Post-heparin plasma lipoprotein and hepatic lipases. Relationships to high density lipoprotein cholesterol and to apolipoprotein CII in familial hyperalphalipoproteinemic and in normal subjects

Post-heparin lipoprotein lipase (PH-LPL)-high density lipoprotein cholesterol (HDL-C) interrrelationships were assessed in 9 subjects with documented familial hyperalphalipoproteinemia (FHA) and in 8 controls to focus on potential biochemical etiologies of FHA and relationships of HDL-C to triglyceride hydrolysis and PH-LPL. FHA subjects had mean HDL-C and HDL2-C levels > twice controls; their PH-LPL levels (mean +/- SEM) (3.14 +/- 2.3 mumol FFA/h/ml) were also > twice that of controls (15.0 +/- 1.6) (P < 0.01), but post-heparin hepatic lipase levels (PH-HL) in the FHA and control subjects did not differ (18.1 +/- 1.6 vs 26.6 +/- 4.3, P > 0.1). For all subjects (FHA and controls) PH-LPL was positively correlated with HDL-C (r = 0.79, P < 0.01) and with HDL2-C (r = 0.90, P < 0.01), but not with HDL3-C (r = --0.02). There were no significant PH-HL and HDL-C interrelationships, P > 0.1. The amount of apo CII (the primary activator of PH-LPL) in HDL2 was greater in the FHA (mean +/- SEM) (16.1 +/- 2.5 microgram/ml plasma) than in control subjects (4.7 +/- 0.9, P < 0.01). There were strong positive correlations between HDL2 apo CII and both PH-LPL (r = 0.79, P < 0.01) and HDL2-C (r = 0.80, P < 0.01). Apo CII as a percentage of HDL2 protein was higher in FHA than control subjects (mean +/- SEM) (1.2 +/- 0.3% vs 0.5 +/- 0.2%, P < 0.01). Apo CII as a percentage of HDL3 protein was similar in FHA and control subjects. We postulate that increased turnover rate of triglyceride-rich lipoproteins due to high LPL activity may be an important factor leading to the elevation of HDL-C in FHA. The highly significant positive correlation between HDL2-C and PH-LPL provides strong clinical evidence for the theory that HDL2 is formed during the hydrolysis of triglycceride-rich lipoproteins. The high concentration of HDL2 apo CII in FHA subjects may be caused by increased catabolism of triglyceride-rich lipoproteins in the presence of high endothelial LPL, with transfer of apo CII from very low to high density lipoproteins.

A micromethod for assay of lipoprotein lipase activity in needle biopsy samples of human adipose tissue and skeletal muscle

A rapid and simple procedure for assay of lipoprotein lipase (LPL) activity in small amounts of human adipose tissue and skeletal muscle is described and validated. The enzyme is eluted from tissues with heparin and the activity is determined from the eluate by measuring the release of [14C]oleic acid from a gum arabic stabilized emulsion of glycerol-tri[14C]oleate in a Tris-buffer medium containing albumin and pooled normal human serum. Reproducible results are obtained with amounts of tissue ranging from 2 to 25 mg. The Km values of the adipose tissue and skeletal muscle LPL for the triolein substrate were 0.74 +/- 0.06 and 0.77 +/- 0.05 mmol/l, respectively. The standard radioactive triolein emulsion was hydrolyzed by adipose tissue LPL at a rate closely similar to rat VLDL-triglyceride labeled in vivo with [1-14C]palmitic acid, suggesting that the experimental substrate behaved in a similar manner to the natural substrate. The LPL activity was much higher in adipose tissue than in muscle. In adipose tissue the LPL activity was 2--4 times higher in women than in men whereas no sex difference was present in the LPL activity of muscle.

