Compartmentation of carbohydrate metabolism in vascular smooth muscle: evidence for at least two functionally independent pools of glucose 6-phosphate

Metabolism of vascular smooth muscle cells in vascular diseases

Vascular smooth muscle cells (VSMCs) are the fundamental component of the medial layer of arteries and are essential for arterial physiology and pathology. It is becoming increasingly clear that VSMCs can alter their metabolism to fulfill the bioenergetic and biosynthetic requirements. During vascular injury, VSMCs switch from a quiescent "contractile" phenotype to a highly migratory and proliferative "synthetic" phenotype. Recent studies have found that the phenotype switching of VSMCs is driven by a metabolic switch. Metabolic pathways, including aerobic glycolysis, fatty acid oxidation, and amino acid metabolism, have distinct, indispensable roles in normal and dysfunctional vasculature. VSMCs metabolism is also related to the metabolism of endothelial cells. In the present review, we present a brief overview of VSMCs metabolism and how it regulates the progression of several vascular diseases, including atherosclerosis, systemic hypertension, diabetes, pulmonary hypertension, vascular calcification, and aneurysms, and the effect of the risk factors for vascular disease (aging, cigarette smoking, and excessive alcohol drinking) on VSMC metabolism to clarify the role of VSMCs metabolism in the key pathological process.

How glycogen sustains brain function: A plausible allosteric signaling pathway mediated by glucose phosphates

Astrocytic glycogen is the sole glucose reserve of the brain. Both glycogen and glucose are necessary for basic neurophysiology and in turn for higher brain functions. In spite of low concentration, turnover and stimulation-induced degradation, any interference with normal glycogen metabolism in the brain severely affects neuronal excitability and disrupts memory formation. Here, I briefly discuss the glycogenolysis-induced glucose-sparing effect, which involves glucose phosphates as key allosteric effectors in the modulation of astrocytic and neuronal glucose uptake and phosphorylation. I further advance a novel and thus far unexplored effect of glycogenolysis that might be mediated by glucose phosphates.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

Imaging brain activation: simple pictures of complex biology

Elucidation of biochemical, physiological, and cellular contributions to metabolic images of brain is important for interpretation of images of brain activation and disease. Discordant brain images obtained with [(14)C]deoxyglucose and [1- or 6-(14)C]glucose were previously ascribed to increased glycolysis and rapid [(14)C]lactate release from tissue, but direct proof of [(14)C]lactate release from activated brain structures is lacking. Analysis of factors contributing to images of focal metabolic activity evoked by monotonic acoustic stimulation of conscious rats reveals that labeled metabolites of [1- or 6-(14)C]glucose are quickly released from activated cells as a result of decarboxylation reactions, spreading via gap junctions, and efflux via lactate transporters. Label release from activated tissue accounts for most of the additional [(14)C]glucose consumed during activation compared to rest. Metabolism of [3,4-(14)C]glucose generates about four times more [(14)C]lactate compared to (14)CO(2) in extracellular fluid, suggesting that most lactate is not locally oxidized. In brain slices, direct assays of lactate uptake from extracellular fluid demonstrate that astrocytes have faster influx and higher transport capacity than neurons. Also, lactate transfer from a single astrocyte to other gap junction-coupled astrocytes exceeds astrocyte-to-neuron lactate shuttling. Astrocytes and neurons have excess capacities for glycolysis, and oxidative metabolism in both cell types rises during sensory stimulation. The energetics of brain activation is quite complex, and the proportion of glucose consumed by astrocytes and neurons, lactate generation by either cell type, and the contributions of both cell types to brain images during brain activation are likely to vary with the stimulus paradigm and activated pathways.

