“ Observations on Lipid Metabolism ”

Based on a Dissertation presented to the Society on 25th October, 1963. Prior to 1936, it was the custom to regard adipose tissue of an accumulation of inert lipid material, possessing little or no metabolic activity, but, in that year, this viewpoint was challenged by the work of Schoenheimer and Rittenberg. On the basis of measurements of the rate of disappearance of labelled fatty acids from the body of the mouse, these workers concluded that the half-life of the total fatty acids in this animal, under the conditions of the experiment, was of the order of three days. Since the depots constitute by far the greatest part of the body fats, it was assumed that the turnover rate observed was that characteristic of the depot fat. The acceptance of these results necessitated the immediate rejection of the long cherished idea that adipose tissue represents an inert lipid store, capable of change only during periods of fasting or of excess ingestion of food. Copyright Royal Medical Society. All rights reserved. The copyright is retained by the author and the Royal Medical Society, except where explicitly otherwise stated. Scans have been produced by the Digital Imaging Unit at Edinburgh University Library. Res Medica is supported by the University of Edinburgh’s Journal Hosting Service: http://journals.ed.ac.uk ISSN: 2051-7580 (Online) ISSN: 0482-3206 (Print) Res Medica is published by the Royal Medical Society, 5/5 Bristo Square, Edinburgh, EH8 9AL Res Medica, Autumn 1964, 4(3): 28-33 doi: 10.2218/resmedica.v4i3.431 “OBSERVATIONS ON LIPID METABOLISM” R. B. L. EWART, B.Sc., M.B., Ch.B., Senior President 1963-64. Based on a Dissertation presented to the Society on 25th O ctober, 1963. Prior to 1936, it was the custom to regard adipose tissue of an accumulation of inert lipid material, possessing little or 110 m etabolic activity, but, in that year, this viewpoint was challenged by the work of Schoenheim er and R ittenberg. On the basis of measurements of the rate of disappearance of labelled fatty acids from the body of the mouse, these workers concluded that the half-life of the total fatty acids in this anim al, under the conditions of the experim ent, was of the order of three days. Since the depots constitute by far the greatest part of the body fats, it was assumed that the turnover rate observed was that characteristic of the depot fat. T h e acceptance of these results necessitated the im m ediate rejection of the long cherished idea that adipose tissue represents an inert lipid store, capable of change only during periods of fasting or of excess in­ gestion of food. T h is “ about face” in belief naturally stim u­ lated m any workers and confirmatory results appeared rapidly in the literature. Thus, Shapiro and W ertheim er evaluated the oxygen uptake of adipose tissue in vitro and took the elem entary precaution, or so it appears now, of expressing their results in terms of the fat-free weight of the tissue in this way demonstrating that “ depot fat” (so-called) ranked amongst the most active tissues in the body when judged by this criterion. In 1942, T u erkischer and W ertheim er reported that all dietary regimens which enhance fat formation also give rise to the deposition of glycogen within the adipose tissue cells. Furtherm ore, these authors pointed out that this accumulation of polysaccharide is associated with high respiratory quotients, often exceeding unity, indicating that active fat syn­ thesis is occurring. M ore recently, isotopic evidence of fat synthesis in adipose tissue in vivo has been obtained by Fararger and G erlach who found that the fatty acids of rat mesenteric lipid have higher specific activities than those of either liver or blood shortly after the injec­ tion of radioactive acetate or glucose. Isolated enzymes of adipose tissue have, to date, been studied relatively little but it may be assumed, from the fact that the tissue is cap­ able of perform ing such reaction sequences as those involved in respiration and fat and glyco­ gen syntheses that m any enzyme systems do occur in this tissue. There is nothing to be gained by further discussion of these systems here— suffice it to say that the main point estab­ lished by the endeavours to demonstrate enzymic activities in adipose tissue is its evident marked specialisation in terms of lipid m etabol­ ism. W h ile most of those enzymes sought can be detected in this tissue, those concerned with fat m etabolism show an activity equal to or exceeding that of their counterparts in the most active tissues of the body. For m any years reports appeared sporadically in the literature of small amounts of 11011esterified fatty acid (N efa) which can be de­ tected in plasma, but until the early 1950 ’s this lipid fraction was widely regarded as an artefact of isolation. A t this time, however, Gordon pointed out that certain observed “ anom alies” in the electrophoretic m obility of plasma pro­ teins may be reproduced in vitro by the addi­ tion of sodium oleate to plasma prior to the application of the separatory procedure. This

Prior to 19 36 , it was the custom to regard adipose tissue o f an accum ulation o f inert lipid m aterial, possessing little or 110 m etabolic activity, but, in that year, this view point was challenged by the work o f Sch oenheim er and R itten b erg.O n the basis o f m easurem ents of the rate of disappearance o f labelled fatty acids from the body o f the m ouse, these workers concluded that the half-life o f the total fatty acids in this anim al, under the conditions of the experim ent, was o f the order o f three days.Since the depots constitute by far the greatest part o f the body fats, it was assum ed that the turnover rate observed was that characteristic of the depot fat.T h e acceptance o f these results necessitated the im m ediate rejection of the long cherished idea that adipose tissue represents an inert lipid store, capable of change only during periods o f fasting or of excess in gestion of food.
