Magnus Hultin

Lipoprotein lipase: regulation and role in lipoprotein metabolism.

Humans and animals deficient in lipoprotein lipase (EC 3.1.1.34; LPL) develop a massive hypertriacylglycerolaemia (Brunzell, 1995). Hence, LPL-mediated hydrolysis of lipoprotein-triacylglycerols is a necessary step in lipoprotein metabolism. The reaction occurs at ‘binding-lipolysis’ sites at the vascular endothelium (Olivecrona & Olivecrona, 1995). Here the enzyme is anchored by interaction with heparan sulfate chains from proteoglycans in the endothelial cell membrane. The reaction creates a high local concentration of fatty acids and these fatty acids can go two ways: immediate metabolism in subjacent tissue cells or transport into the general circulation in complex with albumin. There is evidence that in adipose tissue the uptake of fatty acids from chylomicrons correlates with the local LPL activity (Cryer ef al. 1976). This has led to the concept that LPL activity determines where fatty acids from triacylglycerol-rich lipoproteins are metabolized. Recent studies on mice that express LPL in only one tissue show, however, that lipoprotein metabolism is largely normal (Levak-Frank et al. 1995). The implication is that the fatty acids released by LPL can be transported in the blood to other sites for metabolism. This is in line with data and speculations from Frayn’s group (Samra et al. 1996). They have shown that adipose tissue always releases fatty acids (FFA) but the source differs. In the fasted state the fatty acids are derived from lipolysis of stored triacylglycerols but in the fed state most of them are derived from lipolysis of lipoproteintriacylglycerols by LPL. To study how fatty acids from chylomicrons partition between immediate uptake and recirculation in plasma FFA we have studied the metabolism of labelled chylomicrons in rats, and have analysed the results by compartmental modelling (Hultin et al. 1996). We gave the donor rats oleic acid, which becomes incorporated in the chylomicron triacylglycerols, and retinol, which appears as retinyl esters in the chylomicrons and, hence, provides a label for the core material that remains with the particle after lipolysis. The disappearance curves for both labels extrapolated to a volume that appeared to be larger than the blood volume. To study this more directly we injected ’lcr-labelled erythrocytes (RBC) together with the chylomicrons (Fig. 1). For RBC the distribution volume corresponded to about 5.6% of body weight in both fed and fasted rats. The distribution volumes for the chylomicron labels did not differ significantly between triacylglycerol or retinyl esters or between the nutritional states, but the distribution volumes were 10-25% larger for the chylomicron labels than for the RBC. As the two chylomicron labels behaved similarly, it is likely that it was the whole chylomicron that was distributed into the larger apparent volume. Some of this larger volume may be tissue spaces into which the chylomicrons have immediate access, such as the spaces of Disse in the liver. Some must represent dispersal of chylomicrons to endothelial binding-lipolysis sites. That dispersal occurs is not surprising. As lipolysis occurs at the endothelial sites, it is necessary that the chylomicrons remain at these sites for a significant time period (Olivecrona & Olivecrona, 1995). Harris & Harris (1973) found that with increased amounts of injected chylomicrons, the distribution volume decreased. This suggests that the number of dispersal sites is limited and raises the question of whether one can subsequently change the dispersal.

Rapid Subunit Exchange in Dimeric Lipoprotein Lipase and Properties of the Inactive Monomer

Lipoprotein lipase (LPL), a key enzyme in the metabolism of triglyceride-rich plasma lipoproteins, is a homodimer. Dissociation to monomers leads to loss of activity. Evidence that LPL dimers rapidly exchange subunits was demonstrated by fluorescence resonance energy transfer between lipase subunits labeled with Oregon Green and tetrametylrhodamine, respectively, and also by formation of heterodimers composed of radiolabeled and biotinylated lipase subunits captured on streptavidine-agarose. Compartmental modeling of the inactivation kinetics confirmed that rapid subunit exchange must occur. Studies of activity loss indicated the existence of a monomer that can form catalytically active dimers, but this intermediate state has not been possible to isolate and remains hypothetical. Differences in solution properties and conformation between the stable but catalytically inactive monomeric form of LPL and the active dimers were studied by static light scattering, intrinsic fluorescence, and probing with 4,4′-dianilino-1,1′-binaphtyl-5,5′-disulfonic acid and acrylamide. The catalytically inactive monomer appeared to have a more flexible and exposed structure than the dimers and to be more prone to aggregation. By limited proteolysis the conformational changes accompanying dissociation of the dimers to inactive monomers were localized mainly to the central part of the subunit, probably corresponding to the region for subunit interaction.

