Adaptations of the β-adrenoceptor-adenylyl cyclase system in rat skeletal muscle to endurance physical training

 β-Adrenergic mechanisms may be important in the adaptation of skeletal muscle to endurance training. β-Adrenergic signal transduction was examined in the gastrocnemius muscle of rats submitted to a progressive, 12-week treadmill running program and compared with sedentary controls. β-Adrenoceptor density was significantly lower in exercised rats than in controls. The affinity constant for [¹²⁵I]-(-) iodocyanopindolol binding was not different among the various groups. Adenosine cyclic monophosphate formation was significantly decreased in trained animals when isoproterenol plus guanosine triphosphate or forskolin plus Mn²⁺ were used to stimulate adenylyl cyclase. Immunoblot analyses revealed that the amount of the α-subunit of stimulatory guanine nucleotide-binding protein (G_s,α), both the small and the large isoforms, also decreased with physical exercise. Thus, the present report shows that endurance training results in alterations in β-adrenergic receptor density, adenylyl cyclase activity and G_s protein level in rat gastrocnemius muscle. Received: 18 October 1996 / Received after revision: 11 March 1997 / Accepted: 20 June 1997     

Role of α2-adrenoceptors in brain resistance to total ischemiaadrenoceptors in brain resistance to total ischemia

Neuroprotective effect of epinephrine is revealed against the background of prazosin, β-antagonists and their combinations protect against cerebral ischemia. α₂-Antagonists (but not α₁) block the neuroprotective effect of these combinations, methyldopa, guanabenz, clonidine, and oxymetazoline and decrease brain resistance to ischemia. α₂-Adrenoreceptors participate in the endogenic mechanism of brain resistance to ischemia and in the neuroprotective effect. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 124, No. 10, pp. 413–416, October, 1997     

Adrenergic regulation of lymphocyte proliferative response in cultures with T-cell mitogens

The effects of epinephrine, β-adrenergic agonist terbutaline sulfate, and cAMP phosphodiesterase inhibitor theophylline on proliferative response of peripheral blood lymphocytes from healthy subjects were studied in cultures with phytohemagglutinin and concanavalin A. Both adrenergic agonists inhibited lymphocyte blastogenesis, but the effect of epinephrine was more pronounced. Translated fromByulleten' Eksperimental' noi Biologii i Meditsiny, Vol. 128, No. 8, pp. 207–209, August, 1999     

Chronic sympathetic activation promotes downregulation of β-adrenoceptor-mediated effects in the guinea pig heart independently of structural remodeling and systolic dysfunction

It is uncertain if downregulation of β-adrenoceptor signaling pathway is promoted by an enhanced adrenergic tone at an early stage of cardiac disease, or it develops secondary to detrimental local myocardial changes in advanced heart failure. We examined the integrity of β-adrenoceptor signaling pathway upon chronic infusion of isoproterenol, a β-adrenoceptor agonist, at a dose producing no structural left ventricular (LV) remodeling and systolic dysfunction. Subcutaneous isoproterenol infusion (400 μg kg⁻¹ h⁻¹ over 16 days) to guinea pigs using osmotic minipumps produced no change in cardiac weights, LV internal dimensions, myocyte cross-sectional area, extent of interstitial fibrosis, and basal contractile function. Isolated, perfused heart preparations from isoproterenol-treated guinea pigs exhibited attenuated responsiveness to acute β-adrenoceptor stimulation, as evidenced by reduced LV developed pressure increase, less shortening of LV epicardial monophasic action potential and effective refractory period, and less myocardial cyclic adenosine monophosphate elevation, in response to isoproterenol exposure, when compared to saline-treated controls. Pharmacological responses to forskolin, an activator of the adenylate cyclase catalytic subunit, were well preserved in isoproterenol-treated hearts. Downregulation of β-adrenoceptor-mediated effects upon chronic isoproterenol infusion was associated with markedly reduced stimulatory G-protein α-subunit (G_sα) myocardial expression levels. No change in expression levels of β-adrenoceptors, G-protein-coupled receptor kinase 2, inhibitory G-protein α-subunit (G_iα2), and Ca_v1.2 and K_v7.1 ion channels was determined in isoproterenol-treated hearts. We therefore conclude that sustained adrenergic overstimulation may promote downregulation of myocardial β-adrenoceptor-mediated effects independently of structural LV remodeling and systolic failure, an effect attributed to β-adrenoceptor uncoupling from adenylate cyclase due to reduced G_sα-protein expression.     

