parallel fashion in these tissues.     

Endogenous plasma lipoprotein lipase activity in fed and fasting rats may reflect the functional pool of endothelial lipoprotein lipase

In this study, a correlation was sought between the circulating lipoprotein lipase activity and nutritional state in the rat. In fed rats, the plasma lipoprotein lipase activity was between 30 and 120 munits/ml, whereas after an overnight fast in restraining cages, the lipoprotein lipase plasma levels were between 280 and 500 munits/ml. The plasma lipoprotein lipase activity was inhibited by a specific high titre goat antiserum to rat lipoprotein lipase. No effect of fasting was seen on the plasma hepatic triacylglycerol lipase. 6 h after fasting, adipose tissue lipoprotein lipase decreased maximally, but plasma lipoprotein lipase was not changed and rose only after 16 h. Thus, it seems that most of the lipoprotein lipase activity in the fasting plasma was related to the 3-fold rise in lipoprotein lipase activity in the heart, which may represent total muscle lipoprotein lipase. The increase in heart lipoprotein lipase was due in part to an increase in the t1/2 of the enzyme from 1.2 to 2.9 h. To determine whether the high plasma levels in the fasting rats might result from impaired clearance of the enzyme by the liver, functional hepatectomy was carried out. 15 min after hepatectomy, plasma lipoprotein lipase rose up to 20-fold in fed and about 6-fold in fasting rats. Lipoprotein lipase activity extracted by the liver was calculated to be 30-60 munits/ml in the fed and 171-247 munits/ml plasma per min in fasting rats. An increase in lipoprotein lipase activity in extrahepatic tissues (heart, lung, kidney, diaphragm and adrenal) occurred 30 min after hepatectomy in fed rats. The increase in heart lipoprotein lipase was due to an increase in heparin-releasable fraction. Since no impairment of hepatic clearance of circulating plasma lipoprotein lipase was found, the high fasting plasma lipoprotein lipase activity may be related to an increase in enzyme synthesis, decreased enzyme turnover and an expansion of the functional pool in tissues such as the heart and probably muscle. The present findings indicate that measurement of endogenous plasma lipoprotein lipase can provide information with respect to the size of the functional pool under normal and pathological conditions.     

Long term incubation of cardiac myocytes with oleic acid and very-low density lipoprotein reduces heparin-releasable lipoprotein lipase activity

An exogenous [³H]triolein emulsion was hydrolyzed by intact cardiac myocytes with functional LPL located on the cell surface. This surface-bound LPL could be released into the medium when cardiac myocytes were incubated with heparin. Incubation of cardiac myocytes with VLDL, or the products of TG breakdown, oleic acid or 2-monoolein, did not increase LPL activity in the medium. However, incubation of cardiac myocytes with either VLDL or oleic acid for > 60 min did reduce heparin-releasable LPL activity. In the heart, this inhibitory effect of FFA could regulate the translocation of LPL from its site of synthesis in the cardiac myocyte to its functional site at the capillary endothelium.Abbreviations LPL lipoprotein lipase - TG triacylglycerol - FFA free fatty acids - VLDL very-low density lipoprotein     

The lipoprotein lipase (clearing-factor lipase) activity of cells isolated from rat cardiac muscle.

The total lipoprotein lipase activity recovered in suspension of cells prepared from adult rat hearts was unaffected by the nutritional state of the animals used. The enzyme activity present in the cell suspensions was almost exclusively associated with the cardiac muscle cells present as the major cell type.     

Adipose, muscle and lung tissue lipoprotein lipase activities in young streptozotocin treated rats

Lipoprotein lipase (LPL) activity was studied in adipose, muscle and lung tissues of post-weanling rats 48 and 96 hours after experimentally induced diabetes by streptozotocin administration. Weight gain was reduced, and blood glucose level increased about 3-4 fold above the control level as an indication of the diabetic state. LPL activity in brown and white adipose tissues decreased in diabetic rats to 10-30% of the control level. In soleus muscle the LPL activity was slightly enhanced 96 hours after the streptozotocin injection. In cardiac muscle the LPL activity was markedly increased already 48 hours after the administration of streptozotocin and the increase remained significant until 96 hours. There was in the pulmonary tissue also an increase of LPL activity of diabetic rats, although this was significant only 96 hours after streptozotocin treatment. The results suggest marked tissue specific variation in the LPL activity. Moreover, tissue responses to experimentally induced diabetes vary. In adipose tissue the decrease in the LPL activity suggests that lipid transport to adipocytes is decreased while an increase in skeletal and cardiac muscles and in lung tissue proposes that their lipid utilization is enhanced.     

Activity of lipoprotein lipase in thyroidectomized rats

Total plasma postheparin lipolytic activity as well as lipoprotein lipase activity in plasma was higher after heparin injection in thyroidectomized rats than in controls. In contrast, the activity of liver lipase was lower in thyroidectomized rats. Adipose tissue from thyroidectomized rats contained more lipoprotein lipase activity than adipose tissue from controls as measured both in extracts of tissue homogenates and medium from in vitro incubations of tissue pieces. There were no differences between control and hypothyroid rats in the disappearance of intravenously injected 125I-labeled lipoprotein lipase, but when a low dose of heparin was injected before the labeled enzyme, the disappearance of 125I-labeled lipoprotein lipase was more retarded in thyroidectomized rats. The elimination of heparin itself was slightly retarded by thyroidectomy.     