T h is " about face" in belief naturally stim u lated m any workers and confirm atory results appeared rapidly in the literature.T h u s, Shapiro and W erth eim er evaluated the oxygen uptake of adipose tissue in vitro and took the elem entary precaution, or so it appears now, of expressing their results in terms o f the fat-free w eight o f the tissue in this w ay dem onstrating that " depot fa t" (so-called) ranked am ongst the m ost active tissues in the body when judged by this criterion.In 19 4 2, T u erkischer and W ertheim er reported that all dietary regimens which enhance fat form ation also give rise to the deposition o f glycogen w ithin the adipose tissue cells.Furtherm ore, these authors pointed out that this accum ulation o f polysaccharide is associated with high respiratory quotients, often exceeding unity, indicating that active fat syn thesis is occurring.M ore recently, isotopic evidence o f fat synthesis in adipose tissue in vivo has been obtained by Fararger and G e rlach who found that the fatty acids o f rat m esenteric lipid have higher specific activities than those o f either liver or blood shortly after the injec tion o f radioactive acetate or glucose.
Isolated enzym es of adipose tissue have, to date, been studied relatively little bu t it m ay be assumed, from the fact that the tissue is cap able of perform ing such reaction sequences as those involved in respiration and fat and glyco gen syntheses that m any enzym e systems do occur in this tissue.T h e re is nothing to be gained by further discussion o f these systems here-suffice it to say that the m ain point estab lished by the endeavours to dem onstrate enzym ic activities in adipose tissue is its evident marked specialisation in terms o f lipid m etabol ism.W h ile m ost of those enzym es sought can be detected in this tissue, those concerned with fat m etabolism show an activity equal to or exceeding that of their counterparts in the m ost active tissues o f the body.
F o r m any years reports appeared sporadically in the literature of small am ounts o f 11011esterified fatty acid (N efa) which can be de tected in plasm a, but until the early 19 50 's this lipid fraction was w idely regarded as an artefact of isolation.A t this tim e, however, G ordon pointed out that certain observed " anom alies" in the electrophoretic m ob ility o f plasm a pro teins m ay be reproduced in vitro by the addi tion o f sodium oleate to plasm a prior to the application o f the separatory procedure.T h is provided the first d ue leading to the suggestion th a t N e fa m ig h t be a physiological com ponent o f the circulating lipids, b u t i t was n o t u n til a year had elapsed th a t G ordon and C herkes ascribed to this fraction an im p o rta n t role in the transport o f fats fro m the depots to the tissues fo r oxidation.T h is contention is sup ported by several lines o f evidence.F irst, it h as been shown th a t N e fa injected in to various experim ental animals has a circulating half-life o f the order o f only tw o m inutes, indicating its rapid removal from the blood.