Effect of an α<Subscript>1</Subscript>-adrenergic blocker on plasticity elicited by motor training

Recovery of motor function elicited by motor training after cortical lesions in rats is enhanced by norepinephrine (neurotransmitter mediating α₁-adrenergic function) and downregulated by α₁-adrenergic antagonists. In spite of this, α₁-adrenergic antagonists are used to treat elderly patients with hypertension and prostate hyperplasia in stroke settings. The purpose of this study was to determine the effects of a single oral dose of the α₁-adrenergic antagonist prazosin on training-dependent plasticity in intact humans, a function thought to contribute to recovery of motor function after cortical lesions. We report that prazosin decreased the ability of motor training to elicit training-dependent plasticity relative to a drug-free condition. These data suggest caution when using α₁-adrenergic blockers in rehabilitative clinical settings following brain lesions. Electronic Publication     

Effects of oral creatine supplementation on maximal pedalling performance in older adults

This study was conducted to investigate the effects of giving short-term doses of creatine by mouth to healthy older male subjects, taking into account their training status. A group of 42 volunteers was divided into three: a sedentary group composed of elderly sedentary men [n=14, mean age 70.1 (SEM 1.2) years], a trained group composed of elderly trained cyclists [n=14, mean age 66.4 (SEM 1.4) years] and a young group composed of young sedentary men [n=14, mean age 26.0 (SEM 1.2) years]. In each group, double-blind randomization was carried out: one half was given creatine (3×5 g·day^–1), and the other was given an iso-nitrogenated placebo (3×10 g·day^–1). Before and after the 5 days during which the supplements were given, all subjects performed five all-out 10-s sprints separated by 60-s intervals of passive recovery, seated on a cycle ergometer. Power output, work done and heart rate data were recorded during each sprint. The elderly and the young sedentary subgroups given creatine showed significant (P<0.05) improvements in maximal power (+3.7% and +2.0%, respectively) and work done (+4.1% and +5.1%, respectively) in the subsequent tests. In contrast, no significant change in pedalling performances was observed in the trained elderly subjects. The creatine did not change the exercise and recovery heart rate profiles, in any group. Our study suggested that creatine given by mouth increases the anaerobic power and work capacity of sedentary people of different ages during maximal pedalling tasks. However, the level of physical activity seems to be a determinant of the ergogenic effect of creatine in older subjects. Electronic Publication     

Effects of exercise-training and detraining on fat cell lipolysis in men and women

Summary The effects of training and detraining on adipose tissue lipolysis were studied in 19 healthy subjects (7 women and 12 men) who were submitted to a 20-week aerobic training program. Thereafter, subjects refrained from exercise for a period of 50 days. Suprailiac fat biopsies were performed before training, after training, and at the end of the detraining period. Mean fat cell diameter and epinephrine stimulated lipolysis (ESL) were assessed on collagenase isolated fat cells. Body density through underwater weighing and skinfolds at seven different sites were also obtained. Training significantly increased ESL (P<0.05) in men but not in women. However, ESL values in men returned to pretraining values after the exercise restriction period. No significant changes in women lipolysis were observed under any conditions. Changes in lipolysis were not correlated with changes in body fatness. However, a significant correlation was observed between the increase in ESL produced by training and the subsequent decrease caused by detraining (r=–0.53;P<0.05). The present results suggest that lipolysis in fat cells from the female subjects seems to be insensitive to changes in energy expenditure. Moreover, the present study demonstrates that there are high and low responders in adipocytes ESL to variations in habitual energy expenditure.Supported by FCAC-Québec (EQ-1330, CE-29) and by Natural Siences and Engineering Research Council of Canada (G-0850, A-8150)     

Multiple interactions between the alpha<Subscript>2C</Subscript>- and beta<Subscript>1</Subscript>-adrenergic receptors influence heart failure survival

Background  

Persistent stimulation of cardiac β₁-adrenergic receptors by endogenous norepinephrine promotes heart failure progression. Polymorphisms of this gene are known to alter receptor function or expression, as are polymorphisms of the α_2C-adrenergic receptor, which regulates norepinephrine release from cardiac presynaptic nerves. The purpose of this study was to investigate possible synergistic effects of polymorphisms of these two intronless genes (ADRB1 and ADRA2C, respectively) on the risk of death/transplant in heart failure patients.     