New Aspects on Heparin and Lipoprotein Metabolism

Lipoprotein lipase (LPL) and hepatic lipase (HL) are two enzymes which participate in metabolism of plasma lipoproteins. The enzymes are located at vascular surfaces and are released from their binding sites on injection of heparin. In this paper we give a short overview of the structure of the lipases and their role in lipoprotein metabolism. Earlier studies had shown that low molecular weight (LMW) heparin preparations result in lower LPL activities in blood than do corresponding amounts of conventional heparin. Studies with organ perfusion in rats show that the two types of heparin have similar ability to release the lipases from their binding sites in extrahepatic tissues, but that LMW heparin is less effective than conventional heparin in preventing rapid uptake and degradation of LPL by the liver. After injection of heparin the metabolism of triglyceride-rich lipoproteins is initially accelerated, presumably as a result of the high levels of circulating LPL. Then follows a phase when lipoprotein metabolism is slower than normal, perhaps because endothelial LPL has been depleted by accelerated transport to and degradation in the liver.

Communication in interdisciplinary teams: exploring closed-loop communication during in situ trauma team training

Objectives Investigate the use of call-out (CO) and closed-loop communication (CLC) during a simulated emergency situation, and its relation to profession, age, gender, ethnicity, years in profession, educational experience, work experience and leadership style. Design Exploratory study. Setting In situ simulator-based interdisciplinary team training using trauma cases at an emergency department. Participants The result was based on 16 trauma teams with a total of 96 participants. Each team consisted of two physicians, two registered nurses and two enrolled nurses, identical to a standard trauma team. Results The results in this study showed that the use of CO and CLC in trauma teams was limited, with an average of 20 CO and 2.8 CLC/team. Previous participation in trauma team training did not increase the frequency of use of CLC while ≥2 structured trauma courses correlated with increased use of CLC (risk ratio (RR) 3.17, CI 1.22 to 8.24). All professions in the trauma team were observed to initiate and terminate CLC (except for the enrolled nurse from the operation theatre). The frequency of team members’ use of CLC increased significantly with an egalitarian leadership style (RR 1.14, CI 1.04 to 1.26). Conclusions This study showed that despite focus on the importance of communication in terms of CO and CLC, the difficulty in achieving safe and reliable verbal communication within the interdisciplinary team remained. This finding indicates the need for validated training models combined with further implementation studies.

Depletion of lipoprotein lipase after heparin administration.

Some or most of the turnover of lipoprotein lipase (LPL) occurs by dissociation from vascular endothelial sites in extrahepatic tissues and further degradation in the liver. Heparin greatly enhances this dissociation and delays but does not abolish uptake in the liver, raising the possibility that heparin could lead to accelerated catabolism of functional LPL. To investigate this, we determined time curves for heparin (anti-factor Xa activity) and for LPL and hepatic lipase after injection in rats of two doses of conventional unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH). The high dose (250 U/kg) of both heparins resulted in similar initial levels of LPL activity in plasma, but at 30 minutes the activity with LMWH had declined by more than 80%, whereas with UFH it remained essentially unchanged during this time. In contrast, time curves for heparin activity in blood were similar for the two heparins. The low dose (50 U/kg) led to lower initial levels of LPL activity with LMWH in spite of slower elimination of heparin activity from the blood. These results agree with previous studies that indicate that LMWH has a similar ability as UFH to release LPL, but a lesser ability to delay its removal by the liver. Only slight differences were noted in the time curves for hepatic lipase with the two heparins. To assess the possible depletion of the lipases, we administered a second large dose of conventional heparin. One hour after the first injection, the second injection resulted in lower plasma LPL activities in all four groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of lipoprotein lipase to plasma by triacylglycerol emulsions. comparison to the effect of heparin

It was previously known that lipoprotein lipase (LPL) activity in plasma rises after infusion of a fat emulsion. To explore the mechanism we have compared the release of LPL by emulsion to that by heparin. After bolus injections of a fat emulsion (Intralipid) to rats, plasma LPL activity gradually rose 5-fold to a maximum at 6-8 min. During the same time the concentration of injected triacylglycerols (TG) decreased by about half. Hence, the time-course for plasma LPL activity was quite different from that for plasma TG. The disappearance of injected 125I-labelled bovine LPL from circulation was retarded by emulsion. This effect was more marked 30 min than 3 min after injection of the emulsion. The data indicate that the release of LPL into plasma is not solely due to binding of the lipase to the emulsion particles as such, but involves metabolism of the particles. Emulsion increased the fraction of labelled LPL found in adipose tissue, heart and the red muscle studied, but had no significant effect on the fraction found in liver. The effects of emulsion were quite different from those of heparin, which caused an immediate release of the lipase to plasma, decreased uptake of LPL in most extrahepatic tissues by 60-95%, and increased the fraction taken up in the liver.

Biphasic effects of low-molecular-weight and conventional heparins on chylomicron clearance in rats.