NADH/NAD redox state of cytoplasmic glycolytic compartments in vascular smooth muscle

The cytoplasmic NADH/NAD redox potential affects energy metabolism and contractile reactivity of vascular smooth muscle. NADH/NAD redox state in the cytosol is predominately determined by glycolysis, which in smooth muscle is separated into two functionally independent cytoplasmic compartments, one of which fuels the activity of Na(+)-K(+)-ATPase. We examined the effect of varying the glycolytic compartments on cystosolic NADH/NAD redox state. Inhibition of Na(+)-K(+)-ATPase by 10 microM ouabain resulted in decreased glycolysis and lactate production. Despite this, intracellular concentrations of the glycolytic metabolite redox couples of lactate/pyruvate and glycerol-3-phosphate/dihydroxyacetone phosphate (thus NADH/NAD) and the cytoplasmic redox state were unchanged. The constant concentration of the metabolite redox couples and redox potential was attributed to 1) decreased efflux of lactate and pyruvate due to decreased activity of monocarboxylate B-H(+) transporter secondary to decreased availability of H(+) for cotransport and 2) increased uptake of lactate (and perhaps pyruvate) from the extracellular space, probably mediated by the monocarboxylate-H(+) transporter, which was specifically linked to reduced activity of Na(+)-K(+)-ATPase. We concluded that redox potentials of the two glycolytic compartments of the cytosol maintain equilibrium and that the cytoplasmic NADH/NAD redox potential remains constant in the steady state despite varying glycolytic flux in the cytosolic compartment for Na(+)-K(+)-ATPase.

Energy state, pH, and vasomotor tone during hypoxia in precontracted pulmonary and femoral arteries

To assess effects of smooth muscle energy state and intracellular pH (pH(i)) on pulmonary arterial tone during hypoxia, we measured ATP, phosphocreatine, P(i), and pH(i) by (31)P-NMR spectroscopy and isometric tension in phenylephrine-contracted rings of porcine proximal intrapulmonary arteries. Hypoxia caused early transient contraction followed by relaxation and late sustained contraction. Energy state and pH(i) decreased during relaxation and recovered toward control values during late contraction. Femoral arterial rings had higher energy state and lower pH(i) under baseline conditions and did not exhibit late contraction or recovery of energy state and pH(i) during hypoxia. In pulmonary arteries, glucose-free conditions abolished late hypoxic contraction and recovery of energy state and pH(i), but endothelial denudation abolished only late hypoxic contraction. NaCN had little effect at 0. 1 and 1.0 mM but caused marked vasorelaxation and decreases in energy state and pH(i) at 10 mM. These results suggest that 1) regulation of tone, energy state, and pH(i) differed markedly in pulmonary and femoral arterial smooth muscle, 2) hypoxic relaxation was mediated by decreased energy state or pH(i) due to hypoxic inhibition of oxidative phosphorylation, 3) recovery of energy state and pH(i) in hypoxic pulmonary arteries was due to accelerated glycolysis mediated by mechanisms intrinsic to smooth muscle, and 4) late hypoxic contraction in pulmonary arteries was mediated by endothelial factors that required hypoxic recovery of energy state and pH(i) for transduction in smooth muscle or extracellular glucose for production and release by endothelium.

Intracellular compartmentation of organelles and gradients of low molecular weight species

Intracellular compartmentation of metabolites without intervening membranes is an important concept that has emerged from consideration of the metabolic inhomogeneities associated with a highly organized and structured cytoplasm within mammalian cells. This recognition is primarily due to the development of experimental approaches to measure metabolite or ion concentrations at specific subcellular sites, thereby providing a means to study concentration gradients within the aqueous cytoplasm in intact cells. The presence of mitochondrial clusters has been shown to create gradients of low molecular weight species, such as O2, ATP, and pH, with important implications for substrate supply for function and regulation of cellular processes. Moreover, the existence of kinetically distinct precursor pools has been shown to result in functional compartmentation of biochemical pathways, such as DNA replication and carbohydrate metabolism. The creation of these specialized microzones of metabolism in accordance with their association with cellular organelles or membranal structures may be integral to normal function and regulation of adult mammalian cells.

Effects of insulin and metformin on glucose metabolism in rat vascular smooth muscle