Involvement of muscarinic cholinergic and α2-adrenergic mechanisms in growth hormone secretion during exercise in humans

The purpose of this study was to examine the role of muscarinic cholinergic and α₂-adrenergic mechanisms in growth hormone (GH) secretion during exercise in humans. The GH responses induced during moderate-intensity exercise (using a cycle ergometer at 60% maximal oxygen uptake, V˙O_2max, for 30 min) without treatment (control) and after the administration of a muscarinic cholinergic antagonist (atropine 1 mg) or after an α₂-adrenergic antagonist (yohimbine 15 mg) were compared in seven healthy men. Although, serum GH concentration had increased significantly after exercise in the control experiment [mean peak GH concentration 52.64 (SEM 18.60) ng · ml⁻¹], the increase was suppressed by the administration of either atropine [mean peak GH concentration 8.64 (SEM 7.47)  ng · ml⁻¹] or yohimbine [mean peak GH concentration 17.50 (SEM 7.89) ng · ml⁻¹]. The area under the curve of serum GH concentration against time was significantly lower in the experiment using these drugs [with atropine, mean area 458 (SEM 409) ng · ml⁻¹ · min], with yohimbine mean area 946 (SEM 435) ng · ml⁻¹ · min] than in the control experiment [mean area 3135 (SEM 1098) ng · ml⁻¹ · min]. These results suggest that muscarinic cholinergic and α₂-adrenergic mechanisms are involved in GH secretion during exercise in humans. Accepted: 9 March 2000     

Modulation of acute visceral nociception and bladder inflammation by plant triterpene, α, β-amyrin in a mouse model of cystitis: role of tachykinin NK<Subscript>1</Subscript>-receptors,and K<Superscript>+</Superscript><Subscript>ATP</Subscript> channels

Objective and design: We previously described the visceral antinociceptive property of α, β-amyrin in a mouse model of cystitis induced by cyclophosphamide (CPM). This study examined the contribution of vanilloid-1 (TRPV1), peripheral NK1 receptors to CPM-evoked nociceptive behaviors and bladder edema, and its possible modulation by α, β-amyrin. Methods: The effect of α, β-amyrin (10, 30, and 100 mg/kg, p. o.) and N-acetylcysteine (NAC) on CPM (400 mg/kg, i. p.)-induced cystitis was studied in mice. Sensory deafferentation was done by a high dose capsaicin. The parameters analysed were: CPM-evoked noxious behaviors, bladder edema, vascular permeability, and NK₁ immunoreactivity. To assess the role of K⁺ _ATP channels in α, β-amyrin effect, animals were pretreated with glibenclamide. Results: α, β-amyrin (30 and 100 mg/kg) and NAC significantly (p < 0.01) suppressed the visceral pain-related behaviors and NK₁ immunoreactivity, but bladder edema was reduced weakly. Glibenclamide reversed the effects of α, β-amyrin. Sensory deafferentation by capsaicin significantly reduced the nociceptive responses and the NK₁ immunoreactivity to noxious stimulation by CPM. Conclusions: α, β-amyrin attenuates CPM-induced visceral pain and bladder edema by mechanisms that involve, at least in part, a block either of Substance P release or its receptor function, and partly by opening K⁺ _ATP channels. Received 13 February 2007; returned for revision 13 April 2007; accepted by G. Geisslinger 14 May 2007     

Differential down-regulation of β3-adrenergic receptor mRNA and signal transduction by cold exposure in brown adipose tissue of young and senescent rats

