Adrenergic innervation and vascular patterns in canine adipose tissue

New actions of an old friend: perivascular adipose tissue's adrenergic mechanisms

The revolutionary discovery in 1991 by Soltis and Cassis that perivascular adipose tissue (PVAT) has an anti-contractile effect changed how we think about the vasculature. Most experiments on vascular pharmacology begin by removing the fat surrounding vessels. Thus, PVAT was thought to have a minor role in vascular function and its presence was just for structural support. The need to rethink PVAT's role was precipitated by observations that obesity carries a high cardiovascular risk and PVAT dysfunction is associated with obesity. PVAT is a vascular-adipose organ that has intimate connections with the nervous and immune system. A complex world of physiology resides in PVAT, including the presence of an 'adrenergic system' that is able to release, take up and metabolize noradrenaline. Adipocytes, stromal vascular cells and nerves within PVAT contain components that make up this adrenergic system. Some of the great strides in PVAT research came from studying adipose tissue as a whole. Adipose tissue has many roles and participates in regulating energy balance, energy stores, inflammation and thermoregulation. However, PVAT is dissimilar from non-PVAT adipose tissues. PVAT is intimately connected with the vasculature, which is what makes its role in body homeostasis unique. The adrenergic system within PVAT may be an integral link connecting the effects of obesity with the vascular dysfunction observed in obesity-associated hypertension, a condition in which the sympathetic nervous system has a significant role. This review will explore what is known about the adrenergic system in adipose tissue and PVAT, plus the translational importance of these findings.

LINKED ARTICLES

This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.

Perivascular adipose tissue, vascular reactivity and hypertension

Most blood vessels are surrounded by a variable amount of adventitial adipose tissue, perivascular adipose tissue (PVAT), which was originally thought to provide mechanical support for the vessel. It is now known that PVAT secretes a number of bioactive substances including vascular endothelial growth factor, tumor necrosis factor-alpha (TNF-α), leptin, adiponectin, insulin-like growth factor, interleukin-6, plasminogen activator substance, resistin and angiotensinogen. Several studies have shown that PVAT significantly modulated vascular smooth muscle contractions induced by a variety of agonists and electrical stimulation by releasing adipocyte-derived relaxing (ADRF) and contracting factors. The identity of ADRF is not yet known. However, several vasodilators have been suggested including adiponectin, angiotensin 1-7, hydrogen sulfide and methyl palmitate. The anticontractile effect of PVAT is mediated through the activation of potassium channels since it is abrogated by inhibiting potassium channels. Hypertension is characterized by a reduction in the size and amount of PVAT and this is associated with the attenuated anticontractile effect of PVAT in hypertension. However, since a reduction in size and amount of PVAT and the attenuated anticontractile effect of PVAT were already evident in prehypertensive rats with no evidence of impaired release of ADRF, there is the possibility that the anticontractile effect of PVAT was not directly related to an altered function of the adipocytes per se. Hypertension is characterized by low-grade inflammation and infiltration of macrophages. One of the adipokines secreted by macrophages is TNF-α. It has been shown that exogenously administered TNF-α enhanced agonist-induced contraction of a variety of vascular smooth muscle preparations and reduced endothelium-dependent relaxation. Other procontractile factors released by the PVAT include angiotensin II and superoxide. It is therefore possible that the loss could be due to an increased amount of these proinflammatory and procontractile factors. More studies are definitely required to confirm this.

The microcirculation in adipose tissue inflammation

In most humans, obesity is associated with a chronic low-grade inflammatory reaction occurring in several organ tissues, including the adipose tissue. Infiltration of bone marrow derived leukocytes (granulocytes, monocytes, lymphocytes) into expanding adipose depots appears to be an integral component of inflammation in obesity. Circulating leukocytes invade organ tissues mainly through post-capillary venules in the microcirculation. The endothelium of the post-capillary venules acts as a gatekeeper to leukocyte adhesion and extravasation by displacing on its luminal surface adhesion molecules that bind the adhesive receptors expressed on circulating leukocytes. Several studies investigating the impact of obesity on the microcirculation have demonstrated the occurrence of microvascular dysfunction in experimental animal model of obesity, as well as in obese humans. To date though, working hypotheses and study designs have favored the view that microvascular alterations are secondary to adipose tissue dysfunction. Indeed, a significant amount of data exists in the scientific literature to support the concept that microvascular dysfunction may precede and cause adipose tissue inflammation in obesity. Through review of key published data, this article prospectively presents the concept that in response to nutrients overload the vascular endothelium of the microcirculation acutely activates inflammatory pathways that initiate infiltration of leukocytes in visceral adipose tissue, well before weight gain and overt obesity. The anatomical and physiological heterogeneity of different microcirculations is also discussed toward the understanding of how obesity induces different inflammatory phenotypes in visceral and subcutaneous fat depots.