Glucose metabolism in vascular smooth muscle cells (VSMCs) is characterized by substantial lactate production even in fully oxygenated conditions. Insulin and metformin, an insulin-sensitizing agent, have direct effects on the vascular tissue metabolism. We investigated whether insulin or metformin can induce a switch in VSMC glucose metabolism from lactate production to pyruvate oxidation, by measuring lactate oxidation as determined by the conversion of [1-14C]-D,L-lactate to [1-14C]-pyruvate and subsequent oxidation to acetyl coenzyme A and 14CO2 by pyruvate dehydrogenase (PDH). Lactate oxidation was measured in control rat aortic cultured VSMCs incubated for 30 minutes in media with and without additional glucose compared with VSMCs cultured in the presence of insulin or metformin. The addition of glucose to VSMCs decreased lactate oxidation (4.6+/-1.7 v 9.6+/-2.4 pmol/cell/min, P < .001). In the absence of additional glucose, metformin decreased lactate oxidation in VSMCs compared with controls (4.9+/-1.4 v 9.6+/-2.4 pmol/cell/min, P < .01). Metformin in the presence of glucose caused the greatest decline in lactate oxidation (2.5+/-0.4 pmol/cell/min, P < .001). In contrast to the effects of metformin, insulin increased lactate oxidation both with (12.9+/-1.5 pmol/cell/min, P < .001) and without (17.9+/-4.4, P < .01) additional glucose. This suggests that insulin facilitates VSMC utilization of lactate as a source of pyruvate and energy production even during noncontractile periods.

Oxidation of acetate and octanoate and its relation to glucose metabolism in contracting porcine carotid artery

The oxidation of octanoate and acetate was measured in segments of porcine carotid arteries to ascertain whether the oxidation of exogenous fatty acid substrates (acetate and octanoate) is augmented during contraction induced by K(+)-depolarization. The oxidation of acetate increased from 7 +/- 1 to 14 +/- 2 nmol/min/g (P < 0.01) during sustained isometric contraction. Octanoate oxidation increased from 11 +/- 1 to 14 +/- 1 nmol/min/g (P < 0.05). The rate of oxidation of neither acetate nor octanoate was affected by the presence or absence of glucose either in resting or contracting arteries Acetate or octanoate oxidation could account for the majority of O2 consumption during contraction. Octanoate but not acetate inhibited glucose uptake and glycolysis in resting muscles. In contrast to augmented acetate and octanoate metabolism during contraction, there was a "down-regulation" of glucose metabolism in contracting muscles as evidenced by a decrease in the rate of glucose uptake, glycolysis and lactic acid production during sustained isometric contraction. Thus, contractile activation of vascular smooth muscle is associated with a shifting pattern of substrate utilization. Exogenous acetate or octanoate can serve as the primary oxidative substrate during sustained isometric contraction.

Compartmentation of ATP synthesis and utilization in smooth muscle: roles of aerobic glycolysis and creatine kinase

The phosphocreatine content of smooth muscle is of similar magnitude to ATP. Thus the function of the creatine kinase system in this tissue cannot simply be regarded as an energy buffer. Thus an understanding of its role in smooth muscle behavior can point to CK function in other systems. From our perspective CK function in smooth muscle is one example of a more general phenomenon, that of the co-localization of ATP synthesis and utilization. In an interesting and analogous fashion distinct glycolytic cascades are also localized in regions of the cell with specialized energy requirements. Similar to CK, glycolytic enzymes are known to be localized on thin filaments, sarcoplasmic reticulum and plasma membrane. In this chapter we will describe the relations between glycolysis and smooth muscle function and compare and contrast to that of the CK system. Our goal is to more fully understand the significance of the compartmentation of distinct pathways for ATP synthesis with specific functions in smooth muscle. This organization of metabolism and function seen most clearly in smooth muscle is likely representative of many other cell types.

Local probes and heterogeneous catalysis: a case study of a mitochondria-luciferase-hexokinase coupled system

Biological systems are characterized by a high degree of structural organization. In the intracellular context, this introduces physical constraints which are not considered in the standard biochemical analysis of isolated systems, aimed towards mechanistic studies. A major challenge in cellular biology is thus to integrate the structural and mechanistic information and reach an adequate representation of the modes of operation in situ. We present an approach to this problem which takes advantage of a localized probe to study heterogeneous coupled system, as minimal models for cellular operation. The system consists of ATP production at the surface of mitochondria, and ATP consumption in solution by the hexokinase reaction. Soluble or biologically localized firefly luciferase is used to continuously monitor ATP concentration either in the bulk solution or at the surface of the organelle, respectively. The general system of a surface source and a bulk sink is mathematically modeled, and an analytic steady-state solution for local and bulk ATP is presented. The results are validated by experiment and differ from the expected behavior of an equivalent homogeneous system in solution. The model is further adapted to evaluate the effect of mixing. In addition, two limiting cases of heterogeneous distribution of hexokinase are analyzed, in which the soluble enzyme adsorbs non-specifically to mitochondria, or binds selectively to the site of ATP appearance on the membrane. The results are discussed in terms of their significance to the analysis of bulk measurements in vitro and their relevance to better description of cellular situations.

