The development of peripheral microvasculopathy with chronic metabolic disease in obese Zucker rats: a retrograde emergence?

The study of peripheral vasculopathy with chronic metabolic disease is challenged by divergent contributions from spatial (the level of resolution or specific tissue being studied) and temporal origins (evolution of the developing impairments in time). Over many years of studying the development of skeletal muscle vasculopathy and its functional implications, we may be at the point of presenting an integrated conceptual model that addresses these challenges within the obese Zucker rat (OZR) model. At the early stages of metabolic disease, where systemic markers of elevated cardiovascular disease risk are present, the only evidence of vascular dysfunction is at postcapillary and collecting venules, where leukocyte adhesion/rolling is elevated with impaired venular endothelial function. As metabolic disease severity and duration increases, reduced microvessel density becomes evident as well as increased variability in microvascular hematocrit. Subsequently, hemodynamic impairments to distal arteriolar networks emerge, manifesting as increasing perfusion heterogeneity and impaired arteriolar reactivity. This retrograde "wave of dysfunction" continues, creating a condition wherein deficiencies to the distal arteriolar, capillary, and venular microcirculation stabilize and impairments to proximal arteriolar reactivity, wall mechanics, and perfusion distribution evolve. This proximal arteriolar dysfunction parallels increasing failure in fatigue resistance, hyperemic responses, and O2 uptake within self-perfused skeletal muscle. Taken together, these results present a conceptual model for the retrograde development of peripheral vasculopathy with chronic metabolic disease and provide insight into the timing and targeting of interventional strategies to improve health outcomes.NEW & NOTEWORTHY Working from an established database spanning multiple scales and times, we studied progression of peripheral microvascular dysfunction in chronic metabolic disease. The data implicate the postcapillary venular endothelium as the initiating site for vasculopathy. Indicators of dysfunction, spanning network structures, hemodynamics, vascular reactivity, and perfusion progress in an insidious retrograde manner to present as functional impairments to muscle blood flow and performance much later. The silent vasculopathy progression may provide insight into clinical treatment challenges.

Measuring Mitochondrial Function: From Organelle to Organism.

Mitochondrial energy production is crucial for normal daily activities and maintenance of life. Herein, the logic and execution of two main classes of measurements are outlined to delineate mitochondrial function: ATP production and oxygen consumption. Aerobic ATP production is quantified by phosphorus magnetic resonance spectroscopy (31PMRS) in vivo in both human subjects and animal models using the same protocols and maintaining the same primary assumptions. Mitochondrial oxygen consumption is quantified by oxygen polarography and applied in isolated mitochondria, cultured cells, and permeabilized fibers derived from human or animal tissue biopsies. Traditionally, mitochondrial functional measures focus on maximal oxidative capacity-a flux rate that is rarely, if ever, observed outside of experimental conditions. Perhaps more physiologically relevant, both measurement classes herein focus on one principal design paradigm; submaximal mitochondrial fluxes generated by graded levels of ADP to map the function for ADP sensitivity. We propose this function defines the bioenergetic role that mitochondria fill within the myoplasm to sense and match ATP demands. Any deficit in this vital role for ATP homeostasis leads to symptoms often seen in cardiovascular and cardiopulmonary diseases, diabetes, and metabolic syndrome.

Shifted vascular optimization: the emergence of a new arteriolar behaviour with chronic metabolic disease.

NEW FINDINGS: What is the topic of this review? Altered perfusion distribution at skeletal muscle arteriolar bifurcations and how this is modified by development of chronic metabolic disease. What advances does it highlight? The outcome created is a distribution of erythrocytes in the distal microcirculation that is characterized by increased spatial heterogeneity and reduced flexibility such that mass transport/exchange within the network is impaired, with limited ability to respond to imposed challenges. This advances our understanding of how altered vascular structure and function with metabolic disease impairs perfusion to skeletal muscle at a level of resolution that would not be identified through bulk flow responses. ABSTRACT: This review is based on the presentation 'Shifted vascular optimization: the emergence of a new arteriolar behaviour with chronic metabolic disease', given at the Symposium 'Understanding Complex Behaviours in the Microcirculation: from Blood Flow to Oxygenation' during the Annual Meeting of the Physiological Society at the Aberdeen Exhibition and Conference Centre in Aberdeen, UK in July 2019. The past years of dedicated investigation on linkages between vascular (dys)function under conditions of elevated cardiovascular disease risk and tissue/organ performance have produced results and insights that frequently suffer from limited correlation and causation. Reaching out from this challenge, it was proposed that this may reflect a 'level of resolution' argument and that altered haemodynamic behaviour in vascular networks could be a stronger predictor of functional outcomes than higher resolution measures. Using this approach, we have determined that an attractor that describes the spatial and temporal shift in perfusion distribution at successive arteriolar bifurcations within the skeletal muscle is a strong predictor of functional outcomes within animals and provides novel insight into fundamental mechanistic contributors to altered patterns of intra-muscular perfusion. This article focuses on the applicability and utility of the attractor in models of cardiovascular and metabolic disease risk of increasing severity. We will also discuss the utility of the attractor in terms of understanding the effectiveness of aggressive interventions for reversing established vasculopathy and perfusion impairments.