Noradrenergic projections to the ventromedial hypothalamus regulate fat metabolism during endurance exercise

The regulation of energy metabolism by the central nervous system during endurance exercise was examined. We conducted respiratory gas analysis by functionally paralyzing the ventromedial hypothalamus (VMH), the lateral hypothalamic area, and the paraventricular nucleus of the hypothalamus with local anaesthetic (lidocaine) during treadmill running at a velocity that allowed for efficient fatty acid oxidation. Our results showed that only the lidocaine treatment of the VMH attenuated fatty acid oxidation during endurance exercise. The monoaminergic neural activities at these nuclei during in vivo microdialysis in rats under the same conditions indicated a significant increase in the extracellular concentration of noradrenaline in all nuclei. Similarly, a significant increase in dopamine occurred at some points during exercise, but no change in serotonin concentration occurred regardless of exercise. Disruption of noradrenergic projections to the VMH by 6-hydroxydopamine attenuated the enhancement of fat oxidation during running. Blocker treatments clarified that noradrenergic inputs to the VMH are mediated by β-adrenoceptors. These data indicate that information about peripheral tissues status is transmitted via noradrenergic projections originating in the medulla oblongata, which may be an important contribution by the VMH and its downstream mechanisms to enhanced fatty acid oxidation during exercise.

Sensory and sympathetic nervous system control of white adipose tissue lipolysis

Circulating factors are typically invoked to explain bidirectional communication between the CNS and white adipose tissue (WAT). Thus, initiation of lipolysis has been relegated primarily to adrenal medullary secreted catecholamines and the inhibition of lipolysis primarily to pancreatic insulin, whereas signals of body fat levels to the brain have been ascribed to adipokines such as leptin. By contrast, evidence is given for bidirectional communication between brain and WAT occurring via the sympathetic nervous system (SNS) and sensory innervation of this tissue. Using retrograde transneuronal viral tract tracers, the SNS outflow from brain to WAT has been defined. Functionally, sympathetic denervation of WAT blocks lipolysis to a variety of lipolytic stimuli. Using anterograde transneuronal viral tract tracers, the sensory input from WAT to brain has been defined. Functionally, these WAT sensory nerves respond electrophysiologically to increases in WAT SNS drive suggesting a possible neural negative feedback loop to regulate lipolysis.

Reversal of high pancreatic islet and white adipose tissue blood flow in type 2 diabetic GK rats by administration of the beta3-adrenoceptor inhibitor SR-59230A

Previous studies have shown that the Goto-Kakizaki (GK) rat, a nonobese type 2 diabetes model, has an increased white adipose tissue (WAT) and islet blood flow when compared with control rats. The aim of the study was to examine if these increased blood flow values in GK rats could be affected by the beta(3)-adrenoceptor antagonist SR-59230A. We measured organ blood flow with a microsphere technique 10 min after administration of SR-59230A (1 mg/kg body wt), or the corresponding volume of 0.9% NaCl solution (1 ml/kg body wt) in rats anaesthetized with thiobutabarbital. The GK rat had an increased blood flow in all intra-abdominal adipose tissue depots except for the sternal fat pad compared with Wistar-Furth (WF) rats. However, no differences were seen in the blood perfusion of subcutaneous white or brown adipose tissue. The blood flow was also increased in both the pancreas and in the islets in the GK rat compared with WF rats. SR-59230A treatment affected neither WAT nor pancreatic blood flow in WF rats. In GK rats, on the other hand, SR-59230A decreased both WAT and islet blood flow values to values similar to those seen in control WF rats. The whole pancreatic blood flow was not affected by SR-59230A administration in GK rats. Interestingly, the brown adipose tissue blood flow in GK rats increased after SR-59230A administration. These results suggest that beta(3)-adrenoceptors are involved in regulation of blood flow both in islet and in adipose tissue.