Chylomicrons labeled in vivo with [14C]triglycerides and [3H]retinyl esters were injected in rats at a series of times after administration of conventional unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), or saline. In saline controls the clearance of both chylomicron triglycerides and retinyl esters seemed to follow exponential courses, with half-lives of about 5 and 10 minutes, respectively. Five minutes after administration of LMWH or UFH, the triglyceride clearance rates were dramatically increased and were associated with an increased appearance of the radiolabel in circulating free fatty acids (FFAs). The clearance of [3H]retinol radioactivity, ie, chylomicron particles, was also enhanced 5 minutes after heparin injection. From 75% to 90% disappeared from the circulation within the first 5 minutes. Their continued disappearance was much slower, with a slope similar to that of the saline-treated rats. Hence, it was as if a new, rapid exponent had been added to the disappearance curve that accounted for most of the particle clearance. Injection of chylomicrons 1 hour after the heparins resulted in substantially slower clearance compared with saline-treated controls of both triglyceride and retinol radioactivity in rats given a high dose of LMWH or a low dose of either heparin. Appearance of label in plasma FFAs was also decreased, suggesting that impeded lipolysis was responsible, at least in part, for the impeded chylomicron clearance. Four and 24 hours after heparin injection all studied parameters of chylomicron clearance had returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Dysregulation of subcutaneous adipose tissue blood flow in overweight postmenopausal women

Objective: A putative link between abdominal obesity and metabolic-vascular complications after menopause may be due to a decreased adipose tissue blood flow (ATBF). The present work aimed to analyze possible changes in ATBF with being overweight and menopausal and its putative link to endothelial dysfunction and autonomic nervous system balance. Methods: Forty-three healthy women were classified into four groups according to weight and menopause status. The ATBF was measured by xenon washout while fasting and after oral glucose intake. The nitric oxide synthase inhibitor asymmetric dimethylarginine was used as a marker of endothelial function and heart rate variability-estimated autonomic nervous system activity. Results: Fasting ATBF was decreased in both overweight groups (P = 0.044 and P = 0.048) versus normal-weight premenopausal women. Normal-weight and overweight postmenopausal women exhibited lower maximum ATBF compared with normal-weight premenopausal women (P = 0.015 and P = 0.001, respectively), and overweight postmenopausal women exhibited lower maximum ATBF compared with normal-weight postmenopausal women (P = 0.003). A negative correlation was found between fasting ATBF and asymmetric dimethylarginine (P = 0.015), whereas maximum ATBF was negatively associated with sympathetic-parasympathetic nervous system balance (ratio of the power of the low frequency to the power of the high frequency; P = 0.002). Conclusions: Loss of ATBF flexibility in overweight postmenopausal women may contribute to the metabolic dysfunction seen in this group of women.

Conversion of chylomicrons into remnants

The turnover of chylomicrons in the blood is the sum of several processes. The native chylomicron is synthesized in the intestine out of available substrates. When the chylomicron enters the circulation exchanges of apolipoproteins with other lipoproteins, it also binds to the vascular endothelium where the chylomicron is lipolyzed by lipoprotein lipase. After a short period in the circulation the chylomicron/chylomicron remnant appears to be available for receptor mediated uptake. In this paper several of the processes involved in generation and clearance of chylomicron remnants are discussed.

Subarachnoid haemorrhage induces an inflammatory response followed by a delayed persisting increase in asymmetric dimethylarginine

Abstract Objective. Subarachnoid haemorrhage (SAH) is associated with an inflammatory systemic response and cardiovascular complications. Asymmetric dimethyl arginine (ADMA), an endogenous inhibitor of nitric oxide synthase, mediates vasoconstriction and might contribute to cerebral vasoconstriction and cardiovascular complications after SAH. ADMA is also involved in inflammation and induces endothelial dysfunction. The aim of this study was to evaluate whether and how CRP (marker for systemic inflammation) and ADMA increased in patients during the acute phase (first week) after SAH. The ADMA level was also assessed in the patients in a non-acute phase (three months), and in healthy controls. Methods. A prospective study of 20 patients with aneurysmal SAH. ADMA and CRP were followed daily during the first week after SAH and a follow up sample for ADMA was obtained 3 months later. A single blood sample for ADMA was collected from age- and sex-matched healthy controls (n = 40, two for each case). Results. CRP increased significantly from day 2; 16 (Confidence interval (CI) 10–23) mg/L to day 4; 84 (CI 47–120) mg/L, (p < 0.01). ADMA increased significantly from day 2; 0.22 (CI 0.17–0.27) μmol/L, to day 7; 0.37 (CI 0.21–0.54) μmol/L, p < 0.01. ADMA remained elevated at a 3-month follow-up: 0.36 (CI 0.31–0.42) μmol/L. ADMA in the first sample from the patients (day 1–3); 0.25 (CI 0.19–0.30) μmol/L, was not different from ADMA in matched healthy controls; 0.25 (CI 0.20–0.31), p > 0.05. Conclusion. After SAH, CRP and ADMA in serum increased significantly during the first week and ADMA remained elevated 3 months later.