Second, estimates made o f the arterio-venous differences in plasma N e fa levels across various organs have indicated that, fo r example, the myocardium of the fasted anim al is capable o f rem oving 0.3 M E qu ivalen ts o f N e fa from each litre of the perfusing blood.In order to assess the physio logical significance o f this process, the authors perform ed concurrent measurements o f oxygen uptake and, 011 the basis o f the assumption that the average m olecular w eight o f the fa tty acid taken up is 275 (i.e. a m ixtu re o f fa tty acids q u a n tita tive ly distributed about the h yp o th e ti cal C17 com pound), they arrived at the con clusion th a t the q u a n tity o f N e fa taken up by the heart if sufficient to provide the b u lk o f its energy requirem ent under the conditions o f the experim ent.
T h e next problem requiring explanation was the mechanism by w hich the constancy o f the arterial N e fa level is m aintained, a problem to w hich we shall return later.A t this m om ent, all th a t need be said is th a t all the tissues studied, in clu d in g liver, appeared, on the basis o f a rterio-venous difference studies, to be active in the extraction o f N e fa from the circulating fluid.T h e quest, therefore, was fo r a source o f N e fa and the investigators turned to the adipose depots.T h e y were n o t disappointed fo r i t was found th a t samples of blood from the long saphenous vein, w hich may be regarded as draining the adipose tissue o f the low er lim b alm ost exclusively, showed large negative arte riovenous differences indicative o f N e fa release.
Thus the pattern has emerged in w hich N e fa represents an im p o rta n t, readily available source o f oxidizable material whose concentration in the blood is the resultant o f its rate o f removal by the tissues and the rate o f its lib e ra tio n from the fa t depots.W e may now turn our a tte n tio n to a con sideration o f the concept o f " caloric homeo stasis" .As was noted earlier, the concentration o f N e fa in the plasma is m aintained at a re la tively constant level, between the lim its 0-5-1 m E q per litre , despite the high rate o f re moval by the tissues w hich is observed in the post absorptive state.T h is im plies the opera tion o f some fa irly sensitive co n tro l mechanism capable o f relating the rate o f lib e ra tio n o f N e fa to the somatic requirem ent.I f we return fo r a m om ent to the w ork of G ordon and Cherkes in w hich the rate o f uptake o f Nefa by the myocardium was estimated, we find th a t their results apply o n ly to the fasting state.In fact, G ordon proceeded in 19 57 to repeat the ex periments and obtained sequential blood samples fo r the measurement of arterio-venous N e fa differences before, and fo r some tim e after, the adm inistration o f 100 g. o f glucose together w ith 0.1 u n it o f insulin per Kg. of body w eight to human subjects.In this way he confirm ed the previously obtained results and, in a d d ition , showed th a t the ready avail a b ility o f glucose, w hich may be regarded as a preferred m etabolite, abolishes n o t o n ly the uptake o f N e fa by the m yocardium b u t also its liberation by the adipose tissue.T h is w ork provides an extremely elegant example o f the close correlation existing between those pro cesses leading to the removal o f fa tty acids from the circulating fluids and those leading to th e ir m o b iliza tio n fro m the fa t stores.In general, it may be said th a t all o f those factors w hich in crease the u tiliz a tio n o f glucose, decrease the o u tflo w o f N e fa from the lip id depots.An ingenious explanation o f this phenomenon w ill be quoted elsewhere in this dissertation but, at present, i t w ill suffice to remark th a t the m o b iliza tio n o f fa t shows marked dependence on the state o f n u tritio n , and th a t pre-em inent in this regard is the status o f carbohydrate metabolism.
W e have seen, then, that adipose tissue ap pears to exert its influence on lip id metabolism by adjusting the availability o f N e fa in accord ance w ith somatic requirements fo r an oxidisable substrate.T h is being accepted the central issue qu ite clearly becomes th a t o f the elucida tio n o f the nature o f the mechanism by w hich the adjustm ent is effected under physiological conditions.I t w ould be logical to suppose that the process is subject to hum oral and to nervous control and indeed, evidence th a t such is the case has been obtained by many investigators.