Brain-adipose tissue neural crosstalk

The preponderance of basic obesity research focuses on its development as affected by diet and other environmental factors, genetics and their interactions. By contrast, we have been studying the reversal of a naturally-occurring seasonal obesity in Siberian hamsters. In the course of this work, we determined that the sympathetic innervation of white adipose tissue (WAT) is the principal initiator of lipid mobilization not only in these animals, but in all mammals including humans. We present irrefutable evidence for the sympathetic nervous system (SNS) innervation of WAT with respect to neuroanatomy (including its central origins as revealed by transneuronal viral tract tracers), neurochemistry (norepinephrine turnover studies) and function (surgical and chemical denervation). A relatively unappreciated role of WAT SNS innervation also is reviewed--the control of fat cell proliferation as shown by selective chemical denervation that triggers adipocyte proliferation, although the precise mechanism by which this occurs presently is unknown. There is no, however, equally strong evidence for the parasympathetic innervation of this tissue; indeed, the data largely are negative severely questioning its existence and importance. Convincing evidence also is given for the sensory innervation of WAT (as shown by tract tracing and by markers for sensory nerves in WAT), with suggestive data supporting a possible role in conveying information on the degree of adiposity to the brain. Collectively, these data offer an additional or alternative view to the predominate one of the control of body fat stores via circulating factors that serve as efferent and afferent communicators.

Thematic review series: adipocyte biology. Sympathetic and sensory innervation of white adipose tissue

During our study of the reversal of seasonal obesity in Siberian hamsters, we found an interaction between receptors for the pineal hormone melatonin and the sympathetic nervous system (SNS) outflow from brain to white adipose tissue (WAT). This ultimately led us and others to conclude that the SNS innervation of WAT is the primary initiator of lipid mobilization in these as well as other animals, including humans. There is strong neurochemical (norepinephrine turnover), neuroanatomical (viral tract tracing), and functional (sympathetic denervation-induced blockade of lipolysis) evidence for the role of the SNS in lipid mobilization. Recent findings suggest the presence of WAT sensory innervation based on strong neuroanatomical (viral tract tracing, immunohistochemical markers of sensory nerves) and suggestive functional (capsaicin sensory denervation-induced WAT growth) evidence, the latter implying a role in conveying adiposity information to the brain. By contrast, parasympathetic nervous system innervation of WAT is characterized by largely negative neuroanatomical evidence (viral tract tracing, immunohistochemical and biochemical markers of parasympathetic nerves). Functional evidence (intraneural stimulation and in situ microdialysis) for the role of the SNS innervation in lipid mobilization in human WAT is convincing, with some controversy regarding the level of sympathetic nerve activity in human obesity.

Brain–adipose tissue cross talk

While investigating the reversible seasonal obesity of Siberian hamsters, direct sympathetic nervous system (SNS) postganglionic innervation of white adipose tissue (WAT) has been demonstrated using anterograde and retrograde tract tracers. The primary function of this innervation is lipid mobilization. The brain SNS outflow to WAT has been defined using the pseudorabies virus (PRV), a retrograde transneuronal tract tracer. These PRV-labelled SNS outflow neurons are extensively co-localized with melanocortin-4 receptor mRNA, which, combined with functional data, suggests their involvement in lipolysis. The SNS innervation of WAT also regulates fat cell number, as noradrenaline inhibits and WAT denervation stimulates fat cell proliferationin vitroandin vivorespectively. The sensory innervation of WAT has been demonstrated by retrograde tract tracing, electrophysiological recording and labelling of the sensory-associated neuropeptide calcitonin gene-related peptide in WAT. Local injections of the sensory nerve neurotoxin capsaicin into WAT selectively destroy this innervation. Just as surgical removal of WAT pads triggers compensatory increases in lipid accretion by non-excised WAT depots, capsaicin-induced sensory denervation triggers increases in lipid accretion of non-capsaicin-injected WAT depots, suggesting that these nerves convey information about body fat levels to the brain. Finally, parasympathetic nervous system innervation of WAT has been suggested, but the recent finding of no WAT immunoreactivity for the possible parasympathetic marker vesicular acetylcholine transporter (VAChT) argues against this claim. Collectively, these data suggest several roles for efferent and afferent neural innervation of WAT in body fat regulation.