Dansyl phosphatidylethanolamine-labeled very low density lipoproteins. A fluorescent probe for monitoring lipolysis

The fluorescent phospholipid dansyl phosphatidylethanolamine (DPE) (dansyl, 5-dimethylaminonaphthalene-1-sulfonyl) was incorporated into very low density lipoproteins (VLDL) to form DPE-VLDL. The addition of milk lipoprotein lipase to DPE-VLDL in the presence of albumin resulted in a greater than 3-fold fluorescence increase and a 20 nm blue shift in the wavelength of the emission maxima of the dansyl fluorophore. The lipoprotein lipase-induced fluorescence changes occurred concomitantly with the release of free fatty acids from VLDL. Lipoprotein lipase did not produce fluorescence changes in DPE incorporated into either low or high density lipoproteins. The rate of fluorescence increase in DPE-VLDL was maximal at 37 degrees C, dependent on the concentration of lipoprotein lipase and VLDL, and followed typical Michaelis-Menten kinetics with a Km of 1.0 for lipoprotein lipase. Both the initial rate and the total fluorescence increase correlated well (r = 0.98 and 0.95) with the release of free fatty acids. We conclude that the lipoprotein lipase-induced fluorescence increases in DPE-VLDL provide an accurate, convenient, and the only noninvasive means of following continuously the lipolysis of human VLDL.

Effect of acute vigorous exercise on lipoprotein lipase activity of adipose tissue and skeletal muscle in physically active men

Ten well-trained men ran a distance of 20 km in the morning after overnight fasting. Lipoprotein lipase (LPL) activity was determined from heparin eluates of adipose tissue and skeletal muscle before and after the exercise. The mean LPL activity rose 2.1-fold in skeletal muscle (p < 0.01) and by 20 % in adipose tissue (p < 0.05) during the running. No significant change occurred in serum lipid or lipoprotein concentrations. The plasma insulin decreased and plasma glucagon increased during the exercise. The muscle LPL increment was significantly related to the fall of insulin/glucagon ratio. The results show that during exercise the skeletal muscle is adapted for increased uptake of circulating triglycerides which are either utilized immediately or used for restoration of muscle lipid stores after the end of exercise.

Effects of caloric restriction on lipid metabolism in man: changes of tissue lipoprotein lipase activities and of serum lipoproteins

Heparin-releasable lipoprotein lipase (LPL) activity was measured in biopsy samples of adipose tissue and skeletal muscle of 8 normal healthy females, first during an isocaloric diet and then after 2 and 7 days on a 400-kcal diet. In adipose tissue the LPL activity expressed per tissue weight fell to 38% and to 22% of the initial level after 2 and 7 days' caloric restriction, respectively. In skeletal muscle the LPL activity rose slightly after two days (+24%) but decreased to 49% of the initial value after seven days on diet. The estimated total body LPL activity decreased to 50% and to 20% of the baseline value after 2 and 7 days, respectively, but the relative contribution of skeletal muscle to the total LPL increased from 10 to 30%. The triglyceride and VLDL triglyceride concentrations were not significantly changed during the low calorie diet but the LDL triglyceride increased and the HDL cholesterol decreased significantly (P less than 0.01). It is concluded that substantial restriction of calorie intake results in a decrease of over-all triglyceride removal capacity but in an increase of the fraction removed by skeletal muscle. The decrease of HDL cholesterol is probably a consequence of the low turnover of exogenous and endogenous triglyceride-rich lipoproteins.

Lipoprotein lipase activity in adipose tissue and skeletal muscle of runners: relation to serum lipoproteins

Physically well-trained people generally have lower VLDL-triglyceride and higher HDL-cholesterol levels than sedentary subjects. To examine the underlying mechanisms of this lipoprotein pattern, we measured the lipoprotein lipase (LPL) activity in needle biopsy specimens of adipose tissue and skeletal muscle of competitive runners and of body weight-matched, physically less-active controls. The active sportsmen were either sprinters, whose training program consisted mainly of athletics of short duration or long distance runners undergoing a strenuous endurance exercise program. In sprinters (all males) the serum lipid and lipoprotein concentrations did not differ significantly from those of controls and the mean LPL activities in muscle and adipose tissue were also similar in these two groups. The long distance runners (both sexes), on the other hand, had higher means levels of HDL-cholesterol than the respective controls. The LPL-activity of both adipose tissue (p less than 0.05) and skeletal muscle (p less than 0.01) was significantly higher in male long distance runners than in control males. Female runners had higher muscle LPL activity than controls (p less than 0.01) but in adipose tissue the difference in LPL activity was not significant. Rough estimates calculated for LPL activity present in whole body adipose tissue and skeletal muscle indicated that total LPL activity was 2.3 times higher in male long distance runners and 1.5 times higher in female long distance runners than in the respective controls. In combined groups of male runners and controls, there was a highly significant positive correlation between the serum HDL-cholesterol level and the LPL activity of adipose tissue expressed per tissue weight (r = +0.72, p less than 0.001) or per whole body fat (r = +0.62, p less than 0.001). The group means of HDL-cholesterol and adipose tissue LPL activity in the five cohorts studied (male sprinters, distance runners and controls and female distance runners and controls) were also positively correlated (r = +0.94). It is concluded that endurance training is associated with an adaptive increase of LPL activity not only in skeletal muscle but also in adipose tissue. These changes are not observed in sprinters who are trained by exercises of shorter duration. The high HDL-cholesterol levels of physically well-trained people are probably accounted for, at least partly, by the increased LPL activity and the concomitant rapid turnover or triglyceride-rich lipoproteins.