T h e first piece o f evidence regarding the role of nervous a c tiv ity was provided in 1922 by G o ering who pointed o u t th a t excessive nerve stim u la tio n provokes fa t loss from the adipose tissue situated w ith in the d is trib u tio n o f the affected nerve, whereas paralysis or nerve section results in a marked deposition of fat in the depots, the magnitude of which may be partially or totally masked by the associated atrophy of the somatic musculature.Then, in 1947, Clement reported that unilateral denerva tion of various fat bodies in the rat caused a diminution in the rate of depletion of trigly ceride from the denervated side during fasting.Such observations suggest that the nervous system may exercise a tonic effect upon Nefa liberation from fat depots.This concept has found confirmation in the work of Havel and Gotofen (1960) who investigated the role of the sympathetic nervous system in the metabol ism of free fatty acids and observed that admin istration of hexamethonium, a ganglion block ing agent, causes a reduction in plasma Nefa levels in the fasting dog.From observations such as these, it has been concluded that the sympathetic nervous system exercises a tonic influence on the adipose tissue which may pro vide for a continuous release of Nefa at a level which can be modified by insulin and other Immoral agents.In addition to this, variation in the intensity of sympathetic activity itself might be reasonably expected to exert a direct effect on lipid mobilization.Many humoral agents have been implicated in the regulation of fatty acid exchange in adipose tissue.W e may begin by considering epinephrine.Administration of this hormone to fasting humans has been found consistently to give rise to very rapid and striking increases in the plasma Nefa content which can be attributed to an increased rate of mobilization from the fat depots (as shown by arterio-venous difference studies) (Gordon and Cherkes, 1956).In 1957, Wadstrom observed that 90 minutes after the injection of 0.01 mg. of epinephrine into rabbits there is a decrease in the triglycende content of the fat depots asso ciated with corresponding increases in the levels of glycerol, mono and di-glycerides.This evidence pointed to the conclusion that epine phrine activates lipolysis or, what amounts to the same thing, inhibits resynthesis of trigly ceride in the adipose tissue.More recently, it has been shown by Gordon and Cherkes (1958) that the addition of epinephrine to surviving adipose tissue, in vitro, accelerates the rate at which this tissue releases Nefa into the sur rounding medium.
Shafir et al. have pointed out two interesting facts in relation to the effect of epinephrine on lipid metabolism.In the first place they observed that this hormone gives rise to parallel increases of Nefa and lipiprotein which latter plasma constituent is, in all probability, formed by the liver in response to the increased availability of fatty acids.The second, and perhaps the more intriguing, obser vation made by this group is that epinephrine exerts a more rapid effect upon lipid than on carbohydrate metabolism.Thus it is found that the hormone gives rise to a primary elevation of Nefa levels in plasma which gradu ally return to normal as the blood glucose rises.This observation is in agreement with the earlier results obtained by the same group in 1959 which indicated that the Nefa response to epinephrine can be prevented by simultaneous administration of glucose and insulin.The observation finds confirmation also in the work of Goldfen and H avel which showed that the Nefa response to norepinephrine, administra tion of which does not lead to marked hyperglycacmia, is sustained for a much longer time than that produced by epinephrine itself.The interesting fact emerging from these results is simply that, in time of stress, it is the lipid stores on which the body depends as a primary source of energy.
The second endocrine organ which exercises a profound effect upon lipid mobilization from storage sites is the anterior lobe of the p itu it ary.Some four hormones of pituitary origin have been shown to exert control over fatty acid metabolism; these arc somatotrophin (growth hormone), thyrotrophic hormone (TSH), the corticotrophins (A C TH ) and prolactin.I t should be emphasised that the effects of A C T II and TSH are of peculiar interest in this connection, since, on the basis of in vitro studies, it is known that their action is not dependent upon the presence of their " target organs" , though, in the whole animal, the presence of these organs w ill modify the lipid response observed.
The evidence so far to hand suggests that all four hormones influence lipid metabolism in essentially the same way: somatotrophin, however, by a sort of historical accident, has been studied most intensely and so the effects of this hormone w ill be considered at greater length than those of the other three.
In 1944, Stetton and Salcedo, injected anterior pituitary extract into mice, a procedure known to give rise to a condition of " fatty liver" : they discovered by means of prior deuterium labelling that the excess hepatic lipid had been transported to that organ from the adipose storage depots.