High liver lipoprotein lipase activity in hyperlipemic developing rats from undernourished pregnant mothers

To study the potential relationship between circulating triacylglycerol (TAG) levels and lipoprotein lipase (LPL) activity in the newborn rat liver, pups from undernourished or normal control mothers were nursed by normal dams, and studied at 0, 1, 15 or 30 days of age. Plasma TAG levels and liver TAG concentration increased more in pups from undernourished mothers than they did in controls. At birth, liver LPL activity was similarly high in both groups but, whereas in controls it decreased progressively after birth, in pups from undernourished mothers it remained stable until 15 days of age. Results suggest that the hypertriglyceridemia present in pups from undernourished mothers may be responsible for the sustained high LPL activity in their liver which may enhance the hepatic uptake of circulating TAG.     

beta-adrenergic responsiveness of rats treated chronically with isoproterenol

The effect of chronic administration of isoproterenol on isoproterenol-induced thirst and isoproterenol-induced changes in heart rate and selected organ weights of male rats was studied. Administration of 25 micrograms isoproterenol/kg, s.c., in saline daily for 10 days was accompanied by a significant attenuation of the characteristic increase in water intake following a challenging dose of isoproterenol (25 micrograms/kg, s.c.) on the 11th day. Administration of 25 micrograms isoproterenol/kg, s.c., every 2nd, 3rd or 4th day for 10 days was without significant effect on water intake following isoproterenol (25 micrograms/kg, s.c.) on the 11th day. Administration of 25 micrograms isoproterenol/kg, s.c., every day for 10 days led to a slight increase in cardiac responsiveness to a challenging dose of isoproterenol (25 micrograms/kg) on the 11th day. Chronic treatment with this low dose of isoproterenol for 10 days was also accompanied by a significant increase in the ratio of heart weight to body weight but no significant changes in the ratio of kidney, adrenal, thyroid, spleen, or interscapular brown fat to body weight. Thus, daily administration of the beta-adrenergic agonist isoproterenol for 10 days can alter beta-adrenergic responsiveness in the rat with beta 1 (heart rate) and beta 2 (thirst) mediated responses showing opposite effects. In addition, the results suggest that tests of beta-adrenergic responsiveness must be assessed in terms of the frequency of administration of the agonist.     

Effect of starvation on lipoprotein lipase activity in the liver of developing rats

Liver lipoprotein lipase activity in neonatal (1- and 5-day-old) rats was 2-3-times than in the liver of adult rats. In mid-suckling (15-day-old) or weaned (30-day-old) animals, it was not significantly different from the low activity detected in adult rats. Starvation resulted in a 3-fold increase of lipoprotein lipase activity in the neonatal liver, but did not affect the activity in the liver of mid-suckling, weaned or adult rats. When isolated livers from both 1- and 5-day-old pups were perfused with heparin, a sharp peak of lipoprotein lipase activity appeared in the perfusate. In fed neonates, the peak area accounted for about 70% of the total (released + non-releasable) activity. In starved neonates, the proportion of heparin-releasable activity increased up to about 90%. These results indicate that neonatal rat liver lipoprotein lipase activity is markedly affected by changes in the nutritional status of the animal, and the effect is restricted to the vascular pool of the enzyme, as was reported in extrahepatic tissues from adult rats.     

Restoration of lipoprotein lipase activity in insulin-deficient rats by insulin infusion is tissue-specific

The activity of lipoprotein lipase was measured in white and brown adipose tissues, red vastus lateralis muscle, and heart of rats that have been insulin deficient (streptozotocin, 75 mg.kg-1) for 2 weeks, and that have then received implants of insulin-delivering minipumps (17 U.kg-1.day-1) for 1 or 4 days. Normal glycemia was restored in insulin-deficient animals after 4 days of insulin treatment. Hypertriglyceridemia, but not hypercholesterolemia, was reversed after 4 days of insulin infusion. After 2 weeks of insulin deficiency, fasting lipoprotein lipase activity was lowered in all tissues studied. In white adipose tissue, lipoprotein lipase decreased to 50% of control values. After a single day of insulin infusion, even if tissue weight has not yet been greatly affected, total activity was completely restored to control levels. Enzyme activity in brown adipose tissue was also depressed in deficient animals, and insulin infusion was followed by a slow recovery of activity, to a level intermediate between those of control and insulin-deficient groups. Insulin status had milder effects on lipoprotein lipase activity in vastus lateralis muscle than in the adipose tissues. Deficient rats displayed 60% less activity than controls, and 4 days of hormone infusion only partially restored enzyme activity. There was a large loss of lipoprotein lipase in the heart following 2 weeks of insulin depletion, which was not counteracted by hormone infusion. Thus the speed and extent of recovery of lipoprotein lipase activity following hormone replacement in insulin-deficient animals varied widely among tissues. These findings suggest that insulin is part of the factors that determine the tissue specificity of lipoprotein lipase regulation.     

A comparison of lipoprotein lipase activity and adipocyte differentiation in growing male rats.

During adipose tissue development changes in lipoprotein lipase activity per adipocyte precede significant changes in fat cell size. Lipoprotein lipase activity per adipocyte increases fourfold from the second to seventh postnatal week. Furthermore, when isolated adipocytes and stromal--vascular cells are prepared by collagenase digestion of adipose tissue, there is a progressive shift in enzyme activity during development from the stromal-vascular compartment to the adipocyte fraction. The data support the concept that during normal development a "bed" of preadipocytes is synthesized during the suckling period. The data further suggest a regulatory role for lipoprotein lipase in the control of "lipid-filling" during early postnatal development.