Lack of behavioural effects following intraventricular infusion of somatostatin in the conscious goat

The effect of IV or intracerebroventricular (ICV) administration of somatostatin was studied on the behaviour of conscious goats. The doses of somatostatin infused IV were 100 and 300 microgram for 30 min and 600 microgram for 6 min. The doses infused ICV were 10 and 100 microgram for 30 min and 600 microgram for 6 min. In contrast to earlier reports on experiments with rats ,no behavioral effects whatsoever were seen in goat. IV infusion of 100 to 600 microgram and ICV infusion of 600 microgram of somatostatin caused a difinite reduction in the secretion of insulin and growth hormone, but had no effect on the concentration of blood glucose. The reason why neither IV nor ICV administration of somatostatin had any behavioural effects in the conscious goat, in contrast to the effects in rat, cannot be explained with certainty. This may be due to species specificity, to the amount of somatostatin reaching the central nervous system, or to some metabolic changes in rat but not in goat.

Effect of acute ethanol load on postheparin plasma lipoprotein lipase and hepatic lipase activities and intravenous fat tolerance

This study aimed to examine the possibility that ethanol-induced rise of serum triglyceride concentration in man is partly due to an impaired removal of triglycerides from the circulation. Acute ethanol loads given to normal human subjects after an overnight fast reduced the postheparin plasma lipoprotein lipase activity by an average of 25% but did not influence the postheparin plasma hepatic lipase activity or fractional removal of Intralipid triglyceride. When alcolhol was administered to fed subjects in the evening the postheparin plasma hepatic lipase was significantly decreased in the next morning as compared to corresponding control value but the lipoprotein lipase and Intralipid clearance were not changed. It is concluded that the slight decrease of lipoprotein lipase during alcohol intoxication may contribute to the hyperlipemic effect of ethanol.

Relation of plasma high-density lipoprotein cholesterol to lipoprotein-lipase activity in adipose tissue and skeletal muscle of man

Lipoprotein-lipase activity (LPL) was measured in biopsies of adipose tissue and skeletal muscle of normal human subjects, and the results were related to concentrations of cholesterol and triglycerides in plasma lipoproteins. Adipose-tissue LPL activity was significantly higher in females than in males, whereas no sex difference was observed in skeletal-muscle LPL activity. A highly significant positive correlation was present between the plasma high density lipoprotein (HDL) cholesterol level and LPL activity in adipose tissue (r = +0.66, P less than 0.001) but not between HDL and skeletal-muscle LPL. The results suggest that the activity of LPL in adipose tissue and the rate of catabolism of triglyceride-rich lipoproteins might be one of the factors that determine the concentration of HDL in plasma and at least partly account for the known sex difference in plasma HDL level.

Response of plasma insulin to small doses of tolbutamide in obesity and acromegaly

The early response of plasma insulin (IRI) to successive intravenous doses of 50, 100 and 200 mg of tolbutamide was studied in nondiabetic obese subjects, in acromegalic patients, and in healthy controls. The smallest dose raised the plasma IRI level within two minutes in all subjects. The insulin response was correlated with the basal IRI in controls and in obese subjects but not in acromegalic patients. At each tolbutamide dose level the mean IRI response of obese subjects was about three times greater than in controls. In acromegalic subjects an increased response was observed only after the 200 mg dose of tolbutamide. The results suggest that in obesity the stimulus-secretion coupling of insulin output is normal but the number of secretory units is increased. In acromegaly the insulin release pattern is compatible with an increased number of secretory units which, however, are relatively insensitive to tolbutamide, probably due to an elevated serum growth hormone level.