T h e problems involved in the isolation of pure and homogeneous preparations o f the various p itu ita ry principles made fu rth e r pro gress in this field d iffic u lt and even today, render interpretation a m a tte r open to doubt.However, in 1953, Greenbaum and M cLean reported th a t treatm ent o f rats w ith " p u rifie d " som ato tro p h in caused a very rapid increase in hepatic triglyceride co n ten t.In view o f the fact th a t the horm one is know n to increase the rate o f fa tty acid oxidation in the liver, and bearing in m in d the results o f Stetton and Salcedo above, it seems apparent that the accum ulation o f lip id material in the liver under these circumstances m ust result from an increased rate o f transport o f fa tty acids to this organ from the depot fat.T h a t som atotrophin can increase the rate o f m o b iliza tio n o f N efa from adipose tissue is fu rth e r suggested by the finding th a t the plasma N e fa level o f fasting dogs, already high, may be readily increased tw o-fold by the inje ction o f " p u rifie d " bovine s o m a to tro p in .
The in vivo evidence o f a fat m o b ilizin g action o f s o m a to tro p in is thus fa irly clear cut, b u t u n fo rtunately it has so far proved impos sible to demonstrate this a c tiv ity o f the hor m one in vitro.In 1958, however, W h ite and Lugel in experiments w ith A C T H showed that a) c o rtic o tro p in s arc m ore active than somato tro p h in in pro m o tin g fa tty acid liberation in vivo and b) th a t they also display a pow erful effect in the case o f adipose tissue in vitro .O n the basis o f these observations, W h ite and Lugel proposed the possibility, w hich s till awaits experim ental verification, that somato tro p h in may be inactive in itself b u t that, in the intact anim al, it may be metabolised to a product w hich is capable o f pro m o tin g fa tty acid liberation from the fat depots -the analogy here to the case o f L-thyroxinc is so obvious th a t no fu rth e r com m ent is required.
T h e tw o rem aining hormones, T S H and prolactin, arc n o t so w ell defined as regards their action on lip id metabolism .F o r example, in v itro experiments have indicated that T S H is capable of increasing the rate o f N e fa release from adipose tissue b u t only when present in unphysiologically high concentrations.T he state o f our knowledge concerning the action o f prolactin on fa tty acid metabolism is equally unsatisfactory.Reiss (1947) m aintains that in jection o f this horm one leads to well marked depletion o f the fat stores, b u t, as in the case o f s o m a to tro p in , n o in v itro a ctivity can be demonstrated.O n purely t eleological grounds, Reiss proceeded to argue that it is logical to expect prolactin to exercise a fa t m obilising effect, especially in view o f the fact th a t Shaw and Petersen (1938) have claimed, on the basis of A / V lip id differences across the lactating udder, that more than enough circulating lip id , in w hat form they do n o t say, is abstracted f rom the blood by this organ, to account fo r the entire lip id content o f the m ilk .T h is effect o f prolactin was also claimed on the basis o f H oussay's observation th a t s o m a to tro p in and the corticotrophins can, at least in part, replace prolactin in the maintenance o f lacta tion in the hypophysectom ised anim al.A warning m ust be given at this p o in t concerning the va lid ity o f such " round ab o u t" arguments however, and the w hole status o f the re lia b ility o f observations based upon w ork w ith so called " pure" anterior p itu ita ry principles m ust be examined c ritic a lly before acceptance.
W e may now pass on to a consideration of insulin.Both the concentration and turnover of N e fa in blood are strikin g ly influenced by this horm one.In the norm al fasting animal and in the diabetic anim al, the circulating N efa level is decidedly increased and indeed, in severe k e to tic diabetes the m olar ratio o f fa tty acid to scrum album in may exceed seven, w hich is the m axim um num ber o f fa tty acid m ole cules w hich can be tig h tly bound by one m ole cule o f serum alb u m in , the norm al value being rather less than one (Goodm an and G ordon, 1958).
In 1958 D ole observed th a t the adm inistra tion o f insulin causes a marked fa ll in the plasma N e fa level o f norm al individuals.As in the ease o f epinephrine considered above, the action o f insulin is o f particular interest in that this horm one has been shown (D ole 1958) to exercise an effect more rapidly on circu la t ing lip id than on blood glucose.Once again, this may be regarded as a reflection o f the im portance of lip id metabolism in the liv in g animal.