Learning new tricks from old dogs: beta-adrenergic receptors teach new lessons on firing up adipose tissue metabolism

The three beta AR (beta-adrenergic receptor) subtypes (beta(1)AR, beta(2)AR, and beta(3)AR) are members of the large family of G protein-coupled receptors, each of which is coupled to G alpha s and increases in intracellular cAMP levels. In white adipose tissues, catecholamine activation of the beta ARs leads to the mobilization of stored fatty acids and regulates release of several adipokines, whereas in brown adipose tissue they stimulate the specialized process of adaptive nonshivering thermogenesis. Noteworthy, in most models of obesity the beta AR system is dysfunctional, and its ability to stimulate lipolysis and thermogenesis are both impaired. Nevertheless, selective agonists for the beta(3)AR, a subtype that is found predominantly in adipocytes, have been able to prevent or reverse obesity and accompanying insulin resistance in animal models. Whether this is a viable therapeutic option for human obesity is much debated with regard to the existence of brown adipocytes in humans or their ability to be recruited. Nevertheless, probing the physiological changes in adrenoceptor function in rodent obesity, as well as the process by which beta(3)AR agonists promote a thermogenic shift in fuel use, have yielded unexpected new insights into beta AR signaling and adipocyte physiology. These include the recent discovery of an essential role of p38 MAPK in mediating adaptive thermogenesis, as well as the accessory role of the ERK MAPK pathway for the control of lipolysis. Because these metabolic events were traditionally ascribed solely to the cAMP/protein kinase A system, the integration of these signaling mechanisms may pose new therapeutic directions in the quest to counter the obesity epidemic in our midst.

β-ADRENERGICRECEPTORS ANDREGULATION OFENERGYEXPENDITURE: A Family Affair

The family of adrenergic receptors (ARs) expressed in adipocytes includes three sibling betaARs and two alphaAR cousins. Together they profoundly influence the mobilization of stored fatty acids, secretion of fat-cell derived hormones, and the specialized process of nonshivering thermogenesis in brown adipose tissue. The two types of fat cells that compose adipose tissue, brown and white, are structurally and functionally distinct. Studies on the mechanisms by which individual betaAR regulates these cell-specific functions have recently uncovered new signal transduction cascades involved in processes traditionally ascribed to adenylyl cyclase/cAMP/protein kinase A system. They illustrate how betaAR signaling can orchestrate a coordinated set of intracellular responses for fine control of metabolic balance.

Integrative physiology of human adipose tissue

Adipose tissue is now recognised as a highly active metabolic and endocrine organ. Great strides have been made in uncovering the multiple functions of the adipocyte in cellular and molecular detail, but it is essential to remember that adipose tissue normally operates as a structured whole. Its functions are regulated by multiple external influences such as autonomic nervous system activity, the rate of blood flow and the delivery of a complex mix of substrates and hormones in the plasma. Attempting to understand how all these factors converge and regulate adipose tissue function is a prime example of integrative physiology. Adipose tissue metabolism is extremely dynamic, and the supply of and removal of substrates in the blood is acutely regulated according to the nutritional state. Adipose tissue possesses the ability to a very large extent to modulate its own metabolic activities, including differentiation of new adipocytes and production of blood vessels as necessary to accommodate increasing fat stores. At the same time, adipocytes signal to other tissues to regulate their energy metabolism in accordance with the body's nutritional state. Ultimately adipocyte fat stores have to match the body's overall surplus or deficit of energy. This implies the existence of one (or more) signal(s) to the adipose tissue that reflects the body's energy status, and points once again to the need for an integrative view of adipose tissue function.