Quantification of Mitochondrial Oxidative Phosphorylation in Metabolic Disease: Application to Type 2 Diabetes.

Type 2 diabetes (T2D) is a growing health concern with nearly 400 million affected worldwide as of 2014. T2D presents with hyperglycemia and insulin resistance resulting in increased risk for blindness, renal failure, nerve damage, and premature death. Skeletal muscle is a major site for insulin resistance and is responsible for up to 80% of glucose uptake during euglycemic hyperglycemic clamps. Glucose uptake in skeletal muscle is driven by mitochondrial oxidative phosphorylation and for this reason mitochondrial dysfunction has been implicated in T2D. In this review we integrate mitochondrial function with physiologic function to present a broader understanding of mitochondrial functional status in T2D utilizing studies from both human and rodent models. Quantification of mitochondrial function is explained both in vitro and in vivo highlighting the use of proper controls and the complications imposed by obesity and sedentary lifestyle. This review suggests that skeletal muscle mitochondria are not necessarily dysfunctional but limited oxygen supply to working muscle creates this misperception. Finally, we propose changes in experimental design to address this question unequivocally. If mitochondrial function is not impaired it suggests that therapeutic interventions and drug development must move away from the organelle and toward the cardiovascular system.

The hypertension advantage and natural selection: Since type 2 diabetes associates with co-morbidities and premature death, why have the genetic variants remained in the human genome?

Type 2 diabetes is a major public health crisis around the world. It is estimated that more than 300 million people worldwide have type 2 diabetes. Furthermore, the World Health Organization estimates that deaths from the complications of diabetes will increase by two thirds between 2008 and 2030. Since type 2 diabetes is a major public health crisis, why have the genetic variants for diabetes not been removed from the genome by natural selection? We hypothesize that insulin resistance, a predisposition to type 2 diabetes, and the associated elevation in sympathetic nervous system activity and arterial blood pressure provided an advantage to humans who lived as hunter-gatherers. Specifically, sympathetic hyperactivity stimulates the renin-angiotensin aldosterone system, promotes sodium reabsorption, and increases blood volume, heart rate, stroke volume and peripheral vascular resistance, thus inducing hypertension. The hypertension in turn provides a hemodynamic advantage for hunter-gatherers. Specifically, sympathetic hyperactivity and increased blood pressure increases blood flow delivery to working muscles by increasing cardiac output and shunting blood from non-active tissue. This natural selection for hypertension occurred during the time in human evolutionary history when the lifespan of most individuals was probably 30-40 years, and morbidity and mortality from cardiovascular disorders was limited. Thus, the selection pressure for elevation in sympathetic nervous system activity and blood pressure provided an advantage for hunting and gathering that would be greater than the selection pressure exerted by the manifestations of cardiovascular disease in aged individuals.

Skeletal muscle energetics are compromised only during high-intensity contractions in the Goto-Kakizaki rat model of type 2 diabetes.