Functional compartmentalization of energy production in neural tissue

Previous work in our laboratory has shown that neural trauma results in a disparity between oxidative and glycolytic rates. In non-neural tissue, glycolysis and oxidative phosphorylation have been shown to work independently of one another, a phenomenon known as "energy compartmentalization". We believe that functional compartmentalization of energy production may also occur in the brain with glycolysis providing energy for membrane bound ionic pumps. Spreading depression, induced in rodent brain by topical KCl application, results in K+ shifts. The restoration of K+ gradients is accomplished by energy dependent Na(+)-K+ pumps. If these pumps depend upon glycolysis, blocking glycolysis should prevent reconstitution of normal [K+]e levels. The present series of experiments were designed to suggest that energy compartmentalization may also exist in brain, and that glycolytic energy production is preferentially used by Na(+)-K+ pumps to maintain normal ionic homeostasis by observing the dynamics of spreading depression induced K+ shifts before and after glycolytic blockade. Spreading depression was associated with increased K+ (48.6 +/- 16.6 mM over control) that normalized within 2.9 +/- 0.3 minutes. Following superfusion with a glycolytic blocking agent, spreading depression produced similar increases in [K+]e (40.6 +/- 12.0 mM over control) but time for reconstitution of the normal [K+]e was 400% longer than controls (2.9 +/- 0.3 to 14.9 +/- 2.1 minutes, P less than 0.001). Time required for recovery of EEG was identical pre- and post-blockade. We believe these data suggest that energy compartmentalization may exist in neural tissue and that glycolytic pathways of energy production are functionally tied to membrane Na(+)-K+ pumps.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of two glycolytic enzymes in guinea-pig taenia coli

The bound fractions of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and of fructose 1,6-diphosphate aldolase (ALD) were measured in intact Taenia coli. ALD was approximately 60% bound and GAPDH was approximately 41% bound. Bound ALD activity remaining in chemically demembranated Taenia coli was similar to that in intact tissue indicating a localization to the contractile apparatus. ALD was found to be specifically bound in the demembranated preparation. Chemical demembranation resulted in almost complete loss of all GAPDH activity indicating a localization of bound GAPDH to cellular membranes.

Paradoxical relationship between substrates and agonist-induced contractions of opossum esophageal body and sphincter in vitro

Possible differences in the abilities of esophageal body and lower esophageal sphincter muscles to utilize substrates to support agonist-induced contractions were studied. Strips of longitudinal, circular, and lower esophageal sphincter muscle from the opossum esophagus were first contracted to approximately 70% of the maximal contraction elicited by acetylcholine, histamine, or substance P. The tissues were then exhausted by exposure to 5 x 10(-4) M carbachol and a 15% O2-5% CO2-80% N2 gas mixture for 90 min. They were next reequilibrated with one of a number of alternative substrates and 95% O2-5% CO2 for 3 hr. Responses to the initial agonist doses were again noted and compared to controls. The alternative substrates were: 2-deoxyglucose, glucose, fructose 1-6 diphosphate, pyruvate, lactate, acetate, butyrate, caprylate, histidine, leucine, aspartate, alanine, succinate, acetoacetone, and beta-hydroxybutyrate. The results obtained show qualitative differences in the ability of the three muscle types to use these substrates. More importantly, however, the ability of any one substrate to support contractions was a function of the agonist used to stimulate the muscle. The evidence suggests, therefore, that not all pharmacologic receptors have equal access to intracellular energy sources.