Innervation of mammalian white adipose tissue: implications for the regulation of total body fat

We review the extensive physiological and neuroanatomical evidence for the innervation of white adipose tissue (WAT) by the sympathetic nervous system (SNS) as well as what is known about the sensory innervation of this tissue. The SNS innervation of WAT appears to be a part of the general SNS outflow from the central nervous system, consisting of structures and connections throughout the neural axis. The innervation of WAT by the SNS could play a role in the regulation of total body fat in general, most likely plays an important role in regional differences in lipid mobilization specifically, and may have a trophic affect on WAT. The exact nature of the SNS innervation of WAT is not known but it may involve contact with adipocytes and/or their associated vasculature. We hypothesize that the SNS innervation of WAT is an important contributor to the apparent "regulation" of total body fat.

Regional sympathetic activity in pre-hypertensive phase of spontaneously hypertensive rats

Imbalances in central and peripheral sympathetic nervous system (SNS) activity have been observed in essential and experimental hypertension. This study was carried out in order to evaluate SNS activity in two distinct tissue types of spontaneously hypertensive rats (SHR), compared to Wistar-Kyoto normotensive (WKY) rats, in the pre-hypertensive phase (4-5 weeks of age). Interstitial concentrations of norepinephrine (NE) and other catecholamines were measured by microdialysis in striated muscle, whose tone is controlled by baroreflexes, and in the subcutaneous adipose tissue where sympathetic output mainly controls metabolism. Two groups of SHR and WKY male rats were studied, aged 4-5 weeks, with a mean body weight of 92 and 86 g respectively. Systolic blood pressure (SBP, tail-cuff) values were 113 mm Hg (SD +/- 6.2) in SHR and 108 mm Hg (SD +/- 7.3) in WKY rats (p=0.28,t test). Two microdialysis probes were positioned in the subcutaneous fatty tissue and in the striated muscle of the parascapular region and perfused with Ringers' solution. The dialysate was collected every thirty minutes for 3 hours and analyzed in HPLC-ED to determine the content of NE and other catecholamines. Interstitial levels of NE were higher in SH than in WKY rats in both tissues. Mean NE values from subcutaneous adipose tissue in 4-5 week-old SHR were 1162 +/- 193 pg/ml compared to 496 + 188 pg/ml in WKY rats (p<0.001, t test). Muscle tissue NE levels in SHR were 1241 +/- 337 pg/ml vs. 521 +/- 138 pg/ml in WKY rats (p<0.001, t test). Plasma NE concentrations (279 +/- 61 pg/ml in SHR vs 246 + 69 pg/ml in WKY P = 0.65, t test) were not significantly different between the two strains at this young age. These findings suggest SNS hyperactivity in young SHR, though still normotensive, possibly dissociated from regional components of regulation (baroreceptor control in striated muscle and metabolic control in subcutaneous adipose tissue).

Effect of acute cold exposure on norepinephrine turnover rates in rat white adipose tissue

The present studies were carried out to assess directly sympathetic activity in white adipose tissue in response to cold exposure. Norepinephrine (NE) content and NE turnover rates were determined in epididymal and retroperitoneal adipose tissue from rats exposed to cold (4 degrees C) and controls kept at ambient temperature. Parallel measurements were made in interscapular brown adipose tissue (IBAT), in which activation of catecholaminergic innervation by cold exposure is well known. Exposure to 4 degrees C for 4 h reduced the endogenous NE content by 50% in IBAT and by 30% in both epididymal and retroperitoneal adipose tissues. Compared to warm controls, average values of fractional rates of turnover and cf turnover rates, estimated with alpha-methyl-tyrosine, increased 5-fold in IBAT and 2.5-3-fold in epididymal and retroperitoneal tissues from rats exposed to cold. The present data provide the first direct evidence that white adipose tissue sympathetic activity is increased during acute cold exposure.

Comparison of DNA synthesis in white and brown adipose tissue in rats with ventromedial hypothalamic lesions

Ventromedial hypothalamic (VMH) lesions cause excessive fat accumulation in white adipose tissue (WAT), and brown adipose tissue (BAT); however, little information is available on whether or not cell proliferation occurs in WAT and BAT after VMH lesioning. In this study, we determined the DNA content and thymidine incorporation in unilateral parametrial WAT and interscapular BAT 0, 1, 3, and 7 days after VMH lesioning, and examined the mechanism of increased DNA content in WAT. In rats with VMH lesions, the weight of WAT and BAT had increased significantly at 7 days, and the DNA content and thymidine incorporation of WAT had increased significantly at 3 days and continued to increase for up to 7 days, while those of BAT did not increase for as long as 7 days after VMH lesioning. Restricted food intake according to the pair-feeding method partially inhibited the increased DNA content in WAT. The increased DNA content in WAT was mostly restored but not completely by the administration of anti-insulin antibody, and by administration of propranolol, a beta-adrenergic blocker. The results demonstrated that VMH lesions induced DNA synthesis in WAT early after VMH lesioning, but did not induce DNA synthesis in BAT, and suggested that either hyperinsulinemia or a beta-adrenergic receptor mechanism or both may be responsible for the increased DNA content in WAT.