Type 2 diabetes (T2D) presents with hyperglycemia and insulin resistance, affecting over 30 million people in the United States alone. Previous work has hypothesized that mitochondria are dysfunctional in T2D and results in both reduced ATP production and glucose disposal. However, a direct link between mitochondrial function and T2D has not been determined. In the current study, the Goto-Kakizaki (GK) rat model of T2D was used to quantify mitochondrial function in vitro and in vivo over a broad range of contraction-induced metabolic workloads. During high-frequency sciatic nerve stimulation, hindlimb muscle contractions at 2- and 4-Hz intensities, the GK rat failed to maintain similar bioenergetic steady states to Wistar control (WC) rats measured by phosphorus magnetic resonance spectroscopy, despite similar force production. Differences were not due to changes in mitochondrial content in red (RG) or white gastrocnemius (WG) muscles (cytochrome c oxidase, RG: 22.2 ± 1.6 vs. 23.3 ± 1.7 U/g wet wt; WG: 10.8 ± 1.1 vs. 12.1 ± 0.9 U/g wet wt; GK vs. WC, respectively). Mitochondria isolated from muscles of GK and WC rats also showed no difference in mitochondrial ATP production capacity in vitro, measured by high-resolution respirometry. At lower intensities (0.25-1 Hz) there were no detectable differences between GK and WC rats in sustained energy balance. There were similar phosphocreatine concentrations during steady-state contraction and postcontractile recovery (τ = 72 ± 6 s GK versus 71 ± 2 s WC). Taken together, these results suggest that deficiencies in skeletal muscle energetics seen at higher intensities are not due to mitochondrial dysfunction in the GK rat.

Endothelium-dependent impairments to cerebral vascular reactivity with type 2 diabetes mellitus in the Goto-Kakizaki rat.

Type 2 diabetes mellitus (T2DM) is a prevalent pathology associated with elevated cerebrovascular disease risk. We determined wall mechanics and vascular reactivity in ex vivo middle cerebral arteries (MCA) from male Goto-Kakizaki rats (GK; ~17 wk old) versus control Wistar Kyoto rats (WKY) to test the hypothesis that the diabetic environment in GK, in the absence of obesity and other comorbidities, leads to endothelial dysfunction and impaired vascular tone regulation. Dilation of MCA following challenge with acetylcholine and hypoxia was blunted in MCA from GK versus WKY, due to lower nitric oxide bioavailability and altered arachidonic acid metabolism, whereas myogenic activation and constrictor responses to serotonin were unchanged. MCA wall distensibility and cross-sectional area were not different between GK and WKY, suggesting that wall mechanics were unchanged at this age, supported by the determination that MCA dilation to sodium nitroprusside was also intact. With the use of ex vivo aortic rings as a bioassay, altered vascular reactivity determined in MCA was paralleled by relaxation responses in artery segments from GK, whereas measurements of vasoactive metabolite production indicated a loss of nitric oxide and prostacyclin bioavailability and an increased thromboxane A2 production with both methacholine challenge and hypoxia. These results suggest that endothelium-dependent dilator reactivity of MCA in GK is impaired with T2DM, and that this impairment is associated with the genesis of a prooxidant/pro-inflammatory condition with diabetes mellitus. The restriction of vascular impairments to endothelial function only, at this age and development, provide insight into the severity of multimorbid conditions of which T2DM is only one constituent.

Transient receptor potential vanilloid 4 channels are important regulators of parenchymal arteriole dilation and cognitive function.

OBJECTIVE: Hypertension-associated PA dysfunction reduces cerebral perfusion and impairs cognition. This is associated with impaired TRPV4-mediated PA dilation; therefore, we tested the hypothesis that TRPV4 channels are important regulators of cerebral perfusion, PA structure and dilation, and cognition. METHODS: Ten- to twelve-month-old male TRPV4 knockout (WKY-Trpv4em4Mcwi ) and age-matched control WKY rats were studied. Cerebral perfusion was measured by MRI with arterial spin labeling. PA structure and function were assessed using pressure myography and cognitive function using the novel object recognition test. RESULTS: Cerebral perfusion was reduced in the WKY-Trpv4em4Mcwi rats. This was not a result of PA remodeling because TRPV4 deletion did not change PA structure. TRPV4 deletion did not change PA myogenic tone development, but PAs from the WKY-Trpv4em4Mcwi rats had severely blunted endothelium-dependent dilation. The WKY-Trpv4em4Mcwi rats had impaired cognitive function and exhibited depressive-like behavior. The WKY-Trpv4em4Mcwi rats also had increased microglia activation, and increased mRNA expression of GFAP and tumor necrosis factor alpha suggesting increased inflammation. CONCLUSION: Our data indicate that TRPV4 channels play a critical role in cerebral perfusion, PA dilation, cognition, and inflammation. Impaired TRPV4 function in diseases such as hypertension may increase the risk of the development of vascular dementia.