 Diminished β₃ adrenergic-stimulated thermogenesis in brown adipose tissue (BAT) in senescent rats is restored by a short period of cold exposure. To investigate if the mechanism of this restoration involves preferential up-regulation of β₃-adrenergic signal transduction in senescent compared with young rats, we examined the steady state levels of β₃-adrenergic receptor messenger ribonucleic acid (β₃AR mRNA) and the dose/response relationship for the activation of adenylyl cyclase by the selective β₃-adrenergic agonist, BRL 37344 in BAT membranes from young and senescent rats with or without 2 days cold exposure at 8 °C. β₃AR mRNA declined by 30% with age and by 60% with cold exposure in young rats. In contrast, cold exposure did not down-regulate β₃AR mRNA in senescent rats. BRL 37344-stimulated adenylyl cyclase activity was 38% less in senescent rats and was desensitized by cold exposure decreasing maximum stimulation by 78% and increasing, the dissociation constant 2.5-fold. Cold exposure decreased BRL 37344-stimulated adenylyl cyclase activity to a lesser extent in senescent rats (50%), such that the activity was similar in cold-exposed young and old rats. These data indicate that although cold exposure did not, as we hypothesized, up-regulate β₃-adrenergic signal transduction in the senescent rats, cold exposure preferentially down-regulated β₃AR mRNA and β₃AR-mediated adenylyl cyclase in the young compared with senescent rats. Received: 22 July 1998 / Received after revision: 9 October 1998 / Accepted: 13 October 1998     

Expression of sex steroid hormone receptors in C cell hyperplasia and medullary thyroid carcinoma

Previous studies have shown that C cells are twice as numerous in male than in female thyroids and that C cell hyperplasia (CCH) is much more frequent in men. These findings suggest regulation involving sex steroid hormones through the expression of sex steroid hormone receptors on C cells. To investigate this hypothesis, we performed an immunohistochemical study of estrogen receptors α (ERα) and β (ERβ), progesterone receptors (PR), and androgen receptors (AR) on specimens from a series of 40 patients operated on for a medullary thyroid carcinoma (MTC; n = 28; female 18, male 10) and/or CCH (n = 19; female 6, male 13). ERβ was the only receptor to be consistently expressed in CCH (100%) and MTC (96.5%), whereas ERα was never expressed. PR and AR were rarely expressed in MTC (7 and 14%, respectively). AR was expressed in half the CCH cases (53%), with a trend to male predominance (61% in men vs 33% in women). Our study is the first to describe ERβ expression in CCH. In addition, our findings suggest that CCH, and possibly MTC, might be influenced by sex steroid hormones, namely, estrogens and androgens, through the expression of ERβ and AR on C cells.     

TNF-α mRNA Expression Correlates with TGF-β mRNA Expression <Emphasis Type="Italic">In Vivo</Emphasis>

TNF-α is postulated to play a significant role in regulating TGF-β₁ expression. In lung fibroblasts, for example, TNF-α is supposed to induce TGF-β₁ via AP-1 activation. TNF-α receptor, knock-out mice are resistant to induced fibrosis and over-expression of TNF-α causes increased TGF-β₁ production in mice. Therefore, we investigated whether TNF-α mRNA levels are associated with the TGF-β₁ mRNA levels of blood leucocytes in humans. Quantitative real-time PCR of TNF-α and TGF-β₁ was performed in 118 Germans. Calculations of expression were made with the 2^−ΔΔCT method. When the investigated population was divided in two groups (TNF-α low and TNF-α high) by the median of the determined TNF-α expression, highly significant (p < 0.0001) differences of TGF-β1 mRNA expression were revealed. Additionally, dividing the investigated population into quartiles of the determined TNF-α expression showed significantly different TGF-β1 mRNA expressions. Comparing the determined CT-values of TNF-α in context with these of TGF-β1, a coefficient of determination R ² = 0.4635 was calculated. In this study we demonstrated in vivo a significant association of the relative TNF-α/B2M mRNA expression and the relative TGF-β₁/B2M mRNA expression in 118 Germans.     

Effects of temperature on human L-type cardiac Ca2+ channels expressed in Xenopus oocytes