Adrenergic control of nerve blood flow

The factors that regulate blood flow within peripheral nerve (NBF) are largely unexplored. The presence of norepinephrine (NE)-containing terminals on vasa nervorum suggests that adrenergic regulation may be important. In this study we investigated the adrenergic responsiveness of NBF to agonist and antagonist agents using three separate techniques: (i) laser doppler flowmetry (LDV) using an epineurial probe, (ii) hydrogen clearance (HC) using an endoneurial microelectrode, and (iii) direct video recording (VA) of epineurial microvessels. Selective intraarterial NE delivery induced a phentolamine-reversible fall in NBF. Phentolamine alone increased NBF and lowered microvascular resistance (MR) as measured by HC. Epineurial microvessels had segmental vasoconstriction to bathing solutions of NE as observed by VA. Our findings suggest that NBF responds to adrenergic manipulation, possibly due to a heterogenous distribution of alpha receptors on epineurial vessels that supply the endoneurium.

Exercise-induced increase in dog adipose tissue blood flow before and after denervation

Subcutaneous adipose tissue blood flow was examined during rest and exercise in the inguinal fat pads of four female dogs using the Xe wash-out technique. The experiments were performed before and after denervation of one of the pads. No difference between the resting flows in the two pads could be demonstrated either before or after denervation. The flow increased about two-fold on average from rest to exercise. This response was similar before and after denervation. It is concluded that direct sympathetic innervation is not involved in the regulation of adipose tissue blood flow during exercise.

The effect of intermittent cold treatment on the adipose tissue of the cat. Apparent transformation from white to brown adipose tissue

Young cats (Felis domestica), aged 10-13 weeks, were intermittently exposed to a temperature of -30 degrees C for two periods of 1 hr per day. Animals were sacrificed on the 7th day and adipose tissue from the perirenal, pericardial, axillary, interscapular, and subcutaneous-inguinal depots was examined by electron microscopy and analysed stereologically. All examined depots were morphologically changed after cold treatment. Adipose tissue of perirenal, pericardial, and axillary depots showed a greater decrease in lipid content than the interscapular and subcutaneous-inguinal depots, but other changes were similar. Compared to the control group, which consisted of typical white adipose tissue, the diameter of adipose cells examined after cold treatment was diminished, in extreme cases to 18 micron (from 75 micron in the control group). The number of capillaries per cell was doubled (as evaluated on semithin sections). The most dramatic changes were observed in the mitochondria. Their volume increased to 0.48 micron 3 (from 0.13 micron 3 in the control), and the surface density of mitochondrial cristae per mitochondrial volume increased to 50 micron 2/micron 3 (from 32 in the control). Pleomorphism in mitochondrial size and inner structure and the presence of intramitochondrial electron-dense bodies and crystalline structures led us to conclude that the cold stress induced an increase in the absolute number of mitochondria in the adipose cells. The adipose tissue after cold treatment thus morphologically resembled the brown adipose tissue of cold-acclimated rodents. This implies that the adipose tissue of young cats can change its morphology and function, depending on the requirements of the organism.