Glycogen metabolism during tension generation and maintenance in vascular smooth muscle

To study the regulation of glycogen utilization in vascular smooth muscle, we measured the content of glycogen and glucose 6-phosphate and the activity of the glycogen phosphorylase and glycogen debrancher enzymes in porcine carotid artery. During active contraction, the rates of glycogen phosphorylase and glycogenolysis were as high as expected. Despite this, glycogen content did not decrease to less than approximately 50% of control levels even after sustained contractions. The activity of glycogen debrancher enzyme was found to be limiting glycogen utilization at this point. Although glycogenolysis is closely coordinated with increases in oxidative metabolism concomitant with active contraction, the maximal level of tension obtained after stimulation was not substantially reduced under conditions where glycogen debrancher enzyme was limiting glycogen utilization. On the other hand, the rate of tension generation was increased in these tissues. Thus glycogen utilization is not necessary for maximal force generation per se, but may influence other muscle contractile properties. Finally, during steady-state tension maintenance, glycogen utilization is likely to be regulated by the intracellular concentrations of metabolic intermediates (glucose, glucose 6-phosphate), as it is in skeletal muscle.

Compartmentation of glucose 6-phosphate in hepatocytes

Rat hepatocytes were incubated with 14C-labelled hexoses, and the specific radioactivities of glucose 6-phosphate, glucose 1-phosphate and fructose 6-phosphate were determined. (1) When suspensions of freshly isolated hepatocytes were incubated with [14C]glucose, the specific radioactivities of glucose 1-phosphate and fructose 6-phosphate were severalfold higher than that of glucose 6-phosphate. The ratios of the specific radioactivities decreased with time of incubation. These relationships were also found when incubations were carried out with primary cultures of rat hepatocytes or with crude homogenates of hepatocytes, but not with isolated nuclei. (2) When cells were incubated with [14C]fructose, the ratios of the specific radioactivities were higher than with [14C]glucose, and also decreased with time. (3) Paired incubations were carried out with a mixture of galactose and fructose, with one or other sugar being labelled with 14C. The specific radioactivity of glucose released into the medium was greater than that of glucose 6-phosphate when fructose was labelled, but not when galactose was labelled. Furthermore, glucose 6-phosphate and glucose in the medium differed with regard to the distribution of 14C between C-1 and C-6. These results are interpreted as evidence that glucose 6-phosphate in hepatocytes does not exist as a homogeneous pool, but that subcompartments exist which are associated with glucose phosphorylation, gluconeogenesis and glycogenolysis.

Kinetics of glucose disposal in whole body and across the forearm in man

We reevaluated the concept that the in vivo glucose disposal rate in man is determined by the activity of the glucose transport system. Rates of glucose disposal were determined in whole body and across forearm at four insulin levels (approximately 9, approximately 50, approximately 160, and approximately 1700 microU/ml) and at each insulin level at four glucose levels (approximately 90, approximately 160, approximately 250, and approximately 400 mg/dl). At the lowest insulin level, the Michaelis constants (Ks:s) for glucose disposal in whole body (8.7 +/- 1.1 mM) and across forearm (7.4 +/- 1.4) mM) were compatible with a Ks determined in vitro for the transport system. At higher insulin levels, the apparent Ks increased significantly in whole body (16.2-37.7 mM) and across forearm (20.7-31.2 mM). We interpret the apparent increase of Ks by insulin to reflect a shift in the rate-limiting step from glucose transport to some step beyond transport.

Compartmentation of carbohydrate metabolism in vascular smooth muscle

In vascular smooth muscle, oxidative phosphorylation and glycolysis are independently regulated. Previous studies indicated that the independent regulation of these pathways was related to a compartmentation of carbohydrate metabolism. To further study carbohydrate metabolism, glucose transport and the incorporation of radiolabel from glucose into glycogen and lactate were measured after the oxidative and glycolytic pathways were independently altered. Ouabain stimulated mechanical activity, oxygen consumption, and glycogenolysis, whereas lactate production was decreased. Although glycogenolysis was substantial, glucose was the only substrate for lactate, indicating that intermediates derived from glycogen do not mix with those from glucose uptake. Thus glycogenolysis and glycolysis are carried out by independent enzymatic pathways. Insulin-stimulated lactate production and glucose transport without affecting the other parameters. Again, lactate was produced only from glucose. Phenytoin decreased isometric tension and oxygen consumption, whereas stimulating lactate production and glycogenolysis. Glycogen was the primary substrate for the lactate produced. Our findings indicate that the compartmentation of substrate utilization is ascribable to the coordination of glycogenolysis with increases in oxygen consumption and the coupling of glycolysis to the Na-K-adenosine triphosphatase. The coupling of independent energy providing pathways to specific endergonic processes indicates a mechanism by which cellular energetic efficiency may be optimized.