Obesity and inactivity, not hyperglycemia, cause exercise intolerance in individuals with type 2 diabetes: Solving the obesity and inactivity versus hyperglycemia causality dilemma.

Obesity, a sedentary lifestyle and type 2 diabetes are intricately linked conditions contributing to reduced exercise tolerance, significant morbidity, and premature deaths. It is unknown whether the reported exercise intolerance associated with type 2 diabetes is a direct result of the hyperglycemia, the impact of a relatively sedentary lifestyle, or increased adiposity. We hypothesize that obesity and inactivity, not hyperglycemia, cause exercise intolerance in individuals with type 2 diabetes. An analysis of the literature and results from the Goto-Kakizaki (GK) rat model of type 2 diabetes strongly support this hypothesis. GK rats were not sedentary or obese when compared with Wistar control rats and did not have exercise intolerance. Specifically, despite being hyperglycemic, GK rats demonstrated a longer treadmill run time to exhaustion (150.6 ±â€¯9.0 vs. 77.2 ±â€¯12.9 min), further distance run (1506 ±â€¯90 vs. 772 ±â€¯129 m), more work performed per gram muscle (44.0 ±â€¯2.8 vs. 21.9 ±â€¯3.8 kg*m/g) and a small increase in total vertical work performed when accounting for body mass (116.8 ±â€¯6.3 versus 98.9 ±â€¯15.2 kg*m). These results document preserved exercise tolerance in the non-obese, non-sedentary GK rat supporting the hypothesis that the reported exercise intolerance in models of type 2 diabetes is dependent on obesity and inactivity. Solving the obesity and inactivity versus hyperglycemia causality dilemma is important in understanding the development of type 2 diabetes and implications for future pharmacological and life style interventions.

Effects of altered pyruvate dehydrogenase activity on contracting skeletal muscle bioenergetics.

During aerobic exercise (>65% of maximum oxygen consumption), the primary source of acetyl-CoA to fuel oxidative ATP synthesis in muscle is the pyruvate dehydrogenase (PDH) reaction. This study investigated how regulation of PDH activity affects muscle energetics by determining whether activation of PDH with dichloroacetate (DCA) alters the dynamics of the phosphate potential of rat gastrocnemius muscle during contraction. Twitch contractions were induced in vivo over a broad range of intensities to sample submaximal and maximal aerobic workloads. Muscle phosphorus metabolites were measured in vivo before and after DCA treatment by phosphorus nuclear magnetic resonance spectroscopy. At rest, DCA increased PDH activation compared with control (90 ± 12% vs. 23 ± 3%, P < 0.05), with parallel decreases in inorganic phosphate (Pi) of 17% (1.4 ± 0.2 vs. 1.7 ± 0.1 mM, P < 0.05) and an increase in the free energy of ATP hydrolysis (ΔGATP) (-66.2 ± 0.3 vs. -65.6 ± 0.2 kJ/mol, P < 0.05). During stimulation DCA increased steady-state phosphocreatine (PCr) and the magnitude of ΔGATP, with concomitant reduction in Pi and ADP concentrations. These effects were not due to kinetic alterations in PCr hydrolysis, resynthesis, or glycolytic ATP production and altered the flow-force relationship between mitochondrial ATP synthesis rate and ΔGATP. DCA had no significant effect at 1.0- to 2.0-Hz stimulation because physiological mechanisms at these high stimulation levels cause maximal activation of PDH. These data support a role of PDH activation in the regulation of the energetic steady state by altering the phosphate potential (ΔGATP) at rest and during contraction.

Skeletal muscle performance in metabolic disease: Microvascular or mitochondrial limitation or both?

One of the clearly established health outcomes associated with chronic metabolic diseases (eg, type II diabetes mellitus) is that the ability of skeletal muscle to maintain contractile performance during periods of elevated metabolic demand is compromised as compared to the fatigue-resistance of muscle under normal, healthy conditions. While there has been extensive effort dedicated to determining the major factors that contribute to the compromised performance of skeletal muscle with chronic metabolic disease, the extent to which this poor outcome reflects a dysfunctional state of the microcirculation, where the delivery and distribution of metabolic substrates can be impaired, versus derangements to normal metabolic processes and mitochondrial function, versus a combination of the two, represents an area of considerable unknown. The purpose of this manuscript is to present some of the current concepts for dysfunction to both the microcirculation of skeletal muscle as well as to mitochondrial metabolism under these conditions, such that these diverse issues can be merged into an integrated framework for future investigation. Based on an interpretation of the current literature, it may be hypothesized that the primary site of dysfunction with earlier stages of metabolic disease may lie at the level of the vasculature, rather than at the level of the mitochondria.