 Temperature normally affects peak L-type Ca²⁺ channel (CaCh) current with a temperature coefficient (Q ₁₀) of between 1.8 and 3.5; in cardiomyocytes attenuating protein kinase A activity increases Q ₁₀ whilst activating it lowers Q ₁₀. We examine temperature effects using cloned human cardiac CaChs expressed in Xenopus oocytes. Peak inward currents (I _Ba) through expressed CaChs (i.e. α_1Cα₂/δ_aβ_1b) exhibited a Q ₁₀ of 5.8±0.4 when examined between 15 and 25°C. The nifedipine-sensitive I _Ba exhibited a higher Q ₁₀ of 8.7±0.5, whilst the nifedipine-insensitive I _Ba exhibited Q ₁₀ of 3.7±0.3. Current/voltage (I/V) relationships shifted to negative potentials on warming. Using instead a different CaCh β subunit isoform, β_2c, gave rise to an I _Ba similar to those expressed using β_1b. We utilized a carboxyl deletion mutant, α_1C-Δ1633, to determine the temperature sensitivity of the pore moiety in the absence of auxiliary subunits; I _Ba through this channel exhibited a Q ₁₀ of 9.3±0.3. However, the Q ₁₀ for macroscopic conductance was reduced compared to that of heteromeric channels; decreasing from 5.0 (i.e. α_1Cα₂/δ_aβ_1b) and 3.9 (i.e. α_1Cα₂/δ_aβ_2c) to 2.4 (α_1C-Δ1633). These observations differ markedly from those made in studies of cardiomyocytes, and suggest that enhanced sensitivity may depend on the membrane environment, channel assembly or other regulatory factors. Received: 16 December 1997 / Accepted: 23 February 1998     

An α3β4 subunit combination acts as a major functional nicotinic acetylcholine receptor in male rat pelvic ganglion neurons

We identified major subunits of the nicotinic acetylcholine receptor (nAChR) involved in excitatory postsynaptic potential and intracellular Ca²⁺ ([Ca²⁺] _i ) increase in the major pelvic ganglion (MPG) neurons of the male rat. ACh elicited fast inward currents in both sympathetic and parasympathetic MPG neurons. Mecamylamine, a selective antagonist for α3β4 nAChR, potently inhibited the ACh-induced currents in sympathetic and parasympathetic neurons (IC₅₀; 0.53 and 0.22 μM, respectively). Furthermore, α-conotoxin AuIB (10 μM), a new selective antagonist for α3β4 nAChR, blocked more than 80% of the ACh-induced currents in MPG neurons. Conversely, α-bungarotoxin, α-methyllycaconitine, and dihydro-β-erythroidine, known as blockers of the α7 or α4β2, did not show selective blocking effects on MPG neurons. ACh transiently increased [Ca²⁺] _i which was subsequently abolished in the extracellular Ca²⁺-free environment. Simultaneous recording of [Ca²⁺] _i and ionic currents revealed that ACh increased [Ca²⁺] _i under the conditions of the voltage-clamped (at −80 mV) state, and this resulted from the influx through nAChR itself. ACh-induced [Ca²⁺] _i increase was blocked by mecamylamine (10 μM), but was not affected by atropine (1 μM). RT-PCR analysis showed that, among subunits of nAChR, α3 and β4 were predominantly expressed in MPG. We suggest that activation of α3 and β4 nAChR subunits in MPG neurons induce fast inward currents and [Ca²⁺] _i increase, possibly mediating a major role in pelvic autonomic synaptic transmission.     

In vivo evaluation of visco-elasticity in a biological tube

The urethral response to a sudden forced dilatation was studied by a mathematical analysis of the pressure response in ten healthy women. A total of 60 dilatations, using various sizes and velocities of deformation, were performed in the high-pressure zone. The decay in pressure during relaxation proved to follow an exponential equation of the following form: Y=Z+C_αe^−t/τα+C_βe^−t/τβ, where Z is the equilibrium pressure, C_α and C_β are pressure decay, and T_α and T_β are time constants. The time constants were unaffected by the circumstances of dilatation, whereas all the other parameters were correlated to size or velocity of dilation, or both. The time constants showed a fairly high reproducibility when repeated after one weak. The method is presumed to characterise the tissue composition of the periluminal tissue layers and may prove useful in the evaluation of the normal urethal sphincter function. Furthermore, it may prove of value in the elucidation of the pathophysiology of stress urinary incontinence.     