In ad d ition to the in vivo findings outlined earlier, C h erkes and G ordon, in 1958, measured the rate o f release o f N e fa by epididym al fat bodies obtained from fasting rats, when these adipose tissue fragments were incubated in a m edium containing bovine serum alb u m in as an acceptor o f N efa.I n the absence o f the horm one it was found that the tissue release 1.57µ m o f fa tty acid per gram per h our w hile, w ith the a d d ition o f physiological concentra-tio n o f in su lin a net uptake o f N efa fro m the m edium was observed, a m o u n tin g to 1.03 um per gram per hour.In o th e r words the tissue w hich was releasing Nefa, on the a d d itio n o f in su lin, was persuaded to take up N efa from the environm ent.
O th e r horm ones have been reported to m o d ify the m etabolic a c tiv ity o f adipose tissue b u t th e ir effects rem ain, in general, poorly understood and in the interests o f bre vity they w ill n o t be discussed here.
W e have seen, then, some o f the ways in w hich the uptake and lib e ra tio n o f fa tty acids by adipose tissue may be c o n tro lle d in vivo, b u t tw o very fundam ental questions now present themselves.F irstly, w hat is th e nature o f the stim ulus w hich causes the co n tro l m echanism to come in to play and secondly, how is the ne u ro /h u m o ra l in fo rm a tio n translated in to terms o f biochem ical process?T o n e ith e r of these questions, p a rticu la rly the latter, can d e fin itive answers be given at this tim e b u t in w hat follow s, some a tte m p t w ill be made to cla rify the situation insofar as i t is possible, presently, to do so.
I f we consider the norm al physiological situation o b ta in in g in an organism, then i t is clear th a t the effect o f som atotrophin on lip id m etabolism , in the norm al a d u lt, may be assumed to be m ore or less negligible, though it may be significant in the young anim al in w hich the provision o f adequate am ounts o f oxidisiable substrate to the grow ing tissue is mandatory.In any event, i t seems m ore lik e ly than n o t th a t any physiological effect a ttrib u t able to som atotrophin w ill be to n ic in nature and n o t subject to rapid or marked fluctuation.O n the o ther hand, a ll o f the oth e r com pon ents o f the co n tro l m echanism are capable o f dynam ic variation dependent on tissue require ments from m o m e n t to m om ent.T im s, the m ob ilisation o f N e fa in response to sym path etic nervous stim u la tio n and to the release o f epinephrine by the adrenal m edulla is a biochem ical reflection o f the sensitivity o f the nervous system to various types o f stress, and is m ediated by the a c tiv ity o f the higher nerve centres.In su lin , w hich strongly in h ib its lib e r ation o f N e fa from depot fa t and, as we have seen, may actually prom ote fa tty acid uptake by adipose tissue, is liberated fro m the pancreas in response to the stim ulus o f high blood sugar levels acting directly upon the pancreatic cells.As I have said before, this provides an example of the close correlation existing between carbo hydrate and lip id m etabolism and explains the observed dependence o f plasma N e fa levels on the n u tritio n a l state o f the anim al. T hus, a fasting anim al may be expected to have a lo w blood glucose level associated w ith a high level o f circu la tin g N e fa b u t if an alim entary h y perglycacmia is established, the resulting increase in the level o f circu la tin g in sulin may be held to explain the rapid fa ll in plasma N e fa w hich is observed in such a situation.
T h e mechanism by w hich this final co n tro l is actually m ediated is rather m ore obscure.W h a t then o f N e fa release: in this case the substrate is rather d iffe re n t-n o t lip o p ro te in as in plasma, b u t m ore or less pure neutral fat (triglyceride) a fact th a t appears to rule lip o p ro tein lipase a c tiv ity o u t o f co u rt since the a c tiv ity o f the enzyme toward pure triglyceride is vanishingly small.W e m ust turn, then, to consideration o f the second hypothesis, that dealing w ith rates o f reeste r fication.