Type 2 diabetes mellitus in the Goto-Kakizaki rat impairs microvascular function and contributes to premature skeletal muscle fatigue.

Despite extensive investigation into the impact of metabolic disease on vascular function and, by extension, tissue perfusion and organ function, interpreting results for specific risk factors can be complicated by the additional risks present in most models. To specifically determine the impact of type 2 diabetes without obesity on skeletal muscle microvascular structure/function and on active hyperemia with elevated metabolic demand, we used 17-wk-old Goto-Kakizaki (GK) rats to study microvascular function at multiple levels of resolution. Gracilis muscle arterioles demonstrated blunted dilation to acetylcholine (both ex vivo proximal and in situ distal arterioles) and elevated shear (distal arterioles only). All other alterations to reactivity appeared to reflect compromised endothelial function associated with increased thromboxane (Tx)A2 production and oxidant stress/inflammation rather than alterations to vascular smooth muscle function. Structural changes to the microcirculation of GK rats were confined to reduced microvessel density of ~12%, with no evidence for altered vascular wall mechanics. Active hyperemia with either field stimulation of in situ cremaster muscle or electrical stimulation via the sciatic nerve for in situ gastrocnemius muscle was blunted in GK rats, primarily because of blunted functional dilation of skeletal muscle arterioles. The blunted active hyperemia was associated with impaired oxygen uptake (V̇o2) across the muscle and accelerated muscle fatigue. Acute interventions to reduce oxidant stress (TEMPOL) and TxA2 action (SQ-29548) or production (dazmegrel) improved muscle perfusion, V̇o2, and muscle performance. These results suggest that type 2 diabetes mellitus in GK rats impairs skeletal muscle arteriolar function apparently early in the progression of the disease and potentially via an increased reactive oxygen species/inflammation-induced TxA2 production/action on network function as a major contributing mechanism. NEW & NOTEWORTHY The impact of type 2 diabetes mellitus on vascular structure/function remains an area lacking clarity. Using diabetic Goto-Kakizaki rats before the development of other risk factors, we determined alterations to vascular structure/function and skeletal muscle active hyperemia. Type 2 diabetes mellitus reduced arteriolar endothelium-dependent dilation associated with increased thromboxane A2 generation. Although modest microvascular rarefaction was evident, there were no other alterations to vascular structure/function. Skeletal muscle active hyperemia was blunted, although it improved after antioxidant or anti-thromboxane A2 treatment.

Chronic atorvastatin and exercise can partially reverse established skeletal muscle microvasculopathy in metabolic syndrome.

It has long been known that chronic metabolic disease is associated with a parallel increase in the risk for developing peripheral vascular disease. Although more clinically relevant, our understanding about reversing established vasculopathy is limited compared with our understanding of the mechanisms and development of impaired vascular structure/function under these conditions. Using the 13-wk-old obese Zucker rat (OZR) model of metabolic syndrome, where microvascular dysfunction is sufficiently established to contribute to impaired skeletal muscle function, we imposed a 7-wk intervention of chronic atorvastatin treatment, chronic treadmill exercise, or both. By 20 wk of age, untreated OZRs manifested a diverse vasculopathy that was a central contributor to poor muscle performance, perfusion, and impaired O2 exchange. Atorvastatin or exercise, with the combination being most effective, improved skeletal muscle vascular metabolite profiles (i.e., nitric oxide, PGI2, and thromboxane A2 bioavailability), reactivity, and perfusion distribution at both individual bifurcations and within the entire microvascular network versus responses in untreated OZRs. However, improvements to microvascular structure (i.e., wall mechanics and microvascular density) were less robust. The combination of the above improvements to vascular function with interventions resulted in an improved muscle performance and O2 transport and exchange versus untreated OZRs, especially at moderate metabolic rates (3-Hz twitch contraction). These results suggest that specific interventions can improve specific indexes of function from established vasculopathy, but either this process was incomplete after 7-wk duration or measures of vascular structure are either resistant to reversal or require better-targeted interventions. NEW & NOTEWORTHY We used atorvastatin and/or chronic exercise to reverse established microvasculopathy in skeletal muscle of rats with metabolic syndrome. With established vasculopathy, atorvastatin and exercise had moderate abilities to reverse dysfunction, and the combined application of both was more effective at restoring function. However, increased vascular wall stiffness and reduced microvessel density were more resistant to reversal. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/reversal-of-microvascular-dysfunction/ .