Ethanol-induced alterations of glucose tolerance, postglucose hypoglycemia, and insulin secretion in normal, obese, and diabetic subjects

Ethanol at an average blood concentration of 1 mg. per milliliter enhanced the immediate (first-phase) and prolonged (second-phase) insulin response to an intravenous glucose load in nonfasting normal human subjects. Simultaneously, the glucose disposal rate was increased and the postglucose hypoglycemia was accentuated, resulting in definite hypoglycemic symptoms in some individuals. Oral glucose tolerance was not changed by ethanol administration, but the thirty-minute blood glucose and plasma insulin values were increased, suggesting that alcohol might accelerate the absorption of glucose from the gut. Ethanol given orally during evening hours (1.5 gm. per kilogram) caused a nocturnal hyperinsulinemia and a decrease of blood glucose, but not an actual hypoglycemia. Oral glucose tolerance and plasma insulin response tested the next morning, when ethanol had disappeared from the blood, were not influenced by drinking the previous evening. The K-value of intravenous glucose was increased at this time, however. When alcohol was administered for one week at a dose corresponding to 25 per cent of daily calories and substituting for fat, both the oral and intravenous glucose tolerances were impaired in each subject but the insulin response remained unchaged. In obese nondiabetic subjects, ethanol did not potentiate the early insulin response to intravenous glucose but it increased the second phase of insulin secretion in response to sustained hyperglycemia. In contrast to conditions in nonobese subjects, the glucose disposal rate was not incresed and postglucose hypoglycemia was not accentuated by ethanol in overweight subjects. In insulin-deficient diabetic patients the absent early insulin response could not be restored by ethanol, and the late component of insulin release was little increased by alcohol infusion. Ethanol did not improve the glucose utilization of diabetic patients.

Effect of muscular exercise on insulin secretion

Studies were carried out to define the possible role of insulin in the increased assimilation of glucose caused by exercise. First, plasma immunoreactive insulin (IRI) response in conventional intravenous glucose tolerance tests was measured in normal subjects at rest and during or immediately after ergometer exercise. In all cases the glucose disappearance rate (K value) was increased by exercise but the plasma IRI level assayed at two minutes after glucose showed no constant change from corresponding values at rest. Secondly, the insulin secretion rate was assessed by giving I-131-labeled insulin as a constant-rate infusion and measuring the plasma IRI specific activity. In the fasting state the insulin secretion rate varied from 0.7 to 6.0 mU. per min., and was highest in obese subjects. Increase of blood glucose level to 130-150 mg. per 100 ml. with a constant-rate glucose infusion doubled the insulin secretion rate. In subjects with a normal working capacity moderate exercise of thirteen to twenty minutes' duration did not significantly change the secretion rate whether the muscular work occurred in the fasting state or with glucose infusion. On the contrary, in all subjects with a reduced performance due to coronary heart disease the insulin secretion rate was strikingly increased by exercise and the plasma IRI level rose.

It is concluded that immunoreactive insulin is not the mediator of accelerated glucose metabolism caused by muscular work but that hypoxia of islet tissue may be a stimulus to insulin release.

Effect of acute elevation of plasma glycerol, triglyceride and FFA levels on glucose utilization and plasma insulin

The effects of raised plasma levels of glycerol, free fatty acids (FFA) and triglyceride on the disappearance rate (Kg) of intravenously administered glucose (25 gm.) and on the response of plasma immunoreactive insulin (IRI) to intravenous glucose were studied in thirty-six subjects. Increase of plasma glycerol level by oral glycerol augmented the Kg but not the glucose-stimulated plasma insulin level. Alimentary hyperglyceridemia enhanced the disappearance rate of glucose and the response of plasma insulin to glucose. Heparin injected intravenously during alimentary glyceridemia caused a five-to seven-fold increment of the levels of plasma FFA and glycerol but did not influence either the disappearance rate of glucose or the response of plasma insulin to glucose. Heparin injected in fasting state slowed significantly the disappearance rate of glucose, but did not change the insulin response.