In 1 960, W o o d et al. pointed o u t th a t in rat epididym al fa t tissue, glucose carbon is incorporated in to the glycerol m oeity o f triglyceride at a rate m any times greater than the carbon of glycerol itself: they went on to demonstrate that adipose tissue homogenates require.L-a-glycero-phosphate or one of its phosphorylated precursors for optimum esterification of carbon labelled palmitate, glycerol itself being inactive in this system.If this is equally true of intact adipose tissue, then esterification of Nefa should be limited by the rate of formation of L-a-glycero-phosphate from glucose via di-hydroxy-acetone-phosphate.Now it is known that the triglyceride of adipose tissue undergoes constant hydrolysis ancl re -esterification within the tissue; by what mechanism the hydrolysis is effected is not understood.However, it must be assumed on this evidence that, in the steady state condition, where there is no net uptake on release of Nefa, the liberated fatty acids are rapidly re-esterified.This re-esterification re quires the presence of a supply of l-a-glycerophosphate obtained by the metabolism of glucose via the Em bden-M eyerhof pathway.The equilibrium situation between triglyceride and Nefa within the cells is quite obviously dependant upon a ready supply of glucose.During periods of hypoglycaemia, the Nefa of the adipose tissue might be expected to rise and since intracellular and extracellular Nefa arc in a free translocation relationship this will lead to their liberation into the blood; con versely, after glucose and insulin administration the enhanced respiratory activity of the adipose tissue cells w ill ensure a plentiful supply of dihydroxy acetone phosphate and therefore of L-a-glycero-phosphate so that the fatty acid level of the circulating fluid will diminish as Nefa is withdrawn to be esterified within the cells.
This hypothesis has been found to accord well with experimental data in which the per centage of glucose carbon incorporated into the glycerol moiety of neutral fat has been estimated under varying conditions of availa bility of glucose.It provides an elegant ex ample of the indirect way in which hormonal substances may modify cell metabolism since the action of insulin on fatty acid mobilization appears in this way to be explicable in terms of a primary action of the hormone, notably that of influencing the translocation of glucose across cell membranes.Attractive though the hypothesis may be, however, a note of caution must be sounded-the energy transformations and relationships in systems such as this remain obscure, complicated as they are by the very peculiar solubility properties of the participat ing lipids and lipoproteins and estimations of equilibrium constants for such systems have not, so far, been attempted.
In this article a very few of the ad vances which have been made towards the understanding of adipose tissue and its role in lipid metabolism have been discussed.W e have considered the recognition, some thirty years ago, of the fat depots as dynamic entities, the discovery of the significance of Nefa in metabolism and the possible means by which the control of lipid uptake and release by adipose tissue may be affected under physiolo gical conditions.
W h at has emerged is still a far from coher ent story and we can only wait, perhaps for a further quarter of a century in the hope that the final word may yet be spoken.
the lip o ly tic a c tiv ity o f adipose tissue.T h e im p lic a tio n has been th a t the various com ponents o f the n euro-h u m oral con tro l system m o d ify the a c tiv ity o f tissue lipases.C learly, however, an exactly parallel situation w ould arise if co n tro l was exercised by m o d ifi cations in the rate o f synthesis o f triglyceride w ith in the adipose tissue cells.L e t us examine these possibilities.F irst, the lipase hypothesis.K orn and Q uigley found in 1957 th a t the o nly lipase a c tiv ity demonstrable in m any types o f adipose tissue was that due to heparin activated " lip o p ro te in lipase" .F o r this reason, a great, and perhaps d isp ro p o rtio n ate, num ber o f investigators have studied the behaviour o f this enzyme in relation to fat m o b iliz a tio n and deposition.T yp ica l o f this field o f endeavour, is the w ork o f H ollenberg (1959) w ho reported th a t the a d d ition o f glucose and in sulin to adipose tissue from fast ing animals, restores the capacity o f the tissue, in v itro , to liberate " lip o p ro te in lipase" in response to heparin.From this he concluded th a t the enzyme may be concerned in the accum ulation o f fa t in depots, possibly by in flu e ncin g the rate o f incorporation o f lip o p ro tein fa tty acids in to adipose tissue cells.M a n y fu rth e r studies have been undertaken and, by and large, all im p lica te lip o p ro te in lipase in the uptake o f fat, in the form o f N e fa , by the depots.