Adrenergic innervation in the microcirculation of the bat wing

The distribution of adrenergic nerves to the microcirculation of the wing web of the bat was studied in whole-mount preparations employing glyoxylic acid fluorescence histochemistry. A dense perivascular adrenergic nerve plexus consisting of various-sized nerve bundles surrounded the arteries. The accompanying veins were sparsely innervated by small nerve bundles or single axons. Arterioles also possessed a dense perivascular plexus comprised of a loose network of small axon bundles. Muscular venules accompanying most arterioles were supplied by a small number of varicose axons which surrounded these vessels in a loose network. The majority of the terminal arterioles were devoid of any direct innervation. The proximal segment of some terminal arterioles was innervated by one or two axons which were observed to diverge from the vessels and terminate in the tissue. The true capillaries, postcapillary venules, and lymphatics possessed no direct adrenergic innervation. Surgical sectioning of the appropriate nerve trunks or administration of 6-hydroxydopamine caused complete disappearance of all fluorescent adrenergic axons in the wing web. Faint acetylcholinesterase-positive staining was observed in a perivascular plexus surrounding the arteries and arterioles. A much diminished acetylcholinesterase activity was observed after treatment with 6-hydroxydopamine which implies that this enzymatic activity resides in the adrenergic nerves.

Sympathetic innervation of the jejunal villus

Using fluorescent histochemical methods it has been shown that the noradrenergic nerves in the jejunal villus are associated with the capillaries underlying the basolateral membrane of the epithelium. Noradrenergic fibres were also seen to lie between the epithelial basolateral membrane and the capillaries but were never observed close to the epithelium unless accompanied by an underlying capillary. The distribution of noradrenergic fibres suggests that it is unlikely that noradrenaline diffuses directly from the varicosity to the epithelial basolateral membrane. Noradrenaline may, therefore, act on the capillary itself and in some way affect fluid absorption. However, noradrenaline released adjacent to a capillary might diffuse into the capillary to be distributed at another site along its course.

Morphological studies on the adrenergic innervation of white adipose tissue

White adipose tissue was obtained from the mesentery, epididymis, omentum and subcutis of rats which were fed, fasted or fasted and then refed. Tissue samples were prepared using the glyoxylic acid method to detect adrenergic nerves by fluorescence histochemistry. Other tissue samples were fixed with an aldehyde solution containing sodium molybdate which is specific for catecholamine granules in nerve terminals. Thin and serial thick sections (0.25-0.5 micron) were viewed with a conventional electron microscope and with the high voltage electron microscope. With fluorescence microscopy it was found that most of the blood vessels except veins and venules were richly innervated. The most extensive branching of nerves down to the capillary level was found in the mesentery and epididymal fat of fasted-refed rats. Relatively few adipocytes appeared to be innervated. With electron microscopy, nerve terminals were found distributed with most blood vessels including capillaries, and with some adipocytes. Only 2-3% of all dipocytes were innervated by adrenergic nerves. It is suggested that in the adipose tissue sites studied the major adrenergic innervation is mainly for the supply of blood vessels.

The role of neuronal uptake at alpha- and beta-adrenoceptor sites in subcutaneous adipose tissue

Intravascular noradrenaline infusion may cause vasodilatation or vasoconstriction in subcutaneous adipose tissue, whereas sympathetic nerve activity causes vasoconstriction only. This discrepancy may be due to a differential distribution of alpha- and beta-adrenoceptors in relation to adrenergic nerve terminals in the adipose tissue vessels. In order to test this hypothesis the extent of prejunctional supersensitivity to noradrenaline was studied after blockade the neuronal uptake of noradrenaline with cocaine. In the autoperfused, isolated inguinal canine adipose tissue pretreatment with coacine (200-600 mug close i.a.) increased lipolysis following sympathetic nerve stimulation or close i.a. injection of noradrenaline. Cocaine also potentiated the vasoconstriction induced by nerve stimulation (1-3 Hz) or intraarterial noradrenaline (0.2-2 nmoles) as well as the vasodilatation induced by sympathetic nerve stimulation (1-3 Hz) after alpha-receptor blockade. However, the vasodilatation following close i.a. injection of noradrenaline after alpha-receptor blockade was not changed by cocaine. The results indicate that the functionally important vascular alpha-adrenoceptors in adipose tissue are in close contact with adrenergic nerve terminals, whereas most vascular beta-adrenoceptors seem to be unrelated to the nerve terminals. Thus, the alpha-adrenoceptors in the adipose tissue vessels may be classified as innervated receptors, in contrast to the vascular beta-adrenoceptors which may be more acessible to circulating catecholamines and may be classified as humoral receptors. Furthermore at least some of the beta-receptors on the adipocytes seem to be located close to sympathetic nerve terminals.