Maximal fat oxidation rates in endurance trained and untrained women

The aim of the present study was to examine the differences in fat oxidation between endurance trained (ET) and untrained (UT) women. Eight ET and nine UT women performed a progressive cycle ergometer test until exhaustion. The rate of fat oxidation was similar at low work rates (≤90 W) but was 80–200% higher in ET subjects at 120–180 W. When related to relative exercise intensity, the fat oxidation was similar in the low-intensity domain (≤40% VO₂max), but higher in the ET subjects both at moderate intensities (45–60% VO₂max; +22% vs. UT) and at high intensities (65–80% VO₂max; +35% vs. UT). There was no difference in the maximal fat oxidation rates between the trained and untrained women. The relative exercise intensity that elicited the highest rate of fat oxidation (Fat_max) was 56 ± 3% and 53 ± 2% VO₂max in ET and UT women, respectively (NS). In biopsies from m. vastus lateralis, the activity of the enzymes citrate synthase, β-hydroxy acyl CoA dehydrogenase (HAD), and hormone sensitive lipase was higher in the ET subjects. The HAD activity correlated significantly with fat oxidation at moderate and high intensities. We conclude that the ET women had a higher fat oxidation at moderate- and high-exercise intensities both at same relative and at absolute intensity compared with the UT women. The HAD activity and fat oxidation rates were highly correlated indicating that training-induced adaptation in muscle fat oxidative capacity is an important factor for enhanced fat oxidation. Interestingly, maximal fat oxidation occurred at the same exercise intensity.     

A novel αvβ3-blocking disintegrin containing the RGD motive, DisBa-01, inhibits bFGF-induced angiogenesis and melanoma metastasis

The integrin α_vβ₃ is involved in multiple aspects of malignant cancer, including tumor angiogenesis and metastasis, which makes the receptor a key target for the development of anti-cancer therapies. We report here on the production, the characterization and the in vivo anti-angiogenic and anti-metastatic properties of a novel α_vβ₃-binding disintegrin, DisBa-01, isolated from a cDNA library made with RNAs from the venom gland of Bothrops alternatus. The 11,637 Da-recombinant monomeric form of DisBa-01 displayed an RGD motif and interacted with purified α_vβ₃ integrin in surface plasmon resonance studies, in a dose-dependent and cation sensitive manner. A three-dimensional molecular model of DisBa-01 in complex with α_vβ₃ predicted a large surface of contacts with the β₃ subunit. DisBa-01 inhibited the adhesion of α_vβ₃-expressing human microvascular endothelial cell line-1 (HMEC-1) and murine melanoma cell line B16F10 to vitronectin (IC₅₀ = 555 nM and 225 nM, respectively), and transiently inhibited their proliferation without direct cell toxicity, but did not affect the binding nor the proliferation of a human breast cancer-derived cell line (MDA-MB-231) not expressing α_vβ₃. In vivo, DisBa-01 dose-dependently decreased bFGF-induced angiogenesis in a matrigel plug assay in athymic nude mice (IC₅₀ = 83 nM). When injected intravenously to C57BL/6 mice together with B16F10 melanoma cells, DisBa-01 time- and dose-dependently inhibited lung metastasis monitored by bioluminescent imaging. We conclude that DisBa-01 is a potent new inhibitor of α_vβ₃-dependent adherence mechanisms involved in neo-vascularization and tumor metastasis processes. Oscar H. P. Ramos and Alexandre Kauskot contributed equally to this work.     

Impaired coronary function in Wistar Ottawa Karlsburg W rats—a new model of the metabolic syndrome

The metabolic syndrome is associated with an increased risk for coronary heart disease. The underlying mechanisms are not well understood. The present study was designed to investigate coronary function in Wistar Ottawa Karlsburg W (WOKW) rats, a new animal model of the metabolic syndrome. The responses of coronary artery segments from WOKW and Dark Agouti (DA) control rats of different ages to several physiological vasoconstrictors and vasodilators were measured in a small vessel wire myograph, and potential mechanisms involved in the differential responses between the two strains were investigated. WOKW showed increased α₁-adrenoceptor-mediated coronary constriction at 3 and 10 months of age, as well as seriously blunted β-adrenoceptor-mediated coronary relaxation at 16 months of age. Responses to non-adrenergic agonists were not altered in WOKW compared to DA. The α₁-adrenoceptor-mediated coronary constriction in WOKW was completely blocked by rho-kinase inhibition. Induced hyperinsulinemia did not cause increased α₁-adrenoceptor-mediated coronary constriction or impaired β-adrenoceptor-mediated coronary relaxation in DA. The association between blunted coronary β-adrenoceptor responsiveness and the metabolic syndrome was confirmed in Zucker diabetic fatty rats. We conclude that the metabolic syndrome in WOKW rats is associated with impaired coronary function due to altered adrenoceptor sensitivity. The latter may contribute to inappropriately elevated coronary tone in insulin-resistant subjects, especially when sympathetic activity to the heart is increased.