Chronic, complete cervical6-7 cord transection: distinct autonomic and cardiac deficits.

Spinal cord injury (SCI) resulting in tetraplegia is a devastating, life-changing insult causing paralysis and sensory impairment as well as distinct autonomic dysfunction that triggers compromised cardiovascular, bowel, bladder, and sexual activity. Life becomes a battle for independence as even routine bodily functions and the smallest activity of daily living become major challenges. Accordingly, there is a critical need for a chronic preclinical model of tetraplegia. This report addresses this critical need by comparing, for the first time, resting-, reflex-, and stress-induced cardiovascular, autonomic, and hormonal responses each week for 4 wk in 12 sham-operated intact rats and 12 rats with chronic, complete C6-7 spinal cord transection. Loss of supraspinal control to all sympathetic preganglionic neurons projecting to the heart and vasculature resulted in a profound bradycardia and hypotension, reduced cardiac sympathetic and parasympathetic tonus, reduced reflex- and stress-induced sympathetic responses, and reduced sympathetic support of blood pressure as well as enhanced reliance on angiotensin to maintain arterial blood pressure. Histological examination of the nucleus ambiguus and stellate ganglia supports the profound and distinct autonomic and cardiac deficits and reliance on angiotensin to maintain cardiovascular stability following chronic, complete cervical6-7 cord transection. NEW & NOTEWORTHY For the first time, resting-, reflex-, and stress-induced cardiovascular, autonomic, and hormonal responses were studied in rats with chronic, complete C6-7 cord transection. Loss of supraspinal control of all sympathetic preganglionic neurons reduced cardiac sympathetic and parasympathetic tonus, reflex and stress-induced sympathetic responses, and sympathetic support of blood pressure as well as enhanced reliance on angiotensin to maintain arterial blood pressure. Histological examination supports the distinct deficits associated with cervical cord injury.

Open-Loop Control of Oxidative Phosphorylation in Skeletal and Cardiac Muscle Mitochondria by Ca(2.).

In cardiac muscle, mitochondrial ATP synthesis is driven by demand for ATP through feedback from the products of ATP hydrolysis. However, in skeletal muscle at higher workloads there is an apparent contribution of open-loop stimulation of ATP synthesis. Open-loop control is defined as modulation of flux through a biochemical pathway by a moiety, which is not a reactant or a product of the biochemical reactions in the pathway. The role of calcium, which is known to stimulate the activity of mitochondrial dehydrogenases, as an open-loop controller, was investigated in isolated cardiac and skeletal muscle mitochondria. The kinetics of NADH synthesis and respiration, feedback from ATP hydrolysis products, and stimulation by calcium were characterized in isolated mitochondria to test the hypothesis that calcium has a stimulatory role in skeletal muscle mitochondria not apparent in cardiac mitochondria. A range of respiratory states were obtained in cardiac and skeletal muscle mitochondria utilizing physiologically relevant concentrations of pyruvate and malate, and flux of respiration, NAD(P)H fluorescence, and rhodamine 123 fluorescence were measured over a range of extra mitochondrial calcium concentrations. We found that under these conditions calcium stimulates NADH synthesis in skeletal muscle mitochondria but not in cardiac mitochondria.

Feedback Regulation and Time Hierarchy of Oxidative Phosphorylation in Cardiac Mitochondria.

To determine how oxidative ATP synthesis is regulated in the heart, the responses of cardiac mitochondria oxidizing pyruvate to alterations in [ATP], [ADP], and inorganic phosphate ([Pi]) were characterized over a range of steady-state levels of extramitochondrial [ATP], [ADP], and [Pi]. Evolution of the steady states of the measured variables with the flux of respiration shows that: (1) a higher phosphorylation potential is achieved by mitochondria at higher [Pi] for a given flux of respiration; (2) the time hierarchy of oxidative phosphorylation is given by phosphorylation subsystem, electron transport chain, and substrate dehydrogenation subsystems listed in increasing order of their response times; (3) the matrix ATP hydrolysis mass action ratio [ADP] × [Pi]/[ATP] provides feedback to the substrate dehydrogenation flux over the entire range of respiratory flux examined in this study; and finally, (4) contrary to previous models of regulation of oxidative phosphorylation, [Pi] does not modulate the activity of complex III.

Contractile dysfunction in muscle may underlie androgen-dependent motor dysfunction in spinal bulbar muscular atrophy.

Spinal and bulbar muscular atrophy (SBMA) is characterized by progressive muscle weakness linked to a polyglutamine expansion in the androgen receptor (AR). Current evidence indicates that mutant AR causes SBMA by acting in muscle to perturb its function. However, information about how muscle function is impaired is scant. One fundamental question is whether the intrinsic strength of muscles, an attribute of muscle independent of its mass, is affected. In the current study, we assess the contractile properties of hindlimb muscles in vitro from chronically diseased males of three different SBMA mouse models: a transgenic (Tg) model that broadly expresses a full-length human AR with 97 CAGs (97Q), a knock-in (KI) model that expresses a humanized AR containing a CAG expansion in the first exon, and a Tg myogenic model that overexpresses wild-type AR only in skeletal muscle fibers. We found that hindlimb muscles in the two Tg models (97Q and myogenic) showed marked losses in their intrinsic strength and resistance to fatigue, but were minimally affected in KI males. However, diseased muscles of all three models showed symptoms consistent with myotonic dystrophy type 1, namely, reduced resting membrane potential and deficits in chloride channel mRNA. These data indicate that muscle dysfunction is a core feature of SBMA caused by at least some of the same pathogenic mechanisms as myotonic dystrophy. Thus mechanisms controlling muscle function per se independent of mass are prime targets for SBMA therapeutics.

Androgen receptors in muscle fibers induce rapid loss of force but not mass: implications for spinal bulbar muscular atrophy.

INTRODUCTION: Testosterone (T) induces motor dysfunction in transgenic (Tg) mice that overexpress wild-type androgen receptor (AR) in skeletal muscles. Because many genes implicated in motor neuron disease are expressed in skeletal muscle, mutant proteins may act in muscle to cause dysfunction in motor neuron disease. METHODS: We examined contractile properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles in vitro after 5 and 3 days of T treatment in motor-impaired Tg female mice. RESULTS: Both muscles showed deficits in tetanic force after 5 days of T treatment, without losses in muscle mass, protein content, or fiber number. After 3 days of T treatment, only SOL showed a deficit in tetanic force comparable to that of 5 days of treatment. In both treatments, EDL showed slowed twitch kinetics, whereas SOL showed deficits in the twitch/tetanus ratio. CONCLUSIONS: These results suggest calcium-handling mechanisms in muscle fibers are defective in motor-impaired mice.

Selective inhibition of ATPase activity during contraction alters the activation of p38 MAP kinase isoforms in skeletal muscle.

Muscle contractions strongly activate p38 MAP kinases, but the precise contraction-associated sarcoplasmic event(s) (e.g., force production, energetic demands, and/or calcium cycling) that activate these kinases are still unclear. We tested the hypothesis that during contraction the phosphorylation of p38 isoforms is sensitive to the increase in ATP demand relative to ATP supply. Energetic demands were inhibited using N-benzyl-p-toluene sulphonamide (BTS, type II actomyosin) and cyclopiazonic acid (CPA, SERCA). Extensor digitorum longus muscles from Swiss Webster mice were incubated in Ringer's solution (37°C) with or without inhibitors and then stimulated at 10 Hz for 15 min. Muscles were immediately freeze-clamped for metabolite and Western blot analysis. BTS and BTS + CPA treatment decreased force production by 85%, as measured by the tension time integral, while CPA alone potentiated force by 310%. In control muscles, contractions resulted in a 73% loss of ATP content and a concomitant sevenfold increase in IMP content, a measure of sustained energetic imbalance. BTS or CPA treatment lessened the loss of ATP, but BTS + CPA treatment completely eliminated the energetic imbalance since ATP and IMP levels were nearly equal to those of non-stimulated muscles. The independent inhibition of cytosolic ATPase activities had no effect on contraction-induced p38 MAPK phosphorylation, but combined treatment prevented the increase in phosphorylation of the γ isoform while the α/ß isoforms unaffected. These observations suggest that an energetic signal may trigger phosphorylation of the p38γ isoform and also may explain how contractions differentially activate signaling pathways.